On the RASAero web site Solid Rocket Motor Technical Reports Downloads Page:
http://www.rasaero.com/dl_solid_motor.htm
Part 2 of the Performance Analysis of the Ideal Rocket Motor provides the
nozzle static pressure to ambient pressure ratio where flow separation will
occur in a conical nozzle (Pages 456-458, Equation 3-5), Part 3 provides an
equation for the nozzle static pressure where flow separation will occur in a
bell nozzle (Pages 6-8 of the pdf), that being a bell nozzle without any
special contouring to delay flow separation, or contouring to make the flow
separation more stable. If you have a conical nozzle, or a typical bell nozzle
without special contouring, the equations will give you the nozzle static
pressure where flow separation will occur, and help you avoid flow separation
in the first place.
When I was working on the X-30 National Aerospace Plane (NASP) program, we were
looking at using J-2 rocket engines as auxiliary rocket engines for the X-30.
One question was what nozzle expansion ratio we could tolerate at sea
level/Edwards AFB elevations. J-2 engines were tested at Santa Susana with
full nozzles, but had a lip added to the end of the nozzle to help starting.
Our simple solution was to remove the J-2 nozzle extension, leaving just the
regeneratively cooled nozzle inner expansion section. We later went to a
linear aerospike.
But as part of the study, we got to see a video of an early J-2 engine test at
Santa Susana without the nozzle lip, where the flow separation at starting was
very violent, and destroyed the engine. After watching the video, the big
take-away was avoid flow separation in liquid rocket engine nozzles.
The current professional technology is much more sophisticated, but for this
group my opinion is that continuous flow separation is to be avoided. Flow
separation occurs for a short period of time during start-up for any liquid
rocket engine when the engine chamber pressure is low and building up, but once
the chamber pressure has built up to the nominal operating chamber pressure,
there should be no flow separation.
For those who are interested, the design of the SSME nozzle including the
special contouring relative to flow separation was detailed in a P&W
Engineering Thresholds Journal article. It's been on the web, although all of
the current links appear to be broken.
Charles E. (Chuck) Rogers
CRogers168@xxxxxxx
-----Original Message-----
From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
To: Arocket List <arocket@xxxxxxxxxxxxx>
Sent: Thu, Apr 20, 2017 4:22 pm
Subject: [AR] Re: Rao nozzle
On Wed, 19 Apr 2017, Troy Prideaux wrote:
I'm not exactly sure what this is trying to say. It reads to me like it's
suggesting the TOP nozzle performs better at ground level conditions due to
its tendency to supress the onset of flow separation over an optimised bell
nozzle (optimised to what ???)...
Strictly in terms of performance, flow separation *increases* the
performance of a nozzle in such conditions ie. It eliminates much of the
negative pressure thrust which is a consequence of over expansion.
If the flow separation was asymmetric in nature, that might create
stability issues, but that's not really (directly) a performance related
issue.