Acceleration of spaceships with different masses?

  1. I have been wondering, in films, we can often see small ships flying faster than giant ones.
    Top speed is clearly independant from size and mass, but what about acceleration?
    The thrusters do have frictional losses, the propellant has resistance, waste heat treatment issues and other things can hinder the performance of the engines.

    So if we have two ships, roughly the same designs, technology, will that make the smaller one more agile, or not?
  2. jcsd
  3. russ_watters

    Staff: Mentor

    Smaller vehicles appear to travel faster than larger ones in virtually all contexts due to a size illusion. A vehicle twice as big as another of similar shape but traveling the same speed will tend to appear to be traveling at half the speed because the change in displacement as a fraction of its length is half what it is for a smaller one. This is why the space shuttle appears to launch slowly, when in reality it jumps off the launch pad.

    At the same time, "size" is a tricky concept. People often say that an object with twice the length is twice the size of another, but if all dimensions scale the same, the bigger object is 8x the volume of the smaller one. This can present scale-ability problems by magnifying forces.

    And even just a one-dimensional increase can present problems for human occupants. For example, rotational forces scale with size so you can't rotate a large object at the same angular speed as a smaller one.
    Last edited: Feb 3, 2012
  4. Chronos

    Chronos 10,348
    Science Advisor
    Gold Member

    It's strictly a mass to propulsion issue. Larger vehicles tend to be more massive so it takes more propulsive force for them both to accelerate and change direction. Insects can fly, elephants cannot.
  5. D H

    Staff: Mentor

    Never, ever use scifi films as the basis for understanding what really happens in space now, or what might happen in space in the future. The model for those scifi movies is navies, where big ships are slow lumbering beasts and smaller boats are fast. The physics of space is perceived as being too foreign to the average viewers mind.

    The reality of space travel today is that space vehicles are much more like Mack trucks on ice powered by a moped engine rather than either a speedy little skiff or a lumbering cargo ship.
  6. Ryan_m_b

    Staff: Mentor

    I agree with D H, TV/movie/video-game representation of how things move in space is almost always totally wrong. The craft operate like a bizzare fusion of naval boats and aircraft, the biggest peeve I have is when you have a ship constantly firing it's rockets (with flame and everything) but getting no increase in speed. Then it stops firing and either stops dead or slows down, or sometimes after a long time of boosting in one direction it will stop, flip over and travel back the way it came completely negating any velocity it built up. A perfect example of this is any SF which has small fighter craft that perform dogfights and especially when they dock with a main ship: they boost all the way with no increase in speed and then smoothly decelerate in about half a second when entering the hanger.

    Bottom line is that the makers of these programs find it easier for themselves and the public to envision that space is like flying in atmosphere where you need constant thrust to stay at the same speed (to counter wind resistance) and that acceleration is negligible.
    If you have two ships with the same propulsion the smaller one will be nimbler because it has less mass and thus can accelerate and decelerate faster. But that is presupposing that they have the exact same propulsion.
  7. Are you telling me when the Enterprise drops from Warp 9 and engages impulse power this is scientifically unsound?

    On a serious note, there are a few examples of SF literature where the authors attempt to stay real to science, a sub-genre called Hard SciFi. Obviously it straetched the limits but does aim to stay within known technologies.

    The rama series by Arthur C Clarke is a good example.

    However I woudlnt reccomend reading them to gain an understanding on spaceship dynamics!

  8. Ryan_m_b

    Staff: Mentor

    "Enemy decloaking, quick! Engage the tetranannotonn field and repolarise the hyper-nine arrays then reroute auxiliary radiation through the gamma relays!"
    Literature is very different, hard-SF authors treat things like movement in space in a far more realistic manner. Contrast that to soft-SF which mainly grabs European style Empires and couples them with a USA style navy before putting them in space and making them fight anthropic aliens with l@z0rs.
  9. DaveC426913

    DaveC426913 16,532
    Gold Member

    Look at the problem the other way around.

    If smaller ships weren't more nimble, they would not exist.

    The whole point in making small craft small is that they are nimble. That comes at a huge price of everything else they can't carry with them. Fighters don't have lavatories, kitchens, heavy armour, support personnel, backup systems and a thousand other things that are stripped out of the design (of an otherwise generic craft) for the sole purpose of making it lighter and therefore more nimble.

    Basically, a small craft is stripped of everything except its propulsion and its weapons. That means the ratio of propulsion to total mass is much higher, meaning it has a relatively more powerful engine.

    If you could have a giant fighter as nimble as a tiny fighter, why not have giant fighters?

    So a=F/m

    If you want a big a, with a given size F, the only way to do it is to reduce m.
  10. Ryan_m_b

    Staff: Mentor

    They don't exist. Apart from the space shuttle and a few manned capsules we don't have any real life comparisons for spacecraft, let alone military spacecraft. If we want to have a discussion on that we would have to set up a military situation, the parameters of the technology and then discuss.

