PDA

View Full Version : Mass and acceleartion/manueverability in a zero-gravity environment



Fiender
12-03-2010, 03:41 PM
Okay, so, space story. Ships and stuff.

Is size/mass indicative of a ship's potential speeds and/or rate of acceleration? Basically, would a bigger ship be slower than a smaller ship in space?

Lhun
12-03-2010, 03:45 PM
No.

The only thing defining maximum acceleration of a space vehicle is the ratio of drive size to total size, assuming equal efficiency of all drives. Although generally, bigger drives are more efficient than smaller ones.

On another note, zero-gravity is completely irrelevant here. It just means things don't fall down. The important part is zero-friction.

Fiender
12-03-2010, 04:01 PM
So, no way that a ship's size would affect its movement? Unless its drive wasn't in proportion with that of a smaller ships'?

Lhun
12-03-2010, 04:06 PM
Without friction what matter is how massive the ship is, and how strong the drive.
If you double the mass of the ship you'd half the acceleration. But if you also doubled the size of the drive (since you kept it proportional) the drive provides twice the thrust so the acceleration stays the same.

Jean
12-03-2010, 04:10 PM
I have a question, Lhun. What cause the size matter? Since it's no resistance.

Lhun
12-03-2010, 04:16 PM
Acceleration is proportional to mass. (a=F/m) So twice the size needs twice the thrust to get the same acceleration. Since mass is the only thing relevant in a vacuum when referring to size, that's what's meant.

Fiender
12-03-2010, 04:32 PM
Okay, the point of this topic was for me to see if a larger ship would have difficulty pursuing smaller ships.

It's not impossible, but I'm going to need to finesse the story and logic behind this a bit by saying that the larger ships' engine is not proportional to its size.

I guess another factor is the functionality of the engine, basically how long it takes the engine to reach "full burn" as it were.

Jean
12-03-2010, 04:32 PM
Isn't that equation applied on gravitation body only?

Fiender, acceleration and speed is different, especially in space. When you push mass, nothing slow it down, so the longer you accelerate the faster your ship go, and larger ship mean larger fuel tank, it can accelerate longer than smaller ship.

But you could twist the law of gravity like every sci-fi do. ^__^

Lhun
12-03-2010, 04:39 PM
Okay, the point of this topic was for me to see if a larger ship would have difficulty pursuing smaller ships.

It's not impossible, but I'm going to need to finesse the story and logic behind this a bit by saying that the larger ships' engine is not proportional to its size.

I guess another factor is the functionality of the engine, basically how long it takes the engine to reach "full burn" as it were.The easiest way would be to use some handwavium to explain why the smaller ship has a different type of engine with a better efficiency/thrust than the big ship. (and why that type of engine can't be used in big ships)
Otherwise, it's actually quite likely that a bigger ship has a better top acceleration than a smaller ship. Startup time doesn't help much, since it's just a fixed headstart, which is unimportant in a chase with constant acceleration. (i.e. in an accelerating chase, the vehicle with higher acceleration will eventually be faster and catch up, no matter what their initial start time, distance and velocity)


Isn't that equation applied on gravitation body only?There's no g in the equation.

Jean
12-03-2010, 04:58 PM
Okay, I checked. You're right. :D

Fiender
12-03-2010, 05:03 PM
Well, I have my own idea of how the scenario is supposed to play out:

*Small ships scatter in an attempt to escape, move in different directions.

*Large ship tries to chase down as many as it can before the small ships activate their ftl drives.

*Large ship must launch fighters to ensure the destruction of all smaller ships.


Handwavium can be fun, but I only have so much of it. I'd rather not use up my whole supply on this scene :D

Jean
12-03-2010, 05:08 PM
Handwavium can be fun, but I only have so much of it. I'd rather not use up my whole supply on this scene

Yes, you can write as you want. Sci-fi never make physical sense anyway.

Fiender
12-03-2010, 05:11 PM
Yes, you can write as you want. Sci-fi never make physical sense anyway.

But I'd like to maintain as much relative scientific accuracy as I can.

