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Liosse de Velishaf
06-03-2009, 12:16 PM
If a similar thread shows up under my name, blame it on my crappy internet connection.

I'm trying to come up with a somewhat scientifically accurate "sub-light drive" for use in various sizes of spaceships. Most importantly, a sub-generation survey/exploratory vessel big enough to handle a few thousand hands for a few months. I've caved in and used a blackbox FTL drive, but I want as many other systems as possible to be close to "real". From what I understand, the main three propulsion methods for use on-board a ship are chemical rockets, ion-drives, and possibly solid-state propellant mass drivers. Solar sails would not be feasible in terms of the purpose of the ship. Ion-drives have a high specific impulse, which is great, but they have relatively low acceleration. Chemical rockets have better acceleration as far as I know, but I'd prefer to avoid large quantities of rocket fuel on the ship, for obvious reasons--it can explode. Mass drivgers require large throwable objects... which strikes me as a bit worrisome--what if you run out of them?

I'd love to get some comments and suggestions on the most efficient system.

Albedo
06-03-2009, 12:54 PM
In my opinion any civilisation that's invented faster-than-light travel will probably already have 'conventional' rockets powered by direct fusion or even antimatter. Hell, the engineering behind (fission) nuclear rockets has basically been feasible since the 1960s, they've just always been a bit too, erm, politically incorrect to use.

Project Rho (http://www.projectrho.com/rocket/index.html) is an absolutely invaluable resource on realistic rocketry in science fiction. They have a page full of drive types here (http://www.projectrho.com/rocket/rocket3c2.html).

Liosse de Velishaf
06-03-2009, 01:07 PM
Thanks alot for the link. Much of it is not useful, as its devoted to rocketry, and my ships won't technically be rockets. But the drive material, and much of the general space info looks to be very useful. I'll be up all night again... darnit.

Dommo
06-03-2009, 07:43 PM
Easy. Nuclear Pulse Propulsion.

Allows for ships of up to into the multiple million ton range to be ground launched from earth(assuming you can live with the fallout, or perfectly doable if you had clean nukes). Doable with current technology, and allows for a top speed of somewhere between 5-10% light speed depending on the type of nukes used.

If I were ever going to build a colony on mars, this is how I'd do it. I could take a ship that's larger than an aircraft carrier, and with that much space and cargo ability, it would allow for a viable founding population to start on a colony(say a thousand people), with all of the needed equipment and tools to build a community around the ship and start mining the planet for resources.

http://en.wikipedia.org/wiki/Antimatter_catalyzed_nuclear_pulse_propulsion

http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)

Lhun
06-03-2009, 08:55 PM
Personal favorite: go with an ion drive.
There is no specific reason why an ion drive has to have a very low thrust, that's just current technological limits. If you have a whole quantum-bolognium-singularity power generator on the ship, you have more than enough energy to power a really large ion accelerator. Heck, a big fusion reactor should do the trick.

The upsides are:
Very high thrust-to mass ratio. So you don't need to carry around humongous amounts of reaction mass.

Draws it's power from the ships reactor, not from a fuel/reaction mass combination like a rocket drive, so it's not volatile, and you're a lot more mass efficient. Otherwise your rocket's dead weight while in FTL and the popwer generator for FTL is dead weight while in STL.

Clean. You don't produce radioactive fallout or highly energetic EM radiation like you would with a fusion drive, a nuclear drive, or an antimatter drive. So you could use it on shuttles to land on a planet.
Landing a starship on a planet is a really bad idea anyway. The differences in designs that are optimal for deepspace flight and athmospheric takeoffs are so huge, you'd have to design something like a submacopter. Spaceships should stay in orbit while shuttles do the transfer. On the other hand, if you have a universe where the spaceships carry very thick material armor anyway, they're probably good for planetary landings without being designed for them. If it's armored enough to survive missile hits, it's probably hard enough to survive athmospheric pressure, it's own weight on the ground, and air friction as well.

Improvised weapon. Other than something with a pretty low exhaust velocity, such as a chemical rocket, or something with an unfocused blast such as an Orion drive, an ion drive is basically a big-ass particle accelerator cannon. Might not have the range of a weaponized laser, but it will definitly be the most powerful energy projector on any ship.

In terms of coolness though, the nuclear saltwater drive tops everything imo. The idea is just awesome. :D Though they are dirty as hell, and have the same efficiency disadvantages as chemical rockets.

Addendum:
Or use a photon drive if you have a power generator for your FTL that can produce ridiculous amounts of energy. All the advantages of an ion drive without even needing physical reaction mass.

Liosse de Velishaf
06-04-2009, 01:17 AM
Well, I'm against nuclear pulse propulsion, but nuclear reactors are perfectly okay for the power source. I've been considering using a NIF-like method of nuclear fusion to generate the power, probably with the direct-drive approach.

The FTL drive doesn't rely completely on enormous power generation, but it might manage to power a photon drive. But I'm still leanign towards ion. Its perfectly reasonable for the ship, which would be pretty big, to employ a very large ion drive.

Another question this brings up is what type of drive you would use for something like, say, attitude adjustments. Would small ion drives be effective, or would you be better off with chemical rockets?

Lhun
06-04-2009, 01:46 AM
Well, I'm against nuclear pulse propulsion, but nuclear reactors are perfectly okay for the power source. I've been considering using a NIF-like method of nuclear fusion to generate the power, probably with the direct-drive approach.NIF uses inertial containment fusion iirc. Check out the polywell fusion reactor design, that would scale a lot better for really big reactors.

The FTL drive doesn't rely completely on enormous power generation, but it might manage to power a photon drive. But I'm still leanign towards ion. Its perfectly reasonable for the ship, which would be pretty big, to employ a very large ion drive.Well, a photon drive big enough to provide decent thrust is pretty much the largest application of power i can think of in terms of spaceships. Plus, its usability as a weapon is so much higher than for an ion drive.
It is important to note what kind of ion drive. An ion drive that uses a static charge to accelerate the ions has a very low upper limit for thrust, so you'd have to use a really large number of those (relatively small) drives. Otoh if you use one that accelerates the ions with magnetic coils, you can have one big one, basically a kind of big particle accelerator (though the ions won't get nearly as fast as particles in a cyclotron). I think on Project Rho they're found under the heading of plasma drives not ion drives.

Another question this brings up is what type of drive you would use for something like, say, attitude adjustments. Would small ion drives be effective, or would you be better off with chemical rockets?The probably best choice are arcjets. I.e. the reaction mass is simply heated using electricity from the main generator, and the resulting expansion provides thrust. Drives of that type have lower efficiency than ion drives (need more reaction mass) but require less power and have a higher thrust to power ratio. They're also really small, so it's no problem to plaster them all over a ship as directional thrusters.
You could even use an arcjet as a main engine if you like the concept, the efficiency is not too bad, definitly better than chemical rockets, though that depends on the amount of power you can dump into the reaction mass. A possible modification would be to heat up the fuel using lasers instead of an electric arc, which makes it possible (or easier) to reach extreme temperatures, giving it very good mass to thrust ratio. Though there are some construction problems -you don't want to vaporize your heating chamber walls- which can be explained away by futuristic materials. ;)

Liosse de Velishaf
06-04-2009, 02:00 AM
Polywell might scale better, but I don't think that a net-power gain has yet been proven on a larger model. Correct me if I am wrong.

The arcjets look like a keeper, though. I was originally unsure about power-consumption, but I don't think it would be too much of a problem if I'm running an enormous ion drive anyway. Which I'm planning to have be electromagnetic, not electrostatic, which means it could be classified as a plasma thruster.

Lhun
06-04-2009, 02:21 AM
Polywell might scale better, but I don't think that a net-power gain has yet been proven on a larger model. Correct me if I am wrong.You're not wrong but that's not something that actually needs to be tested. Since volume scales as cube of the radius and surface area as square, the actual question is how small you can build fusion reactors and still get a net power output, not how big. It's the same for tokamak designs, that's what makes ITER so interesting, it's the first tokamak that actually got enough funding to be built in a reasonable size. And it looks like tokamaks need to be really big to get a net power output. In theory, polywell might still provide power at pretty small sizes, a meter or so, but there's no question you'd get a net power gain in a big one.

The arcjets look like a keeper, though. I was originally unsure about power-consumption, but I don't think it would be too much of a problem.If you can power an ion drive, you can power arcjets. After all, all that inreased efficiency of an ion drives comes from simply pumping more power into the reaction mass. That's basically the tradeoff for all drives. The more reaction mass you burn, the less power you need, and vice versa.

Liosse de Velishaf
06-04-2009, 02:31 AM
Okay. I'll tentatively assign polywell fusion reactor(s) as the power system. Though some secondary system power schemes might run better on other generators.


The ion drive would do best with a dedicated generator, right? Then you wouldn't have to run through all that power re-routing crap that's so popular in scifi, like on star trek. Not that I'm holding up Star Trek as a useful example for a hard sci-fi system.

BTW, what are the major dangers of this sort of fusion reactor?

Lhun
06-04-2009, 03:27 AM
Okay. I'll tentatively assign polywell fusion reactor(s) as the power system. Though some secondary system power schemes might run better on other generators.As far as i know, inertial containment fusion reactors are more interesting for small scale, while continuous reactor designs are better for big ones. Though polywell is not the only possibility there, tokamak is the most common example and should scale just as well. Heck, even some technobabble like weber uses (gravity containment something) would be possible. The interesting point here is that starting fusion needs quite a lot of energy, so when you want a large power output over a long period of time, it's the better choice to leave the fusion process running continuously, as opposed to fusing small pellets of fuel repeatedly.

The ion drive would do best with a dedicated generator, right? Then you wouldn't have to run through all that power re-routing crap that's so popular in scifi, like on star trek. Not that I'm holding up Star Trek as a useful example for a hard sci-fi system.That's probably best asked of an electrical engineer with some experience in high power systems, but as far as i'm aware there's no problem with having a single reactor powering everything. Though a very useful feature of such a reactor would be to have variable power output, that shouldn't be a problem for a sufficiently advanced fusion reactor. It should be able to function with various levels of fuel input, and thus various levels of energy output (and heat generation).
As far as i know, "power rerouting" is just so much bullshit anyway. You can't expect of any normal type of machinery to work better if you just pump more energy into it. Most would burn out instead. Or at least blow the fuses. ;)
And if you don't have enough electricity to run everything that's hooked up to your generator/battery/power outlet, you usually deal by simply switching one off. ;)
Though some (smaller) backup power systems can be sensible for other practical purposes. For example starting a fusion reactor would require quite a bit of energy. So you'd want at least some good capacitors to store enough for that. And you'd probably also want to be able to run life support independently, in case there's some problem with the main reactor that is still repairable but takes time.
The best choices i can see here are either capacitors, which would need to be a lot more efficient than the ones we have today, though even now there's research into that so i'd call it plausible speculation to have really good capacitors. Or, second choice, fission reactor. The advantage of those is that they're pretty foolproof, require refueling only every few dozen years, and are safe. Toshiba currently has some designs in testing that could provide 200kw for around 40 years and is about 24m in size. Would be perfect for running lifesupport on a ship whenever the fusion reactor is powered down. Though that assumes pretty big ships already, for a small sized ship, a simple fossil-fuel powered generator might be the best choice, though miniaturization might be the most reliably predictable of all technological advances.

BTW, what are the major dangers of this sort of fusion reactor?Not a lot of dangers really. Fusion reactors of all kinds do not explode. Since the big problem in getting them to run is to actually contain the fuel so that fusion can happen, any major damage will cause the fusion to fizzle out resulting in no explosion whatsoever. If whatever type of containment happens to be switched off abruptly (magnetic in tokamak, electrostatic in polywell) you still have a big ball of hot plasma though. Which will immediatly expand, burn anything in it's path, while slowly getting colder. But if you have decently thick walls around the room where the reactor is placed, it should only damage what's inside. Heck, you probably have enough radiation shielding around such a reactor anyway.
The reactor "overheating" or some such thing, as often seen on StarTrek, is also not a big problem. If it's decently designed, it should be possible to switch off the fuel to the reactor with the press of a single button, making the plasma ball go *fzzt* almost immediately. Obviously, you'd design such a system with many redundancies. Heck, you could even have a simple manual valve on the fuel lines somewhere, in case absolutely everything else fails and you need to switch it off.
Depending on what fuel is used for fusion that might be volatile. The easiest type of fusion is with hydrogen isotopes, which means hydrogen tanks on board. The cleanest type of fusion would be boron-proton, still requiring some hydrogen tanks, and lot better containment, since the plasma needs to be about 12 times as energetic for the fusion to start. "Cleanest" because it releases a lot less harmful radiaton than deuterium fusion. Which is the biggest hazard of fusion reactors, most types of fusion release neutrons and gamma rays, which will turn most materials they hit into radioactive isotopes with varying half-lifes. So after some time, pretty much every part of the machinery exposed to the fusion plasma will be highly radioactive. Not a problem in space, just chuck it in the nearest sun and replace the parts, but people definitly shouldn't walk around inside the (switched off) reactor without protection.

