[AR] Re: Orions and PDEs (was Re: More MAX delays.)
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Fri, 31 Jan 2020 01:23:42 -0500
On Fri, Jan 31, 2020 at 01:28:20PM +1100, Troy Prideaux wrote:
Funny that’s kinda my initial impression of how they worked, but this
thread (particularly with the mention of rotary PDEs and some of the
terminology) had me second guessing :)
The puzzle for me with rotary PDEs is how you'd get the pressure low
enough that you could inject propellant at a lower pressure. With the
speed of a detonation wave being thousands of meters/second, injecting
propellants and having them mix thoroughly before the detonation wave
returns is a challenge already, even if you're injecting them from a
higher-pressure source. But the idea is supposed to be that you can
inject them from a lower-pressure source, because if you did the
normal thing and used a higher-pressure source, you'd just have a
normal engine, except for the bragging rights from using an uber-cool
form of combustion. ("You know how most people try to minimize
combustion instability? Well, we try to maximize it. [sound of chest
thumping]")
So the idea has to be that you use some sort of resonances in the
waves bouncing around to create moving low-pressure zones where
propellants are dragged in from a source pressure that is considerably
lower than the average chamber pressure. And then you probably need
some sort of mechanical valving to prevent your propellant manifold
from getting filled with exhaust gases during the majority of the time
when the low-pressure zones aren't there. And that mechanical valving
has to operate at ridiculously high speeds, opening and shutting
hundreds or thousands of tiny injection orifices. (Tiny check
valves?) And all this has to not just work at steady state but allow
reliable startup somehow.
Or you could give up on that part of the scheme, put in a high
pressure pump, and just settle for the bragging rights. It's not like
any of the politicians or MBAs who are funding you will know the
difference.
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