[AR] Re: Orions and PDEs (was Re: More MAX delays.)
- From: Jim Davis <jimdavis2@xxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Mon, 27 Jan 2020 15:47:56 -0600 (GMT-06:00)
Looking up various cycles, I see the ideal Brayton cycle quoted as
having the usual efficiency formula (T2-T1)/T2, the maximum for any
thermodynamic cycle.
That's the efficiency for the Carnot cycle; the ideal Brayton cycle efficiency
is
eff = 1- 1/PR^(Y-1/Y)
where
PR = pressure ratio
Y = ratio of specific heats
The ideal Humphrey cycle efficiency is
eff = 1- 1/PR^(Y-1)
For a given amount of heat addition the constant volume process entails a
smaller entropy rise than the constant pressure process. This translates to a
higher pressure after expansion in a nozzle of a given length or the ability to
use a longer nozzle to expand to the same pressure.
Translating the theory into practice has, unfortunately, proven challenging.
Jim Davis
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