Lhun
12-03-2010, 05:20 PM
Yes, you can write as you want. Sci-fi never make physical sense anyway.Only on TV and in movies. Written SF is much better at realism. While there's still many shades of SF hardness, even the softest space opera pays much more attention to being realistic (and more importantly: consistent) than ST or SW.


Well, I have my own idea of how the scenario is supposed to play out:

*Small ships scatter in an attempt to escape, move in different directions.

*Large ship tries to chase down as many as it can before the small ships activate their ftl drives.

*Large ship must launch fighters to ensure the destruction of all smaller ships.


Handwavium can be fun, but I only have so much of it. I'd rather not use up my whole supply on this scene :DI recommend checking out the space combat thread(s) in the SF/F forum.
And of course Project Rho: http://www.projectrho.com/rocket/

To give a quick overview of the most common misconceptions/important differences though:
Chase scenarios are interesting, since a constant acceleration is a big advantage for the fleeing side.
Because of frictionless physics, one has to have a serious acceleration advantage to catch multiple scattering ships with a single one (unless they don't reach any breakoff point and the chase can take as long as necessary)
Space fighters make no sense whatsoever.
There is no stealth in space.
There is no terrain in space.

Drachen Jager
12-03-2010, 09:20 PM
Technically, all else being equal a larger ship would be faster than a small one. Some things are just more efficient as they get larger. The surface skin would be one of the biggest weight components but, as the ship gets larger the skin size squares whereas the internal size cubes meaning more space left over for drives and other things as the ship gets bigger.

lbender
12-03-2010, 11:17 PM
Everyone's been talking about acceleration and speed in a straight line. If I were in a smaller ship being pursued by a larger one, I would attempt to take advantage of what a smaller ship might do better, such as changing directions. A smaller ship should be able to turn more quickly. Straight line momentum to be overcome would be much less with a smaller mass at the same speed. In the scenario you've proposed, assuming you aren't in deep space, there should be plenty of asteroids, etc. to dodge around.

Lhun
12-03-2010, 11:46 PM
Everyone's been talking about acceleration and speed in a straight line. If I were in a smaller ship being pursued by a larger one, I would attempt to take advantage of what a smaller ship might do better, such as changing directions. A smaller ship should be able to turn more quickly. Straight line momentum to be overcome would be much less with a smaller mass at the same speed.Acceleration is acceleration. To change direction you need to apply acceleration, and how a big a ship is that accelerates at 1g doesn't matter.

In the scenario you've proposed, assuming you aren't in deep space, there should be plenty of asteroids, etc. to dodge around.Just as there's no stealth in space, there's nothing to hide behind in space. Asteroids are so small and widely dispersed that there's no way to use them as cover. Dense collections of asteroids can't exist because they condense to form planets.

Hallen
12-04-2010, 12:51 AM
Everyone's been talking about acceleration and speed in a straight line. If I were in a smaller ship being pursued by a larger one, I would attempt to take advantage of what a smaller ship might do better, such as changing directions. A smaller ship should be able to turn more quickly. Straight line momentum to be overcome would be much less with a smaller mass at the same speed. In the scenario you've proposed, assuming you aren't in deep space, there should be plenty of asteroids, etc. to dodge around.
So, like Lhun says, but using different terms, size doesn't matter, but mass does. And mass doesn't matter as long as you assume that more mass = bigger ship = bigger engines = proportional thrust (when compared to a smaller ship).

You can do some manipulations and suggest that your propulsion systems don't scale up very well and therefore give your bigger ships less proportional thrust. Then they'd have both an acceleration deficit and a maneuvering deficit (which is just a sideways acceleration deficit). (but maybe they can carry more/better weapons?)

Of course, if the combatants come from different places and the technology is somewhat different, then one could have an advantage over the other. Not all engine manufacturers are equal.

So, in other words, if you're going to have a space battle with little ships vs big ships, you'll need to deviate from real physics with handwavium. The OP is already doing this with the FTL drive, so it isn't a big deal. Just be consistent.

