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Meatball
06-14-2010, 05:00 AM
Hey folks,

Got a question that I've been mulling over for a while and thought you guys might have some good answers or insight into.

I think Helios 2 is the fastest manmade object in space so far at 150,000 miles per hour. I know theoretically a ship can travel up to the speed of light in space since there's no resistance in space to slow you down (assuming you don't run into something...)

My question is how fast do you think you could actually make a ship go under some type of propulsion power before the engine itself tears the ship apart?

For example, if you were to take a jet engine and strap it a balsa wood airplane, the engine itself would be too powerful for the airplane and would destroy it.

So, assuming materials that are available today, would there be some sort of max speed a ship could accelerate/decelerate from without using any other type of forces like gravity, etc to effect it's velocity?

Of course, now that I think about it, if you just impart small amounts of thrust continuously, that would build up a lot of speed, so that might be the answer to my question.

What's everyone's thoughts?

Shadow_Ferret
06-14-2010, 05:05 AM
Well, I think if there was such a powerful engine the engineers would try to make the ship structurally sound enough to handle it.

rmgil04
06-14-2010, 05:39 AM
There was a show on Discovery Channel called The Universe. They had an episode where scientists discussed/presented some theories about traveling near the speed of light. The closer you get, the more power you need (increaasing exponentially). IIRC, the only type of thrust they said might reach the speed of light would be some type of ion propulsion. However, the acceleration would be very slow.

In the book I'm writing, I use the theory of controlled particle collisions to create an artificial black hole to get from one place to another faster than light.

FOTSGreg
06-14-2010, 09:33 AM
Theoretically, the absolute maximum speed of a spacecraft is 99.99 ad infinitum 9's percent of the speed of light. You cannot actually attain 100% of the speed of light without using more energy than the combined output of the entire universe.

Properly designed spacecraft will not experience most of the effects of what we think about here on Earth when the engines torch off (or whatever it is they do). There is nothing to resist the "push" of the engines or warp the actual structure of the spacecraft itself. If the engines "push" they will provide thrust to the entire craft in some manner. It could be made of balsa wood and still have a "jet engine" (problems with the concept begin with the fact that jet engines operate by pushing air through a portion of the engine at high speed where it is combined with fuel explosively - something that cannot happen in space) attached if the attachment is strong enough to absorb the push.

It's been therorized by some science fiction writer and scientist friends of mine that a "field effect" drive might have a top speed limited by the effect of a sort of "drag" effect (wrong word, I know) or resistance of the local ions, dust, gas, etc. in the vacuum against the thrust of the drive. Essentially, the field itself might encounter enough local material or draw in such amounts of material that the infall against the field ahead of the spacecraft effectively becomes so large it negates the thrust of the drive itself. This is, of course, only theoretical (although some research on Bussard ramjets and local hydrogen densities suggests that Bussard ramjet driven ships might have a theoretical top speed).

I think I got most of that correct but Lhun will correct me if I missed the boat anywhere (thanks ahead of time, Lhun).

Lhun
06-14-2010, 02:15 PM
For example, if you were to take a jet engine and strap it a balsa wood airplane, the engine itself would be too powerful for the airplane and would destroy it.

So, assuming materials that are available today, would there be some sort of max speed a ship could accelerate/decelerate from without using any other type of forces like gravity, etc to effect it's velocity?Speed has no effect at all on a vehicle, neither in space nor on earth. Acceleration does, but sustained acceleration can theoretically get a vehicle close to the speed of light.

You cannot actually attain 100% of the speed of light without using more energy than the combined output of the entire universe.More actually. ;) To accelerate an object with rest mass to or beyond lightspeed would theoretically take an infinite amount of energy. It's more of a theoretical problem than one of how much energy is available in the universe.

I think I got most of that correct but Lhun will correct me if I missed the boat anywhere (thanks ahead of time, Lhun).Damn, i'm predictable.

Pthom
06-15-2010, 12:32 AM
Speed has no effect at all on a vehicle, neither in space nor on earth. Acceleration does, but sustained acceleration can theoretically get a vehicle close to the speed of light.
What matters, practically, is how fast you want to get to this state of "almost light speed." A solar sail might do the trick but would take longer than the lifespans of the occupants.

Damn, i'm predictable.Yes, you are. So where's the corrections? :D

Lhun
06-15-2010, 03:04 AM
Yes, you are. So where's the corrections? :DSurprisingly (:tongue) there's really nothing to correct here. Though i just have to add that when writing about solar sail powered spaceflight, despite the temptation, please do not start using nautical terminology. A solar sail does not work like a nautical sail, and never, ever will a solar sail ship be able to beat to windward with respect to the light pressure.

Meatball
06-15-2010, 05:16 PM
Awesome info/answers so far, thanks everyone!

To clarify a bit, I'm really looking to figure out how fast someone could actually get to another planet in our own system, say Jupiter or Saturn as opposed to a long range trip that is slowly building up speed.

That's why I was thinking more of some type of direct thrust system as opposed to a sail or something like that.

Lhun
06-15-2010, 05:26 PM
There's no way to answer that without the specifics of the drive. Although in general, if you have magical anti-inertia technology, it can be arbitrarily fast, if not, no matter what the drive, 1g sustained acceleration is pretty much the maximum.

Meatball
06-16-2010, 04:20 PM
What would 1G acceleration actually equate to in velocity?

