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Homework Help: Ideal Rocket Mass Distribution

  1. Jun 23, 2015 #1
    Note: I have noted that one of the formulas I have provided does not show up on my webpage in preview mode and so have also made it as an attachment.

    Quick summary: I have found two different websites that offer the equation that I am looking for in different forms, but I am starting to think that the equation is actually incorrect. So I'm really asking if any rocket engineers could assure me that the equation I am using is indeed incorrect and (if I'm lucky) point me to a reference where I can get find the actual equation. I have actually spent a long time (two days) looking for a better formula but the web has not proven very helpful and the books I have read have not gone into enough detail for actual calculations. I do not actually have a textbook (one does not exist) because this is not strictly coursework.

    1. The problem statement, all variables and given/known data

    Find the ideal mass distribution and total mass of a three stage rocket assuming:

    1. A specific impulse of 300 seconds for all three stages
    2. A ##\Delta## V of 9000 m/s
    3. A dry mass fraction for each stage of 0.35
    4. There are three stages
    5. Payload is allowed to be left as an unknown

    2. Relevant equations
    https://www.physicsforums.com/cid:F649DCE6-2FDC-453C-830C-5D2F46A91771@guardedsystems.com [Broken]

    M = final mass of the rocket (all three stages added together)
    A = payload mass
    S = dry mass fraction
    Vf = final velocity: (considering the rocket starts from 0 m/s) is ##\Delta## V
    C = exhaust velocity

    https://math.la.asu.edu/~nbrewer/Fall2007/MAT267/RobertWagner/RobWagner%20Footnote%20181.html [Broken]

    - This assumes a constant inert-mass fraction throughout the rocket, which for my analysis is a given. It is clean because it uses a final formula rather than me having to partially derive one by adding up all of the stages of the rocket.

    ## V_{exh} = I_{sp} * g_0## (For a rocket blasting off earth) (I'm using 9.8 for ##g_0## for now)

    3. The attempt at a solution

    I can calculate the ##V_{exh}## to be 2940 using formula 2.
    The rest is just plugging into formula 1 from which I can get a ratio between the final mass of the rocket and the mass of the payload. Here's the calculation:

    ## M = A \bigg( \Big((1 - .35) e^{9000/(3 * 2940)}/\big(1 - .35*e^{9000/(3 * 2940)}\big)\Big)^3 - 1 \bigg) ##

    The end result is that M = 240805 A or that the total mass is 240 thousand times the payload. The Saturn V has a mass that is only roughly 17 times its payload (borrowed from mass data on Wikipedia here: https://en.wikipedia.org/wiki/Saturn_V). When I make ##g_0 ## only 9.3 I suddenly get a negative number for my mass payload ratio. This is telling me that I'm working with a very finicky formula and that it is probably incorrect.

    I have done this equation in a more tedious fashion using a stage by stage analysis provided here:

    Unfortunately this has led me to the same unrealistic answer, but prevents me from assuming my formula is incorrect (since two different sites seem to agree on the process).

    Thanks for taking the time to read this.

    Attached Files:

    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Jun 23, 2015 #2
    Sorry for the trouble. Actually my assignment provider changed the dry mass fraction because he realized that the parameters we were given were not feasible. Everything is resolved now. I hope this thread did not cause any difficulties. Thanks for the time.
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