Liosse de Velishaf
06-04-2009, 03:40 AM
Okay. I found an earlier thread where dommo spouts off about polwell, so hopefully that will help.

On the subject of type, I've heard that tokomak is much more expensive than polywell is projected to be. Since the ship itself is built by a trade organization for survery-exploratory missions, I'd imagine that lower costs would be preferable.

I still have a lot more research to do, now that I have some good choices laid out in front of me, though. There's just so much more than the propulsion to consider in terms of what combination of systems would work best.

Vincent
06-04-2009, 03:54 AM
Easy. Nuclear Pulse Propulsion.

Allows for ships of up to into the multiple million ton range to be ground launched from earth(assuming you can live with the fallout, or perfectly doable if you had clean nukes). Doable with current technology, and allows for a top speed of somewhere between 5-10% light speed depending on the type of nukes used.

If I were ever going to build a colony on mars, this is how I'd do it. I could take a ship that's larger than an aircraft carrier, and with that much space and cargo ability, it would allow for a viable founding population to start on a colony(say a thousand people), with all of the needed equipment and tools to build a community around the ship and start mining the planet for resources.

http://en.wikipedia.org/wiki/Antimatter_catalyzed_nuclear_pulse_propulsion

http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) (http://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29)

Yeah, this method is a favorite of mine. How else can you a 10,000 ton lunker constructed like a battleship into orbit?

Liosse de Velishaf
06-04-2009, 03:58 AM
Yeah, this method is a favorite of mine. How else can you a 10,000 ton lunker constructed like a battleship into orbit?

By building it there?

Not that building it like a battleship is necessarily a good idea in the first place.

Vincent
06-04-2009, 04:25 AM
By building it there?

Not that building it like a battleship is necessarily a good idea in the first place.

Sure, but this way you get to do it with 1950s tech. Which is all sorts of retro-cool.

Liosse de Velishaf
06-04-2009, 04:30 AM
Sure, but this way you get to do it with 1950s tech. Which is all sorts of retro-cool.


We had space battleships in the 1950s?

Albedo
06-04-2009, 03:37 PM
Though that assumes pretty big ships already, for a small sized ship, a simple fossil-fuel powered generator might be the best choice, though miniaturization might be the most reliably predictable of all technological advances.

Wouldn't you need to carry massive amounts of oxidiser as well as fuel to run a fossil fuel-burning engine though? I don't see how it would offer any advantage over something like a souped-up version of the nuclear batteries we use on satellites now.

White-Tean
06-04-2009, 04:08 PM
Oh gosh, I love this thread, thanks for starting it.
It's actually been something I've been needing to research for my own SF - there may be a tiny bit set in actual space, most of it is on the planet but I'd like my main character to be able to have a better than layman understanding of space travel technology and other forms of advanced technology. I mean, I'm working with the assumption that the average person (in this futuristic novel) won't have any deeper understanding of how starship propulsion systems work - because microwaves have been around for ages and are used everyday and most people only have a vague understanding of how they work.

Anyway, much props to everyone and their vastly greater knowledge of science than myself.
:D

Lhun
06-05-2009, 01:20 AM
Wouldn't you need to carry massive amounts of oxidiser as well as fuel to run a fossil fuel-burning engine though?No different than carrying fuel for a chemical rocket. ;) If you have one of these as engine/directional thrusters you could even use the same fuel tank.

I don't see how it would offer any advantage over something like a souped-up version of the nuclear batteries we use on satellites now.Compared to nuclear batteries, chemical fuel delivers a whole lot more power in a shorter time. The only advantage nuclear batteries have is a really long lifetime. If you want a decent mass/energy ratio from nuclear fuel, you need to start a chain reaction.

dgiharris
06-05-2009, 01:33 AM
Looks like you got some decent advice on this.

Sub-light drives aren't really a problem, we have that technology already. It is merely a matter of fuel and propulsion.

The biggest problem will be acceleration and decceleration. You will need some type of inertia dampner or you will be forced to accelerate and deccelerate at a slower rate that can be tolerated by your passengers, probably 1G.

just use the simple physics equation of velocity equaling acceleration to figure out how long it woudl take you to accelerate and deccelerate to whatever % of c you are shooting for.

Another problem would be fuel. You could 'refuel' at your local neighborhood gas giant or you could develop some theoritical fuel that has a kick ass weight to thrust ratio.

And as has been mentioned above, nuclear propolusion or something equivalent would be realistic.

Mel...

Liosse de Velishaf
06-05-2009, 04:44 AM
Looks like you got some decent advice on this.

Sub-light drives aren't really a problem, we have that technology already. It is merely a matter of fuel and propulsion.

The biggest problem will be acceleration and decceleration. You will need some type of inertia dampner or you will be forced to accelerate and deccelerate at a slower rate that can be tolerated by your passengers, probably 1G.

just use the simple physics equation of velocity equaling acceleration to figure out how long it woudl take you to accelerate and deccelerate to whatever % of c you are shooting for.

Another problem would be fuel. You could 'refuel' at your local neighborhood gas giant or you could develop some theoritical fuel that has a kick ass weight to thrust ratio.

And as has been mentioned above, nuclear propolusion or something equivalent would be realistic.

Mel...

*sigh* I didn't even bother to bring up acceleration. Choosing from realistic sub-light engines is bad enough. I haven't decided whether to use an inertial dampener or not. I probably will. As much as it feels like cheating, realistic acceleration just complicates things so much, at least for the story I'm planning. This is especially true since the main two methods I'm considering for FTL require acceleration.

As an example (and everyone please feel free to correct my calculations):

In order to make a trip to Alpha Centauri in the timescales I'm planning on using, it has to be done in about four hours. If I did the math right, that's 8760*c. Ignore for the moment that that's impossible by physical laws, and you understand the issues of acceleration involved. No way that's happening without some kind of excuse, like a massive inertial dampener. I'm not interested in using an actual hyperspace system. My FTL exists at least partially in normal space.

Though, for sub-light travel, I don't imagine significant percents of c are practical anyway. I'm honestly not bothered by differences in intrasystem travel time. For my puposes, 8 minutes an AU is not all that much better than 32, or 64, or even 128. Also inter-system would have to be FTL already, to meet time constraints.

Vincent
06-05-2009, 05:18 AM
*sigh* I didn't even bother to bring up acceleration. Choosing from realistic sub-light engines is bad enough. I haven't decided whether to use an inertial dampener or not. I probably will. As much as it feels like cheating, realistic acceleration just complicates things so much, at least for the story I'm planning. This is especially true since the main two methods I'm considering for FTL require acceleration.

As an example (and everyone please feel free to correct my calculations):

In order to make a trip to Alpha Centauri in the timescales I'm planning on using, it has to be done in about four hours. If I did the math right, that's 8760*c. Ignore for the moment that that's impossible by physical laws, and you understand the issues of acceleration involved. No way that's happening without some kind of excuse, like a massive inertial dampener. I'm not interested in using an actual hyperspace system. My FTL exists at least partially in normal space.

Though, for sub-light travel, I don't imagine significant percents of c are practical anyway. I'm honestly not bothered by differences in intrasystem travel time. For my puposes, 8 minutes an AU is not all that much better than 32, or 64, or even 128. Also inter-system would have to be FTL already, to meet time constraints.




At speeds of a light year a hour, you're out of the realm of real physics anyway, and I can't imagine that mundane matters like acceleration and deceleration come into the equation. I don't think you really can ignore that. An inertial dampener larger than the known universe wouldn't help you. You're cruising into the realm of the exotic, on engines powered by weapons grade balonium, where you get to make the rules.

If that's your thing.

Liosse de Velishaf
06-05-2009, 07:08 AM
At speeds of a light year a hour, you're out of the realm of real physics anyway, and I can't imagine that mundane matters like acceleration and deceleration come into the equation. I don't think you really can ignore that. An inertial dampener larger than the known universe wouldn't help you. You're cruising into the realm of the exotic, on engines powered by weapons grade balonium, where you get to make the rules.

If that's your thing.


It was one option. The FTL was already bolognium anyway. It's the sub-light that has a chance to be somewhat realistic.

Ziljon
06-05-2009, 07:37 AM
What about the Bussard Ramjet Engine? (http://en.wikipedia.org/wiki/Bussard_ramjet)

Lhun
06-05-2009, 12:16 PM
It was one option. The FTL was already bolognium anyway. It's the sub-light that has a chance to be somewhat realistic.Imo it's important not to ignore STL physics, even if the ships are going FTL. One example why it's important for consistency are weapons. If some kind of STL drive can easily, quickly and most important of all cheaply get a vehicle to significant portions of c, every weapon used in that universe will be some kind of kinetic kill missile. And if they're big portions of c, they're doomsday weapons.

Dommo
06-05-2009, 07:18 PM
I always liked the "fold space/wormhole" types of drives. Drives that could instantaneously take you a place within a certain range, but were restricted to being used in deep space far from objects of large mass(e.g. you could only jump within about 1,000,000 miles of the earth, and it was a lot farther for bigger massed objects). I've been trying to outline a science fiction that used that idea. It made the moons of gas giants strategically valuable from a defensive perspective, and it made stealing large asteroids and putting them in orbit around the earth an important part of defensive procedures(pushed the jump horizon farther from earth).

In the universe I'm constructing, there's no artificial gravity on ships, and they are propelled by conventional means(typically nuclear pulse for large ships, powerful ion drives/nuclear rockets for smaller ones, and of course rockets). Jumping requires a "gate ship" which is a specialized ship that has enormous power generating capabilities, and tears space open to another location in space, and will open up a portal on the other side somewhere in the general vicinity of the target(within a few hundred thousand miles). This requires stupid amounts of energy and can only be held open for a few minutes at a time between week long recharges, however if a second gate ship put on the other side, and the wormhole permanently connected, the energy requirement drops to where the gate can be left open continuously.

Liosse de Velishaf
06-05-2009, 10:41 PM
Imo it's important not to ignore STL physics, even if the ships are going FTL. One example why it's important for consistency are weapons. If some kind of STL drive can easily, quickly and most important of all cheaply get a vehicle to significant portions of c, every weapon used in that universe will be some kind of kinetic kill missile. And if they're big portions of c, they're doomsday weapons.


That's why I'm trying to use a realistic method, because they won't be able to hit significant fractions of c quickly. What do you define as big portions of c?


Dommo- I've been considering two methods for FTL communication. The drive method does not allow for communication. The first is wormholes, like you are using for ships. But depending on how the wormhole physics in the story is constructed, that could be obscenely expensive, or otherwise too cheap. The other way is based on the propagation of light in a medium. Still working on that, since by our current science, this doesn't allow for transmission of information. But I thought it might be interesting to allow for an analogy to Cherenkov radiation pollution caused by FTL communication.

Dommo
06-05-2009, 10:46 PM
I'd say anything up and over .1c .

Liosse de Velishaf
06-05-2009, 11:32 PM
I'd say anything up and over .1c .


Well, I was thinking of a practical max-out of .3c. Is that too much?

Dommo
06-06-2009, 12:06 AM
Reaching that velocity is possible, but at that speed you'd definitely be approaching RKV kill velocities. Heck anything going any substantial fraction of light speed I'd say would be in that territory.

Liosse de Velishaf
06-06-2009, 12:19 AM
Okay, but I'd imagine a self-propelled manned vessel to be a very expensive RKV. The idea of an RKV being used by a resource-hungry group seems a bit off anyway. Why destroy what you're trying to get control of?

I mean, I guess it'd be no harder for a civ at that level to lauch an asteroid or something, but it would kind of defeat the purpose of the conflict anyway. RKVs are good for relatively static targets, like a planet, but they wouldn't be much use in a ship-on-ship battle. And if you destroy the other planet, what do you gain by winning the conflict?

Dommo
06-06-2009, 12:46 AM
But the reality is that some planets I don't care if I trash.

Suppose I wanted a mars like world because it had a really high concentration of rare metals. My enemy has mines and other stuff on the planet, and is defending it. It'd be easier for me to lose a ship in the fleet, and have it just smack into the world and kill everything on it. I don't care about the condition of the planet, as long as I can mine it.
On the other hand a planet like earth, I might want the world in a "pristine" state. After all I want to have a sweet place to live in the bahamas, so I'd probably not use an RKV attack.

