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Rocket in space

  1. Apr 26, 2009 #1
    1. The problem statement, all variables and given/known data

    A rocket is travelling with a speed 20 km/s in a non-gravity space. To fix the direction of motion, it turns on an engine, which pushes the gasses with constant speed 3 km/s w.r.t. the rocket perpendicularly to the direction of its motion. The engine is on till the mass of the rocket declines for 1/4 of the initial.

    For what angle has the rocket changed the direction of motion and what is its final speed?

    2. Relevant equations

    x-axis: d( p(rocket) ) = - d( m(gasses) ) v(gasses) cos( alpha+d(alpha) )

    dm/dt * v(rocket_final) + dv/dt m( rocket_new )= - dm/dt * v(gasses) (cos(alpha)cos(d(alpha)) - sin(alpha)sin(d(alpha)) )

    dm/dt * v(rocket_final) + dv/dt ( m- dm/dt *t) = - dm/dt * v(gasses) *cos(alpha)cos(d(alpha))

    3. The attempt at a solution

    I tried to express the change in momentum of the rocket w.r.t. the angle of motion like above for both x- and y- axis, but I get unknown velocity and angle.
  2. jcsd
  3. Apr 26, 2009 #2


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    Taking the direction of motion of the rocket to be the x-axis, write the momentum conservation equation in the y direction. This will give you the final velocity of the rocket in the y direction.

    Now, you know both the x and y components of the final velocity, and you can calculate the angle that the velocity makes with the x-axis using trigonometry. (Hint: Draw a picture. What is tan(θ) in terms of Vx and Vy?)
  4. Apr 26, 2009 #3
    Thank you, dx.

    But the problem I see is, that the direction of gasses also changes with moving rocket. Therefore also Vx changes. The whole momentum does not go into Vy.
  5. Apr 26, 2009 #4


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    The direction of motion changes, but the orientation of the rocket stays the same. So the direction in which the gas is ejected is the same.
  6. Apr 26, 2009 #5
    What refers orientation of the rocket to, dx?

    I thought of this problem again and maybe this is a solution.

    Let us say, that the rocket would push the gasses with a given Vr=3 km/s in front of itself. Then its final Vx would be 19,2 km/s.

    On the other hand, if the rocket would push the gasses with a given Vr perpendicularly to the initial direction of motion, its final Vy would be 0,86 km/s.

    Therefore in any case 0<Vy<0,86 and 19,2<Vx<20. Therefore the angle for which it turns is
    0< theta < 2,6 degrees.

    Using the fact about the angle, we can see, that the change in Vx in x-axis can be neglected, since sin(2,6)~0,045. Also since cos(2,6)~0,998, we can take V=Vr for the relative velocity of gasses in y-axis during turning.

    Therefore we can really assume that the gasses are ejected perpendicularly to the initial direction of rocket's motion.
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