On a secondary note. I'm debating some type of 'jump' technology using natural phenomena and debating whether I should use black holes or wormholes. I'm not sure how I could make a black hole able to transport someone without mashing them to bits, while wormholes are hypothetical and feel like a cop-out :) Any thoughts?

Pthom
06-16-2010, 08:46 PM
Acceleration is defined as an increase in speed: the rate at which something increases in velocity. The longer you accelerate (at whatever rate) the greater your velocity becomes. Up to the speed of light.

Regarding transport through black holes or wormholes, from a scientifically factual standpoint, it's impossible. You might consider posing that question in the main forum. :)

rmgil04
06-16-2010, 10:40 PM
What would 1G acceleration actually equate to in velocity?
Could you be more specific? On Earth, I'd say 1G sounds like 1 unit of Earth gravity or 9.8 m/s^2.



On a secondary note. I'm debating some type of 'jump' technology using natural phenomena and debating whether I should use black holes or wormholes. I'm not sure how I could make a black hole able to transport someone without mashing them to bits, while wormholes are hypothetical and feel like a cop-out :) Any thoughts?

In the Lost Fleet, ships travelled to jump points near stars. They couldn't jump to their version of FTL in open space. In the novel I'm writing, I use artificial black holes as a conduit to FTL space. The transition is very stressful on crew. As the ship passes through the event horizon, shields protect the ship and crew from being torn apart by the gravity, but the crew still feel the effect of gravity trying to pull them apart.

Meatball
06-16-2010, 10:49 PM
I'm moving the wormhole/black hole question to another thread (http://absolutewrite.com/forums/showthread.php?t=183089).

As for being more specific about the 1G acceleration, I'm basically trying to determine how quickly a ship could get to other objects in the solar system, say Jupiter or Saturn for example, if they were able to apply a constant 1G acceleration. Though I'm still not positive why a ship couldn't go faster than 1G other than some crew discomfort.

Assuming they don't go past 1G, I assume they'd only be able to accelerate to the half way point and then they'd need to reverse the 1G acceleration to stop at the correct location.

small axe
06-17-2010, 02:12 AM
You might search their website or past issues of Discover magazine for a few years back, I recall they had an excellent overview article on possible propulsion systems.

Lhun
06-17-2010, 02:42 AM
Could you be more specific? On Earth, I'd say 1G sounds like 1 unit of Earth gravity or 9.8 m/s^2.Anywhere. 1g is defined as the gravitational acceleration of earth. The earth not an earth.
As for being more specific about the 1G acceleration, I'm basically trying to determine how quickly a ship could get to other objects in the solar system, say Jupiter or Saturn for example, if they were able to apply a constant 1G acceleration. Though I'm still not positive why a ship couldn't go faster than 1G other than some crew discomfort.The human body evolved to live on earth which has a 1g gravity, no more, no less. Big deviations from that will cause serious health problems, not just discomfort. With a superbly trained astronaut crew, long periods of different gravity are possible, if it doesn't deviate too much, but for normal passengers it's not a fun ride. Free fall is a problem even then, but a good fraction of normal gravity can do much to prolong the maximum safe time.
On the other side however, there are many additional consideration. For one, while 2g doesn't sound like too much (it's less than you can experience in a sports car after all) it's a very different thing to be under 2g 24/7 for a while than experiencing it for a few seconds. Even for a reasonably fit man or woman, weighing twice as much is a huge strain, the average modern couch potato would be pretty much immobilized.
And that's just the immediate biological concerns.
If you drop something in 2g, it accelerates twice as fast. You're not going to catch it, since your reflexes are as slow as before. So you better never drop anything. And don't trip and fall, you'll break a bone much more easily. Having a hammer fall on your foot will likely result in broken toes. Everything weighs twice as much, so something that could normally be carried by one person now needs two.

Assuming they don't go past 1G, I assume they'd only be able to accelerate to the half way point and then they'd need to reverse the 1G acceleration to stop at the correct location.That's true no matter what acceleration you use. Accelerating hard and coasting is much less efficient than constant acceleration. So, you need to take half the distance, calculate the time it takes to travel under 10m/sē acceleration (rounding up for convenience) and double it. Wiki has all the relevant distances on the pages for the respective planets. As s=0,5atē, the time it takes is t=2(s/a)^0,5, units are metre for s, metre/second for a and second for t.

benbradley
06-17-2010, 03:14 AM
Awesome info/answers so far, thanks everyone!

To clarify a bit, I'm really looking to figure out how fast someone could actually get to another planet in our own system, say Jupiter or Saturn as opposed to a long range trip that is slowly building up speed.

That's why I was thinking more of some type of direct thrust system as opposed to a sail or something like that.

What would 1G acceleration actually equate to in velocity?
How long can your ship supply 1g acceleration? The formula for velocity in this context is acceleration times time. 1g is 32 feet per second squared. If you accelerate (from a dead stop) at 1g for 1 second, you'll be going at 32 feet per second.

The Space Shuttle goes into orbit in about ten minutes, and accelerates at something like 3 to 4 g's. At the end of ten minutes it's going at about seven miles per second. I think the limitation on acceleration is more because it has astronauts inside that can only take so much acceleration than other design limitations. I've heard of missiles (for delivering nuclear warheads) having acceleration of ten g's, designed to deliver their payload to the target as quickly as possible, as in crossing oceans in a few minutes.

But yes, if there's a human on board, acceleration is limited to what the human can take. Fighter pilots trained to take g forces can take more than an average person, but it's still limited to the 2 to 4 g range, depending on the duration (lower g's for longer duration - I doubt anyone could handle the Shuttle's acceleration for hours, even if it had that much fuel) and training, or about 1 g if passenger comfort is a must.

