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Homework Help: Multi-stage Vs. single stage rockets

  1. Nov 27, 2016 #1
    1. The problem statement, all variables and given/known data
    Hello all
    I have to show why there advantageously can be employed multi-stage rockets

    2. Relevant equations
    How do i do this best, a graph/plot?

    3. The attempt at a solution
    i have done some calculations for a single stage rocket and a multistage rocket, but how can i show this more over all?
  2. jcsd
  3. Nov 27, 2016 #2
    It is very difficult to follow your equations. If you would like to make a more general statement use variables rather than numbers. Also define what those variables mean.
  4. Nov 27, 2016 #3
    Hello i have the equation with variables:
    Should i explain this mathematicaly, and if so can you give a little hint? :)

    Attached Files:

  5. Nov 27, 2016 #4
    Now you need to relate the two situations. What if the single stage and the two stage have the same total mass. Also say they have the same payload mass and fuel mass. The part that is missing is the mass of the structure. Say they have the same structure mass, but with the two stage rocket you get to throw away a large fraction of that for the second stage. Can you show the final v is greater in the two stage?
  6. Nov 27, 2016 #5
    Ps. In your first stage you do not burn all the fuel, so some of the fuel mass should appear in the denominator in the log.
  7. Nov 27, 2016 #6
    i have done it with structure mass and so, im not sure on how to relate the two situations, if they have the same payloat its mp =mp and same fuel mass; mf=mf1+mf2 - and the same structure mass then its ms=ms1+ms2?

    and why is there fuel left in the first stage?
  8. Nov 27, 2016 #7
    The fuel left after the first stage is the fuel carried in the second stage. The second stage has to have fuel!

    To relate them write the single stage eqn using the same terms assuming things are equal because you want to compare similar rockets.

    Same amount of structure: not ms, but ms1 + ms2,
    Same amount of fuel, so not mf, but mf1+mf2

    Now with both equations in the same terms can you prove algebraicly that the two stage velocity is greater than the single stage?
  9. Nov 27, 2016 #8
    Mann you are so great :D

    but im a bit unsure, because, in the 2. stage rocket the u, could be different in each stage, so upload_2016-11-27_20-5-48.png
    i have taken the part that goes out with each other, and i can see it must be larger.
  10. Nov 27, 2016 #9
    What you wrote in the first line in each kind of rocket is correct. I think the math leading to the second line in each case is flawed. That's ok. You only need the first line of each. Now put them on a line together with ">?" between them and do algebra until you prove it's true. You should be able to prove it's true as long as ms1 > 0. (And to be sure I'm not asking the impossible I did work it out myself)
  11. Nov 27, 2016 #10
    ok, just so im not going on a wild goose chase, i put the equations like so: upload_2016-11-27_21-5-31.png
    and work it out for ms1 > 0
    and ofc again thank you for helping
  12. Nov 27, 2016 #11
    Just do algebra. The inequality will get simpler and simpler until all you are left with is ms1>0. So if ms1>0 then the inequality was true from the top.
  13. Nov 27, 2016 #12
    okay thank you, i dont know why i think it is more difficult then it is... you know rockets... no, but really thank you for your help and your patience, i really appreciate it. i will go do some algebra :D
  14. Nov 27, 2016 #13
    Hint: the u cancel. Then exponentiate both sides and remember exp(a+b) = exp(a) exp(b)
  15. Nov 27, 2016 #14
    can you give a little hint, im having som trouble with the u, i always end up with a u, the u cancel a u on the other side, but what about the last u? because if the 2 u's are different, that could be a possibility in a multi stage rocket right?
  16. Nov 27, 2016 #15
    Well, you have to do legal algebra. If you don't know algebra then this isn't going to be easy. All terms have a u. When you divide both sides of the equation by u you must divide all terms by u according to the distributive property. You shouldn't wind up with a second u. Please think carefully about doing proper algebra.
  17. Nov 27, 2016 #16
    oh yes i see now :D much easier, now i get the correct solution, totaly forgot about dividing all the terms :D
    Thank you so much you were a great help :D
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