Drachen Jager
12-04-2010, 02:48 AM
There is one aspect of manoeuvrability that might be hurt by being larger. Assuming your primary thrust is in only one direction you need to turn to change direction. If your engines are capable of really high outputs a large ship might be torn apart by centrifugal force when executing a very fast turn whereas a smaller ship could manage easily.

Wouldn't do you too much good in the long run (because, as mentioned above there's nothing to play slalom with), but it might give you a second or two lead.

Lhun
12-04-2010, 03:06 AM
There is one aspect of manoeuvrability that might be hurt by being larger. Assuming your primary thrust is in only one direction you need to turn to change direction. If your engines are capable of really high outputs a large ship might be torn apart by centrifugal force when executing a very fast turn whereas a smaller ship could manage easily.

Wouldn't do you too much good in the long run (because, as mentioned above there's nothing to play slalom with), but it might give you a second or two lead.Nah. You have to keep in mind that the centrifugal force you experience, and are used to, in cars and other common vehicles doesn't actually come from the turning, It comes from the effects of inertia pushing you along the previous vector while you use ground friction to change your vector. I.e. it's not the turning of the car, it's the car driving in a curve/circle.
A frictionless vehicle on the other hand experiences nothing of the sort. The only centrifugal force there results from the actual turning. While it is true that a smaller ship can, because of that, turn faster than a bigger ship, that's not relevant. The time spent turning in a given direction is a tiny fraction of the time spent accelerating.
Also keep in mind that large changes in vector only reduce the effective speed of the fleeing vehicle. Since there's no terrain to follow, the pursuer can always follow in a straight line, any changes in vector by the fleeing vehicle will make it easier to catch up.

benbradley
12-04-2010, 04:38 AM
The easiest way would be to use some handwavium to explain why the smaller ship has a different type of engine with a better efficiency/thrust than the big ship. (and why that type of engine can't be used in big ships)
Do all these ships have one or more human occupants? Acceleration in manned ships is limited by what a human can take. The Space Shuttle gives about 3 g's to its occupants over the ten minutes or so it takes to get into orbit. Fighter jets can go up to 5 g's or perhaps even more, but there's only so much even a well-trained pilot can take.

Well, I take that back - here this famous fighter jet passenger gets peaks of 7.6 g's (click or double click video for full screen):
http://premium.nascar.com/pr/video/cup/2006/03/04/cup.junior.blueangels.nascar_frameset_exclude.html
Other passengers (obviously not quite in as good a shape, perhaps not following the 'tense your legs' instructions) have temporarily passed out at lesser g's.

Missiles with nuclear bombs have acceleration as high as ten g's.

Jean
12-04-2010, 05:18 AM
Do all these ships have one or more human occupants? Acceleration in manned ships is limited by what a human can take. The Space Shuttle gives about 3 g's to its occupants over the ten minutes or so it takes to get into orbit. Fighter jets can go up to 5 g's or perhaps even more, but there's only so much even a well-trained pilot can take.

Well, I take that back - here this famous fighter jet passenger gets peaks of 7.6 g's (click or double click video for full screen):
http://premium.nascar.com/pr/video/c...t_exclude.html (http://premium.nascar.com/pr/video/cup/2006/03/04/cup.junior.blueangels.nascar_frameset_exclude.html )
Other passengers (obviously not quite in as good a shape, perhaps not following the 'tense your legs' instructions) have temporarily passed out at lesser g's.

Missiles with nuclear bombs have acceleration as high as ten g'sG matters on gravitation body (ie Earth) only. In space, there is no problem with G force.

benbradley
12-04-2010, 06:36 AM
G matters on gravitation body (ie Earth) only. In space, there is no problem with G force.
Okay, if you say so ...


ETA: F=MA everywhere I've been...

Xelebes
12-04-2010, 11:08 AM
G matters on gravitation body (ie Earth) only. In space, there is no problem with G force.

G is a unit of measure for acceleration, not gravity itself.

Jean
12-04-2010, 11:13 AM
But in space, crew won't be knocked out by G force like on jet plane. That's what I wanted to say.

