[AR] Re: Orions and PDEs (was Re: More MAX delays.)
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Mon, 27 Jan 2020 15:19:33 -0500
On Mon, Jan 27, 2020 at 08:34:39AM -0600, Jim Davis wrote:
It seems rather puzzling, though, where such an advantage might come
from.
It works on a different thermodynamic cycle. A PDE approximates
constant volume heat addition, meaning it is working on the Humphrey
cycle (characterizes by constant volume heat addition, constant
pressure heat rejection) rather than the Brayton cycle (constant
pressure heat addition, constant pressure heat rejection) of
conventional rocket and gas turbine engines. Expansion is (close to)
isentropic in both cycles.
That last sentence explains why it can be on par with a conventional
engine, but not where an advantage would come from.
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.
So I'm afraid that to get this higher efficiency figure for a PDE
they're smuggling in an assumption of higher pressure and thus higher
T2. While a natural assumption in a way -- detonations do have very
high pressures -- it isn't really fair: there's no theoretical limit
to the pressure used in conventional rocket engines (and some,
e.g. the SSME, have actually been built to run at very high
pressures), and in a PDE you still have to build the engine strong
enough to survive that peak pressure.
(This is a different way of looking at what I was stating earlier as
being a consequence of engines operating in air. In vacuum, expansion
can be so complete that T1 is quite low, thus considerably lowering
the improvement one can get by elevating T2.)
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