00:00The second clip is at least as important. It shows the ignition of a vacuum raptor
00:05after a longer cooling phase simulating a drift phase. So exactly in the state that an HLS engine
00:13will be in before the descent. The problem isn't the ignition itself. The problem is
00:19where is the propellant in the tank, what state is it in and how many gas bubbles,
00:24how much liquid, how much plasma mixture. However, these are things that are difficult or even
00:31impossible to simulate on Earth. Nothing can replace a good old real-world test. In space,
00:37there is no up or down. Propellant doesn't slosh around. It spreads out. That's why ignitions
00:44after long coast phases are one of the trickiest challenges in spaceflight.
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