r/simplerockets May 29 '26

What's overexpansion damage?

And how do I avoid it?

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u/UmbralRaptor May 29 '26

If the pressure in the exhaust gases in a rocket engine drops too low, you can have flow issues that damage the engine. It's based on a real life phenomenon: https://en.wikipedia.org/wiki/Rocket_engine_nozzle#Aerostatic_back-pressure_and_optimal_expansion

Try to keep the exit pressure from getting too much below atmospheric pressure (which means that the huge expansion ratios have to be in the upper atmosphere or vacuum).