Re: SSTO to LEO, 80,000 pound payload or Bust. Jeff Findley achieves skunkitude.

In article <1142180574.345817.8690@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
<ianparker2@xxxxxxxxx> wrote:
I think it is fairly obvious, given all other things that a
stoiciometric mixture (in vacuo of course) gives the maximum energy.

A stoichiometric mixture does indeed give maximum energy release, assuming
that the flame isn't hot enough to cause dissociation of the reaction
products (not a safe assumption for rocket engines, by the way).

But that's thermal energy. Converting thermal energy to kinetic energy is
a different story, especially in very rapid expansion. Some exhaust gases
are good at that, and others aren't; notably, H2 is considerably better
than H2O. Which is why most rocket systems yield best performance running
fuel-rich, and LOX/LH2 in particular gives best results running *very*
fuel-rich -- the penalty in energy release per kilogram is more than paid
back by more efficient energy conversion in the nozzle.

The stoichiometric ratio for LOX/LH2 is 8:1, but maximum exhaust velocity
typically requires running about 4:1 -- with only about half the LH2
actually burning. Real rockets actually tend to run around 6:1, because
LH2 is so ridiculously bulky that if you go still richer, you lose more in
extra tank mass than you gain in improved exhaust velocity.

4 parts O to 1 H by weight is stoiciometric.

Uh, no. Atomic weight of O is 16, atomic weight of H is 1, and you need
two Hs per O. So stoichiometric is 8:1.

The Shuttle is 3 parts O to 1H.

Uh, no. 616_t of LOX, 103_t of LH2, essentially exactly 6:1.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | henry@xxxxxxxxxxxxx
.

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