Direct injection precooling

T.Neo

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This silly little idea was inspired by the precooled SABRE engines. The premise is; instead of running the fuel (or in the case of SABRE, fluid cooled by the fuel) through channels in front of the engine, why not directly inject it into the air, to cool the inlet surface and more effectively cool the air entering the engine?

If hydrogen embrittlement is too much trouble, maybe a fuel such as methane, or even a heavier hydrocarbon, would be a better option. I'm not sure what this would do to compressors; maybe it might be a better option for some kind of scramjet.

Would this work/be practical/be applicable? Does it have a shred of usefulness anywhere? I'm inclined to think not, hey... there's always the infinite monkey theorum. :facepalm:

KEY:
Blue: Air coming into the engine.
Green: Fuel injected directly into incoming air.
Light blue: Cooled air/propellant mix.
Yellow: Combustion.
Orange & Red: Exhaust.
 
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Urwumpe

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The problem is: How do you mix it into the air flow, without badly ruining exactly this airflow?
 

RisingFury

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I did a few calculations about a year ago on a similar concept for a ramjet - injecting compressed butane into the airstream to cool the air. As the butane expands, it cools and it cools the air as well. After that, the air would be re-heated to increase pressure. The problem with ramjets is that they suffer from low compression ratio at low airspeeds. My calculations indicated an additional compression ratio of around 4...

I found a few problems with that idea though... for one, the turbulence in the airflow would suck, second, the butane-air mixture would need some time to actually cool the air down. The engine would then become quite long. The third is that there's water vapor in the air, which would condense and freeze. I didn't calculate the amount of butane needed for an ~hour long flight and did not estimate the tank dimensions and it's mass. Butane would need to be stored at high pressure before the flight - compressing it in flight probably wouldn't work, because it would heat up and you couldn't get rid of the heat fast enough.
 

RGClark

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This silly little idea was inspired by the precooled SABRE engines. The premise is; instead of running the fuel (or in the case of SABRE, fluid cooled by the fuel) through channels in front of the engine, why not directly inject it into the air, to cool the inlet surface and more effectively cool the air entering the engine?

If hydrogen embrittlement is too much trouble, maybe a fuel such as methane, or even a heavier hydrocarbon, would be a better option. I'm not sure what this would do to compressors; maybe it might be a better option for some kind of scramjet.

Would this work/be practical/be applicable? Does it have a shred of usefulness anywhere? I'm inclined to think not, hey... there's always the infinite monkey theorum. :facepalm:

KEY:
Blue: Air coming into the engine.
Green: Fuel injected directly into incoming air.
Light blue: Cooled air/propellant mix.
Yellow: Combustion.
Orange & Red: Exhaust.

There was a proposal to use water to inject into the airf flow to cool the engine to allow airbreathing at near hypersonic speeds. This would increase the mass load that had to be carried in addition to the dry mass but would increase the thrust. There must have been some reason why it was not deemed sufficient to use the kerosene to inject into theair flow. Perhaps because it did not provide sufficient cooling.
See here for this concept called Mass Injection Pre-Compressor Cooling (MIPCC):

RASCAL — A DEMONSTRATION OF OPERATIONALLY RESPONSIVE SPACE LAUNCH.
http://www.responsivespace.com/Papers/RS2%5CSESSION%20PAPERS%5CSESSION%208%5CLOPATA%5C8004P.pdf

Bob Clark
 

Urwumpe

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There was a proposal to use water to inject into the airf flow to cool the engine to allow airbreathing at near hypersonic speeds.

Proposal? The MiG-25 does that!

But it uses a different kind of engine, which is the difference there. The MiG-25 does water-methanol injection to permit flying at Mach 3 for a short time with relatively normal low-bypass turbofan engines.



This cooling is activated already above Mach 1.5
 
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