The main limitation in moving between planets within the solar system is fuel. Fuel-efficient travel with chemical engines involves relatively short burns with long times of coasting in space, The Apollo astronauts took about 2 or 3 days to go to the Moon, and planned trips to Mars involving current chemical fuel rockets likewise takes rocket burns that last for minutes or hours, and coasting times of months. The rocket burns just change the ship's orbit around the Sun enough that it becomes an ellipse that crosses the target planet's orbit (and are timed so the ship and the planet are both at the crossing at the same time).

If you have enough fuel (think of the Space Shuttle, but with many of those big external tanks instead of just one), you can accelerate all the way, and go "straight" there (okay, maybe not perfectly straight, but the optimum path would be a lot closer to straight). Actually, you'd accelerate halfway there, then decelerate the other half, so you're at a stop when you get there instead of whizzing by or hitting the planet at some high speed. The amount of fuel needed for this goes up by some huge amount (at some power or exponentially, I forget the exact equation, that's why I have books) as your acceleration goes up and your travel time goes down. In short, it becomes hideously expensive (as if space travel weren't already expensive enough). But doing that, you can reduce your travel time between planets from months to days, or in the best cases hours.

Lhun
06-17-2010, 03:58 AM
The amount of fuel needed for this goes up by some huge amount (at some power or exponentially, I forget the exact equation, that's why I have books) as your acceleration goes up and your travel time goes down. In short, it becomes hideously expensive (as if space travel weren't already expensive enough). But doing that, you can reduce your travel time between planets from months to days, or in the best cases hours.The reason for that is inertia. Any additional fuel and reaction mass also need to be accelerated. I.e. let's say one unit of fuel accelerates you to a speed of one. If you take two instead, your first unit of fuel will only accelerate you to a speed of 0,5 since your ship was twice as heavy. The second unit of fuel will then accelerate you to a total of 1,5 which means you doubled the fuel and got only 50% more total speed out of it. Take three units of fuel and it adds up to 0,3+0,5+1 and so on. And that's taking straight acceleration in a line. If you have to take twice that fuel to brake down at the end again, it just gets worse. This is the calculation for chemical drives, since there the fuel(+ reaction mass) is the single most massive part of the ship. And that's why chemical drives are far, far from the theoretical maximum speed/acceleration.
With something like a fusion powered plasma drive or even arcjet, one can get reaction mass efficiencies that allow for constant acceleration, and fuel is even essentially infite.

Meatball
06-17-2010, 06:53 AM
Alright, so, dropping back to our 'theoretical' situation of being able to accelerate at a constant 1G. Tell me if I'm figuring out the travel time from Earth to Jupiter right.

Looking at the Brachistochrone equation over on Project Rho (http://www.projectrho.com/rocket/rocket3i.html), the calculation is:

T = 2 * sqrt[ D/A ]

Where:
T = transit time in seconds
D = distance in meters
A = acceleration in m/s2 - (1G is 9.8 m/s2)
sqrt[x] = square root of x

The distance from Earth to Jupiter varies from 628311058.2 km at it's closest to 927506800.2 km at it's furthest (I'm going off the 4.2 AU - 6.2 AU listed over on WikiAnswers (http://wiki.answers.com/Q/What_is_the_distance_between_Jupiter_and_Earth_in_ astronomical_units), don't know how accurate that is)

So, at it's closest...

T = 2 * sqrt [628311058200 / 9.8] comes out to 506412.3745949117 seconds or 5.8 Days.

At it's furthest...

T = 2 * sqrt [927506800200 / 9.8] comes out to 615283.8403235252 seconds or 7.1 Days.

Not sure if I'm doing that right, but, 5-7 days? Wouldn't that be nice? :)

Lhun
06-17-2010, 11:45 AM
Almost correct. If earth and Jupiter are both at Perihelion the distance even shrinks to 3,9AU, but that's rare of course.

For the calculation, keep in mind you only accelerate half the way, then brake again, so the complete calculation isn't 2(s/a)^0,5 but 4(s/2a)^0,5 i.e. the time needed for half the distance, multiplied by two.

Pthom
06-17-2010, 01:04 PM
I believe you can accelerate for more than half the way. At the beginning of the trip, your ship is full of fuel. At the end of the trip, it's nearly empty, so because the mass to decelerate is less, it can be done in a shorter distance.

Lhun
06-17-2010, 01:40 PM
1g is 1g. If you have constant acceleration you need to switch direction halfway. You need less energy to accelerate less mass at the same rate of course.
But it's not going to be too relevant, because fuel mass is mostly a factor for chemical and other similarly inefficient engines, and those couldn't keep up a constant acceleration anyway. Those would just accelerate at the start, and then coast along.

Maraxus
06-17-2010, 04:32 PM
If the ship was moving like this with no Star Trek Inertial damper and artificial gravity ... they would obviously fly not like in a plane with the direction of travel forward but with the direction of travel "up". Would the contact pressure due to acceleration be distinguishable from gravity by anything, if they accelerated with 1g?

