[AR] nozzle separation (was Re: fiberglass conduit)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Wed, 12 Jan 2022 00:02:54 -0500 (EST)
On Wed, 12 Jan 2022, Troy Prideaux wrote:
Speaking of Cf from the nozzle, for long burn time single stages,
there’s a staggering amount of theoretical altitude available from
dialling up the expansion area of nozzle exit according up RASAero II
which raises the question of flow separation and how much one can
tolerate in a relatively small and stout nozzle during the early phases
of flight?
Potentially a lot, with the caveat that it's a poorly-explored subject.
The idea that flow separation is invariably destructive and must be
avoided at all costs is a recent superstition. As I think I've mentioned
before, the center ("sustainer") engine of the classical Atlas usually ran
separated at sea level.
Aside from some lingering concerns about unsteady or oscillating
separation, the biggest issue is that separated performance can be
substantially worse than what you'd see from a nozzle cut off at the
separation point. (Simple engine-performance models might not predict
this accurately -- actually I'm not sure anyone knows how to predict this
accurately.) The separated plume can entrain air well enough to pump the
pressure in the surrounding volume of nozzle down below ambient, actively
reducing net thrust. And of course the shape of the flow-attached part of
the nozzle usually isn't the same as an optimal nozzle for that pressure
ratio.
It does seem like something that would be worth exploring.
Henry
Other related posts:
