totm nov 2023 SpaceX Discussion Thread

[sevenperforce]

I've been speculating a lot about the gas press system and COPVs, and I finally got livingjw to weigh in on the question over at the NSF forums. For those of you who aren't familiar, John is an extremely well-connected rocket scientist based in Florida who knows more than any one person fairly ought to.

A couple of points:

• Autogen press gas needs to be piped straight from Raptor to the tanks at the highest temp possible, because the hotter the gas, the more space it will occupy and the lower ullage gas mass is required to maintain pressure.
• GCH₄ and GOX COPVs are absolutely necessary. They supply the igniters, spin up the turbopumps  at Raptor start/restart, and (eventually) will feed the hot-gas thrusters. They also provide ullage pressure to the header tanks prior to Raptor restart.
• For the COPV tanks, you want the GOX and GCH₄ to be as cold as possible (without condensing) in order to store as much gas mass as possible, with pressure as high as possible, in tanks that are as light as possible.
• Because there's essentially unlimited heat in the preburner exhaust, the valves running from the regenerative coolant loop into the vaporization loop can be "throttled" to send more or less autogen press gas out. So there's a certain minimum amount running through a regulator straight to the tanks, and whatever is left would be passed through a coolant loop (probably using the CH₄ downcomer) to reduce temperature before pushing into the COPVs.

Here's a more detailed Raptor diagram that shows the hot press gas going out from vaporizing heat exchangers (in the fuel and oxidizer turbopump exhaust) to press the tanks, as well as the cold gas lines coming in from the COPVs. Not shown: the regulator that sends part of the hot gas into the tanks and the rest through a coolant loop and into the COPVs.

Spoiler

This diagram suggests the use of helium for turbopump spinup but I believe John thinks they will use cold GOX and GCH₄ for that. This makes sense to me as well. They will need GN₂ to purge, though. Consumables, then, are GN₂, LOX, and CH₄.

John pointed out that it's also possible to have a secondary vaporization line which sends the liquid methane and LOX through a shorter heat exchanger to vaporize only without superheating, and push this directly into the COPVs to avoid needing the coolant loop. I'm not sure which is more likely. 

He also argues that if you need to "restart" a Starship after a lengthy coast, you can have an electrical pump to send LOX and liquid methane directly into the COPVs and then use solar-powered electric resistance heaters to boil the contents and repress the COPVs. Very efficient, but I feel like incomplete vaporization might be a problem there, though. A faster method would be to use a dedicated burner and heat exchanger to produce warm GOX and GCH₄ on demand.

7 minutes ago, RealKerbal3x said:

Seems like SN9 got a new aft flap after the tipping incident

That's a good sign.

[RealKerbal3x]

6 minutes ago, sevenperforce said:

That's a good sign.

Disregard that, I can still see damage on other photos. A new flap did arrive onsite though, it might be a replacement.

[tater]

If we see Raptors getting pulled, that's probably a bad sign, but if they take the fins off... good sign.

[tater]

Maybe it buffs right out?

 

[RealKerbal3x]

1 hour ago, tater said:

If we see Raptors getting pulled, that's probably a bad sign, but if they take the fins off... good sign.

Honestly things are looking alright for SN9 already. They're not immediately attacking it with cutting torches, and new flaps have arrived, which suggests it's repairable.

Edited December 15, 2020 by RealKerbal3x

[Flavio hc16]

32 minutes ago, tater said:

Maybe it buffs right out?

 

so: tomorrow they swap the flaps/buff out the dents and on the 17th they move it to the pad for testing?

[RCgothic]

Eric Berger thinks SN9 may be ok to ship to the launch site Thurs, maybe:

 

 

 

[tater]

That or they need a closure to move SN8's nosecone.

[Jaff]

Petition (this might take done explaining) to give SN9 a new nickname. 
 

Fidget. 
 

after fidget the bat in Disney’s “Basil the great mouse detective” the one with the crippled wing & peg leg

 

 

[JoeSchmuckatelli]

Speculation; throwing SN9 skyward and re-doing the belly flop, drift and land attempt is data gathering.  Regardless of whether it lands; SN9 will be a success if they get a second look at how the concept launches, flips, falls, flies, and (maybe) lands.