That sounds cool and I can imagine a mean captain on the way to Alpha Centauri giving his crew some hours of 1.1g if they have been unproductive and maybe a bit of 0.9g on Sundays (a journey which would take about 6 years and I have no idea how much fuel but definitely too much)

Oh and one thing I remember: The turn point has to be very exact in this case. Shouldn't be much problem for a computer but it's something that happens, when the atomic clock and the ship sensors tell the navigation computer, that they have reached exactly half point, not when the captain awakes and thinks: Hey, today it's roughly 3 years anniversary of the mission, we should turn and deaccelerate.

benbradley
06-17-2010, 09:39 PM
I believe you can accelerate for more than half the way. At the beginning of the trip, your ship is full of fuel. At the end of the trip, it's nearly empty, so because the mass to decelerate is less, it can be done in a shorter distance.

1g is 1g. If you have constant acceleration you need to switch direction halfway. You need less energy to accelerate less mass at the same rate of course.
But it's not going to be too relevant, because fuel mass is mostly a factor for chemical and other similarly inefficient engines, and those couldn't keep up a constant acceleration anyway. Those would just accelerate at the start, and then coast along.
Actually we're talking about chemical engines "with enough fuel to accelerate the whole way." Yes, it would be a big f'ing humongous fuel (and oxygen) tank(s) and not practical, but if you Absolutely Positively Had To make the trip in days rather than months...*

I can see what Pthom is saying, if the engines create constant force (presuming chemical rocket engines are on-and-off devices rather than throttleable), the acceleration will increase as fuel is burned, because the mass to be accelerated will be less.


If the ship was moving like this with no Star Trek Inertial damper and artificial gravity ... they would obviously fly not like in a plane with the direction of travel forward but with the direction of travel "up". Would the contact pressure due to acceleration be distinguishable from gravity by anything, if they accelerated with 1g?
Other than practical limitations such as engine noises, it would be indistinguishable. Einstein showed this with a thought experiment:
http://www.astronomynotes.com/relativity/s3.htm

* Such a rocket would likely be cheaper with nuclear power and the reactant being steam or ions shot out the engine at high velocity, but for this argument we're just blindly scaling up current chemical rockets, and especially their fuel tanks.

Lhun
06-18-2010, 12:43 AM
I can see what Pthom is saying, if the engines create constant force (presuming chemical rocket engines are on-and-off devices rather than throttleable), the acceleration will increase as fuel is burned, because the mass to be accelerated will be less.Chemical rockets are some of the easiest to create with adjustable thrust so on/off is a non-issue. And the reasons to not have accelerations much beyond 1g have been mentioned above.
* Such a rocket would likely be cheaper with nuclear power and the reactant being steam or ions shot out the engine at high velocity, but for this argument we're just blindly scaling up current chemical rockets, and especially their fuel tanks.Scaling up is what star drives are all about. Nuclear salt water rockets or an orion drive are pretty cool concepts (well, you don't want to use a NSWR for planetary take-offs) and with those a trip under a constant 1g is possible, even with current technology. The most important part about them is the significantly increased fuel efficiency and high thrust, as well as increased reaction mass efficiency. Without going nuclear, you have to pick either of the two. Anyway, even when not going nuclear though, any engine with high reaction mass efficiency will likely not lose a significant part of it's mass so. The theoretical limit would be a photon drive.

Pthom
06-18-2010, 04:46 AM
Given that this is the "Science Fact" subforum, can someone describe a propulsion system for space ships that does NOT consume mass?

Lhun
06-18-2010, 05:52 AM
Given that this is the "Science Fact" subforum, can someone describe a propulsion system for space ships that does NOT consume mass?As i mentioned, a photon drive is the theoretical limit to reaction mass efficiency (requiring none).
However, any other propulsion system that can expel the reaction mass at relativistic velocities doesn't require significant reaction mass. Thrust is proportional to exhaust mass as well as exhaust velocity, so if you can accelerate your reaction mass enough, you don't need a lot of it. Even ion drives don't require significant reaction mass, but there is a rather small limit on the amount of thrust you can get out of one, because you can't run them with a lot of reaction mass.
Fusion drives don't require significant reaction mass as well, and even the mass of the nukes for an orion drive isn't going to be the biggest part of a ship.
Nuclear salt water rockets are still less efficient, and although they have enough thrust to provide constant acceleration where a chemical rocket can't, i suspect 1g over several weeks is enough that fuel starts to become a dominating part of the mas of a ship powered by NSWR drives. I would really have to run the numbers on that though, it's just a guess.

Ibelong
06-19-2010, 02:08 PM
Hey folks,

Got a question that I've been mulling over for a while and thought you guys might have some good answers or insight into.



The only correct answer (IMHO) for fiction is, as fast as you want it to go.

According to Steve Hawkins (if you haven't read any of his stuff you should, the guy is very easy to understand and he has a brilliant mind) due to the constraints of matter it can only go so fast.
I am paraphrasing here, it's been a while since I read his work, the Universe in a Nutshell, but from what I recall, matter cannot and never will be able to exceed the speed of light. The faster it goes the slower it goes? Or something like that.

Anyhow...from what I gathered, extreme distance space travel borders on the impossible. At least going by the current known laws of physics.

Pthom
06-20-2010, 06:47 AM
I prefer using unobtainium (http://www.projectrho.com/rocket/index.html) only for fiction. :)

Dommo
06-20-2010, 07:52 AM
The name is "Stephen Hawking", and the max speed anything with mass can attain, is just less than the speed of light. There may be ways to bend that rule to an extent (e.g. if warping space is possible), but for all intents and purposes it's been experimentally proven to be true.

benbradley
06-20-2010, 10:19 AM
The name is "Stephen Hawking", and the max speed anything with mass can attain, is just less than the speed of light. There may be ways to bend that rule to an extent (e.g. if warping space is possible), but for all intents and purposes it's been experimentally proven to be true.
Also, it was an earlier scientist who came up with the speed of light as an absolute speed limit thing, some guy named Albert something...

pdknz
06-22-2010, 09:59 PM
If the ship was moving like this with no Star Trek Inertial damper and artificial gravity ... they would obviously fly not like in a plane with the direction of travel forward but with the direction of travel "up". Would the contact pressure due to acceleration be distinguishable from gravity by anything, if they accelerated with 1g?

