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Homework Help: Spacecraft landing on an alien planet

  1. Feb 22, 2010 #1
    1. The problem statement, all variables and given/known data

    A spacecraft of mass m0 is descending with velocity v0 to land on an alien plant where the value of g is 1/6 of g on the earth. In order to land safely (meaning the final velocity upon landing is zero), fuel has to be burnt at a constant rate dm/dt=-k, where k is a constant. How far above the surface of the planet should one begin firing the spacecraft (assume constant deceleration)

    2. Relevant equations

    m = m0 - kt

    3. The attempt at a solution

    I am trying to use my knowledge of rocket motion. But i am having a hard time picturing the problem. Any comment/help will be great.
  2. jcsd
  3. Feb 22, 2010 #2
    starting with
    v - v0 = vexln(m0/m)

    v= final velocity
    v0 = initial velocity
    vex= exhaust speed relative to spacecraft
    m0= initial mass
    m = final mass

    because final velocity has to be zero and assuming constant exhaust speed, i simplified the above expression to get t.

    t = m0(e-v/vex -1)/ke-v0/vex
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