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Is that why you see several engines not functioning during flight tests? The issue is that the controllers have to maintain a crazy unstable balance of the mix

Is that why you see several engines not functioning during flight tests? The issue is that the controllers have to maintain a crazy unstable balance of the mixture to keep these things running due to the dual preburn cycle and cryogenic storage, that any unforeseen circumstance can lead to failure. In the days of Falcon, where Space X was attracting people willing to disrupt the industry, I would have been in the "its possible" camp. Nowdays, with everything going on, I would place a large chunk of money on the fact that they will never get it to be reliable enough. They haven't even gotten it to orbit yet, despite the massive experience they have.

WalterBright 6mo ago
The rockets were designed to be tolerant of a couple of the engines not functioning. This is actually good engineering, as perfect reliability is very expensive.
- ActorNightly 6mo ago
  >This is actually good engineering, Lol. First its "engines are reliable", then its "well actually they are not, but the starship can function with a few out" The Elon simps never cease to amaze me. An engine should not go out by itself. Some catastrophic event can take out an engine, and then if you design your rocket to fly with a few out, thats fine. But if the engines just are so unstable that they fizzle out, thats a huge risk because that means any non planned event can cause more engines to fizzle out, leading to loss of vehicle.
  - 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 ago
      The 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 ago
      The 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 ago
        I 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.
  - WalterBright 6mo ago
    > An engine should not go out by itself. Airliners have twin engines, and are designed to be able to fly one one. Why? Because engines fail now and then. Every flight critical part on an airliner has a redundant workaround, sometimes two. Redundancy is a cost-effective way to successfully deal with unreliable parts. When I go dirt bike riding in the wilderness, I only ride with a buddy. I wear protective gear. I carry a phone, water, and a minimal survival kit. I also text a photo of the trail map to a friend before hitting the trail. Small things, but potential life savers.
    - ActorNightly 6mo ago
      Yeah and if you look at all the jet engines produced, the amount of failures are extremely tiny compared to total flight hours. Not the same for raptor engines.
      - inglor_cz 6mo ago
        The ratio was different in the late 50s when jet engines were new. Experience counts.

WalterBright 6mo ago

The rockets were designed to be tolerant of a couple of the engines not functioning. This is actually good engineering, as perfect reliability is very expensive.

ActorNightly 6mo ago
>This is actually good engineering, Lol. First its "engines are reliable", then its "well actually they are not, but the starship can function with a few out" The Elon simps never cease to amaze me. An engine should not go out by itself. Some catastrophic event can take out an engine, and then if you design your rocket to fly with a few out, thats fine. But if the engines just are so unstable that they fizzle out, thats a huge risk because that means any non planned event can cause more engines to fizzle out, leading to loss of vehicle.
- 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 ago
    The 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 ago
    The 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 ago
      I 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.
- WalterBright 6mo ago
  > An engine should not go out by itself. Airliners have twin engines, and are designed to be able to fly one one. Why? Because engines fail now and then. Every flight critical part on an airliner has a redundant workaround, sometimes two. Redundancy is a cost-effective way to successfully deal with unreliable parts. When I go dirt bike riding in the wilderness, I only ride with a buddy. I wear protective gear. I carry a phone, water, and a minimal survival kit. I also text a photo of the trail map to a friend before hitting the trail. Small things, but potential life savers.
  - ActorNightly 6mo ago
    Yeah and if you look at all the jet engines produced, the amount of failures are extremely tiny compared to total flight hours. Not the same for raptor engines.
    - inglor_cz 6mo ago
      The ratio was different in the late 50s when jet engines were new. Experience counts.