That sounds cool and I can imagine a mean captain on the way to Alpha Centauri giving his crew some hours of 1.1g if they have been unproductive and maybe a bit of 0.9g on Sundays (a journey which would take about 6 years and I have no idea how much fuel but definitely too much)

Oh and one thing I remember: The turn point has to be very exact in this case. Shouldn't be much problem for a computer but it's something that happens, when the atomic clock and the ship sensors tell the navigation computer, that they have reached exactly half point, not when the captain awakes and thinks: Hey, today it's roughly 3 years anniversary of the mission, we should turn and deaccelerate.


This raises an interesting issue. If, hypothetically, a ship could accelerate at 1g constantly, it would reach light speed in about a year (353 days), disregarding relativity effects, which would be significant. At lightspeed there would be no subjective passage of time, so the observed time enroute to Alpha C (or indeed to anyplace else) would be acceleration plus deceleration time, something less than two years. The actual travel at or near lightspeed would not be noticed by the crew.

pdknz
08-27-2010, 06:15 AM
Given that this is the "Science Fact" subforum, can someone describe a propulsion system for space ships that does NOT consume mass?

OK, I'll give it a shot. Look up at the night sky and imagine how many stars there are in your line of sight. Let's call it a big number, and say that the mass in that line of sight is another big number. The equation for gravitational attraction is (copied from Wikipedia)--


Every point mass (http://absolutewrite.com/wiki/Point_mass) attracts every single other point mass by a force (http://absolutewrite.com/wiki/Force) pointing along the line (http://absolutewrite.com/wiki/Line_(mathematics)) intersecting both points. The force is directly proportional (http://absolutewrite.com/wiki/Proportionality_(mathematics)) to the product (http://absolutewrite.com/wiki/Product_(mathematics)) of the two masses (http://absolutewrite.com/wiki/Mass) and inversely proportional (http://absolutewrite.com/wiki/Proportionality_(mathematics)) to the square (http://absolutewrite.com/wiki/Square_(algebra)) of the distance between the point masses:[2] (http://absolutewrite.com/forums/#cite_note-Newton1-1)
http://upload.wikimedia.org/math/2/2/f/22f17757801c9ae9afd6dd9374f877b1.pngwhere:

F is the magnitude of the gravitational force between the two point masses,
G is the gravitational constant (http://absolutewrite.com/wiki/Gravitational_constant),
m1 is the mass of the first point mass,
m2 is the mass of the second point mass, and
r is the distance between the two point masses.
So, if there was no stars, say behind you, you would be falling toward the ones ahead, at some effortless acceleration determined by the root-mean average distance of all the mass ahead. (Technobabble, I know, but what I mean is that those stars are attracting us all the time. The reason we don't fall directly into them is that we are surrounded by other stars, equally attractive.)

So. The space drive is just a gadget that cancells gravity in one direction. If you use the stretched rubber sheet model of gravitational fields, it would mean that you somehow rip the rubber sheet in two, and it snaps you toward one edge. Bullshit, I know, but elegant bullshit. My hero/MC just has to put the grav polarizer in an old pressurised junk airplane, hit the switch and ride off into the starshine.

Bottom line, if you can mess with gravity, the rest is easy.

zerospark
08-27-2010, 08:32 AM
There's been some quasi-fringe science musings about a Mach-Lorentz thruster (http://nextbigfuture.com/2010/02/mach-effect-propulsion-research-update.html) that works on similar principles, using the gravitational mass of the entire universe as tread on the proverbial space-tire.

Also possibly of interest, in a similar vein to the photon drive:

Neutrino Propulsion for Interstellar Spacecraft (http://arxiv.org/abs/physics/9811009)

Dark Matter as a Possible New Energy Source for Future Rocket Technology (http://arxiv.org/abs/0908.1429)

As "reaction-less" drives go, odds are it's not going to happen. The best you could hope for in hard SF terms is a propellant-less drive (not to be confused with reaction-less) like those, paired with a mother of an energy source (i.e., black holes or antimatter, both of which are massive engineering nightmares).

Arxiv has a few other papers that may be of interest, with regards to speculative propulsion technologies, in the vein of modifying physical constants or using Visser-esque wormholes (which would lead to a kind of FTL, as long as you don't travel outside your wormhole network). Unfortunately Tsiolkovsky and Einstein are a hard team to beat when it comes to restrictions on space travel.

pdknz
08-29-2010, 09:25 PM
The only correct answer (IMHO) for fiction is, as fast as you want it to go.

(snip of Stephen Hawking paragraph)

Anyhow...from what I gathered, extreme distance space travel borders on the impossible. At least going by the current known laws of physics.