It's sort of an economics issue. Is it cheaper and more practical to lose a small ship, or risk losing more by taking the time to carefully attack a planet?

Lhun
06-06-2009, 01:47 AM
There's no need to sacrifice people and ships. If you can reach .3c, that means one kilogram of matter has a kinetic energy of 8.1 PJ. (Petajoules, not pyjamas)
That's almost 2 Megatons. The biggest nuke ever built was the Tzar Bomba, built and tested by the USSR, it was detonated at 50 Megatons, and built to have a maximum yield of 100. To get the same energy you simply need a ship to coast at .3c towards a planet, toss a 50l bucket of wastewater overboard, and take a turn so they don't hit the planet themselves.
Note: Just newtonian calculation, the relativistic mass is actually higher, increasing the energy even more, though only by a few percent at this "low" speed.
And that's just using stuff launched overboard, if you have drives that manage to get you up to .3c, you can bet that every military in the universe is going to research ways to make that drive small enough to fit it on a missile.

On an additional note: Relativistic weapons are very impressive in terms of energy output, but because of the same thing are also easy, well simply, to defend against. Anything tossed in the way of an incoming relativistic missile will have enough relativistic velocity to turn it into a rapidly expanding ball of plasma on impact. If you intercept it far enough from the target so that the plasma has thinned to harmlessness when it reaches the target you successfully survived. (this is why armor is still useful when you have kinetic weapons)

Liosse de Velishaf
06-06-2009, 06:02 AM
But the reality is that some planets I don't care if I trash.

Suppose I wanted a mars like world because it had a really high concentration of rare metals. My enemy has mines and other stuff on the planet, and is defending it. It'd be easier for me to lose a ship in the fleet, and have it just smack into the world and kill everything on it. I don't care about the condition of the planet, as long as I can mine it.
On the other hand a planet like earth, I might want the world in a "pristine" state. After all I want to have a sweet place to live in the bahamas, so I'd probably not use an RKV attack.

It's sort of an economics issue. Is it cheaper and more practical to lose a small ship, or risk losing more by taking the time to carefully attack a planet?


On the other hand, if you go around trashing planets, even the neutrals are going to start worrying about you.

Plus, at inter-system distances, you don't even need all that powerful of a drive to take out a planet. If you have a drone, it doesn't really matter what speed you accelerate at, so you can hit max. Plus RKVs aren't big on course changes. So, as Lhun says, it's not all that hard to stop them.


But I think the real point here is that we already have drives that are powerful enough to hit high fractions of c, at least over large distances and long periods of time. So maybe a .3c drive makes it easier and qucker, but it's not as if a month or two here and there, or even a year, honestly, is going to put much of a dent in anyone's schedule. What amount of acceleration would it take to move to .3c in a year?



Getting back on topic, is that .1c figure still reasonable? I think Pluto maxes out at about 49 AU from the sun, which should make about 2.722 days to reach the sun at .1c. Which I guess really isn't all the much if we've got an FTL drive to bring us to a star system, anyway. Now, even .05c is still reasonable. I'd probably have to jiggle a few parts of my story timeline, but it's not all that bad. What would be a reasonable amount of time to make it to that speed? Because I think at one g, it's about .9684 years. Which is not exactly going to work. At 10g it's about 35.37 days. What would my acceleration have to be to make it 49au in ten days? I know it's a hell of a lot, but I don't know the exact number because I can't remember the equation.

Albedo
06-06-2009, 06:28 AM
What would be a reasonable amount of time to make it to that speed? Because I think at one g, it's about .9684 years. Which is not exactly going to work. At 10g it's about 35.37 days. What would my acceleration have to be to make it 49au in ten days? I know it's a hell of a lot, but I don't know the exact number because I can't remember the equation.

You can use this nomogram (http://www.projectrho.com/rocket/TransitNomogram03b.pdf). Get a ruler and find the values you know on the four right-hand columns (you can ignore the two on the left), and it will give you the other values, but annoyingly doesn't have max velocity. It's a bit imprecise but will give you a good general range. This chart is for a ship that's accelerating constantly, by the way.

Liosse de Velishaf
06-06-2009, 06:47 AM
You can use this nomogram (http://www.projectrho.com/rocket/TransitNomogram03b.pdf). Get a ruler and find the values you know on the four right-hand columns (you can ignore the two on the left), and it will give you the other values, but annoyingly doesn't have max velocity. It's a bit imprecise but will give you a good general range. This chart is for a ship that's accelerating constantly, by the way.


Thanks.

But I think I only need the calc equations that relate velocity to acceleration. Someone tell me if this is right:

(v-vi)/a=t


Now all I should have to do is plug in the desired final velocity, and the acceleration, and I can get the time.

So:

(14989622.9m/s2-0)/9.81=t

Which makes t=17.7 days.

So then, I need to get the position after 17.7 days. That needs the equation:

s=si+vi((delta)t)+1/2a((delta)t)2

And that's how far I traveled during acceleration. It should be possible to manipulate that equation by plugging in t=5.4444 days, and s=49au, and vi=0, to find what acceleration I need to make the trip in that time. Is that right?

Except I should need only .05c. So I need to find the distance I travel while accelerating to that, and then subtract, then recalculate the time to finish once I'm up to speed.

Dommo
06-06-2009, 07:06 AM
Well if you use nuclear pulse propulsion(with no crew, just a big slug RKV) up to 10% light speed in a few hours.

Seriously, I saw a calculation once that showed how an Orion type vessel, assuming you could accelerate at 100g(no humans on board) could make it into orbit in only a handful of nuclear blasts. Plus nuclear pulse produces ungodly amounts of thrust. Using thermonuclear warheads, I could propel an asteroid with a billion tons of mass into a world if I wanted to pretty easily if I took the time to configure an orion drive.

Really there's no comparison in drives performance wise in terms of thrust and ability to reach a high speed. Plus this is the ONLY drive system that increases in efficiency as the ship gets bigger(ship can capture more of the explosion), and it's one that could be built in a year or two with today's technology. For interplanetary travel, where you actually want to land on planets(or launch from them) it'd be the only practical choice.

Liosse de Velishaf
06-06-2009, 07:27 AM
I guess it's lucky then that I'm not using today's technology.

Besides, you only need to accelerate at 1g to hit .05c in 17.7 days... assuming I did the calculations right. Surely an NPP drive isn't the only way to manage that. Not if it can hit .1c in a few hours.

So even if the drive was only 1/100th as powerful as an NPP drive, you could use it. I'm pretty set on avoiding NPP if I can help it.

Dommo
06-06-2009, 08:16 AM
Oh yeah there's probably other ways to achieve that velocity, however you're not going to do it to any "large" object very easily unless you use something like NPP that has monstrous amounts of thrust.

Try accelerating a 10,000,000 tons of freight and spaceship with an Ion drive. Sure it'll eventually get going, but it might take years and years to get up to speed. The reason I'm such a fan of NPP is simply the size of the ships. It makes it possible for interplanetary commerce to be economical. For example, suppose on earth we have space elevators. With an orion type vessel, you can literally ground launch from a nearby planet(say mars), with 5 million tons of cargo, and deliver it quickly and economically. Think round trip of like two or three months at most. No other propulsion system can perform at that level, the combination of useful cargo, and speed.

I think even with sophisticated technology, the orion would still be hugely popular, if only as a means of shuttling large cargo ships around at an interplanetary level. They might not even land, just drop into orbit and offload locally through a space elevator or something, but they can get up to speed so quickly, and move so much mass so easily, that it'd make absolute sense from an economical perspective. Plus relatively speaking they'd be cheap and easy to make.

For interstellar travel, unless you've got a lot of antimatter(which could get an orion up to 80% C), it'll be a slow go(between .08-.15 c), but still doable especially if you've got cryo abilities, or are willing to make a generational ship. Plus the orion could even be armored with 20 foot thick steel to protect it from collisions with dust/micro meteors while zipping along. I've yet to see how that problem is dealt with in other propulsion systems when the ships have to be built light weight.

Don't get me wrong. For interstellar travel Orion isn't the best platform(unless you've got anti-matter then it's the cat's ass), because it just can't quite go fast enough compared to other possible options(.1 c sucks compared to say .3 or .5 c). But at the interplanetary level it truly does appear to be the be all end all.

Dommo
06-06-2009, 08:24 AM
http://www.projectrho.com/rocket/rocket3c2.html

Good sight with some quality tables and info.

Liosse de Velishaf
06-06-2009, 08:27 AM
Well, I'm cheating for interstellar travel, anyway. :D


If there's no way around orion, there's no way around it. But rho says it's crap for orbit-to-orbit, and the uneven acceleration might work great for cargo hauling, but what about live humans?

Dommo
06-06-2009, 08:40 AM
I really, really, really wish we would do one orion launch. Kill off most of the USA nuclear arsenal, and launch an 10,000,000 ton ship, that required about 1500 megaton sized warheads. Even fallout would be minimized if we launched from an armored landing pad covered in graphite. We'd just need one launch, and it could carry enough people to set up a permanent outpost where ever we want inside the solar system.

What a good way to get rid of surplus nukes, and finally do some real exploration in a ship large enough to be a small city(perhaps 400m in diameter and about the same in height). Heck it could even LAND on other worlds, and take off again. *sigh* If only the political impossibilities could be surmounted :(

Liosse de Velishaf
06-06-2009, 08:51 AM
Maybe so, but wouldn't it be best to have more than one kind of engine? Wouldn't a constant thrust be better for changing orbits? The great thing about NPP is that it delivers an enormous amount of thrust very quickly. The problem with NPP is that it delivers an enormous amount of thrust very quickly. It's got no finesse.

Also, how would the effectiveness of the graphite platform change as the ship gets farther and farther from it?

Dommo
06-06-2009, 08:57 AM
Thermonuclear weapons are actually fairly clean. Sure there's some fallout, but the danger of the stuff is greatly mitigated when there's no other "stuff" for the explosion to scatter(dust, debris, etc.). The problems with fallout occur when blasts occur at ground level, as it causes the contamination of nearby objects. As long as the launch pad is say 10 km in diameter or so, the overall level of contaminants wouldn't be that bad. In fact thermonuclear weapons are considerably cleaner than regular atomic weaponry, so from the perspective of an orion it'd be best to use a massive ship from an environmental and practical standpoint(a suitably large orion wouldn't need a shock absorber, as the mass of the ship would be high enough to limit the acceleration, thus you'd have more useful payload).

But for changing orbits, I think for a small orion ship(few thousand tons) a secondary drive would make a lot of sense(as it'd be light enough that the small amounts of thrust could do their thing in a reasonable amount of time), but for a big boy you'd need the nukes.

Liosse de Velishaf
06-06-2009, 09:05 AM
Try attacking this from another angle. Assuming an in-space start, what would be the maximum practical size for an ion-drive ship?

Dommo
06-06-2009, 10:35 AM
Well with the maximum projected thrust of a single large ion engine being about 10,000 N, that means that it could accelerate a something with a mass just over 1,000 Kg at about 1g.

So assuming you've got a 500 ton space craft, you could accelerate at about .02 m/s^2. Extrapolating this acceleration out, you'd gain about 50 km/s of velocity per month, and peak out at about 4 months with a speed of 200 km/s(based on the exhaust velocity which is your limiter). I'd say south of 1000 tons, but probably with a manned craft, no more than a 100-200 tons(to keep acceleration and deceleration times reasonable). It might be possible to push it if you devote more of the ships mass to propulsion, but that'd be at the cost of useful cargo/passenger capacity.

Orion on the other hand has a working thrust of 400,000,000 N, and an exhaust velocity of about 10,000,000 m/s(or about 10,000 m/s or roughly 50x faster than an Ion drive). Assuming my orion ship has a mass of about 100,000 tons (or a about 100,000,000 kg), my orion would accelerate at about 4 m/s(assuming smooth thrust). Thus in a perfect world, it'd take about 1 month to reach its top speed, and another to slow to a stop. This is for a fission based orion, that could hit a peak speed of about 3% light speed(which is what the original orion design was based around, as they weren't going to be building 100,000 ton ships and a thermonuke would be a wasted with a small vessel since its pusher plate isn't big enough to harness that much of the explosion).

