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so I found out that SR-71 Blackbird is using what's called "turboramjet" and I find the idea a bit appealing since they say with such engine, the Blackbird is more fuel efficient at it's top speed. I know that the mechanism for such engine is extremely complicated, but what's on my mind is not making an engine like Blackbird's, more to the point of having a direct supersonic fresh airflow to the afterburner to keep the fuel consumption slightly lower thanks to more oxygen to keep the afterburner effectively burn(because I heard somehwhere afterburners burns 3 times more fuel than dry thrust, I hope this can reduce it to only twice or even 1.5 times original dry thrust fuel consumption and have the same performance). Is it possible to do such thing? what would be the main problem to such engine/design? can it produce more thrust/more efficient in fuel consumption? and for the sake of calculations, let's say the turbofan engine in question is GE F-414-EPE, on afterburner, in supersonic speed.

Albert G.
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  • Are you asking whether the turbofan you mention could be modified to become a ramjet (or well, ram-fan)? There's a lot of fluff to your question which makes it hard to condense your main point. – Sanchises Oct 08 '16 at 08:45
  • not necessarily a ramjet, more on to make a direct fresh airflow to enhance the combustion process in afterburner, the jet engine is still running during that time unlike Blackbird's turboramjet – Albert G. Oct 08 '16 at 09:20
  • A turbofan already has bypass air, which could be considered 'fresh' air for the afterburner. So I'm not sure what else you're looking for. – Sanchises Oct 08 '16 at 11:37
  • you're right, I might have to edit the question a lot...

    I'm was looking for a way to put direct supersonic airflow into the afterburner, since turbofan's bypass air is not really supersonic, I'd like to see if I can add more air for that

     – Albert G. Oct 08 '16 at 12:24
  • Burning fuel in super sonic flow is hard. It's in active research but hasn't made into any service aircraft yet. Also your question makes it a much much harder because it implicitly requires the turbine exhaust be super sonic, and potentially also super sonic in the main combustion chamber. Most jet engines, including turbofan, turbo jet, or rocket engine doesn't have any super sonic flow until the nozzle. If you want supwersonic flow from the intake, through compressor, combustion chamber, turbine, into afterburner, plus supersonic bypass. I think 20 years of R&D from now is quite optimistic. – user3528438 Nov 15 '17 at 00:37

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The J-58 took compressed air from the compressor at stage 4 and piped it directly into the flow aft of the turbine. This cooled the exhaust stream that entered the afterburner, so the starting temperature there was lower and the density higher, which increases efficiency and thrust. Note that the intake of the SR-71 slowed the flow down to Mach 0.4, so all internal flow was subsonic. Only when the hot exhaust gas in the afterburner expanded again would the flow speed increase to supersonic speeds again.

This is only possible because the intake would already compress the air by a factor of nearly 40 when flying at Mach 3.2. This precompression scales with $$p_0 = p_{\infty}\cdot\frac{(1.2\cdot Ma^2)^{3.5}}{\left(1+\frac{5}{6}\cdot(Ma^2-1)\right)^{2.5}}$$ so the precompression is much lower (less than 6) for a typical F-414 maximum speed of Mach 1.8.

($Ma$ = Mach number, $p_0$ = ram pressure, $p_{\infty}$ = atmospheric pressure).

The gains in efficiency scale with the ratio of starting and exhaust temperatures (measured from absolute zero), so the increase in efficiency is much lower than what you seem to hope for.

Make sure that flow speed at the entry of the afterburner is decidedly subsonic. Supersonic flow at the start of the afterburner section will only delay ignition until the flow exits through the nozzle. All afterburners have rings inside, called flameholders, which cause local separated flow, so some burning gas is always present to ignite the newly arriving fuel-air mixture.

Peter Kämpf
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