[AR] Re: Orions and PDEs (was Re: More MAX delays.)
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 28 Jan 2020 23:57:15 -0500
On Tue, Jan 28, 2020 at 07:57:14PM -0600, Jim Davis wrote:
On 1/28/2020 2:16 PM, Norman Yarvin wrote:
The thing is, if the pressure were higher after expansion, wouldn't
that have to mean it was higher before expansion, too?
Yes, Indeed it would. Constant volume heat addition will result in a
higher pressure and a higher temperature than the same amount of heat
being added at constant pressure. The difference is that the constant
volume heat addition results in a lower entropy increase than the
corresponding constant pressure heat addition. This means the Humphrey
cycle rejects less heat which means more of the heat added is doing work
which means the cycle is more efficient.
And if you
pumped up your conventional engine to that same higher pressure,
wouldn't it have the same efficiency?
Indeed, it might have higher efficiency. But we're comparing cycles
with the same pressure ratio.
That's what I'm arguing against; it seems like the wrong comparison.
That "pressure ratio" is the compression ratio, and the compression
stage isn't a process that actually exists in a rocket engine; it's
just a theoretical part of those cycles. The ratio that exists in a
rocket engine is the expansion ratio; that seems like the proper
standard of comparison. When you're actually building a combustion
chamber, it is of no consequence that the 'official' high pressure is
the one before combustion: you still have to design it to deal with
the much higher pressure after combustion.
(Pump-fed rocket engines do involve compression, of course, but the
pumps are normally pumping liquids, which thermodynamically falls not
into the "heat engine" category but almost purely into the "doing
work" category, since liquids have minimal compressibility. And since
they are denser than gases, not nearly as much power is required in
the first place, compared to what is required to run a compressor in a
jet engine of the same thrust.)
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