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Aluminum Rocket Questions

  1. Jun 3, 2006 #1
    Hi, I have some questions about using Aluminum as a rocket fuel with
    According to my calculuations, V_ex = sqrt(2*Ed) where V_ex = exaust
    velocity at 100% efficiency and Ed = energy density of the propellant
    in J/kg. This is proportional to sqrt(T/m_w) where T is temperature and
    m_w is molecular weight.

    Let's try this with LH2-LOX first: H2O has a molecular weight of
    18.02e-3 kg/mol and enthalpy of formation of 242e3 J/mol. V_ex =
    sqrt(2*242e3/18.02e-3) = 5182.57 m/s.
    Dividing by 9.8 we get 528 s so at 88% efficiency we get 465 s which is
    the actual specific impulse.

    Next let's try this with Al-LOX. V_ex = sqrt(2*1675.7e3/101.96e-3) =
    5733.2 m/s.
    Dividing by 9.8 we get 585 s, so it beats the space shuttle main
    engines! At 88% efficiency we get 514 s. This is enough to reach space
    in 2 stages very easilly.

    We can only conclude from the above that the combustion temperature of
    Al-LOX is very, very hot. How can I find the temperature exactly, to
    determine if the Al_2_O_3 product will be in gasseus form or not?

    My questions are:
    1. How can I find the ideal mixture ratio of oxygen to aluminum? Why
    don't LH2-LOX rockets just have twice as much H_2 as O_2??? Why 4 times
    the O_2 as H_2 instead? It seems to me this reduces the combustion
    temperature, thus increasing the mass ratio at the expense of the
    exaust velocity (specific impulse). Will an molten Aluminum-fueled
    rocket need excess LOX too?? How can I find the ideal ratio? I want to
    compute the volume of tanks but to do this I need to know the
    Oxygen-Aluminum ratio.

    2. After passing through the nozzle, will the Al_2_O_3 exaust be a
    liquid? Who ever heard of a liquid exaust rocket, except
    water-compressed-air rockets! But my understanding is that as the
    exaust passes through the nozzle it cools and speeds up, like a heat
    engine. I imagine then the exaust will be liquid, molten Al_2_O_3!
  2. jcsd
  3. Jun 3, 2006 #2


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    https://www.physicsforums.com/showthread.php?t=122759 [Broken]
    Last edited by a moderator: May 2, 2017
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