microsat wrote:In testing our experimental motors for ignition/re-ignition in space we built an 200mm vacuum chamber and simply use an aluminum plate with a silicon gasket. When the vacuum is applied, it holds the cap on yet when we do ignition, the cap blows away harmlessly.
Yes, this is the simplest way to do it, but I prefer the “chamber” (pipe) to be mounted horizontally with fall-away covers on both ends. Then, even when the engine fails catastrophically during a test (and you know it will now and then), the pipe can not become pressurized by the explosion because both ends open instantly. While the pipe might be shattered by an engine failure, it will not itself explode like a pipe bomb. 200 mm, however, sounds like a tight fit. The bigger the pipe (sorry, “chamber”), the better. Your safety is in your hands.
If you are planning a high-altitude balloon launch, a cryogenic vacuum test is the most authentic for simulating the long slow climb to launch altitude. Just chill the engine with dry ice (frozen carbon dioxide) well before the test.