[AR] Re: airbreathing engines (was Re: Re: to the stars, soon)

  • From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
  • To: Arocket List <arocket@xxxxxxxxxxxxx>
  • Date: Tue, 5 Apr 2016 15:49:37 -0400 (EDT)

On Tue, 5 Apr 2016, Ian Woollard wrote:

Up to hypersonic speeds, jet engines beat the ass off rockets for
efficiency. Weight, no question, rockets are mcuh lighter, but they don't
hold a candle for efficiency.

And you are probably the only person in the world who cares. :-) That simply isn't an important figure of merit for real propulsion.

Usually the key figures of merit are Isp -- *fuel* efficiency, not energy efficiency -- and propulsion-system mass (and cost).

Basically, any supersonic jet engine is an inlet (basically a De Laval
nozzle run in reverse), a burner, and another De Laval Nozzle. De Laval
Nozzles and inlets are pretty damn efficient, they're pretty much adiabatic.
You don't lose a lot.

And transferring momentum to air passing at high speed still requires the engine to burn more and more fuel -- so the Isp deteriorates -- as you go faster. This isn't about losses (although they usually do make the situation even worse), it's about the nonlinearity of kinetic energy. Please stop obsessing about the "efficiency" of such systems; nobody but you cares. What matters is the lower Isp and higher engine mass, which are both theoretically predictable and observed facts.

(And characterizing a supersonic intake as "a De Laval nozzle run in reverse" is a thundering oversimplification if there ever was one.)

      jet must accelerate x lb of air by y ft/s to produce thrust z...

No, the faster you go, the more air is going into and out of the engine per
second...

Sorry, wrong, fundamental misconception (although a common one). In practice, particularly at ramjet speeds and above, to keep airframe loads and heating tolerable, you must increase altitude so air density drops *faster* than the speed increases. (Remember that dynamic pressure scales with the square of speed, and heating roughly with the cube.) Which means that unless the intake can somehow get larger and larger, it will deliver less mass flow, not more.

Henry

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