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Impulse and Momentum of a rocket

  1. Jan 8, 2015 #1
    1. The problem statement, all variables and given/known data
    Small model rocketry engines are sized by specific impulse. One common size is a C6 engine. With each letter increase in engine size. The "C" engine is 10.0 N -s specific impulse. The "6" indicates that the engine produces an average thrust of 6 newtons.

    How much time does the engine burn?

    2. Relevant equations
    P=mv
    J=F*T
    F*T=M(Vf-Vo)

    3. The attempt at a solution
    Just started this unit today worked through the rest 10 problems only one that i have no clue about.
     
  2. jcsd
  3. Jan 8, 2015 #2
    My attemps were using all the equations posted and not making progress tried using kenimatics but dont have distance i think vo and vf is 0 but i know thats wrong because it would make it not move
     
  4. Jan 8, 2015 #3

    haruspex

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    You are given an impulse and a force, and you want to determine a time. Which of the three equations you posted has such variables?
     
  5. Jan 9, 2015 #4

    collinsmark

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    I'm sorry if I'm about to confuse you, I must point out an inconsistency in the problem statement.

    The problem statement gave the impulse as 10.0 N⋅s. Yet it called that "specific impulse." Both of those facts cannot be correct. I'm guessing that the problem statement should have said, "The 'C' engine is 10.0 N⋅s total impulse." There is a difference between total impulse and specific impulse.

    Let me run some definitions by you:

    Total impulse:
    This is the average force of the engine times the amount of time the engine burns. This is the one that is consistent with your relevant equations. It has units of force times time, thus in the SI system, it has units of N⋅s (agrees with the problem statement, in-so-far as the units are concerned).

    Specific impulse:
    This is the impulse generated by one unit of fuel, divided by one unit amount of fuel. And now this one gets a little more complicated because you can express the unit "amount of" fuel in either mass or weight. If the unit amount of fuel is measured in terms of mass, then specific impulse has units of velocity (such as m/s). If the unit amount of fuel is measured in terms of weight, the specific impulse is expressed in units of time (such as seconds). Specific impulse is a good measure of how efficiently the engine uses fuel, but has no bearing on the total amount of fuel, or total amount of impulse.

    Small model rocketry engines are typically specified according to their total impulse, not their specific impulse. This also agrees with the units given in the problem statement. So if I had to guess, cross out "specific" in the problem statement and replace it with "total."

    [Edit: by the way, if the problem actually did mean specific impulse (albeit with wrong units), there isn't enough information to solve the problem. That's another indication that "total" impulse is what was intended to be written.]
     
    Last edited: Jan 9, 2015
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