If you used a thermonuclear orion, lets assume that you're going to get about 10x more thrust(4 billion N), and the top speed is perhaps 10% c(or about 30,000,000 m/s or 30,000 km/s). With a ship with about 1 million tons in mass or (1 billion KG), you'd accelerate at the same 4 m/s. It'd take about 3 months to hit top speed. So with a thermonuclear orion, there's a balance that needs to be struck between useful payload(e.g. no shock absorber, just a pusher plate thus more weight can go into cargo) and the rate of acceleration(e.g. a huge ship has more useful payload but accelerates slowly, a small ship wouldn't be survivable for a human crew/might not efficiently capture the explosion).

I hope this kind of puts things into perspective. Having a high exhaust velocity is great, but if it lacks the "oomph" to push hard enough to actually get something moving then it's not that useful for human travel. Out of all known or proposed forms of propulsion, only Orion has a high top speed, and the thrust to push a monster sized ship.

Check out the bbc documentary "To mars by a-bomb". They had this sucker figured out, and if we'd had the political will(and the military wouldn't have tried to show kennedy an orbital nuclear battleship), we'd probably have surveyed the solar system by now.

Liosse de Velishaf
06-06-2009, 10:54 AM
Many thanks, Dommo. It does indeed put things into perspective. I might still be able to get away with shuttles or patrol craft running on ion drives, but something the size of my planned sub-generational ship is going to need a bit more power.

That thermonucleardrive is still a bit slower than I'd hoped. But I might be able to get around that by adjusting the drive to cargo ratio. I figure that any speed above .05c is probably impractical for interplanetary travel anyway (and my time-plots are calculated at .05c anyway), so the Orion meets my speed requirements, if not my dislike of nuclear propulusion.

Dommo
06-06-2009, 11:02 AM
I think powerful ion drives would be great for shuttles that were going to and from ships and space stations, as they're compact, and the ships are small enough that they could accelerate decently. Ion drives would also be very suitable for probes and the like. Anything small is a good candidate for an ion drive.

As I said, if you can get anti matter for your orion, then you've got, what I'd consider the "ultimate" in space propulsion. 80% C, billions of newtons of thrust, and ships that could be the size of good sized asteroids(hell you might hollow out a 10 mile wide asteroid and use it as a ship). Unfortunately antimatter appears to be impossible to acquire in the quantities needed :(

However I think that as a giant vehicle of commerce the orion can't be beat at the interplanetary level. Interstellar level, if you've got antimatter the orion is the IDEAL interstellar vessel, as it's got the cargo capacity to bring a self sufficient colony, and it can be armored enough to survive impacts with dust at relativistic velocity.

Orion just gets me worked up because as an engineer, I can see this working. They had almost all the bugs worked out and researched, they even had scale model working prototypes that used conventional explosives. Given 5 more years, we'd have had a nuclear spaceship that could have taken us wherever we wanted in the solar system. This is something that at the flick of a switch we could build, and probably do it faster than NASA currently is with the new rocket they're building, and we could go where we can't go with chemical rocket technology. What's amusing is that NASA recently reacquired all of the stuff from Dyson's son, so that they'd have the know how to build one.

Lhun
06-06-2009, 03:12 PM
So even if the drive was only 1/100th as powerful as an NPP drive, you could use it. I'm pretty set on avoiding NPP if I can help it.Nuclear salt water drives outperform orion drives in every regard (an quite dramatically) but are no cleaner. Fusion drives do the same, but are also not very clean. (secondary radioactivity as in fusion reactors is a problem. The same goes for Antimatter Beam drives. Though of the three, the NSW drive is the only one we could build today, the others need fictional (but realistic) advances in technology.
The big problem with the orion drive is that most of its effectiveness for takeoff comes from all the the free reaction mass flying around. (air). In a vacuum, you lose all that, and have to rely only on the reaction mass you built into the nukes. Still useable, but mostly because the nukes just provide ridiculous amounts of energy.
If you're going SF with your power generator however, there are a lot more options than those three. From arcjets to iondrives, many types of drives use the basic concept of feeding electricity and reaction mass to the drive, accelerating the reaction mass up with the electricity to provide thrust. These can all (or nearly all) be scaled up to provide pretty much any thrust, as long as you have a generator that provides enough electricity.
And, most efficient of all, the photon drive is scalable to any dimension (and any thrust) since it's basically just a laser. (or lightbulb). Of course, if you have a photon drive capable of accelerating ten kilotons at 1g, you'd better not point it at a planet, you might cut it in half accidentally.

Dommo
06-06-2009, 09:59 PM
According to the site I listed, the Nuclear Salt Water rocket is comparable to an orion, but the pollution problems are even MORE severe than that of an orion. As in so bad, you wouldn't even want to launch from a largely uninhabited world. An orion would be far cleaner.

It's also a bit slower than an orion as well, peaking out at like 3% light speed. A thermonuclear orion is able to go about 3x as fast, and has more thrust while ground launching, and comparable thrust while in space. The main advantage I see to this propulsion system is the political aspect of it. There's no nuclear weaponry involved, so it's a lot more likely to get built.

One thing that could be done with an orion is to loft it up a few hundred thousand feet up with say a few dozen conventional rockets then at high altitude use nukes(this would prevent like 99% of the fallout). Sure you'd lose a few low earth orbit satellites but most of the important ones are in higher orbits.

http://www.youtube.com/watch?v=V1vKMTYa40A

Lhun
06-06-2009, 11:39 PM
According to the site I listed, the Nuclear Salt Water rocket is comparable to an orion, but the pollution problems are even MORE severe than that of an orion. As in so bad, you wouldn't even want to launch from a largely uninhabited world. An orion would be far cleaner.Well, for a decent comparison i'd need an explanation of what exactly they mean by the Orion MAX in the table. The numbers for normal fission and fusion bombs aare far lower, and i couldn't find a reference of what that's supposed to be. If it's the mentioned antimatter based Orion it'll obviously go higher. Don't know how they arrived at the maximum performance for the Orion.
Also keep in mind that many drive types could increase in performance with some simple (yet totally SciFi) technological advances. One of the biggest problems for all kinds of plasma/fusion drives is that you can't run them with as much power as you want, or your drive melts down. Some bolognium can fix that easily.

One thing that could be done with an orion is to loft it up a few hundred thousand feet up with say a few dozen conventional rockets then at high altitude use nukes(this would prevent like 99% of the fallout). Sure you'd lose a few low earth orbit satellites but most of the important ones are in higher orbits.The real fallout problems are the longliving isotopes anyway, even if you start from the ground, pick a big enough desert and the hot stuff will have decayed before it gets blown out of the desert anyway.
Tho long lived but colder isotopes might not be a problem with a few orions, but having a star-faring civilization with a few ships taking off every week is a totally different scale.

Liosse de Velishaf
06-07-2009, 12:34 AM
I hate pet theories...:)

But even if the ion drive were scalable with power, how much power are we talking? And keep in mind that I'm going for as much realism as possible. Earlier, Lhun proposed using a polywell generator. How much power would that be capable of giving to the ion drive? Say I go so far as to plug in four of them per drive (however many drives I have--is one enormous one the most efficient method?). Using the most effective type of propellant, what would be the max thrust that an ion drive could give me with no more than four generators? If it's still less than the orion, then I just have to trade off environmentalism for power.

Dommo
06-07-2009, 12:57 AM
Ion drives are kind of capped in how much thrust they can put out.

You can go fast but it'll take forever. I mean current ion drives can put out like a half ounce of thrust. Seriously.

I'd say your viable options are as follows for ships already in space.

Small ships/probes(less than 500 tons) - Ion drive, laser sail, chemical rockets etc.
Medium Sized Ships(greater than 500 < 5000) - Nuclear Salt Rocket, Gas core nuclear Rocket, NERVA(prototype nuclear rocket made in the 70s).
Large Ships(5000-20000tons)- Nuclear Salt Rocket, Orion varieties.
Massive Ships(20000+tons)- Nuclear Salt Rocket, Orion

For lightweight ships, and Ion drive is pretty useful since it's cheap, compact, and can accelerate things up to speed in a reasonable amount of time. At the medium to low end of the large range of the ship scale, Nuclear Salt Rockets are the way to go as orions are horribly inefficient at lighter weights(although they accelerate like a bat out of hell). However as a ship gets bigger an orion gets more and more efficient, and for "massive" million ton vessels the orion is the way to go.

What's amusing to me is that NASA is actually researching Orion again under the name "External Pulsed Plasma Propulsion". Basically they're trying to shed all the words involving nuclear so that they can acquire funding for this technology, since it's like the one option that will for sure work for moving large amounts of stuff across the solar system.

Lhun
06-07-2009, 01:00 AM
I hate pet theories...:)

But even if the ion drive were scalable with power, how much power are we talking? And keep in mind that I'm going for as much realism as possible. Earlier, Lhun proposed using a polywell generator. How much power would that be capable of giving to the ion drive? Say I go so far as to plug in four of them per drive (however many drives I have--is one enormous one the most efficient method?).Yes. The power output of a spherical burning chamber scales faster than the requirements in material, energy etc. Works the same for fusion as for any other fuel.

Using the most effective type of propellant, what would be the max thrust that an ion drive could give me with no more than four generators? If it's still less than the orion, then I just have to trade off environmentalism for power.Since you can always use several drives that question is not answerable as such. If you have a ship running on a fusion generator and are environmentally careful i'd suggest a two-part solution. Use arcjets or chemical rockets for takeoffs/landings, and use a fusion drive for interplanetary travel. The arcjets/chemical rockets need relatively big amounts of reaction mass, but that could be picked up at a station or fuel depot before landing on a planet so you don't need to carry it around in FTL. Heck, you could mine an asteroid for it (arcjet only of course) Note that this requires a highly advanced (though theoretically possible) arcjet, not one of the low powered ones we've already built. For maximum effect you'll want to heat the reaction mass to plasma temperatures, which pretty much requires lasers to heat it up in addition to the electrical arc, and huge amounts of power.
A fusion drive is slightly cleaner than an Orion (biggest problem is secondary radioactivity) but that wouldn't be noticed in interplanetary space anyway, there's a lot of hard radiation going around there.
You can refer to the Project Rho site to get some numbers. Obviously, the thrust and weight numbers are not static, double the number of drives, double the thrust (and weight). Or use a bigger one.

Lhun
06-07-2009, 01:08 AM
Ion drives are kind of capped in how much thrust they can put out.

You can go fast but it'll take forever. I mean current ion drives can put out like a half ounce of thrust. Seriously.That depends a bit on the type of drive. For the ion drives that use static electrical charges to accelerate the ions it's a hard limit. You can increase neither the density of the plasma nor the electric charge beyond a common limit or you get arcing, which kinda ruins the concept.
But if you use magnetic fields to accelrate the plasma it becomes a question of the power/weight ratio of your generator, not so much of the drive system.

Small ships/probes(less than 500 tons) - Ion drive, laser sail, chemical rockets etc.
Medium Sized Ships(greater than 500 < 5000) - Nuclear Salt Rocket, Gas core nuclear Rocket, NERVA(prototype nuclear rocket made in the 70s).Also known as NTR. Though i'm not sure how clean a high-powered NTR drive would be. The exhaust should be pretty hot from secondary radioactivity.

Large Ships(5000-20000tons)- Nuclear Salt Rocket, Orion varieties.
Massive Ships(20000+tons)- Nuclear Salt Rocket, OrionDon't forget fusion and photon drives. While an IC fusion drive quite similar to an Orion, only with much hire "bomb" frequency and much smaller bombs, it'd be perfectly realistic (imo) to speculate on so far unkown scientific examples that result in a continuous fusion drive. Heck, who would've thought a ball-shaped fusion reactor design like the polywell was possible if you'd have asked them twenty years ago?

What's amusing to me is that NASA is actually researching Orion again under the name "External Pulsed Plasma Propulsion". Basically they're trying to shed all the words involving nuclear so that they can acquire funding for this technology, since it's like the one option that will for sure work for moving large amounts of stuff across the solar system.As long as they're starting those things out in Nevada. ;)

Dommo
06-07-2009, 01:30 AM
The heat issue is the real problem with most of the "internal" types of engines. Orion solves this by allowing the pusher plate to ablate or partially disintegrate, rather than try to shed all of the heat. Sure that means you need to periodically replace the pusher plate, but it does solve a pretty nightmarish problem.

I do think you can utilize a slew of Ion drives, to improve the acceleration issue, there's still the problem of the inherent lack of thrust. Lets say that each Ion drive has a mass of like 5 tons with the engine and propellant(lets say the each put out 10000N of thrust). That means the engine can accelerate at about 2 m/s^2, so the rest of your ship is eating into that 2 m/s^2 max acceleration. Now to be honest, all you need for max acceleration for a drive to be useful is probably a .1 m/s^2, however anything higher than that would be extremely useful for doing things like shuttling around between a ship and a space station.

