Rocket engines make spaceflight possible, but, testing a rocket engine for spaceflight appears to represent an engineering challenge. Engines designed to operate in a vacuum may behave differently from those operating inside an atmosphere, and you might think that no vacuum pump could extract the exhaust gasses fast enough to stop the pressure building up and ruining the test conditions.
However, as it turns out building a test stand which maintains high altitude conditions is easier than you would imagine since the rocket exhaust can be exploited as part of a pumping mechanism to keep the chamber evacuated.
Includes a small section of Cody's Lab showing the problems with igniting a rocket engine in a vacuum chamber.
https://www.youtube.com/watch?v=76JM03a6WH4
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