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Comment by votingprawn | Hacker News Reader

> The Lockheed SR-71 for instance, derives a fair percentage of thrust just from the shape of the airframe, so the pilots actually throttle back a little once they reach Mach 2+!

That sounds...unlikely. Do you have a source for this?

I have a suspicion you might be conflating the "turboramjet" aspect of the engine design, with the air-frame actually generating thrust.

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I believe he's referring to the the high pressure acting against the aft-facing region of the inlet compression centerbody. Kelly Johnson famously claimed that at cruise, the inlet contributed more to thrust than the engines did. That was a bit of a deliberately provocative statement, made with an element of jest, and based on the accounting conventions one uses for for thrust. The pressure distribution necessary to achieve these results would not be possible without the engine serving as a pump for the inlet-captured mass flow, and providing the high-pressure boundary condition at the outlet of the inlet.

Ah, so I guess it comes down to whether you consider the inlet and exhaust nozzles to be a part of the engine, or just a part of the air-frame.

Personally I'm of the mindset the inlet and exhaust nozzles are an integral part of the installed engine, particularly in this case given the rather unique intake/engine/exhaust design.

Yes, agreed. If we were discussing hypersonic airbreathing configurations, for example, no one would be making those kinds of statements. Once you get into the realm of high speeds, the integration between the propulsion components becomes necessarily deeper and tighter, and trying to parcel out that kind of credit is really more of a basis for engineers to rib each other over during happy hour, rather than a rigorously defensible position.

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