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Michkov t1_it726lc wrote

Yep you are pretty much on the right track there. The latter part of the nozzle is mainly to match the gas pressure to the ambient pressure at the exit. Because as you point out the two should match up to provide the most thrust. The terms you want to look into are over/underexpanded plumes.

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zekromNLR t1_it8ywem wrote

Needing to match exit pressure to ambient pressure is also why increasing the chamber pressure gets you a higher efficiency in atmosphere, because it lets you use a greater expansion ratio for the same exit pressure. For example, the Rocketdyne F-1 and the SpaceX Merlin are both kerosene-oxygen rocket engines that use a gas generator cycle, and both have relatively similar specific impulse in a vacuum (F-1 304 s, Merlin 311 s). But where the F-1 has 7 MPa of chamber pressure, the Merlin has 9.7 MPa, which translates into a significantly higher specific impulse at sea level (F-1 263 s, Merlin 282 s).

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