So here's my idea of an Ion drive powered ship.

Total mass of about 300 tons.

Has a total amount of thrust of 40000 N, and the engines/propellent weighs in a 10 tons a pop, so there are 4 engines so 40 tons. The polywell reactor, and its fuel weigh in at 60 tons. Ship structure weighs in at 100 tons. Passengers/supplies/cargo takes up the last 100 tons.

So with a ship that big, and 40000N of thrust would require 5 GW of reactor power. (extrapolating out on wiki's design for a prototype hydrogen ion thruster, which requires 7.5 MW for every 60 N of force, for a total of 5 GW of reactor power). Let's say the top speed of the ship is about 200 km/s, and it would accelerate at about .13 m/s^2(not to shabby). It could hit its top speed in about 17 days, or so. If this ship existed it could easily do quick back and forth runs from mars and earth pretty quickly in a few months time for a there and back trip. The power requirement for the ship, while huge, is still within the realms of possibility for a large fusion based energy supply.

Dommo
06-07-2009, 01:35 AM
I definitely agree with you Lhun. I think a continuous variant on the orion is probably possible, IF they can figure out how to achieve a pure fusion weapon(which would probably be a nightmare form a proliferation standpoint, but would fix 99% of the fallout problem with the orion). If that can be done, then it might be possible to sort of create a continuous nuclear blast that just points out the tailpipe as long as you feed it fuel.

I definitely think there's a lot of room for improvement, but I think that by using what we have now as a "baseline" it sort of can show where those improvements might come from. Me and you are both aware the biggest problem with any of the fusion based systems is the heat issue. I mean seriously trying to shed the heat from a multigigawatt powerplant ain't gonna be easy. I mean look at the cooling towers at a typical nuclear plant or even at coal fired plant.

Lhun
06-07-2009, 03:07 AM
I definitely agree with you Lhun. I think a continuous variant on the orion is probably possible, IF they can figure out how to achieve a pure fusion weapon(which would probably be a nightmare form a proliferation standpoint, but would fix 99% of the fallout problem with the orion). If that can be done, then it might be possible to sort of create a continuous nuclear blast that just points out the tailpipe as long as you feed it fuel.Well, a pure fusion weapon isn't the biggest problem. You can use inertial containment fusion, the stuff that's being researched at NIF as a reactor type. It's pure fusion, but it's still pulsed. The real hard part is to have a continuous fusion reactor that's open to one side to procide the thrust. Heck, we have a hard enough time keeping the plasma confined so it doesn't fizzle out, continuously blowing it into space is definitly not going to make it easier. ;)
Hm, now that i mentioned it, i think a pulsed IC fusion drive might actually be the closest thing to what Liosse needs. Basically like an Orion drive, just that instead of a nuke, you drop a small pellet of frozen hydrogen isotopes and ignite it with a laser. Since the pellets have to be small (or you need a HUGE laser) you might have an egg-carton shaped pusher plate so you can use several small fusion explosions at once, instead of a single big one.
Depending on how far you can miniaturize the laser (and pusher plate), you should be able to make that drive pretty small.

I definitely think there's a lot of room for improvement, but I think that by using what we have now as a "baseline" it sort of can show where those improvements might come from. Me and you are both aware the biggest problem with any of the fusion based systems is the heat issue. I mean seriously trying to shed the heat from a multigigawatt powerplant ain't gonna be easy. I mean look at the cooling towers at a typical nuclear plant or even at coal fired plant.Yep. And it's not even the worst drive type. Can you imagine the heat issue with a photon drive, when you need an energy output of 300MW per single newton thrust?

Lhun
06-07-2009, 03:20 AM
I do think you can utilize a slew of Ion drives, to improve the acceleration issue, there's still the problem of the inherent lack of thrust. Lets say that each Ion drive has a mass of like 5 tons with the engine and propellant(lets say the each put out 10000N of thrust). That means the engine can accelerate at about 2 m/s^2, so the rest of your ship is eating into that 2 m/s^2 max acceleration. Now to be honest, all you need for max acceleration for a drive to be useful is probably a .1 m/s^2, however anything higher than that would be extremely useful for doing things like shuttling around between a ship and a space station.It's best to have two types of drives on a ship for exactly that purpose. Since pretty much all drives that provide decent efficiency have horrible thrust, and vice versa, you'll want one drive for close quarters maneuvering, and one for flying large distances. The drives that provide thrust and efficiency (Orion, NSWR, Fusion, AM Beam) are the kind you'll definitly not want to use around space stations.
So what's interesting for a maneuvering drive is one that has a very high thrust to mass ratio, so you can keep it as small as possible. Efficiency doesn't matter much since you're not going to accelerate beyond a few m/s. Chemical rockets are actually a good choice here for ships that use drives that don't require large power generators (Orion, AM Beam etc.). If you have a big reactor anyway, you can use an arcjet instead, which works with almost anything as fuel, unlike a chemical rocket. Provided you have enough spare power to make it more effective than a chemical rocket of course. Oh, and it definitly needs to be of a type that can either freely change thrust, or be quickly switched on and off.
While a NSWR would be the ultimate in maneuvering drives, i'd not use it for the same reason i'd not use an Orion drive close to a station or other ships.

So with a ship that big, and 40000N of thrust would require 5 GW of reactor power. (extrapolating out on wiki's design for a prototype hydrogen ion thruster, which requires 7.5 MW for every 60 N of force, for a total of 5 GW of reactor power). Let's say the top speed of the ship is about 200 km/s, and it would accelerate at about .13 m/s^2(not to shabby). It could hit its top speed in about 17 days, or so. If this ship existed it could easily do quick back and forth runs from mars and earth pretty quickly in a few months time for a there and back trip. The power requirement for the ship, while huge, is still within the realms of possibility for a large fusion based energy supply.Hm, i'm not going to do it tonight, but i think i'll try to calculate what the possible power output of a fusion reactor of a given size could be. I mean, i have a vague idea of the dimension involved, but i have actually never read or calculated some hard numbers on how realistic all those thousand of SF stories that use them are. Heck, how much fuel can you actually burn in a fusion reactor of a given size?

benbradley
06-07-2009, 03:23 AM
...
Depending on how far you can miniaturize the laser (and pusher plate), you should be able to make that drive pretty small.
Yep. And it's not even the worst drive type. Can you imagine the heat issue with a photon drive, when you need and energy output of 300MW per single newton thrust?
Speaking of lasers and photon drives... LEDs and solid-state lasers (as in the now-ubiquitous red handheld laser pointer) have been dropping in cost and (somewhat) rising in efficiency (they're predicted to outstrip CF bulbs for general lighting soon). I can imagine (for sufficient amounts of imagination) big square silicon wafers that take in electric power and output monocromatic light, all at a normal to the wafer, at something like 99.999 percent efficiency. They're run at whatever power they can be run at and still remove the generated heat...

Dommo
06-07-2009, 05:38 AM
Actually what you're talking about Lhun, is called the "mini-mag" orion. It uses tritium or deuterium and does just what you said. It's not as efficient or powerful as the thermonuclear orion (e.g. can't ground launch with it and has a lower top speed), but it'd still be a great power plant for an interplanetary ship(albeit you couldn't easily move the kind of loads you can move with a conventional orion).

I've been wondering about the fusion reactor fuel burn rates as well, as it works on an entirely different principle than other energy production methods.

Lhun
06-07-2009, 07:24 AM
Actually what you're talking about Lhun, is called the "mini-mag" orion. It uses tritium or deuterium and does just what you said. It's not as efficient or powerful as the thermonuclear orion (e.g. can't ground launch with it and has a lower top speed), but it'd still be a great power plant for an interplanetary ship(albeit you couldn't easily move the kind of loads you can move with a conventional orion).Well i know them as Inertial Containment Fusion drives, named just like the IC-Fusion reactors based on the same principle. Heh, here project rho agrees with my naming convention. (i disagree with them on the naming of ion drives)
Anyway, i see no reason why an IC-Fusion drive should be less powerful than an Orion, and it should definitly have a higher efficiency. On Project Rho's list, the IC-MAX has about ten times the power of the Orion MAX, though it is still a mystery to me how they arrived at those maximum numbers. (Both for the IC-Fusion and the Orion drive)
Not to mention the figure of 1000 tons drive mass for the IC-Fusion drive. What the heck is all that? The mass for the laser? And likewise the 8 tons drive mass for the Orion. Can't be the pusher plate, much to low, and if it's not the pusher plate, what else is there actually having mass in an orion drive?

I've been wondering about the fusion reactor fuel burn rates as well, as it works on an entirely different principle than other energy production methods.I've also heard some pretty interesting, if extremely theoretical, ideas of how to extract energy from the fusion. Thermal is the easiest way to go of course, but it might be possible to design a Tokamak so you siphon charged particles to create a charge, and apparently a Polywell might generate a rotating magnetic field which could be used directly to turn a generator. Anyway, even standard turbines have efficiencies in the double digits, so after figuring out the rate a fusion generator burns fuel, simply subtracting one order of magnitude should give a ballpark estimate of electricity output.

Heck, i went and searched a little anyway. (5 am here)
I found a back-of-envelope calculation at polywelltalk that arrives at a reactor radius of about 3m for a 10GW reactor. And checking up in ITER (i remembered seeing figures there) gives a planned fuel consumption of about 2.5 tones of fuel for running a 10GW reactor for one year.
Now, both of those figures were just grabbed from (albeit trustworthy) websites, so i have no idea how the numbers are calculated. For example the fuel consumption figure might be for 24/7 operation for a year, or it might include a realistic downtime similar to other power plants. But it gives a pretty good overall estimate of the matter.
Conclusion one: fuel weight for fusion reactors is totally trivial. More likely than not, the actual reactor on the spaceship will weigh more than the fuel, unless it's designed for seriously long term missions.
Conclusion two: Given the necessary materials, a fusion reactor can produe nearly unlimited power. Probably not enough to power a 1G photon drive, (i'd have to make some calculations on that and i'm still lazy) but powering arcjets or plasma/ion drives is not even remotely a problem.
The net power output of the reactor scales with something like the square of the radius, so the only serious limit is how powerful you can make the reactor before the materials melt. Some speculation here wouldn't be unrealistic. Anyway, 10m radius reactor is almost 100GW, which is a serious amount of power. Enough to power a whole country pretty much.
If you have no materials that can survive such energies, you can use several reactors if necessary, however that obviously doesn't scale as well. Though two or three 10GW reactors wouldn't be unrealistic, assuming that's the highest you can go without meltdown. Afaik, a 10GW reactor would be possible with todays material technology.

Dommo
06-07-2009, 10:09 AM
The 8 ton drive mass is the mass of a nuclear warhead. Remember the orion has no real "engine" per se, it simply uses the mass of a nuclear warhead surrounded by plastic. The nuke itself probably weighs about a half ton, but the rest of the bomb is just "filler" used to actually "push" on the plate.

Some of the numbers I wonder about. They used to have links to some of the NASA papers and whatnot linked on the site. For some reason a bunch of the papers are down, but I think the orion stuff is a bit mixed up. The thrust is about 1/10th of what should be (should be in the range of 4 billion N) from what I can remember on the paper I saw with a bomb that was like 1 megaton in size.

That's pretty incredible the power output to fuel use of the a fusion reactor. That'd definitely solve the world's energy issues in a heartbeat. But I agree with you about the problem of heat. With that kind of energy density you're looking at INSANE amounts of heat being produced. I have no idea how anyone could deal with it in a compact ship. I also agree with you on the 10 GW reactor being possible. I figure a large fission based plant can produce probably 5 GW, so an a reactor based on an energy source far more dense and powerful should definitely be capable of achieving better once the engineering issues are worked out.

Liosse de Velishaf
06-15-2009, 11:23 PM
So assuming we're using a 400,000,000N Orion drive, what would be the best method for the crew to handle the acceleration, and what would the best acceleration to use be?

Dommo
06-16-2009, 04:02 AM
The ideal way for the crew to handle the acceleration is to have a massive ship. Simply make a ship that's so large that the nuke simply can't accelerate it rapidly, that solves the problem in the most elegant manner. However for something not in the million ton range, you're looking at a giant shock absorber. That way you can reduce the acceleration to something that's a bit more survivable.

Pthom
06-16-2009, 04:23 AM
As Larry Niven wrote about riding in an Orion-type craft in the book "Footfall": Wham! Wham! Wham!

I think I will wait for the continuous fusion drive.

