In article ,
Iain McClatchie wrote:
HS ... although there is also some risk of explosion.

I know that premixed LOX/liquid hydrocarbon is an explosive. But premixed
gas-phase O2/CH4 is reputed to be fairly tame. Certainly if you dilute it
with 4 parts N2 and run it at 1 atm it's pretty reasonable.

Depends on the mixture ratio. :-) Explosions in coal (etc) mines are
usually methane, and they can be pretty bad. But they, of course,
involve a very large volume of mixture.

HS Gaseous fuel mixes burn much faster if you stir some of the combustion
HS products back into the incoming mix; it's thought that they catalyze
HS otherwise-slow steps in the combustion process.

I didn't know this.

It's one of those occasional cases where for some reason, the full,
accurate explanation for something is found only in specialist literature,
while the accounts of the underlying theory in the engineering-design
books are oversimplified to the point of being actively misleading.

(The classic example of this is the notion that the molecular weight of a
chemical rocket's exhaust is important. All the rocket-design books say
so. Only when you read a book written by a propellant chemist -- Clark's
"Ignition!" being the canonical example -- do you find the real story.)

I know that one other thing flameholders do is provide
streams of mixture below flame speed, so that the flame always has a chance
to burn back down the flow and doesn't get blown off.

That's significant, although you would think that it could be overcome by
use of a hypergolic fuel, or continuous laser-spark ignition.

Do you know if the H2 in the SSME / RL10 / etc boils before it gets to
the injector face?

Yes and no. It doesn't boil, in the sense of a phase transition, because
pressure is well above hydrogen's critical pressure throughout -- there is
no sharp phase transition at such pressures. (Use of such pressures is
deliberate: boiling in the cooling passages is problematic because it's
unstable, with the constant danger that a brief increase in heat flux will
cause a transition to film boiling, which greatly *reduces* heat transfer
and generally leads to rapid destruction of the chamber wall.)

However, the hydrogen temperature at the injector is high enough that the
behavior is gas-like, with relatively low density and high flow velocity.
The injectors exploit this, with each element providing a slow-moving jet
of LOX at the core of a fast-moving jet of hydrogen, and the velocity
difference providing very effective atomization and mixing of the LOX.
(And the ends of the "LOX posts", the tubes that introduce the LOX into
the hydrogen stream, tend to function as flameholders.)

HS People have run RL10s on methane and propane without difficulty, using
HS the same basic injection strategy (gaseous fuel, liquid oxidizer).

Ah, but with methane the liquid volume ratio is completely different.
Did they get good Isp out of the RL10 running LOX/CH4?

They did, although they needed minor modifications to the RL10 to make it
work properly with methane or propane.
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