re: SpaceX - big badda boom!

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Upper stage boom. Wonder why they were filling that part for a main stage test firing. Maybe you can't just fill one LOX tank.

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They do a fully integrated test fire, so all payloads are loaded, the rocket is fully fueled, and all controls and guidance are running. The differences between launch and test pretty much all come after t-0.00, mainly the thrust profile is a quick ramp up to 100% power for a second or two and the hold-downs/umbilicals don't release during the test. If anything with the launch vehicle turns up anomalous during the static test fire, like the upper stage detonating, they'll know it before launch.

The shuttle didn't do a static test fire, but the 6 or so seconds between main engine start and SRB ignition while the rocket stack flexed and rocked back through vertical was essentially the static test fire for the stack.