Marcus
06-16-2009, 04:55 AM
Sterling motors are a long lost jewel of simplicity that has all but been forgotten.

http://en.wikipedia.org/wiki/Stirling_engine

Isotope H3 is only available on earth from the decay of nuclear weapons, but our Moon has collected Millions/billions of tons of it over the lifetime of our solar system from the solar wind blown by our sun.

H3 gives a viable way to produce a Fusion reaction without the harsh lose of efficiency caused by Neutron expulsion. We've had fusion reactions in labs, but its not viable at this point without H3 because the chamber walls of the reactor are constantly bombarded by neutron expulsion, requiring the reactor chamber walls to be changed out at exorbitant costs... H3 solves this hiccup, and guess what, our moon has a HUGE supply of the stuff.
http://www.betterhumans.com/blogs/dry_observer/archive/2006/01/28/4193.aspx


Mr Sevastyanov predicted that nuclear reactors capable of running on helium 3 would soon be developed and said that just one ton of the isotope would generate as much energy as 14 million tons of oil.

Use the H3 for the fusion, use the sterling for the heat dissipater, attach an energy based drive to the ship and there you go. ION drives are ridiculously simple.

My ideal energy based star ship engine for us humans (sub light speed) would be a "Maser" type system that focuses magnetic energy to push or pull against planetary bodies.. the problem with this is shielding the occupants from the Astronomical magnetic forces the Maser drive would require... Magnetic fields dissipate at a rate equal to the inverse square law, which makes them Less than ideal for a drive system... but if you have energy and a shielding system, it would be perfectly clean, I just wouldn't advise being under the ship when it tries to land on the surface of a planet! If your wondering what kind of magnetic power i'm talking about.. current CAT scan machines have magnets powerful enough that the Techs that work on them have a time limit they can spend inside the field, otherwise it makes you "drunk"... The Maser drive would be billions of times more powerful than this... But then again I guess the Maser drive isn't really practical today...
For intra stellar movement using inertia i recommend the Lofstrom Loop to get off the solar bodies surface.
http://en.wikipedia.org/wiki/Launch_loop
Once its in orbit using the Lofstrom loop it will get picked up by this device.
http://www.astronomyandspace.com/businesstechnology/technology/tether_tech_030618-1.html
Both of the above named objects are completely build able using todays technology. We just have to invest in the infrastructure to get it done. :)

This is a subject that i've studied up on over the past couple years and I know of a few other far flung ideas, but these are the ones that are grounded with good science... maybe with the exception of the Maser drive, thats my baby lol :)
Maser- The maser is based on the principle of stimulated emission proposed by Albert Einstein in 1917. When atoms have been put into an excited energy state, they can amplify radiation at the proper frequency. By putting such an amplifying medium in a resonant cavity, feedback is created that can produce coherent radiation.
http://en.wikipedia.org/wiki/Maser

hope everyone enjoys the links, this is one subject I feel halfway informed on.

Pthom
06-16-2009, 05:07 AM
Marcus, don't you mean He3 (an isotope of Helium) and not H3 (which would be, I suppose, some strange molecule of Hydrogen)?

Marcus
06-16-2009, 05:25 AM
Marcus, don't you mean He3 (an isotope of Helium) and not H3 (which would be, I suppose, some strange molecule of Hydrogen)?

sorry my eyes are bleeding right now... you got it, I wasn't really being careful with the post. Time for more eye drops.. sigh... thanks for pointing that out.. I'm sure there is another error or two... I'm waiting for someone to start a semantics fit over the use of the word Maser lol :)

Liosse de Velishaf
06-16-2009, 07:19 AM
The ideal way for the crew to handle the acceleration is to have a massive ship. Simply make a ship that's so large that the nuke simply can't accelerate it rapidly, that solves the problem in the most elegant manner. However for something not in the million ton range, you're looking at a giant shock absorber. That way you can reduce the acceleration to something that's a bit more survivable.

No, it's a ginormous ship. So no absorber. But at a useful rate of acceleration, what would be the best position for crew members to be in? Are we talking strap in, current astronaut style, what?

Marcus
06-16-2009, 09:15 AM
ok so are you avoiding the inertial compensator because you don't know a viable way to explain it, or are you avoiding it because the tech level isn't there?

If you want i can PM you a way to explain a fairly simple way to make an inertial compensator explanation that would be Very believable... and wouldn't be Super high tech either...

In my work the setting is only like 30 years in the future and they just don't have the understanding on M theory to make it work yet.. but the advent of the power source i introduced makes a LOT of things possible.

Liosse de Velishaf
06-16-2009, 11:28 AM
ok so are you avoiding the inertial compensator because you don't know a viable way to explain it, or are you avoiding it because the tech level isn't there?

If you want i can PM you a way to explain a fairly simple way to make an inertial compensator explanation that would be Very believable... and wouldn't be Super high tech either...

In my work the setting is only like 30 years in the future and they just don't have the understanding on M theory to make it work yet.. but the advent of the power source i introduced makes a LOT of things possible.

I'm avoiding it because as far as I am aware, there is not any sort of realistic explanation for how one could be made. ICs are pure handwavery. If there was a real method, the tech level in my story would certainly be high enough to make use of it.

Dommo
06-16-2009, 03:47 PM
No seriously a shock absorber would work just fine. Think something analogous to the shocks on a car, only scaled up greatly. They basically "smooth" out the bumps from the nukes going off, and they should work even in relatively small orions(like 500 tons) just fine.

Marcus
06-16-2009, 10:26 PM
I'm avoiding it because as far as I am aware, there is not any sort of realistic explanation for how one could be made. ICs are pure handwavery. If there was a real method, the tech level in my story would certainly be high enough to make use of it.

I've spent the last two years self educating myself on the ins and outs and particulars of modern day physics. Without getting into specific equations, a simple grasp on M theory, which I consider myself to have, can magic up a device that uses a high frequency (really high freq, think on the order of a hundred million billion) to interact with atoms on the quantum level. (you'll need at least a basic quantum computer i'd think as well) This interaction can be metaphored as someone in a swimming pool who has an ultrasound machine that is tuned to their bodies physical density pushing them from one end of the pool to the other and not affecting the other objects in the pool that arn't equal to the swimmers physical density. This is the simplest and closest compensator you can use a technical description for. the device on the ship pushes their bodies mass the same way you make a wave bigger in the bathtub. Resonant frequency is a wonderful thing and can be used to do a lot in the world of science fiction IMHO... This would probably be uncomfortable to have to sit through for a really long period, but if you incorporated a device such as this, you can magic up your own description for the specific limitations it has. Your not going to be able to go from zero inertia to .3c in minutes or days, but you SHOULD be able to maintain a high acceleration that would otherwise not be possible.

hope you think this is at least somewhat thought provoking. :Shrug:

Liosse de Velishaf
06-17-2009, 12:32 AM
Marcus, I'll look into it.

Dommo, your post said that the most elegant way to handle the issue was to have a massive ship. The ship is very massive. Would a shock absorber really provide all that much extra benefit?

Marcus
06-17-2009, 01:33 AM
the shock absorber was only for ships that where smaller than a certain size. he gave examples of ton ranges... anything smaller than the largest class would need some kind of shock absorber i believe he was meaning to get across. the reason he brought up the absorber again is because there was a doubter of the effectiveness of the absorber after his initial post. At least this is my own estimate of the way the discussion went :)

Dommo
06-17-2009, 02:03 AM
Marcus basically hits it on the head.

If you've got a massive ship it's so big that the acceleration experienced by it absorbing a nuclear blast isn't that large, and in fact could be relatively small, and thus not harmful to people. In the same sense, if you've got a little ship and it's experiencing the force from a nuke, if you don't have a way to "slow down" the collision(think how a crumple zone on a car works), the ship would have some serious acceleration(like turn a human to mush acceleration).

This does create some interesting options in terms of how you could launch an orion, or the capabilities of an unmanned system. I saw a calculation once where a 5000 ton unmanned orion could literally be launched into space with only *3* nuclear bombs. However to do so would require the ship to experience about 100g. It also suggests that an unmanned orion ship could accelerate up to 10% lightspeed pretty damn quick(possibly inside of a day). Compared to a manned ship, that might have a maximum continuous acceleration of say 3 g's, and might take a few weeks to get up to speed.

blacbird
06-17-2009, 02:10 AM
Compared to a manned ship, that might have a maximum continuous acceleration of say 3 g's, and might take a few weeks to get up to speed.

A manned ship with a few weeks of sustained 3g acceleration would be pretty tough on the crew, quite possibly fatal. Think of weighing, say 500 pounds compared to what you weigh now, and, in particular, triple the weight of your head. The strain on the vascular and respiratory systems would be pretty severe, methinks. The maximum long-term sustained g-force a human could reasonably endure and still be functional would be an interesting thing to research. I'm guessing you wouldn't want anything beyond about 1.5g.

caw

Marcus
06-17-2009, 02:58 AM
You can always make your ships accelerate at a rate that humans can't handle and send your crew ahead at a slower accelerate only to meet up at some point a week or two into the voyage. This pretty much negates having to deal with acceleration rates except for the purpose built crew-shot ships.

Pthom
06-17-2009, 03:08 AM
Okay, what's the smallest practical thermonuclear device for use in such a ship? Is it possible to have very many of them and drop them into the combustion chamber rapidly enough that the shock is perceived by the crew and cargo as merely a vibration rather than the wham, wham, wham Niven describes?

benbradley
06-17-2009, 03:35 AM
Okay, what's the smallest practical thermonuclear device for use in such a ship? Is it possible to have very many of them and drop them into the combustion chamber rapidly enough that the shock is perceived by the crew and cargo as merely a vibration rather than the wham, wham, wham Niven describes?
Now that you mention that, having a pusher/deflection plate (instead of having the bombs explode on the end of the ship) would be less of a problem with smaller bombs exploded at a higher frequency. Instead of a spring between the deflector and the main ship, one could use a hydraulic "shock absorber," a piston/cylinder with computer-controlled fluid pressure that would very effectively turn the varying accelerations of the bomb pulses into a smooth acceleration for the cargo and passengers. A higher frequency means a lower stroke distance is needed to convert the bursts into smooth acceleration.

Without such a mechanism, a ship that varies from zero to two+ gees is going to be bad for the crew, even if just on an emotional response level, no matter whether that variation is at 1 Hz or 500 Hz or something in between.

benbradley
06-17-2009, 03:42 AM
A manned ship with a few weeks of sustained 3g acceleration would be pretty tough on the crew, quite possibly fatal. Think of weighing, say 500 pounds compared to what you weigh now, and, in particular, triple the weight of your head. The strain on the vascular and respiratory systems would be pretty severe, methinks. The maximum long-term sustained g-force a human could reasonably endure and still be functional would be an interesting thing to research. I'm guessing you wouldn't want anything beyond about 1.5g.

caw
One thing that would surely help is being immersed in water (except for one's face, of course). I recall a story (pretty sure it was by Heinlein but I don't recall the story name) where this very thing was done to get a manned ship out somewhere on a high-G emergency flight.

blacbird
06-17-2009, 03:44 AM
You can always make your ships accelerate at a rate that humans can't handle and send your crew ahead at a slower accelerate only to meet up at some point a week or two into the voyage. This pretty much negates having to deal with acceleration rates except for the purpose built crew-shot ships.

Not clear to me what energy advantage this would have. Eventually, when the two meet, you've accelerated the same amount of mass to the same velocity, which by any laws of physics I'm aware of means you've expended exactly the same amount of energy.

caw

Lhun
06-17-2009, 03:44 AM
Wow, this thread got far. Too bad i don't have the time to always hang out here.

Okay, what's the smallest practical thermonuclear device for use in such a ship? Is it possible to have very many of them and drop them into the combustion chamber rapidly enough that the shock is perceived by the crew and cargo as merely a vibration rather than the wham, wham, wham Niven describes?There is no size limit for the design, except the smalles practical nuke you can build. If you want to go very small, you can use an IC-Fusion drive, which is for most practical purposes the same as an Orion, except the technical details of the drive. With an IC-Fusion drive you can theoretically go as small as a bomb consisting of two atoms of fuel. ;)
The smallest for the fission (or fission/fusion) orion would be the smallest amount of fissionable material you can still compress down to a supercritical mass.

Marcus
06-17-2009, 04:08 AM
Not clear to me what energy advantage this would have. Eventually, when the two meet, you've accelerated the same amount of mass to the same velocity, which by any laws of physics I'm aware of means you've expended exactly the same amount of energy.

caw

Its easier to move a smaller ship, smaller mass, less energy to get it up to speed. If you have a Very Large ship that you want to get up to speed faster than the crew can handle, then you send the crew ahead a week in advance at a slower acceleration rate. You get the Very Large ship up to speed with little or no limitation on how fast you can get it up to speed.

