** The Official Space Flight Thread - The Space Station and Beyond **

They need more power from a single engine, if they re-orient a bit higher if the second engine doesn't work then they have more time to throttle the first engine up and try to catch it on one.

Think it was a bit of a mistake having SN10 that close though, wonder how much time they're going to lose checking it over for parts of SN9..
 
Trig said:
They need more power from a single engine, if they re-orient a bit higher if the second engine doesn't work then they have more time to throttle the first engine up and try to catch it on one.

Think it was a bit of a mistake having SN10 that close though, wonder how much time they're going to lose checking it over for parts of SN9..
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It doesn’t have any of the engines fitted to SN10 currently, I’d imagine they’ll be able to know if it took any impacts. But yeah, it might not have been the best decision. The cryo check or whatever it’s called which SN10 will do next will tell
 
I also dont get, if its got 3 engines, why not re-start all 3, then run them at a lower throttle, rather than trying 2, or restart all 3 and then turn one off when its not needed, or leave it at 0 throttle ready to throttle up if needed..
 
Trig said:
I also dont get, if its got 3 engines, why not re-start all 3, then run them at a lower throttle, rather than trying 2, or restart all 3 and then turn one off when its not needed, or leave it at 0 throttle ready to throttle up if needed..
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sadly rocket engines all have varying degrees by which they can be throttled, it’s not a 0-100% range, an engine might only be able to go down to 50% or whatever.

So they have to manage descents etc with the limits of the thrust from the engines, I don’t know the ins and outs of the raptors or the starship test profile for today’s test or how it will operate when they’re all installed
 
They appear to start 1 engine during the initial flip and then the 2nd shortly after to control the descent. I would guess 3 at the same time would knacker the flip and provide too much thrust.

Edit: If the whole body's a fuel tank, wouldn't the fuel be all over the place during the flip? Perhaps the engines are just starved of fuel?
 
Pigeon_Killer said:
They appear to start 1 engine during the initial flip and then the 2nd shortly after to control the descent. I would guess 3 at the same time would knacker the flip and provide too much thrust.

Edit: If the whole body's a fuel tank, wouldn't the fuel be all over the place during the flip? Perhaps the engines are just starved of fuel?
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They have a small header tank at the top that it switches to I believe for this very reason.
 
I think the header tanks are also important for centre of gravity too.
We’re so used to the success of the Falcon9 and Merlin engine we’ve kind of forgotten that the Raptor is an all new high efficiency engine. I mean this engine has only flown what 5 times and the restart from flip is a new thing on a new engine the failures are part of the learning.

Also isn’t it great watching something explode :D
 
Trig said:
I also dont get, if its got 3 engines, why not re-start all 3, then run them at a lower throttle, rather than trying 2, or restart all 3 and then turn one off when its not needed, or leave it at 0 throttle ready to throttle up if needed..
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If fuel flow from the header tanks is the issue like it was in the first failure then starting 3 is going to be even less likely than 2. Also this is all being done by Starship's computers, I'd imagine by the time it knows engine 2 hasn't ignited properly its too late to do anything else.
I'm not sure they throttle low enough that 3 engines at lowest power would allow it to hover and gently land. They might have to do what the Falcon 9 does and do a suicide burn as that engine produces more thrust that the mass of the rocket at that point. Its incredible really that they have to time the burn perfectly so the velocity hits zero just as the rocket reaches the ground, it can't hover.
 
Should remember of course, they never explode, they just have rapid unscheduled disassembly.
 
They obviously aren't too dumb, but I'd be interested in knowing why they are doing things the way they are..
 
Because the engines don't work the way you think they do. Throttling them is hard (Elon's exact words), igniting all 3 would be too much power, and they may not even have the pressure for all 3 at that point.

They only need 1 engine to land, the 2 engine burn is for the flip and to kill a bit more speed.
 
Turbines and compressors often have very narrow efficient working bands. So the turbine compressors (the fuel pumps) for his full flow engines can’t be dialled down to far before they stop delivering the flow and pressure necessary for the preburners and main combustion chamber would be my guess. I’m a mech eng and work on industrial gas turbines and compressors so it’s an informed guess.
 
VaderDSL said:
sadly rocket engines all have varying degrees by which they can be throttled, it’s not a 0-100% range, an engine might only be able to go down to 50% or whatever.
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IIRC they (currently) can only throttle down to 40% (of full thrust) with the Raptors. I believe Elon mentioned in the past that 50% on the Raptor is hard and getting down to 25% would be extremely difficult/nie impossible.

Rumour has it that they might do the "flip" earlier, start all three engines and throttle one back/shutdown the failed engine for landing. But supposedly it would most likely involve a header tank redesign before they can do it so it might not be doable for SN10.

Pigeon_Killer said:
Edit: If the whole body's a fuel tank, wouldn't the fuel be all over the place during the flip? Perhaps the engines are just starved of fuel?
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My understanding is that it's a pressurised propellant tank (sits inside) where they 'back fill' the tank with nitrogen/helium to help with propellant starvation and "sloshing".
 
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