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By far the most common reason for engines to go out is when they don't have enough fuel, and that is mostly caused by faults in ullage, which is what I mentione
by inglor_cz 6mo ago
By far the most common reason for engines to go out is when they don't have enough fuel, and that is mostly caused by faults in ullage, which is what I mentioned in my GP comment to be a significant problem. Once you spent most of your fuel, relighting the engines is not easy.
Ullage is a plumbing matter, though. External to the engine itself and its intrinsic (un)reliability.
On the ascent burn, where fuel is plentiful, Raptor flameouts have become way less frequent over time. The # of engines failing on the Super Heavy during the ascent burn is 0/33, 1/33 and 0/33 during the last three IFTs. Not bad for a test vehicle.
- WalterBright 6mo agoThe Saturn V's had a big problem with "pogoing" which they never solved. The Soviet moon rocket apparently was abandoned because of unstable coupling among the grid of engines that powered it. SpaceX must have a handle on these problems, I haven't heard anything about it.
- ActorNightly 6mo agoThe full flow cycle of the raptors means they do 2x preburn for oxidizer and fuel to spin the turbines to generate the pressure required for the pressure ratio. If a small part of this fails for whatever reason, or is out of sync, either engine doesn't start or goes kaboom. Ascent isn't the problem its the relighting of them on the way down.
- inglor_cz 6mo agoI agree, relighting is tricky, not just because of the limitations that you describe, but because there is a lot less cumulative experience with relighting of engines than simply lighting them at the launch. We (as the entire humanity) have seen tens of thousands of rocket motors lit on ascent, orders of magnitude than those relit. Raptors don't seem to be prone to kabooms during flight, probably because the command unit switches them off in case of any suspicion.