Xelebes
12-04-2010, 11:24 AM
But in space, crew won't be knocked out by G force like on jet plane. That's what I wanted to say.

What makes you say that?

Lhun
12-04-2010, 11:27 AM
G is a unit of measure for acceleration, not gravity itself.Actually, the proper unit for acceleration is m/sē. g is simply used for easier comparison as the standard gravitational acceleration on earth.


But in space, crew won't be knocked out by G force like on jet plane. That's what I wanted to say.There's no gravitational acceleration in free-fall, but acceleration still has the same effects as anywhere else. Frictionless isn't the same as inertia-less.

Xelebes
12-04-2010, 11:28 AM
Measuring things in terms of Gs is what helps determine the jerk and jounce, what a pilot and crew experience in high velocity manoeuvres.

Xelebes
12-04-2010, 11:30 AM
Actually, the proper unit for acceleration is m/sē. g is simply used for easier comparison as the standard gravitational acceleration on earth.


Of course. Didn't mean to say it was anything else. I used an indefinite article as opposed to the definite article. :)

Anaximander
12-06-2010, 03:40 PM
But in space, crew won't be knocked out by G force like on jet plane. That's what I wanted to say.
Yes they will. No gravity means no weight, not no mass. They still have mass, so they still have inertia, which means they'll still be crushed into their seats when they accelerate hard, and their organs will still be squished about.

Don't be confused by the term 'g force'. The g refers to multiples of 9.81m/s^2. It doesn't mean it's caused by gravity - it's just a helpful way of understanding how strong the force is by relating it to something familiar. So, if you're in space and you're getting 1g, it'll feel like you're in Earth gravity with 'up' being the direction you're accelerating in. If you're getting 3g, you'll feel like you're three times as heavy as normal.

Drachen Jager
12-06-2010, 10:11 PM
Nah. You have to keep in mind that the centrifugal force you experience, and are used to, in cars and other common vehicles doesn't actually come from the turning, It comes from the effects of inertia pushing you along the previous vector while you use ground friction to change your vector. I.e. it's not the turning of the car, it's the car driving in a curve/circle.

Take a bucket of water. Twirl your arm around so that the water stays in the bucket even when it's upside-down. You didn't have to twirl it very fast did you? That's about a 1 metre radius circle. If you have a ship that's 100 metres from farthest points it's going to experience 100 gees of force to spin at that same speed. If you have crew spread throughout the ship you CANNOT turn very quickly without squishing some of them. If it's Imperial Star Destroyer sized it may take minutes re-align the ship to a new heading whereas something the size of a car could turn in a second.

Think about it before you answer sometimes, honestly you annoy me with these quick-fire poorly thought out responses. Even the turning radius of the car you use in your example would be well within what most people would describe as a "large" spacecraft. Most cars have a turning radius smaller than the space shuttle.

The second part of your comment was already addressed in my original.

Lhun
12-06-2010, 11:35 PM
Take a bucket of water. Twirl your arm around so that the water stays in the bucket even when it's upside-down. You didn't have to twirl it very fast did you?That depends on what you'd call very fast. To get 1g in the horizontal, you need about 30 rpm. I'd call that not very fast for a centrifuge, and pretty damn fast as hell for a vehicle.
That's about a 1 metre radius circle. If you have a ship that's 100 metres from farthest points it's going to experience 100 gees of force to spin at that same speed. If you have crew spread throughout the ship you CANNOT turn very quickly without squishing some of them.Centrifugal gravity is proportional to the square of the rotation rate. A ship a hundred times as long only need to slow it's rotation rate by a factor of 10 to have the same g force at the ends. A 200m ship (thus 100m spin radius) can do a 180° turn in just 6 seconds if it restricts the turn acceleration to a very possible 3gs at the ends.

