[AR] Re: Orions and PDEs (was Re: More MAX delays.)
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sun, 26 Jan 2020 18:48:07 -0500
On Sun, Jan 26, 2020 at 05:31:51PM -0500, Henry Spencer wrote:
Try Bratkovich et al, "An Introduction to Pulse Detonation Rocket
Engines", Joint Propulsion Conference 1997, AIAA 97-2742. There may be
better and/or more recent references -- it's not an area I really follow.
Last I heard, they typically claim a 10-15% advantage in Isp, and
sometimes actually demonstrate that :-) [...]
It seems rather puzzling, though, where such an advantage might come
from. The usual expansion in a bell nozzle is reasonably modeled as
isentropic, and normally that means it's the best that can be done.
(Setting aside the complicated issues surrounding nozzle operation in
air, that is, and considering only expansion into vacuum.)
Speeds quoted for detonation waves in explosives do often exceed
exhaust velocities, but those are speeds of _waves_: the material
itself doesn't move that fast, not even right at the shock front.
Also, explosive unevenness of thrust opens up an enormous number of
ways to mismeasure it, so even laboratory demonstrations can be
questioned.
Or maybe it's wrong to "set aside" the issues surrounding operation in
air; maybe that's where the advantage lies and it's simply the higher
pressure of the detonation which gives it that advantage. In that
case you could also get it by building a higher-pressure conventional
engine, as in the SSME. And that might even be practical. (I snipped
Henry's comments on the impracticality of pulse detonation engines,
but they seem thoroughly correct.)
Other related posts:
