[AR] Re: Orions and PDEs (was Re: More MAX delays.)
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Thu, 30 Jan 2020 13:53:16 -0500
On Wed, Jan 29, 2020 at 07:50:04PM -0500, I wrote:
Uh, now we're back to where we started. What thermodynamic
advantages, other than an artifact of using an irrelevant performance
metric?
Thinking about it, I can see the advantage for jet engines, which
seems to be the usual context for discussions of pulse detonation
engines. If you use combustion to compress the fuel-air mix from,
say, 6 bar to 30 bar, you're saving all the shaft horsepower that
would otherwise go into that compression, and you can use that
horsepower to drive the fan in your turbofan setup -- the usual 'jet
engine' these days being a turbofan. That's indeed a clear win, if
you can pull it off.
But rockets don't spend nearly as much energy on compression as jet
engines do: they're pumping liquid, not gas, and that's much lower
volume. They also don't have the fan; if you were to extract
mechanical horsepower from a rocket engine, there'd be nothing to
spend it on that could boost thrust -- or at least nothing comparable
to leaving the energy in the combustion chamber and letting it boost
thrust that way.
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