If it's Imperial Star Destroyer sized it may take minutes re-align the ship to a new heading whereas something the size of a car could turn in a second.Even if that was the case (and it's not) it'd hardly matter. Because of the frictionless environment, space vehicle are very sluggish at changing a vector anyway, the time spent turning into the desired direction is of no importance. After building up a vector for an hour, it takes another hour to come to a relative stop again, if the ship takes 6 seconds or 60 seconds to turn around before braking doesn't matter. Flying a narrow slalom course (the only scenario where turning speed would matter) is extremely pointless in space, which is just a big, big vacuum with nothing to fly a slalom around.
Think about it before you answer sometimes,Sage advice, thank you. Let me return the favour and suggest you not only think before answering, but also take a little time and do some fact-checking with a calculator.
honestly you annoy me with these quick-fire poorly thought out responses.Thanks.

Even the turning radius of the car you use in your example would be well within what most people would describe as a "large" spacecraft. Most cars have a turning radius smaller than the space shuttle.Turning radius is a term that makes no sense whatsoever when applied to spacecraft. A turning radius is the result of a vehicle that can use friction to change its vector. Every spacecraft can turn on a point, but needs to apply thrust to change the vector. The distance required to change the vector is the same as the distance covered while accelerating, unless the vehicle didn't accelerate with max thrust.

Drachen Jager
12-07-2010, 12:03 AM
Lhun, you're not half as smart as you think you are. Please stop giving people bad advice and "correcting" information which is already correct.

I am done talking to you now.

Lhun
12-07-2010, 05:24 AM
Lhun, you're not half as smart as you think you are.Oh, but you have now idea just how smart i think i am. :tongue

Please stop giving people bad advice and "correcting" information which is already correct.I'm more than happy to provide anyone interested with the relevant formula to do the calculations themselves and see which set of "correct informations" deserves the name. In the current case that would be:
a = 0.011 * rē * l
Where a is the resulting centrifugal acceleration in m/sē (9,8m/sē=1g), r is the rotation in rpm and l is the distance to the centre of the rotation. Or there's this handy Calc: http://www.artificial-gravity.com/sw/SpinCalc/SpinCalc.htm
Anyone interested in how the formula derives can simply look up "centrifugal force" in wikipedia.
I am done talking to you now.That's really too bad. Guess i'll have to keep posting just for everyone else then.

KQ800
12-29-2010, 04:13 AM
..Lhun is quite correct. However, I would point out that "turn" and "rotate" might not be synonymous regarding vehicles in the mind of all readers.

A space ship cannot turn like an aircraft in a wide arc without wasting copius amounts of fuel. It will rotate so that the engines main thrust is in a new direction, and then start accelerating.

KQ800
12-29-2010, 04:31 AM
Okay, so, space story. Ships and stuff.

Is size/mass indicative of a ship's potential speeds and/or rate of acceleration? Basically, would a bigger ship be slower than a smaller ship in space?

This depends on how much leeway you will give the constructors. If you use a mass drive, such as a rocket engine, this works by throwing mass in one direction to push mass in the other.

This type of drive is very limited in that you push the fuel along with the ship, and that means that you must have more fuel and so on. In this case only the engines capacity in proportion to the remaining mass determines how fast a ship will accelerate.

But if you are prepared to accept either artificial gravity or inertia dampers in your universe you can have an engine generate a node of gravity in front of the ship, and then let the ship "fall" towards it. Kind of like pulling yourself along by your bootstraps. Or lessening the ships mass ( afaik not physically possible) so that a mass drive plus inertia damper will result in dramatically improved acceleration.

In this case size might translate to acceleration in that only larger ships could generate energy to charge the dampers.

Also, things to keep in mind, final speed is dependant on fuel and time, not acceleration. A low acc ship with lots of fuel can eventually reach a high speed. A ship with fast acc will reach that speed in shorter time. this means that a small ship with good acceleration might still not get away from the bigger "slower" ship if they don't have the fuel to keep their acc up long enough.

Aerial
12-29-2010, 04:33 AM
But in space, crew won't be knocked out by G force like on jet plane. That's what I wanted to say.

