Post by Quadibloc Post by Peter Trei Post by Quadibloc Post by Quadibloc Post by Peter Trei
But I still don't understand is how it saves fuel. Perhaps it gets more complete
combustion while still within the engine.
One thing is that the fuel actually explodes instead of just burning. That would
already seem to imply the fuel is engaging in activity more relevant to thrust.
...oh dear, that's just silly. While one doesn't usually think of a gas stove,
say, as providing thrust, when something explodes, that's just expansion due to
heat happening quickly.
2 H2 + O2 -> 2H2O yields a fixed amount of heat pretty much regardless of
conditions. The only thing I can think of that makes this engine more fuel
efficient is if more of the combustion products can be directed out the back
of the rocket, instead of burning outside the bell in a bright flame.
I was thinking about it, and a different possible inefficiency occurred to me.
It could be that the rotating detonation rocket works better because more of the
heat goes into expanding the combustion products, thus providing thrust... and
less into heating up the rocket engine itself instead.
Typically, liquid fueled rocket engines are regeneratively cooled.
Propellants are circulated through the combustion chamber and nozzle
walls before being burned, so heat lost through these surfaces pre-heats
the propellants and the energy is recovered during combustion. Once
the engine is up to full power its temperature stabilizes.
Among liquid hydrogen/ liquid oxygen rocket engines, the Aerojet-Rocketdyne
RL10B-2 has the highest efficiency I've seen listed. It has a specific
impulse of 465 seconds, which works out to an exhaust velocity of 14,700
feet per second, or 4,490 meters per second.
Liquid hydrogen has an energy of combustion of about 120 megajoules per
kilogram when combined with oxygen and the resulting H2O remains in
vapor form, as it does in a rocket. Burning one kilogram of hydrogen produces
nine kilograms of water vapor. If all the energy of combustion of one
kilogram of hydrogen goes into kinetic energy of the resulting exhaust,
this works out to a theoretical maximum exhaust velocity of 5,160 meters
per second, or 17,000 feet per second. This is fifteen percent higher than
with the best conventional engines, so some improvement is possible,
but not a great deal.