The Shuttle SSME was also over expanded being ground start to orbit. The
verniers were too but of negligible consequence.
Ken
On Sat, Apr 29, 2023 at 8:37 PM Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
On Sat, 29 Apr 2023, Michael Kelly wrote:
Atlas sustainer could have been a lot better, but it was designed to run
at sea level (they didn't want to risk an air start early in the
program).
Indeed, the whole point of "stage-and-a-half" was to get much of the
benefit of staging without having to do in-flight engine starts.
You can tolerate a certain amount of overexpansion at sea level, but you
don't want the flow to separate. That beats the crap out of the nozzle.
Interestingly, Sutton's "History of Liquid Propellant Rocket Engines" says
that the Atlas sustainer -- or at least the final version of it -- in fact
usually ran separated at sea level. They pushed overexpansion a bit too
far, but fortuitously got stable, symmetrical separation. (Separation in
itself typically isn't that bad; what really beats up nozzles and gimbal
actuators is when the separation is asymmetrical or wanders around.)
Henry