It's both fortunate and a bit sad that as fiction writers we can posit almost anything. Fortunate because it leaves a lot of story latitude, sad because we really, really wish it was true. I want to point out a couple of SF technology web sites I stumbled across (assuming that they are common knowledge here, but)--

http://www.stardestroyer.net/Empire/Tech/
Is a fansite that compares the technology of Star Wars and Star Treck. Remember the scene where Han Solo tells ObiWan Kenobi that "she's fast enough for you, old man." Toward the bottom of this (http://http://www.stardestroyer.net/Empire/Tech/Propulsion/Propulsion2.html) page, it turns out that their trip to Alderaan in Millenium Falcon must have been at a speed of something "in excess of 10 million c." As far as I know, that's the most over the top claim for how fast a space ship can go. The Star Trek vessel Enterprise was a comparative creeper, taking "decades to cross their galaxy."

There is some good stuff in this site about how much energy is involved in the various activies that take place in the story, and about the range of the vessels. It turns out that the energy density required is way more than the mass equivalent of the vessels. That is, if you converted the Death Star to pure energy, it still wouldn't be enough to produce the effects that came out of the Industrial Light and Magic labs. Bottom line is that they use magic, possibly disguised by a thin layer of technobabble.

http://www.projectrho.com/rocket/rocket3c2.html
The project rho site is a much more realistic site, based on either known principles or reasonable projections thereof. It includes a timetable for the estimated levels of future technology. Great site. There's one page about FTL travel, referring to handwavium (http://http://www.projectrho.com/rocket/rocket3v.html), which basically amounts to magic, used by the author to violate the laws of science as we know it. There are a lot of ways to speculate on how reality isn't really there, but they basically all equate to handwavium. This section makes a pitch for saving all the sanity you can when you write about it.

Julie Worth
08-29-2010, 09:52 PM
This raises an interesting issue. If, hypothetically, a ship could accelerate at 1g constantly, it would reach light speed in about a year (353 days), disregarding relativity effects, which would be significant. At lightspeed there would be no subjective passage of time, so the observed time enroute to Alpha C (or indeed to anyplace else) would be acceleration plus deceleration time, something less than two years. The actual travel at or near lightspeed would not be noticed by the crew.

You would approach arbitrarily close to c, assuming you have enough fuel. And once you got close enough, you could theoretically cross the galaxy in almost no time at all, if you measure it by ship time. A photon, which travels exactly at c, does not experience time, and travels from one end of the universe to the other in an instant, by its own reckoning. So the speed of light is not a real limit (at least to the people on the ship), it's discovering an almost unlimited source of fuel.

Julie Worth
08-29-2010, 09:54 PM
Toward the bottom of this (http://http://www.stardestroyer.net/Empire/Tech/Propulsion/Propulsion2.html) page, it turns out that their trip to Alderaan in Millenium Falcon must have been at a speed of something "in excess of 10 million c."

This isn't ridiculous if you measure velocity using ship time, which dilates without limit as you approach the velocity of light.

benbradley
08-29-2010, 11:18 PM
This isn't ridiculous if you measure velocity using ship time, which dilates without limit as you approach the velocity of light.
Yeah, but it messes with the story and plotline when they come back from across the galaxy and everyone they knew has been dead for tens of thousands of years (or if very long-lived have been patiently waiting that long). That's another sort of ridiculous, unless the author, the characters, and almost certainly the readers know this teeny little fact before starting the trip.

benbradley
08-29-2010, 11:19 PM
Given that this is the "Science Fact" subforum, can someone describe a propulsion system for space ships that does NOT consume mass?
Um, light sail?

ETA: Okay, that was already discussed...

Julie Worth
08-29-2010, 11:31 PM
Yeah, but it messes with the story and plotline when they come back from across the galaxy and everyone they knew has been dead for tens of thousands of years ...

It is hard to understand how you could ever have an interstellar economy if everyone you know is dead every time you go anywhere.

zerospark
08-30-2010, 01:50 AM
In real terms, you wouldn't have that at all.

Humans aren't adapted for the realities of interstellar flight and colonization. As trite as it may be, you'd need something best qualified as posthuman, and it wouldn't resemble anything we know as civilization now. Not only are our individual bodies and minds not up to the task, the institutions we build are also insufficient.

Physical trade between solar systems would be so expensive that you'd need a mother of a rare specimen to make it worth the trouble. Information can be beamed and whatever your thing is can be fabricated on site for far less, although you still have the light-speed lag to deal with.

On the other side of that, any given solar system would have enough material and energy resources to make billions to trillions of people very, very rich and very very comfortable if they were used efficiently. I think a lot of space travel stories have gotten so hooked on the idea of going to the stars that they forget how much mass and energy a given star can provide.

Julie Worth
08-30-2010, 03:48 AM
Humans aren't adapted for the realities of interstellar flight and colonization. As trite as it may be, you'd need something best qualified as posthuman, and it wouldn't resemble anything we know as civilization now. Not only are our individual bodies and minds not up to the task, the institutions we build are also insufficient.


I can imagine an advanced civilization sending out several ships a year. At first these would be to nearby star systems with a round trip of just a few years, and then more far flung systems (but still just a few years ship-time, due to time dilation). After a start up period, there would be constant stream of returning craft, some containing exotic species and perhaps even civilized life from alien worlds. Perhaps they would even try to direct the evolution of other worlds. Crewing these ships wouldn't be a problem. Any race that's interested in doing this has its share of adventurers, and besides, what greater adventure is there than taking a time machine into the future? Because, that's essentially what this is.

pdknz
08-30-2010, 04:58 AM
It is hard to understand how you could ever have an interstellar economy if everyone you know is dead every time you go anywhere.

