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Supersonic Flow, Kinetic-Heating And Sturdiness

Tue Jan 01, 2008 2:22 am

One problem which I've heard about on supersonic airplanes, particularly ones that spend long amounts of time at supersonic speeds, like supersonic bombers (XB-70, B-1A, Tu-160), high-speed recon aircraft (SR-71), and supersonic-transport concepts (L-2000, B-2707, HSCT, AST).

Now, I know airframe shape plays a significant role in the kinetic heating of the plane. A highly-streamlined clean design doesn't reach as high a temperature for the same mach number as one that's not as cleanly designed. But I'm wondering if it's more than that why fighter planes can do Mach 2.5 or so with aluminum skin and an SST to achieve Mach 2.5 would need at least some titanium in it's design. Especially considering that fighter planes actually are not as cleanly designed as a larger supersonic aircraft to the best of my knowledge, relying more on extreme thrust to weight ratio than extroardinary aerodynamic cleanliness except some old interceptor designs not that they are high-drag designs.

My only guesses are that

-Fighter planes are a hell of a lot sturdier, with thicker skin, and a thicker internal support structure -- they're designed to pull 9 G's or more, in the olden days it was 7.33 G's: Way more than an airliner which requires at least 3.75 to 5.0. The guess is that since fighters are so much sturdier, that even as they weaken a bit, there's still so much strength there that it still will hold up until a higher speed.

-Large supersonic-aircraft fly much longer at supersonic-speeds than fighter-sized and as a result fully heat-soak when fighters don't. But the thing that has me wondering is if fighters are smaller and less massive than the larger planes, wouldn't they heat-soak faster?

Andrea Kent
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RE: Supersonic Flow, Kinetic-Heating And Sturdiness

Tue Jan 01, 2008 3:54 am

Well heat is measured in either joules per second or BTU's per hour, so yeah more time spent in a high heat environment (like the compressed air in a mach cone) means the vehicle/object will get hotter. But to a point; you cannot boil a gallon of water with a 10W heat source, the water will absorb to a point and maintain a contant temperature above ambient as the water would radiate the rest of the heat. Does it happen to metals and ceramics and composites, sure and they are certainly mass/surface area dependant.

While fighters are much smaller than those few supersonic airliners we know of, they rarely go supersonic, much of their flight and mission profiles are subsonic. Supersonic is only for dashing or intercept or aborting a mission.

Add to the fact that because fighters aren't designed to sustain supersonic flight for much longer than as much fuel they carry, I'd suppose they don't need to worry about overheating the fuselage. Think about it, say I'm in an F-15 doing 0.8 and throttle up to 1.6, the stagnant compressed air temp out there is 34% higher, but that doesn't mean jet will immidiately reach that temp, it takes a bit of time to heat up.

No doubt, the design of the aircraft will vary the effect of compressed air as the heat source and vary the time required to heat up the plane.
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RE: Supersonic Flow, Kinetic-Heating And Sturdiness

Tue Jan 01, 2008 5:28 pm

You'll find that the tech specs of a high speed military aircraft will state that their maximum loads will be lower at high supersonic speeds than at subsonic speeds. The typhoon for instance is only going to let you pull 6g at 1.6M whereas it will do over 9g at .9M. I dont think the heating will be much of an issue for the main structural parts but remember the comet that had the nose cone dent in when it was in flight? I think the skin around the nose would be the biggest worry in this case.

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RE: Supersonic Flow, Kinetic-Heating And Sturdiness

Tue Jan 01, 2008 8:07 pm

I would expect the parts most quickly to heat (nosecone, leading edges, etc.) are designed with more heat-resistant materials then other areas (general fuselage, wing panels) for aircraft designed to "dash" at supersonic speeds versus maintain sustained supersonic cruise.

Your best shot at seeing the differences are likely between the supersonic cruise B-1A and the supersonic dash B-1B since it the same airframe (in general) designed for two separate supersonic operating envelopes.

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