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Newtons third law regarding spacecraft engines

  1. Aug 16, 2011 #1
    Hello PF members,

    I was wondering whether newtons third law was exclusive to spacecraft thrust; space being empty and without enough particles for a force to push against. That in order to produce thrust in space, you would need to carry the fuel that you plan to eject with you (disregarding the solar sail; good luck if it gets punctured during a microscopic meteor shower). Is this notion correct or are there other methods of space flight.

    I had an idea some time ago, that instead of ejecting fuel out of an engine, you built a device which emitted vibrations in a certain direction much like a stereo speaker. would something like this work in space, or are there not enough particles in space to generate a vibration?
  2. jcsd
  3. Aug 16, 2011 #2
    Welcome to Physics Forums.

    I would not say "exclusive to spacecraft thrust" as the law has universal application. You are correct, though, that to accelerate a body in one direction another body must accelerate in the opposite direction.

    If this "engine" were to push on any particles at all, then the engine will be pushed in the opposite direction. The acceleration, though, will be inversely proportional to the mass of the engine (and the ship attached to it). It could be miniscule.

    I think you really mean to ask "Is there enough mass to push off and get a significant acceleration?" I suspect the answer is no.

    This might help clarify: the Deep Space One probe had an ion drive that ejected about 7 x 1018 xenon ions per second to get a thrust equal to the weight of a sheet of paper.
  4. Aug 17, 2011 #3
    You guessed right. There's not enough gas in space for sound energy to propagate from a vibrating system. But with sufficiently large scoops one can gather up useful reaction mass to propel a ship. The trick is doing so without creating too much drag.
  5. Aug 18, 2011 #4
    I suppose as a side note, a future space insustry would be fuel harvesting.

    Another thought occured to me. Can the sudden mass displacement be used as a directed motion. Eg: wheel with a central axis on a ramp. If you place mass at the center of the axis, the wheel will roll uniformly down the ramp. If on the other hand you place the weight on the circumference of the wheel, it bounces down the ramp. Will this work in space or do you need the presence of gravity?
  6. Aug 18, 2011 #5
    I am not sure how you see the "bounces" as applying to space ship propulsion, but you are right that gravity i(as well as the force the incline applies upwards on the wheel) is responsible for the behavior. Also note that the potential energy you put into the wheel and weight while lifting them to their starting points on the ramp is the actual source of the kinetic energy of the wheel's motion.
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