For both SN8 and SN9 my brain is screaming at me "surely they need more than 3 engines?!" Is the ultimate expectation that the engines will never fail, because it sure seems like they can't deal with losing even a single engine. I'm sure this is completely incorrect given that the issue for SN8 was tank pressure.
The issue is that the Raptor engines can only throttle down to 40% which represents a large thrust-to-weight ratio when you have basically just an empty tank.
There is also a startup time for each engine, so if one fails Starship will likely hit the ground before they can light another.
There are enough engines that a failure on ascent can be handled gracefully but landing is another story.
Ah ok, this sort of clarifies the issue, which is that you can't really run more engines at lower thrust, and if you deal with the tank pressure issues from S8 then I imagine it should be able to go down on 2 engines alone without catastrophic issues.
With respect to failure on ascent, I imagine in that case they just send up another starship and transfer in orbit (coming down has always been the hard part).
There could be a delicate dance somewhat similar to what Soviet lunar lander was planned to do at launch: start all engines, and if all is well, shut down those backups. Here with Raptors one could imagine launching two engines, but if one runs well - during the startup sequence - shut the other down mid-sequence.
But this is totally question to Raptor creators. Might be not really feasible or even meaningful.
It's my understanding currently that a 'production' starship will have three central engines optimized for sea level, and additionally three vacuum-optimized engines installed around its inside base perimeter.
Indeed. Those engines are currently missing partially because there's no reason to lose more engines than strictly necessary and also because they only need that much thrust when the fuel tanks are completely full.
If they decide to lift the StarShip from sea level completely full they will need to use only sea level engines (not vacuum optimized) or risk engine damage. Of course in production the booster will put the StarShip (upper stage) well above sea level.
> because it sure seems like they can't deal with losing even a single engine
Define "deal with". This rocket did 100% of what any rocket was ever supposed to do, before SpaceX came along and redefined what a rocket is supposed to do: go up.
"Deal with" == "Not blow up" == "No RUD" in the event of a single engine failure.
Manned space flight kind of has this kind of important additional requirement that things like ICBMs don't, which is that they can't just "go up" the have to come back down and not end in a ball of fire.
Unless I am much mistaken what what blew up was a first stage, not a manned stage. I do not believe that SpaceX plans on doing suicide burns to retrieve humans from flight.
What blew up is the second stage, the stage that would contain the astronauts. If this landing profile is also the profile they will use for a manned starship is unknown.
It feels to me like there is no backup plan for engine failures, but they can take so many paths here that I don't think anyone knows except people within SpaceX.
Example paths:
* Don't launch it with people until it has landed unmanned so frequently that everyone trusts it.
* Land cargo versions like this to get max payload, while manned versions light a lot sooner, with possibly more engines, and sacrifice some fuel for increased reliability.
* Catch starship just like the booster in case of failure, with a longer travel to support higher speeds and lower G's.
* Add a parachute or increase the flap sizes to lower terminal speed so much that it is survivable to land without engines.
The test articles for the first stage are still being built and are expected to land similar to the falcon 9, which the big difference that Elon is talking about them being catched by a tower instead of landing on their own legs. This seems far enough away though that I would not be surprised if that changes radically.
Falcon 9 lands using only a single engine and it has been a long time since a merlin engine failed to reignite on landing.
It'll be interesting to see what exactly happened with SN9, there's some clear video footage of piece flying off the rocket right as it was doing the flip maneuver.
They only need 2 of the 3 engines to perform the landing flip and 1 of the 3 engines to perform the landing. So they do have some redundancy, and the computers were very likely programmed to light the third engine when the second failed. That this didn't happen was either due to debris from the second engine taking out the third during its failure or the failure being in a common area like the downcomers or that the third engine was still too hot -- the third engine was the one that was used to hover at apogee. Or some other reason.
