[AR] Re: Rao nozzle
- From: "" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "crogers168" for DMARC)
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 25 Apr 2017 12:41:02 -0400
...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...
< One caution: Cornelisse et al, "Rocket Propulsion & Spaceflight
< Dynamics", 1979, says the exact pressure depends somewhat on details like
< wall roughness, pressure gradient, viscosity, and (especially) presence of
< solid particles.
Agreed. The equations are meant to be rules of thumb, a good starting point to
avoid flow separation, if one has the design goal to avoid flow separation.
Our simple solution was to remove the J-2 nozzle extension, leaving just
the regeneratively cooled nozzle inner expansion section.
< Um, as I understand it, the J-2 didn't have a nozzle extension. The lower
< half had some help from turbine-exhaust film cooling, but unlike the F-1,
< that section was still tube-wall regen, rather than being otherwise
< uncooled.
Yep, good catch. It wasn't just film cooling.
< However, shortening a regen nozzle isn't necessarily a big deal. The
< shortened RL10s for DC-X likewise had a shorter version of the original
< RL10's all-regen nozzle.
< Modern high-altitude engines sometimes deliberately put a manufacturing
< joint in the nozzle at a suitable point, making shortening easy.
You can see from J-2 drawings where the manufacturing break was, and where we
planned to remove the rest of the nozzle.
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.
< I.e., "we tried it once and it failed badly, so no variation on it could
< possibly work". Hundreds of Atlases flew successfully with sustainer
< nozzles running separated at liftoff, although as I noted, this was never
< widely advertised. Avoiding separation certainly would make things
< *easier*, and on a cost-constrained program that might be deemed
< mandatory, but one should be aware that there is a choice there and
< today's conventional wisdom is not the only way.
The SSME nozzle of course is the best example of sustained, stable flow
separation at sea level operation, to allow a high expansion ratio for high
altitude/vacuum operation. It can be done, and the dual bell nozzle concept is
based on a stable flow separation point at low altitude, and then full nozzle
flow at high altitude/vacuum operation.
The question is really is the payoff in performance worth the potential
development risk. For an SSME running from lift-off to orbit one can see why
they chose a nozzle with a stable flow separation point at low altitude. Same
with the sustainer motor on the Atlas, runs all the way to burnout on an ICBM
trajectory. For a small start-up company, with a two stage liquid rocket, the
advantages for the Stage-1 nozzle might not be worth the development risk.
The performance benefit of a high expansion ratio nozzle with a stable low
altitude flow separation point can be quantified. You adjust the nozzle
expansion ratio during the trajectory simulations, typically in a couple
discrete step changes, low altitude with stable flow separation (low expansion
ratio), and then a step change to full flow in the nozzle (high expansion
ratio). That performance benefit should then be weighed relative to the
development risk.
Charles E. (Chuck) Rogers
CRogers168@xxxxxxx
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