Well said. I got the feeling in reading through the SW vs ST web site that the plot was the driver--George Lucas wanted to get on to the next scene, so the issue of travelling from waythehellandgone out on the rim to the galactic center was allocated two scenes and about five minutes of script. The how and why was never considered until the geek story fans came along and tried to make sense of it. The result was that they ended up with a backwoodsy bootlegger's pickup truck that could go 10 million c and short range fighters that can go 140,000 light years without refueling.

The only problem with that scenario comes for those few of us who keep asking, "Yeah, but what if it could really happen?"

Sometimes it seems that the fantasy is more real than the reality.

Oddly enough, there is a long flame war on the blog associated with that site where they argue about the "science" of the spaceships. That sort of thing goes back to at least the Sherlock Holmes series--there are still people arguing about various contradictions or implications of the SH "gospel", and he didn't even bother to violate the laws of physics.

So tantalizing, but so remote.

zerospark
08-30-2010, 07:14 AM
Regarding SW and their hyperdrive, there is a real astrophysicist who did some of the background work on that (the geek story fans, as you say ;)) and came up with some rather interesting ideas.

The long story short is that tachyonic motion isn't exactly forbidden by relativity; what happens is given FTL, Relativity, and Causality, you can Pick Any Two.

Relativity is a given barring some bizarre circumstances, and most physicists have no reason to throw out causality, as it introduces some potential problems to do so. That's good science, in any case.

If you're writing a SF universe, things can be a little different. Throwing out causality could lead to some very real headaches, depending on how the universe chooses to respond to time travel, but it's not strictly ruled out. The only issue is that, since relatively rules out the idea of an absolute universal reference frame, when you start dealing with objects traveling at different velocities you wind up with the potential to wind up in your past light-cone and screw things up. FTL travel doesn't mandate going back into your own past, but it does allow it with the right conditions, and since the jumps would have no absolute frame, you'd get to your destination pretty quickly - then wind up in your own past if you didn't adjust things by some local clock on the way back home.

Stross did this in Singularity Sky and Iron Sunrise; the FTL drives allow travel into your past light-cone, on paper. It's just that if you do that the psychotic AI god will nuke your solar system to protect its own history. Stephen Baxter used the idea in Exultant, where strategic back-time FTL jumps were considered a viable battle plan.

The other option is to require a special absolute frame of reference that was the same for all observers (think "hyperspace"). This...doesn't exactly invalidate relativity, but would expand on it in the same way that relativity expanded on Newton's classic gravitation. It would require that relativity hold in our universe, as we've verified this to fine detail, so you'd have to come up with some alternate universe or different layer of reality that is only accessible under very odd conditions. This would have the advantage of preventing any unwanted backwards time travel, at least.

As to how you'd go about making these things happen, well...that's something we don't even have the theory to theorize about.

pdknz
09-01-2010, 06:23 AM
You would approach arbitrarily close to c, assuming you have enough fuel. And once you got close enough, you could theoretically cross the galaxy in almost no time at all, if you measure it by ship time. A photon, which travels exactly at c, does not experience time, and travels from one end of the universe to the other in an instant, by its own reckoning. So the speed of light is not a real limit (at least to the people on the ship), it's discovering an almost unlimited source of fuel.

I'm really enjoying this discussion, so I hope nobody will mind if I try to drag it out a little farther.

I tried once to calculate how much energy it would take to accelerate a ship to light speed, and then to deccelerate again. This was during some lucid moment when I thought I could do calculus, but briefly, if you integrate the equation
KE (kinetic energy) = (mV^2)/2, with the velocity going from 0 to c, you get e = (mc^2)/2

(That's almost certainly wrong, but the result is intuitively elegant, e.g.--)
So, if you have total conversion from mass to energy, the mass ratio of the ship is 100%. That is, a ship can't carry anything except fuel, and still have enough energy to get up to lightspeed and back.

Oddly enough, it seems that there is a mathematical model that says the energy associated with velocity peaks at c, and is less at higher speeds. The almost sounds like uhh, FTL might work, if you can just get over the hump.

zerospark
09-01-2010, 08:42 AM
The Lorentz factor (gamma) is 1/SQRT(1-(v/c)^2)

Obviously moving exactly at c would mean dividing by zero, which we all know destroys the universe. For values of > c, you wind up with an imaginary number (SQRT(-1), or i) for your Lorentz transformation.

What that means is that...well, nobody really knows what it means. If FTL is possible, it would be weird. You certainly couldn't accelerate to that kind of velocity, and in fact it's questionable as to what "velocity" would even mean in that state, given that you've shattered the relationship between space and time.

Still, strange or not tachyonic movement isn't ruled out in the strictest sense, nor is the possibility that you could make some kind of state transition over the asymptote to the "other side of light speed".

ProtoMatic
10-29-2010, 07:09 AM
Hey folks,

Got a question that I've been mulling over for a while and thought you guys might have some good answers or insight into.

I think Helios 2 is the fastest manmade object in space so far at 150,000 miles per hour. I know theoretically a ship can travel up to the speed of light in space since there's no resistance in space to slow you down (assuming you don't run into something...)

My question is how fast do you think you could actually make a ship go under some type of propulsion power before the engine itself tears the ship apart?

For example, if you were to take a jet engine and strap it a balsa wood airplane, the engine itself would be too powerful for the airplane and would destroy it.

So, assuming materials that are available today, would there be some sort of max speed a ship could accelerate/decelerate from without using any other type of forces like gravity, etc to effect it's velocity?

Of course, now that I think about it, if you just impart small amounts of thrust continuously, that would build up a lot of speed, so that might be the answer to my question.

What's everyone's thoughts?

