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Hydrogen-Oxygen Rocket Engines

  1. Oct 25, 2006 #1
    Hi !
    I remember an experiment in school where two volumes of Hydrogen and one of Oxygen were mixed in a test tube set upside down on (at the time !!) a platter full of mercury.
    A spark in two electrodes on top of the tube would trigger an IMPLOSION and the mercury would jump upward to fill the volume. Miracle: in a residual space probably due to the imperfection of the ratio of the mixture, small droplets of WATER were visible!
    My question concerns Hydrogen/Oxygen rocket engines. I never got a satisfying answer to this question:
    I understand that the usual mixtures of propulsion create an instantaneous violent & important expanding volume of mainly CO2 being expelled on the back of a rocket and create a reaction thus a propulsion.
    But how something that I understood apparently wrongly as a reduction / IMPLOSION can push a rocket ??
    Jacques D. Paris France
    Thx to your old folks for coming o:)
  2. jcsd
  3. Oct 25, 2006 #2


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    2 parts Hydrogen combusted with 1 part Oxygen forms water. H_2 O remember? The water left in the tube was the PRODUCT of the combustion of the gases. Why did the Mercury fill the tube? Because the volume occupied by the water was less then that occupied by the original gases.

    The combustion of O and H releases a LOT of energy, it is this energy which propels the space shuttle. The white plume of exhaust gases is WATER vapor, not CO_2
  4. Oct 25, 2006 #3
    Thanks for your answer Integral. In:
    I was refering to most others ie:Black Powder generating expulsed gases.
    Bye and thanks again.
  5. Oct 25, 2006 #4


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    For the experiment in the OP, the water condenses (as Integral said) because even though the reaction produces a lot of energy for the material consumed, it isn't a lot compared to the heat capacity of the test tube and mercury, so the water vapor quickly cools and condenses.
  6. Oct 25, 2006 #5


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    Let's look at this in more detail using the ideal gas law.

    The chemical reaction is 2H2 + O2 = 2 H20.

    The pressure of an ideal gas is given in many places, for instance

    PV = nRT, or if we count in molecules, rather than moles, PV = NkT

    For our example, it's convenient to count in molecules, N, rather than moles, n, so the correct formula is PV = NkT.

    Now, when the above reaction occurs, N goes down - N is intially 3, and goes down to 2.

    But T goes up - way up. T starts at 300K at room temperature, and increases to about 3000K (per http://hypertextbook.com/facts/1998/JamesDanyluk.shtml )

    If we hold the volume constant, we can ask what happens to the pressure. P = NkT/V, where V is a constant. So P is proportional to the product of N T. The decrease in N by a factor of 2/3 is more than made up by the 10x increase in T.

    I would predict that high speed photos would show the mixture initally expanding (pushing the mercury down, before it cooled off and contracted).
  7. Oct 25, 2006 #6
    when 2 things bond the energy used to create the bond is released =]
  8. Oct 26, 2006 #7
    Thanks pervect and everyone.
    This talks to my down to ground way of understanding things, :wink: . ( I do not have a formal scientific education or diploma just curious of everyhing)
    I was of course aware that energy is pushing the rocket It cannot be denied :approve:
    Over time, it is usually the only answer I got but it did not feed my curiosity. I would grumble using my sparse (or spare) neurones something like...
    """So, if one would violently dissipate a tremendous amount of Joules from enormous capacitors in a suppositely as big gas discharge flash device thus generating an intense radiation of this energy light in the reflector's direction would this crazy lampost be pushed violently backward"""
    Last edited: Oct 26, 2006
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