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A question about rockets

  1. May 16, 2009 #1
    I have a question about rockets. Consider a rocket, with a perfect dynamic fulcrum and center of balance (lets say its a magic rocket ship maybe.) Anyway, this rocket never rotates in flight, and has continuous and stable thrust through the flight. Would it continually bounce up and down or continually fly upwards?

    http://xs139.xs.to/xs139/09206/rocket472.jpg [Broken]
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 16, 2009 #2

    DaveC426913

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    You lost me...
     
  4. May 16, 2009 #3
    Basically if you shot a rocket at a 45 degree angle would it fly up then fall down due to gravity (accelerating downwards even though there is no change in engine thrust)
     
  5. May 17, 2009 #4

    cjl

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    If the direction it was pointing was constant (after launch, it remained pointed at 45 degrees, regardless of flight path angle), its acceleration would depend on its thrust to weight ratio. If the thrust were greater than the square root of two times its weight (~1.414 times its weight), it would accelerate upwards along a line, at a shallower angle than 45 degrees. If its thrust were less than that, it would be unable to get off the ground.
     
  6. May 17, 2009 #5
    Cool, so it wouldn't fall down. I thought that was the case but unfortunately that is the harder one to simulate.
     
  7. May 17, 2009 #6

    russ_watters

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    It isn't that hard: you have two force vectors, just add them together into one. That's the direction the rocket will accelerate in.
     
  8. May 17, 2009 #7

    DaveC426913

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    It would fall down when the thrust ran out. It would fall in an arc...

    drdede: I think there's some misconceptions happening here, but I can't pin them down. Can you elaborate on your scenario and how you see it playing out from beginning to end?
     
  9. May 17, 2009 #8
    If the thrust is constant, and it is carrying the fuel it is burning, then the thrust to mass ratio is constantly improving, and the acceleration is constantly increasing.
     
  10. May 17, 2009 #9
    Yes! Moreover, since the mass is decreasing, the component of the resultant force on the rocket caused by gravity is decreasing. So, even though the angle of the rocket is fixed, the rocket's trajectory will curve upwards.

    So neither picture is correct. The rocket won't oscillate (why did you think it would?), nor will it travel in a straight line. Of course, there are still a bunch of unvoiced assumptions here. Is the curvature of the Earth significant, for example.
     
  11. May 18, 2009 #10

    cjl

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    That is true. A lot of the solution is dependent on which assumptions you make. In my post for example, I was assuming effectively constant mass, which is clearly not the case for most rockets.
     
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