Yes, they would. Force is mass x acceleration, period. Your ship is a mass and your human body inside the ship is a mass. If you apply an acceleration, either positive or negative, that body experiences a force. We usually express this force in g's because that unit gives the most intuitive understanding of what the number means. But you will absolutely experience g's in maneuvers in space, just none of it due to a planet's gravitational pull.

Aerial

Aerial
12-29-2010, 04:38 AM
This depends on how much leeway you will give the constructors. If you use a mass drive, such as a rocket engine, this works by throwing mass in one direction to push mass in the other.

This type of drive is very limited in that you push the fuel along with the ship, and that means that you must have more fuel and so on. In this case only the engines capacity in proportion to the remaining mass determines how fast a ship will accelerate.

[snip]

Also, things to keep in mind, final speed is dependant on fuel and time, not acceleration. A low acc ship with lots of fuel can eventually reach a high speed. A ship with fast acc will reach that speed in shorter time. this means that a small ship with good acceleration might still not get away from the bigger "slower" ship if they don't have the fuel to keep their acc up long enough.

Practically speaking, fuel is a HUGE limiter on maximum accelerations and speeds. Infinite acceleration is fine in concept, but acceleration requires thrust, which requires fuel. Every spacecraft that has to carry fuel onboard (i.e. not solar sails, etc) will have a computer algorithm whose sole purpose is to compute fuel remaining versus distance to travel versus acceleration versus time versus course to determine what paths the ship is capable of traveling without running out of fuel in the middle of nowhere.

Aerial

Lhun
12-29-2010, 04:49 AM
But if you are prepared to accept either artificial gravity or inertia dampers in your universe you can have an engine generate a node of gravity in front of the ship, and then let the ship "fall" towards it. Kind of like pulling yourself along by your bootstraps. Or lessening the ships mass ( afaik not physically possible) so that a mass drive plus inertia damper will result in dramatically improved acceleration.Any kind of inertialess or reactionless drive will require copious amounts of handwavium, because ignoring inertia and/or conservation of momentum means fucking with the most basic laws of physics. For example: the kind of reactionless gravity generator you describe is an instant infinite energy machine, and one that's actually very easy to engineer. Apply artificial gravity to one half of a flywheel.
Similarly, without inertia it's impossible to determine the actual kinetic energy of an object. Make flywheel inertialess, spin it up to lightspeed using only infinitely small amount of energy, switch on inertia and watch explosion with infinitely large energy output tear it apart.
Handwavium like this needs to be applied carefully, with very clear limitations because it can make the whole world of the story fall apart, or make the character look like they're carrying idiot balls for not seeing easy solutions to their problems. (Just look at how many StarTrek episodes need to make the transporters unusable with some harebrained excuse because a single "beam me up scotty" would solve the problem of the week)

Addendum: My suggestion for a SpaceOperaEngine(tm) would be to have some kind of handwavium energy generator that can supply infinite energy but limited power. While that breaks the 1LoT, it does so in a fairly unimportant way, after all there's not much difference to, say, a fusion reactor, except that you don't need to carry fuel. Use it to power a normal reaction engine (still carrying normal reaction mass) which can accelerate the reaction mass to a very high percentage of lightspeed. So, in effect you have ships that don't break any important laws of physics in a major way, get interesting accelerations, interesting ranges, but are still limited in their capabilities.
Of course, it absolutely does break normal economics, unless you limit the free infinite energy by horrendous maintenance costs for the handwavium generator. But then, pretty much any civilization capable of serious space travel has the technological capabilities to evolve a post-scarcity economy.


In this case size might translate to acceleration in that only larger ships could generate energy to charge the dampers.The one big thing that makes small vehicles faster than big ones on earth is friction. Without friction, there's no reason why any kind of engine would make a small vehicle faster. Generally, bigger is more efficient (less overhead) and no matter what the engine, you'd expect bigger vehicles to be faster than smaller ones. The best way to handwave around this, if one absolute must recreate midway in space, is to have two kinds of engine, a fast one with a maximum size and a slow one with minimum size. (space fighters are still dumb, but at least they're faster than the big ships now)

benbradley
12-29-2010, 06:56 AM
Oh my, how cantankerous this thread has become since my last post ... anyway:

Practically speaking, fuel is a HUGE limiter on maximum accelerations ...
Actually, maximum acceleration is determined by the design of the rocket engine, not how much fuel you have. Higher acceleration does use fuel at a faster rate, so the same amount of fuel won't last as long as using a smaller engine giving a lower acceleration. But the design of the rocket engine, and of course the ability to design the rocketship/missile/payload to handle the acceleration, are the limitations.

