Supersonic Engines

TrinidadGT20

TrinidadGT20

Vice President of Awesome
What is different about an engine on a supersonic plane versus that of a commercial airliner? Could, say a GE90, provide mach 1+ speed on an airframe that is capable of supersonic flight? Obviously, I'm a complete ignoramous in this subject. I'm just curious...

As always thanks in advance for any replies.
 
Supersonic flow and high-bypass fans don't get along well. The power is probably there to get something like a 777 to just ofver the speed of sound (maybe with a dive), but fans produce thrust with a relatively low pressure differential (think EPR, but not). Once you've got flow being mechanically compressed at the face of the fan, the engine has to work a lot harder to generate a sufficient differential pressure to keep pace. Supersonic flow is not as big a deal for a turbojet, because all the air going into the engine is going to be compressed to pressures several times higher than you could ever hope to get with supersonic flow stagnation.

I have a sneaking suspicion that this was a major hurdle for the Sonic Cruiser. You'd have to have engines that can get you off the ground in 6,000 feet (fans are better at this), but still get you 6,000 miles away at trans-sonic speeds (jets are better here).
 
It could use a system like what's implemented on the F-14. As the airspeed increases, inlet doors begin to close to keep the speed of the inlet air below Mach 1.
 
Every aircraft with conventional jet engines that can exceed the speed of sound is designed so that the air entering the engine is first slowed to subsonic speed. There also has to be a mechanism to limit the amount of airflow entering the inlets. As you go faster and more air is stuffed into the inlets, you don't need as big of an air intake.

It is true that at extremely high speeds turbojets hold the advantage, but this is only at speeds well in excess of Mach 1. All modern fighters have a turbofan engines, although the bypass ratios are usually between 1:1 and 2:1. Commercial transport engines are around 5:1 or even higher for the newer designs. Any transport designed for speeds near Mach 1 would surely have turbofan engines.

I was puzzled by the whole sonic cruiser concept, because they wanted to operate right around Mach 1 and due to transonic drag rise, it is better to go either slower than the speed of sound, or a little bit faster. There is actually more drag right at Mach 1 than there is if you go faster.
 
From what I have learned, any jet engine needs a subsonic flow into the compressor stage. It can be performed with either many oblique shocks or one normal normal in the inlet. Flow past an oblique shock is generally supersonic still, but the flow after a normal shockwave is subsonic. Subsonic flow is then fed to the compressor.
The compressor needs a subsonic flow as I havent heard of many compressor blade (or prop) that will take supersonic flow. Once you have flow seperation on the compressor blades, the compressor section as a whole will stall, and you loose power to that engine.
In short, I dont think the GE90 would operate at supersonic airflows. Unless some inlet was made to induce a normal shock on the supersonic flow, I cant see it happening with a fan face like that, and no ability to process a supersonic flow. Interesting topic though.
Go pick up a book called Aerodynamics for Naval Aviators; it's a true gem.
 
I read a book written by a guy who used to fly the SR-71 (really good book if you get a chance to read it) called Sled Driver. He said that the big cones on the front of the SR-71 engines were there to slow the air going into the engine down below the speed of sound. They moved forward or backward according to the speed of the aircraft, but sometimes they would not work right and one engine would have a large drop in thrust. Needless to say when you're going that fast it can get pretty violent. Again, really cool book if you are looking for something good to read.
 
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I read a book written by a guy who used to fly the SR-71 (really good book if you get a chance to read it) called Sled Driver. He said that the big cones on the front of the SR-71 engines were there to slow the air going into the engine down below the speed of sound. They moved forward or backward according to the speed of the aircraft, but sometimes they would not work right and one engine would have a large drop in thrust. Needless to say when you're going that fast it can get pretty violent. Again, really cool book if you are looking for something good to read.

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In the book "Skunk Works" by Ben Rich, they talk about "unstarts" or something like that. Its when there is a disruption of airflow as a result of those cones being in the wrong position and results in a flame out. Interesting stuff
 
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It could use a system like what's implemented on the F-14. As the airspeed increases, inlet doors begin to close to keep the speed of the inlet air below Mach 1.

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A lot of aircraft implement systems like that. the MiG-21's nose cone is movable to slow the airflow down.
 
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