The advantage is it makes the Very large ship a Whole lot simpler to accelerate if you Don't have to worry about the crew. The smaller Crew-Shot ship can use Ion or chemical rockets, and the smaller ship is the one that gains the sophistication of having to maintain the crews LIVES :) Once you get to where your going you put the crew back into the crew-shot ship and then you can start decelerating the larger ship later and harder at the other end..

Its like riding a top fuel dragster. The thing accelerates at several G-s, but thats not the real problem, its the Decelerating that is the killer on the eye-balls. Theres been more than one nitro methane driver who had to retire because his retinas were become detached, and that didn't happen from the acceleration, it happened from the deceleration.

think of it like this. It takes your ship a total time of one week to make a voyage once its up to speed. But you have the limitation of the CREW for how hard you can accelerate and decelerate to get up to and down from cruise speed. That one week trip takes three weeks to accomplish because you have to accelerate for a full week with the crew on board, then another week on the other end with the crew on board, suddenly you have a one week trip that takes three weeks.
Now picture a Very Large ship that accelerates up to speed in one day, ship picks up the crew, one week later gets to its destination, they evacuate for the deceleration, and the ship decelerates in another day. Then you have a ship that can make the full one way trip in 9 days instead of 21 days. That's worth something in my opinion.

Its not an energy advantage, its an economics advantage :)

Pthom
06-17-2009, 04:08 AM
The smallest for the fission (or fission/fusion) orion would be the smallest amount of fissionable material you can still compress down to a supercritical mass.
Which is ... ?

Lhun
06-17-2009, 04:19 AM
Which is ... ?Dependent on techonology level. A.K.A. I have no idea. I think the smallest nuke we can produce currently is something around half a kiloton.

Edit: Googled it and holy cow was i wrong on that.
http://en.wikipedia.org/wiki/Davy_Crockett_(nuclear_device)
That thing (while still suicidal) has a yield of 10-20 tons.

Lhun
06-17-2009, 04:32 AM
Its easier to move a smaller ship, smaller mass, less energy to get it up to speed. If you have a Very Large ship that you want to get up to speed faster than the crew can handle, then you send the crew ahead a week in advance at a slower acceleration rate. You get the Very Large ship up to speed with little or no limitation on how fast you can get it up to speed.Not in space, with no friction, there's no difference between accelerating a small or a big ship. Since the biggest problem with interstellar travel is actually the crew, this is not a very useful plan. If you're going to seperate crew and cargo, it makes a lot more sense to send the cargo ahead, since you can send that at a lower speed as it doesn't get bored if it takes 3 years to reach the destination.
Having the crew actually travel longer than the cargo is exactly the wrong way around.

Its like riding a top fuel dragster. The thing accelerates at several G-s, but thats not the real problem, its the Decelerating that is the killer on the eye-balls. Theres been more than one nitro methane driver who had to retire because his retinas were become detached, and that didn't happen from the acceleration, it happened from the deceleration.There's no difference between acceleration and deceleration in space. If you have a drive capable of that, you accelerate half the way, turn around, and decelerate until you're at your destination, if not, you have some time of coasting at a fixed velocity in between.

Now picture a Very Large ship that accelerates up to speed in one day, ship picks up the crew, one week later gets to its destination, they evacuate for the deceleration, and the ship decelerates in another day. Then you have a ship that can make the full one way trip in 9 days instead of 21 days. That's worth something in my opinion.Unfortunately what you're missing here is that there's no real point in ever having the crew get aboard the ship if sending cargo fast is your point. And if sending the crew is the point, using a single ship is more efficient.

Its not an energy advantage, its an economics advantage :)Not really.

Marcus
06-17-2009, 04:32 AM
Okay, what's the smallest practical thermonuclear device for use in such a ship? Is it possible to have very many of them and drop them into the combustion chamber rapidly enough that the shock is perceived by the crew and cargo as merely a vibration rather than the wham, wham, wham Niven describes?

If you had a viable fusion system, you could use powerful magnetics to contain the fusion, and at one end of the fusion bottle you have a magnetic path for the fusion material to be shoot out the rear of the ship. Instead of a traditional nozzle that we understand on our mighty rockets, we replace this with a magnetic containment sphere, with a pinhole escape path that the ship used for propulsion. There might even be a way to capture the spent fuel after it cools down and recycle it. The configuration for this would put the engine at the front a looooong ship and the exhaust particals are collected as they travel a magnetic path once they cool down and lose their thrust giving force.

This system would give you continuous thrust, but the problem is that your going to be burning fuel in the same way rockets do. So your going to have to refuel pretty much at every stop depending on how efficiently your Continuous Fusion Drive is.

benbradley
06-17-2009, 04:41 AM
If you had a viable fusion system, you could use powerful magnetics to contain the fusion, and at one end of the fusion bottle you have a magnetic path for the fusion material to be shoot out the rear of the ship. Instead of a traditional nozzle that we understand on our mighty rockets, we replace this with a magnetic containment sphere, with a pinhole escape path that the ship used for propulsion. There might even be a way to capture the spent fuel after it cools down and recycle it. The configuration for this would put the engine at the front a looooong ship and the exhaust particals are collected as they travel a magnetic path once they cool down and lose their thrust giving force.
It's not the temperature that gives the thrust (though temperature may be indirectly involved in providing it), it's the velocity, and in space there's nothing to slow down the exhaust particles. Collecting them would decelerate the ship as much as sending them out the exhaust accelerates it.

Lhun
06-17-2009, 04:56 AM
<snip>The configuration for this would put the engine at the front a looooong ship and the exhaust particals are collected as they travel a magnetic path once they cool down and lose their thrust giving force.No. Doesn't work for the same reason you cannot attach a magnet at the end of a long stick and pull yourself with it. If you catch the particles, you get their impulse, which will negate all impulse you gained when releasing them. All that setup manages is a rocket that pulls itself apart.

This system would give you continuous thrust, but the problem is that your going to be burning fuel in the same way rockets do. So your going to have to refuel pretty much at every stop depending on how efficiently your Continuous Fusion Drive is.Any drive that is even remotely realistic has to have reaction mass. A reactionless drive violates at least as many laws as an FTL drive does.

Dommo
06-17-2009, 05:08 AM
I agree Lhun.

The main reason you'd consider using immense amounts of acceleration is when it improves the economics of the ship, or where you don't have a crew.

For example, suppose I'm launching an orion from the ground. I want to minimize fallout, so I choose to leave the ship unmanned, and I make sure the ship/cargo is designed to handle high g-loads. This allows me to launch an orion with far fewer bombs, which is both good for the environment, and good for ship economy(e.g. I've got more fuel to use to get where I want to go). Then I simply ship up the crew separately and have them board the ship while in orbit.

Another example is that I can get far more useful cargo space if I don't use a shock absorber system because that system has a lot of mass. This means that in a ship that requires 15% of its mass to be shock absorbing in order to reduce the acceleration to a tolerable level for humans, perhaps I can reduce that to 5% of its mass. In a ship that weighs 50,000 tons, that nets me 5000 tons more cargo I can take a long. Not to mention that having rapid acceleration means I can move things farther, faster(gets up to speed quicker), and cheaper(fewer bombs needed for acceleration and deceleration), which in terms of interplanetary commerce would be a big deal.

Marcus
06-17-2009, 05:11 AM
Not in space, with no friction, there's no difference between accelerating a small or a big ship.

I want to give your post credit, but the opening sentence is just flat wrong. In my post I specifically talk about MASS. Your saying that to accelerate a golf ball in space is the same as accelerating a planet? Accelerating a 5 ton ship IS easier than accelerating a million ton ship.


Since the biggest problem with interstellar travel is actually the crew, this is not a very useful plan. If you're going to seperate crew and cargo, it makes a lot more sense to send the cargo ahead, since you can send that at a lower speed as it doesn't get bored if it takes 3 years to reach the destination.
Having the crew actually travel longer than the cargo is exactly the wrong way around.

I'm talking about Simplifying a Very large ship so that it doesn't have to deal with keeping the crew alive under Heavy acceleration to get up to cruise speed. In the OPers posts he talks about not having an inertial compensator. If your objective is JUST to get a crew somewhere, then awlrighty then, your point here is valid. However, being able to get your Very Large ship up to speed without worrying about the crew Does have advantages. We are not talking about a Honorverse here, we are talking about people traveling extra solar with very specific limitations.



no difference between acceleration and deceleration in space. If you have a drive capable of that, you accelerate half the way, turn around, and decelerate until you're at your destination, if not, you have some time of coasting at a fixed velocity in between.

I'm familiar with this description, I think your picturing Weber's ships that have been described in Great detail in his books. Nothing wrong with this. Hey, you are Right to point out there is no difference in the two in space, its only a change in inertia depending on the direction of the change. I was going off the fact that I'm thinking about the Mass of a Very Large ship and somewhat limited tech.


what you're missing here is that there's no real point in ever having the crew get aboard the ship if sending cargo fast is your point. And if sending the crew is the point, using a single ship is more efficient.

So ships never break down and require repairs? They never require being maintained while en-route to their destination?
My suggestion to send the crew ahead at a manageable acceleration rate ahead of their main ship was never meant to be More efficient. It was meant to give the OPer the idea that he can Still accelerate his Very Large ship however fast he wants to, crew separation under Heavy acceleration is the only real way to accomplish this.


not really.

I'm a bit torn by the Complete denouncing of what I was talking about. One thing that I attempt to accomplish is to give due its due. If you truely think the idea of seperating the crew from the main ship while the main ship recieves Heavy acceleration is just dumb, :Shrug: , IDK, I really don't know how to explain it if you don't get it. :Hug2:

Lhun
06-17-2009, 05:25 AM
I agree Lhun.

The main reason you'd consider using immense amounts of acceleration is when it improves the economics of the ship, or where you don't have a crew.
<snip>The Orion scenario you sketched is a bit special, since an Orion drive requires a huge amount of mass to reduce acceleration (in small ships), and has increased efficiency if you use big nukes. Usually it's the other way around, whatever (other) drive system you use. Higher acceleration will require a bigger drive with more thrust, and will be less efficient (because the thrust is higher, not because the drive is bigger).

Though for a cargo-only Orion ship, i don't think the shock absorber will be an issue either. For ground launches you'd probably use really massive ships that can utilize very big three-stage nukes to minimize the radiation you release into the athmosphere. To get that mass you'd just launch ships with more cargo, instead of small ships with big shock absorbers. Once in space, radiation doesn't matter anymore so you can repackage the cargo into whatever size is most feasible for interstellar cargo transport.

Marcus
06-17-2009, 05:28 AM
No. Doesn't work for the same reason you cannot attach a magnet at the end of a long stick and pull yourself with it. If you catch the particles, you get their impulse, which will negate all impulse you gained when releasing them. All that setup manages is a rocket that pulls itself apart.
Any drive that is even remotely realistic has to have reaction mass. A reactionless drive violates at least as many laws as an FTL drive does.

So when i have particals traveling away from their heat source they will never lose their energy and start to slow down as they travel away from the reaction that caused them to fuse?

I did mention a VERY long ship right? By the time the particals reach a distance where they have and cooled down their movement will no longer be linear away from the ignition source. They will be traveling around like gas in a container, completely random. I'm not talking about having a chamber on the other end, it Has an opening, and the matter being excited to give this thrust is collected at some limited way as it reaches the "random" gaseous movement state at the rear of the ship.

Dommo
06-17-2009, 05:44 AM
I agree with you again Lhun about the orion.

I just don't ever see the practical reason for making the ship small, except for economic ones(e.g. can't afford the 50 billion pricetag of a big orion, have to settle for something smaller). It makes sense to me with the orion just to go big or go home, since not only does the ship get more efficient with size, but it's mechanically A LOT simpler to engineer.

Although as I said the small orion does have some interesting aspects about them. The idea of accelerating a 5000 ton mass to like .1 C in like 3 or 4 hours is a pretty interesting prospect(particularly as a KE weapon).