Here's something with really good acceleration:

The Sprint accelerated at 100 g (http://en.wikipedia.org/wiki/G-force), reaching a speed of Mach (http://en.wikipedia.org/wiki/Mach_number) 10 in 5 seconds.
http://en.wikipedia.org/wiki/Sprint_%28missile%29

Aerial
12-29-2010, 07:01 AM
Actually, maximum acceleration is determined by the design of the rocket engine, not how much fuel you have. Higher acceleration does use fuel at a faster rate, so the same amount of fuel won't last as long as using a smaller engine giving a lower acceleration. But the design of the rocket engine, and of course the ability to design the rocketship/missile/payload to handle the acceleration, are the limitations.


That's true. I was more trying to comment on the idea that you could have as much acceleration as your engine is capable of, whenever you want it and for however long you want it, which isn't true because fuel management will probably be the determining factor in what you can do. I just assumed there were physical and mechanical limits on the engine thrust capability and the structure, and didn't mention that aspect.

So your answer is definitely more complete :-)

Aerial

benbradley
12-29-2010, 07:11 AM
Practically speaking, fuel is a HUGE limiter on maximum accelerations and speeds. Infinite acceleration is fine in concept, but acceleration requires thrust, which requires fuel. Every spacecraft that has to carry fuel onboard (i.e. not solar sails, etc) will have a computer algorithm whose sole purpose is to compute fuel remaining versus distance to travel versus acceleration versus time versus course to determine what paths the ship is capable of traveling without running out of fuel in the middle of nowhere.

Aerial
Rereading your post, it appears you're saying fuel is a limiter on delta-v (http://en.wikipedia.org/wiki/Delta-v), which it certainly is. If I'm thinking right, the amount of fuel and mass of the ship (and other details, such as the mass of the fuel and how much energy is extracted by burning it) completely determine the max delta-v a rocket-powered ship has.

If I'm thinking wrong, I have no doubt one or more posters will correct me, because after all, This is The Internet. :)

Lhun
12-29-2010, 09:00 AM
Rereading your post, it appears you're saying fuel is a limiter on delta-v (http://en.wikipedia.org/wiki/Delta-v), which it certainly is. If I'm thinking right, the amount of fuel and mass of the ship (and other details, such as the mass of the fuel and how much energy is extracted by burning it) completely determine the max delta-v a rocket-powered ship has.Yes. Acceleration is determined by the engine design, some have variable thrust, some don't, some have different energy and/or reaction mass efficiencies at different thrusts. dV on the other hand is mostly determined by the amount of reaction mass and fuel that's being carried. (also by the drive setting used if it's one of the drive types that has varying efficiency)
Generally though, SciFi in general and space opera especially assumes extremely large values for dV, usually much larger than the maximum practical travelling speed. It's somewhat necessary to have spaceships that travel like ships, not planes. This is more interesting for missiles, and other small vehicles with very limited range. (range being expressed in dV in space) For example civilian intrasolar vehicles might not even have enough range to make it from the inner to the outer planets in a single trip without refueling, and the range of a missile is determined by the distance at which the target has enough time to build up an evasive vector greater than the missiles dV.

benbradley
12-29-2010, 09:40 AM
This is a completely different way to travel in interplanetary space using very small amounts of delta-v (once it has left Earth, essentially mid-course corrections) to get from planet to planet. The drawback/tradeoff is the large amount of time it takes (as if ballistic courses between planets wern't slow enough):
http://www.sciencedaily.com/releases/2002/07/020718075904.htm