Does carbon nanotube technology count as available today? It exists today, it's just very hard to make. IF you could make it into a load bearing space-ship structure, the people inside would pulp from the acceleration long before the ship gave up.

Ion drives work on the pulse principle, and apparently requires very little fuel. But realistically, anything requiring that you bring fuel is unrealistic for travel in space. It's also kind to the passengers.

If you want engines you'll probably want Tokamak (http://en.wikipedia.org/wiki/Tokamak) reactors. With those puppies you can power almost anything.

Other than that, solar sails seems to be going well, and will have a slow acceleration.

I'll come back if I can think of anything else.

RichardLeon
11-07-2010, 11:57 PM
There are various more or less serious suggestions for FTL, but it seems they're likely to be unstable, and may cook anyone travelling at speed.

See e.g. http://www.technologyreview.com/blog/arxiv/23292/

But the limits of relativity only apply if the assumptions of relativity apply - specifically that spacetime is a smooth manifold with certain properties.

If it turns out that spacetime is derived from some more fundamental process, the rules may change. Since it obviously is derived - hence the scramble for quantum gravity - what is and isn't possible remains an open question.

Physicists like to talk about causality as if it's a fundamental limitation, but there are already events that appear to violate or stretch linear causality, so I'd be suspicious of taking that as an absolute.

One of my favourite fictional FTLs is is an obscure book called the Gameplayers of Zan by M.A. Foster, who suggested that space, time, energy and mass are derived phenomena of some more fundamental matrix. The characters in the book build a ship that manipulates the matrix directly, making FTL travel and free energy possible by means that are more like sailing than pushing.

The catch with this technology, which leads to a significant plot point, is that the 'engine' can never be turned off. The ship has to be flown while docked to keep it in place, otherwise it turns into a huge planet-destroying bomb.

PeterL
11-11-2010, 06:05 PM
What would 1G acceleration actually equate to in velocity?

That's the way to go. At an acceleration of 1 g, after an hour one would be going 35.28 kps, after 1 days it would be 846 KPs, AND after a week 6773 Kps, which it pretty darned speedy. It would take a while to be going really fast, but if one had to turn over and decelerate at the halfway point that would be fine.


On a secondary note. I'm debating some type of 'jump' technology using natural phenomena and debating whether I should use black holes or wormholes. I'm not sure how I could make a black hole able to transport someone without mashing them to bits, while wormholes are hypothetical and feel like a cop-out :) Any thoughts?

You might want to read Timemaster by Robert L. Forward. But remember that such thigs are onlt for long distances. If you are playing in a single system, then using slingshot effects would be more reasonable.

Lhun
11-11-2010, 06:16 PM
That's the way to go. At an acceleration of 1 g, after an hour one would be going 35.28 kph, after 1 days it would be 846 KPH, AND after a week 6773 KPH. It would take a while to be going really fast, but if one had to turn over and decellerate at the halhway point that would be fine.I think you dropped a decimal there somewhere. An hour of 1g acceleration gets you 35 km/s not km/h

PeterL
11-11-2010, 06:23 PM
I think you dropped a decimal there somewhere. An hour of 1g acceleration gets you 35 km/s not km/h

No, I just slipped on which time unit I was using.

Lhun
11-11-2010, 06:40 PM
No, I just slipped on which time unit I was using.Ah.
Anyway, a constant 1g acceleration is pretty slow for interstellar distances (but then, all STL travel is) but you can make interplanetary ones in a reasonable amount of time. Think long sea-voyage. I.e. Earth-Mars 49-107 hours, Earth-Jupiter 140-170 hours.

PeterL
11-11-2010, 08:48 PM
Ah.
Anyway, a constant 1g acceleration is pretty slow for interstellar distances (but then, all STL travel is) but you can make interplanetary ones in a reasonable amount of time. Think long sea-voyage. I.e. Earth-Mars 49-107 hours, Earth-Jupiter 140-170 hours.

Yes, it is a damned good acceleration for interplanetary trips, and it will get a ship to a very respectable percentage of c in less than a year.

The problem with getting close to c is that one has to find an energy source that could accelerate something there. At 1g the energy problem doesn't seem as bad, but it's still an almost infinite amount of energy, and then there's the problem of mass increasing as one nears c, so it would take innfinite energy to get to c, and more energy than exists in the universe to get to just 99.99999% of c. I worked all that out the second time I read Tau Zero, and mostly forgot about it later.

jake62
02-23-2011, 07:27 PM
If you read the material on the Billy Meier UFO case (the most documented UFO case), you will find where he documents the Pleiadian space travel. When they want to go long distances fast they have a device that transforms the ship and all aboard into particles.

Once in particles they travel at the speed of thought.

Billy described the time when they travelled at thought speed. He described it kind of like the afterlife. He couldn't hear anything, colors were more vivid, peace and tranquillity, etc.

The process was all controlled by their computer like devices. I bet it is one hell of a long logarithm.

Sarpedon
02-23-2011, 11:30 PM
Solar sails. (propulsion that doesn't consume mass)

And why can particles travel at the speed of light? I'm currently composed of particles, and I can't travel at the speed of light.

Skyler
02-23-2011, 11:37 PM
Once in particles they travel at the speed of thought.

How fast is the speed of thought?

Pthom
02-24-2011, 02:15 AM
How fast is the speed of thought?
Slower than the speed of light.

movieman
02-24-2011, 06:32 AM
How fast is the speed of thought?

Around 250mph, I believe. So it'll be a long trip.