Lhun
06-17-2009, 05:44 AM
I want to give your post credit, but the opening sentence is just flat wrong. In my post I specifically talk about MASS. Your saying that to accelerate a golf ball in space is the same as accelerating a planet? Accelerating a 5 ton ship IS easier than accelerating a million ton ship.Correct, but not relevant for your example though. You accelerate the same mass anyway, in your model a big part in an unmanned cargo ship and a small part in a manned crewship. There is no difference between the total energy you will need to do than wether you do it seperate or on a single ship. Which means that for sending one part of the mass you wish to transport ahead, you need to use a drive that is bigger than it proportionately would be. Which means more expensive. Sure, by building bigger drives you can always go faster, but separating one big ship's mass into two small ships makes them neither faster nor more efficient.
The only thing that determines the speed of a ship is the proportion of it's mass you allocate to the drive. Size doesn't matter. A small ship with 30% drive will be exactly as fast as a big ship with 30% drive. (unless one uses a worse drive design obviously, which is besides the point though)

I'm talking about Simplifying a Very large ship so that it doesn't have to deal with keeping the crew alive under Heavy acceleration to get up to cruise speed. In the OPers posts he talks about not having an inertial compensator. If your objective is JUST to get a crew somewhere, then awlrighty then, your point here is valid. However, being able to get your Very Large ship up to speed without worrying about the crew Does have advantages.Not if you need to send the crew along anyway. Again: the biggest time problem in interstellar travel is the crew. If you have figured out a way to get the crew to your target at a cozy 1g acceleration, there's no reason to keep their luggage back and then shoot it at them. Just use a big ship with 1g acceleration. Having the cargo accelerate much harder to catch up with the crew will be a lot less efficient than simply taking it along.

<snip> I was going off the fact that I'm thinking about the Mass of a Very Large ship and somewhat limited tech.I simply don't see the connection here. Whatever drive you use to accelerate, you use to decelerate. The ship being small or large doesn't really matter.

So ships never break down and require repairs? They never require being maintained while en-route to their destination?If you need a crew, you need a crew. If you can do without, then do without. Simply a question of available technology.

My suggestion to send the crew ahead at a manageable acceleration rate ahead of their main ship was never meant to be More efficient. It was meant to give the OPer the idea that he can Still accelerate his Very Large ship however fast he wants to, crew separation under Heavy acceleration is the only real way to accomplish this.Yes, and there really is not much point to doing that. Sending the cargo ahead is useful. Then crew spends less time in space. Sending the crew ahead is not useful. Then you can just build the crewship larger, and put the cargo on it as well.

I'm a bit torn by the Complete denouncing of what I was talking about. One thing that I attempt to accomplish is to give due its due. If you truely think the idea of seperating the crew from the main ship while the main ship recieves Heavy acceleration is just dumb, :Shrug: , IDK, I really don't know how to explain it if you don't get it. :Hug2:I get the point you're trying to make, but it just doesn't work out that way. Again: separating cargo and crew makes sense if you send the cargo ahead. Use lower acceleration, lower speed, maybe a hohmann transfer orbit instead of a brute force manuever etc. pp. Save a lot of money on transportation by having the cargo take longer. Like you could use a sailship instead of a motorboat to transport goods on earth for lower fuel costs but longer transport times. Doesn't make sense here of course since fuel is not a significant enough cost factor, though there are some prototype ships already swimming around that try various systems to use wind when possible (my favorite is the giant kite). In space, fuel is the only significant cost. Not so much because of the price of your fuel, but because of the percentage of your ships mass that has to be fuel. And sending stuff slower allows you to have more cargo and less drive aboard. Which means it's most economical to send stuff as slow as otherwise feasible. If it's precious alien egg delicacies from alpha centauri that might be short (though they'd probably handle acceleration badly) but if you're sending iron ore, who cares if it takes 5 years?
The crew on the other hand will not be exstatic (or even willing to board) at the prospect of spending 5 years in a tin can in deep space. If there is anything you want to minimize travel times for it's the ships that actually have crew aboard.
So, sending the crew slowly ahead, and the luggage quickly after them is exactly the wrong way around.

Lhun
06-17-2009, 05:50 AM
So when i have particals traveling away from their heat source they will never lose their energy and start to slow down as they travel away from the reaction that caused them to fuse?Fusion is not so much caused by a reaction as simply by two particles colliding at high enough energy.

I did mention a VERY long ship right? By the time the particals reach a distance where they have and cooled down their movement will no longer be linear away from the ignition source.If there is nothing altering their direction (and what would there be in space?) each and every particle will have the exact same direction and speed as when it left the fusion chamber. As long as the exhaust stream is pretty dense the particles might still collide, but as their general direction is downstream, that wouldn't slow them down.
They will be traveling around like gas in a container, completely random.Relative to their own reference frame yes. But relative to the ship they still have the same speed as when they left the fusion chamber. What would slow them down?
I'm not talking about having a chamber on the other end, it Has an opening, and the matter being excited to give this thrust is collected at some limited wayThat doesn't change the principle. It'd be the same as having several drives, and one of them closed off at the bottom. Whatever portion of the particles you collect will be the portion of thrust that will not propel you forward but instead try to pull your ship apart.

as it reaches the "random" gaseous movement state at the rear of the ship.It really doesn't. Thing's don't stop in space unless stopped by something.

Lhun
06-17-2009, 06:01 AM
I just don't ever see the practical reason for making the ship small, except for economic ones(e.g. can't afford the 50 billion pricetag of a big orion, have to settle for something smaller). It makes sense to me with the orion just to go big or go home, since not only does the ship get more efficient with size, but it's mechanically A LOT simpler to engineer.I agree that given somewhat realistic (or even pretty unrealistic) spaceship technology the only reasons to use multiple smaller ships instead of a single big one have to be non-technological.

Although as I said the small orion does have some interesting aspects about them. The idea of accelerating a 5000 ton mass to like .1 C in like 3 or 4 hours is a pretty interesting prospect(particularly as a KE weapon).Hell, at .1c, that thing has a hundred freaking megaricks!. That's not a kinetic weapon, that's a doomsday device.

Dommo
06-17-2009, 06:06 AM
I agree with Lhun. Usually in most forms of propulsion there's a trade off.

Either you get High Speed, low thrust or you get low speed High thrust. The first is usually pretty energy efficient(Ion Drive), and the latter isn't(Chemical Rocket). It's comparable in some ways to how cars have gearing. Orion(nuclear) and Orion-minimag(which uses exploding deuterium pellets), are the only known breakers of this rule, and have relatively high speed, and immense amounts of thrust, with the nuclear being the one with enough thrust to ground launch.

It makes sense to send things that aren't that time critical(e.g. sending massive amounts of iron ore, or some other type of non-perishable), using the first method. Who cares if it takes me 5 years to get it to its destination, as long as it gets there cheaply.

However with people/pets/etc., you need to get to your destination as fast as reasonably possible. In those cases it makes sense to trade off efficiency for more rapid acceleration, because unlike iron ore, it's difficult for people to survive spending multiple years in microgravity, and they get bad cases of cabin fever.

It's sort of a trade off, and combinations of propulsion methods are sometimes useful. For example using a chemical rocket to get to about 1/2 of the max speed of your ion drive makes sense, since it cuts in half the time remaining for the ion drive to peak out(which might only take 1 month instead of three months). I just like Orion because it can do it all :P.

Marcus
06-17-2009, 06:11 AM
Hell, at .1c, that thing has a hundred freaking megaricks!. That's not a kinetic weapon, that's a doomsday device.

on the note of kinetic weapons... NO payload. I wouldn't want to see what this thing does first hand.

In a final test flight, the 12-ft.-long aircraft hit 7000 mph nearly Mach 10. In other words, the X-51 is not just some lab experiment; it's being designed from the start to deploy.

http://www.popularmechanics.com/technology/military_law/4203874.html?page=3

Lhun
06-17-2009, 06:12 AM
Orion(nuclear) and Orion-minimag(which uses exploding deuterium pellets), are the only known breakers of this rule, and have relatively high speed, and immense amounts of thrust, with the nuclear being the one with enough thrust to ground launch.I don't actually see any reason why the IC Fusion drive would have different specs than the Orion. Heck, given the 10ton fission device that's apparently possible i don't even think it has an edge in miniaturization. The only noticeable difference should be that an IC Fusion drive needs a decent power generator aboard, while the Orion requires fissile materials for the bombs as well as fuseable.

Dommo
06-17-2009, 06:21 AM
As a weapon you've got to admit Lhun that's a pretty sick use for an orion. Make a pusher plate on the back of a medium sized asteroid. Put a bunch of nukes on board, and you could seriously raise some hell.

Think about it, if my asteroid needs to travel 50 million miles, it could cover that distance VERY quickly. Just making a ballpark estimate and some back of the envelope calculations, and ignoring the time it takes for it to accelerate up to speed(which would make this an integration problem that I'm too lazy to do), it could travel that distance in about 45 minutes. THAT IS NUTS! Realistically it'd be a bit slower than that(try a few hours), but still, trying to stop 100,000 tons of planet smashing fist of god, when it's basically a solid hunk of iron asteroid, is basically impossible. If a projectile hit the earth at that speed it'd probably completely liquefy the crust, and annihilate all life on earth.

Dommo
06-17-2009, 06:30 AM
http://www.ebaumsworld.com/video/watch/933662/

Here's a video of a scale model test of the orion. It's pretty neat.

Dommo
06-17-2009, 06:36 AM
Seeing that video, it makes me wonder if there are any conventional explosives that might have a higher energy density than current rocket fuels. Maybe it might be possible to build an orion type ship(albeit way less powerful/fast), using conventional bombs to launch it?

Lhun
06-17-2009, 06:45 AM
As a weapon you've got to admit Lhun that's a pretty sick use for an orion.Well, i said it's a doomsday device. ;)

but still, trying to stop 100,000 tons of planet smashing fist of god, when it's basically a solid hunk of iron asteroid, is basically impossible. If a projectile hit the earth at that speed it'd probably completely liquefy the crust, and annihilate all life on earth.Stopping a massive kinetic weapon isn't actually as hard as it seems. The big advantages of the defender are that such weapons aren't exactly maneuverable, that they bring all the bang you need and that they're not exactly dark. What you want to do to stop them is simply shoot a different kinetic weapon straight at them. The resulting explosion will turn the projectile into a big, quickly dissipating cloud of plasma. If you managed to hit it far enough away from the target, the plasma will have dissipated enough to be harmless. Otherwise you'll have various levels of damage, depending on how far away you hit it. In the almost worst case scenario of successfully intercepting it, but very close to the planet, it would still boil of the athmosphere but cause much less physical damage. People in airtight underground shelters could survive, even if the planet is unusable.
If you intercept it far enough from the planet, you'll just get a few very brilliant northlights and induced currents in power and communication lines.
If you can actually use a kinetic weapon of similar size to intercept an incoming one, the resulting plasma will lose a lot of its punch and be pretty harmless, since the specific impulses will cancel each other out. Not perfectly, obviously, but you'll get a lot less ions headed to the planet, some reaching relative stop, and some even travelling backwards.
Though that's a lot more expensive than using an interceptor that's just big enough to vaporize the asteroid.

Lhun
06-17-2009, 06:58 AM
Seeing that video, it makes me wonder if there are any conventional explosives that might have a higher energy density than current rocket fuels.Higher energy density is difficult, the highest possible would be hydrogen-fluorine but hydrogen-oxygen is not far behind anyway. Nanotechnology offers some ways to significantly increase burning speed and efficiency of otherwise bad explosives (those with seperate substances that are mixed), but i don't think you can get far beyond the power of the best monomolecular explosives. Nitroglyzerin is already pretty close to the theoretical limit.

Maybe it might be possible to build an orion type ship(albeit way less powerful/fast), using conventional bombs to launch it?I don't think that'd actually be a good design. If you think about it, the Orion is horribly inefficient. The produced energy isn't focused, and more than half of the produced energy is wasted anyway since the explosion is circular. If you use chemicals, you can burn them inside a chamber and use an exhaust nozzle to focus the exhaust stream which will greatly increase the efficiency. The orion is so much superior to chemical rockets because that loss of efficiency (heck, let's call it one order of magnitude) absolutely pales in comparison to the power output you get out of thermonuclear fusion (or even fission) when compared to chemical energy stored in explosives.
By the same reasoning i'd expect a continuous fusion drive to have an efficiency at least 4 times as high as an IC Fusion/Orion, though the problem with that is that there's no practical idea so far how to build one.

blacbird
06-17-2009, 09:10 AM
Its not an energy advantage, its an economics advantage :)

Okay, we agree it's not an energy issue. I guess I would also question the economics advantage, but that's more a political/social/practicality issue than a scientific one.

caw

Marcus
06-22-2009, 01:05 PM
worth checking out because its just wiki--facts! lol

http://en.wikipedia.org/wiki/Spacecraft_propulsion

Marcus
07-16-2009, 08:22 AM
ok just ran across this after much web surfing.

very interesting

http://en.wikipedia.org/wiki/Variable_specific_impulse_magnetoplasma_rocket