    For instance: is a "space-fighter" really useful? Like you say it's essentially a seat, an engine and some guns but considering it is so small how much fuel can it hold? If it doesn't hold much fuel it is either limited to slow speeds to prolong the engagement or can only fight for a small amount of time. If you add more fuel you make it less nimble. A bigger craft with more fuel might be less nimble but it could have other advantages like being able to perform over long distances for long times and could be packed with more weaponry.
  11. So true and so funny how they re-engineer their engines to have double the power and then on a subsequent episode they forget they can do this! I actually laughed in the office and got a few strange looks reading this!

    Yes thats very true - it really does seem to be analogues of medievel stlye empires - on a galactic scale. I am still a sucked for a good alien battle and often read things that are totally unplausable! I also read fantasy and the line between the two is very blurred nowadays!
  12. D H

    Staff: Mentor

    You aren't using your imagination.

    There are plenty of reasons other than nimbleness that would require smaller ships. Atmospheres, for example. Not squashing the inhabitants of the planet to be visited, for another.

    Suppose that in a galaxy far, far away, the Frobozz Magic Intergalactic Spaceship Company manufactures spaceships that can go from star to star. Quickly. They've found ways around our supposed light speed limit and supposed relativistic rocket equations. The downside: The machinery for the magical Frobozz drive is huge, so huge that even the smallest Frobozz spaceship is the size of a small moon. Landing such a beast on a planet is out of the question. They bring smaller ships for such tasks. These smaller ships are restricted to our supposed limits, using technology that is not that far ahead of ours. They are slow, but they can land on a planet.
  13. If we talk about military applications, for orbital defence, you dont need big range (not as big as interplanetary travel at least). And yes, small ones can enter into places where big ones cant.

    "If you have two ships with the same propulsion the smaller one will be nimbler because it has less mass and thus can accelerate and decelerate faster. But that is presupposing that they have the exact same propulsion."

    Exactly same like same proportion to the size, or same power output?
  14. DaveC426913

    DaveC426913 16,532
    Gold Member

    I specifically avoided using the word space, since the principle applies to any craft. Space, land, sea, air.
    Which is why we have learned to use carriers to hold the small craft with their limited fuel supplies. That way we maximize the utility of the craft to its purpose: maneuverability at highest premium (except actual payload).
    Last edited: Feb 3, 2012
  15. DaveC426913

    DaveC426913 16,532
    Gold Member

    OK, you're taking it a little far. :uhh:

    We have today, without reference to hypothetical giant motors, good reasons for why we design some craft large and some craft small. And there's pretty much only two reasons (at least, that I can think of) for paying the price of a making a small craft when a larger one could do more.
    1] to avoid detection.
    2] to make it more nimble
  16. DaveC426913

    DaveC426913 16,532
    Gold Member

    It's a general principle.

    Yes, we are presupposing that they have the exact same propulsion for the sake of the principle. That is not an error in logic, it is a issue of isolating the variables so as to identify the principle.

    The issue is: a less massive craft before throwing any other factors in will be more maneuverable.

    Yes, you can add all sorts of other factors in there that will compete with the 'more maneuverable' mandate for the craft's design, such as needs a gunner, needs long-range capability, needs ECM technology. But that doesn't change the principle of less mass = more maneuverable.
  17. "I am still a sucked for a good alien battle and often read things that are totally unplausable! "

    If a little OFF is enabled, Space Battleship Yamato was more realistic, and i really liked the aliens in it, they were really alien.
  18. Small ships are definitely more maneuverable than big ones since they can suddenly swearve without suffering the structural stress that a larger ship would under the same velocities. Also, less power is needed to overcome small ship's inertia whether that inertia be stationary or dynamic. It's easier to get a speedboat going at a good clip than it is to get an ocean liner. It's also easier to divert or stop one. Example? The Titanic. Even with engines reversed and with steering rudder to its limit it was too little to either divert it or slow it down significantly in order to avert disaster. Too much momentum too little power. All in accordance with what Newton tells us:

    Each sci fi writer appeals to a different audience. Isaac Asimov readers enjoy him because he is a stickler for scientific accuracy and goes to great lengths to make his stories as believable as possible. The genre is called hard sci fi as opposed to soft sci fi or sci fi/fantasy mixes where magic is thrown in.

    I personally don't enjoy films which assume viewer ignorance of basic laws of nature. Ships in the vacuum of space flying as if in an atmosphere and ignoring the exigencies of action and reaction and momentum is one example.
    Last edited: Feb 4, 2012
  19. if anyone is interested in how to try and make space ships realistic, you should read this website

    it's a nifty site where he talks about how to make a good space ship for a sci-fi book or something. Talks about weapons and how to make the ship go, etc.
  20. Ryan_m_b

    Staff: Mentor

    I've seen this site before and think it's excellent, especially this section

    Too many authors don't think through the ramifications of what they propose when they invent a plot device.

    EDIT: Relevant to what we have been talking about:
  21. Ryan_m_b

    Staff: Mentor

    The "faster" a ship is depends on the amount of fuel and the efficiency of the engine. A million tonne ship with a specific impulse of 10,000 seconds carrying a million tonnes of fuel will have a higher top speed than a one tonne ship with a specific impulse of 1,000 seconds carrying ten tonnes of fuel (although with the same amount of thrust it will take longer to get up to speed).
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