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Rocket Science - need confirmation

  1. Mar 17, 2013 #1
    1. The problem statement, all variables and given/known data
    given a graph of the thrust of an engine, we need to determine what mass to make our rocket so that it goes 15m into the air. Here is the graph:
    http://www.321rockets.com/content/image/26402/800/A8_thrustcurve.jpg

    I just need some confirmation on if I'm doing it right.

    2. Relevant equations

    Fnet= Ft+Fg=ma


    3. The attempt at a solution
    Using the section of 0.0s-0.1s for refernce:
    - Find the average thrust force during this time interval (my teacher talked about how later we would use impulse for this, but we haven't learned that yet, so I calculated average force using the inital force for that interval (0N) and the final force for that interval (5N). Add together and divide by two, giving me 2.5N. This means during the first 0.1s of the rocket's flight, the average thrust force acting upon it is 2.5N.
    -I add this with the force of gravity to determine the net force. Net force also equals ma, so i divide that number my the mass for the acceleration value.
    - Using the initial velocity (0 for this segment), the time (0.1s) and the acceleration, i use the big 5 kinematic equations to find the displacement and the final velocity. This final velocity is the initial velocity of the next segment. The displacement is D1.
    - Keep finding D2,D3,..etc and add them all together, hopefully giving me a TOTAL distance of 15m. But I know that the last displacement (rocket going up but the engine is no longer thrusting) will only have a net force of mg, so i take that into consideration when calculating.


    good?
     
  2. jcsd
  3. Mar 17, 2013 #2
    Anyone? Please I need help ASAP!
     
  4. Mar 17, 2013 #3

    rude man

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    Gold Member

    I don't know about using average force.

    Instead: how about
    ∫F(t) ds = mgh, h = 15m where F(t) includes thrust and gravity.
    ds = v dt
    dv/dt = F(t)/m
    dv = (1/m)F(t) dt
    find v(t) by numerical integration
    Then substitute for ds in the above integral to solve for m, again numerically. Two numerical integrations!

    BTW this is not the first time this probem has been posted, and I remember someone pointing out that the problem does not state the loss of m during burn, so presumably you will choose to ignore it.
     
  5. Mar 17, 2013 #4
    Is that calculus? I won't be learning that for another 2years in the school system :/
     
  6. Mar 17, 2013 #5

    rude man

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    That makes things difficult, meaning for me.
     
  7. Mar 17, 2013 #6
    Is my method workable though? I'm assuming it won't be as efficient or accurate, but will it still give me accurate enough data to use?
     
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