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Rocket Fuel Problem

  1. Mar 7, 2012 #1
    1. The problem statement, all variables and given/known data
    A warlock testing a rocket broom takes off from a standing start and flies at fixed elevation. The warlock and fueled rocket system start out with mass Mi. The rocket fires with exhaust speed u, losing mass at a constant rate R, and we’ll assume air drag is negligible. If the rocket broom is to reach a velocity equal to 0.6u, what minimum fraction of the initial mass must be devoted to fuel?


    2. Relevant equations
    Conservation of momentum



    3. The attempt at a solution
    I really don't even know how to start. I looked at this topic but I do not understand how the fuel has initial momentum, or how to apply any of that in another situation
     
  2. jcsd
  3. Mar 8, 2012 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Bugeye :bugeye:! Welcome to PF! :smile:
    The question means that the difference in speed between the rocket and the fuel that's just gone is u.

    So call the mass "m" (as a function of t), and write a formula for what happens when a mass dm of fuel is expelled in time dt …

    what do you get? :smile:

    (all the other smilies say "hi!" too :yuck: :cry: :cool: :confused: :zzz: :redface: :frown: :eek: :uhh: :shy: :grumpy: :!!) :mad: :wink: o:) :tongue2: :surprised :devil: :biggrin: :blushing: :tongue: :rolleyes: :rofl: :approve:)
     
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