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The SSME engine nozzle was optimized for an exhaust pressure of around [COLOR=#A40000 said:2 psi[/color], i.e. about halfway along its typical trajectory. It could have been designed for higher expansion and more net thrust in vacuo, but at a weight penalty. The result is a trade between the vehicle empty weight vs. total impulse that the engine could deliver. When starting, the main engines were operated at about 100% of rated sea level thrust, which normally would result in a nozzle that would have separated flow along the nozzle wall, due to shock waves forming there. A tweak to the design (slight change in angle of the wall near the exit plane) results in a near-wall pressure closer to 4 or 5 psi, while the core flow remains at about 2 psi. As the exit pressure drops, the nozzle runs more "full", with less of the flow passing the exit plane at a non-90 degree angle, resulting in lower cosine losses, and higher effective exit velocity. ]
btrueblood said:The SSME flight nozzles would ring like a bell (but much more violently) when starting into atmospheric pressure...