They've probably already learned several things that are wrong with the SN8/9 design - but seeing as it's built; probably just worth launching to get data (rather than tear apart and fix / upgrade).

[tater]

I think they'd also try and fix the fuel pressure issue if it can be mitigated without a major effort (opening valves, etc).

[tater]

 

[sevenperforce]

9 hours ago, tater said:

I think they'd also try and fix the fuel pressure issue if it can be mitigated without a major effort (opening valves, etc).

I wish I could get Elon's attention to ask how the CH₄ header tank is pressurized. There are three possibilities, really:

• Header tank pressed via main tank.
  • Valves between header and main are closed at MECO
  • Residual ullage in header provides inlet pressure for Raptor restart
  • Valves between header and main opened immediately after restart
• Header tank pressed direct from Raptor via secondary line.
  • Additional pressure to header from Raptor after valves between header and main are closed at MECO
  • Residual ullage in header provides inlet pressure for Raptor restart
  • Autogen press gas from Raptor maintains pressure after restart
• Header tank pressed by Raptor and by COPV via redundant lines.
  • Additional pressure to header from COPV after valves between header and main are closed at MECO
  • COPV line to header maintains ullage pressure after Raptor restart
  • Autogen press gas from Raptor takes over once spun up

I think the first possibility would definitely explain what we saw happen with SN8. There was just enough ullage in the header to restart Raptor, but with turbopump damage; the subsequent kick-flip produced enough inertia to maintain inlet pressure dynamically, but once the second engine shut down, the autogen press gas flowing into the main tank couldn't pressurize it fast enough to maintain pressure across the main-header valves.

The second possibility would also explain it if the ullage pressure just wasn't sufficient to keep things operating until the autogen press gas started flowing. They could fix this by pressurizing the header tanks higher at MECO...I'm sure the header tank can handle 8-12 bar. I don't think Raptor will mind having a higher inlet pressure.

The third possibility is clearly the safest but would require additional plumbing if they haven't already set it up that way.

[magnemoe]

1 hour ago, sevenperforce said:

I wish I could get Elon's attention to ask how the CH₄ header tank is pressurized. There are three possibilities, really:

• Header tank pressed via main tank.
  • Valves between header and main are closed at MECO
  • Residual ullage in header provides inlet pressure for Raptor restart
  • Valves between header and main opened immediately after restart
• Header tank pressed direct from Raptor via secondary line.
  • Additional pressure to header from Raptor after valves between header and main are closed at MECO
  • Residual ullage in header provides inlet pressure for Raptor restart
  • Autogen press gas from Raptor maintains pressure after restart
• Header tank pressed by Raptor and by COPV via redundant lines.
  • Additional pressure to header from COPV after valves between header and main are closed at MECO
  • COPV line to header maintains ullage pressure after Raptor restart
  • Autogen press gas from Raptor takes over once spun up

I think the first possibility would definitely explain what we saw happen with SN8. There was just enough ullage in the header to restart Raptor, but with turbopump damage; the subsequent kick-flip produced enough inertia to maintain inlet pressure dynamically, but once the second engine shut down, the autogen press gas flowing into the main tank couldn't pressurize it fast enough to maintain pressure across the main-header valves.

The second possibility would also explain it if the ullage pressure just wasn't sufficient to keep things operating until the autogen press gas started flowing. They could fix this by pressurizing the header tanks higher at MECO...I'm sure the header tank can handle 8-12 bar. I don't think Raptor will mind having a higher inlet pressure.

The third possibility is clearly the safest but would require additional plumbing if they haven't already set it up that way.

You can keep the pressure equal even with main tank to header valves closed simply by letting the gas flow into both tanks. 
Now once you have good ullage as in braking burn open the valves to let exec methane to enter the header tank. Here you might want the main tank pressure to be slightly higher than the header tank to push the methane in.

Now this might be relevant for landing tests there you want extra fuel for the landing burn, it let you do the flip higher giving you more time to stabilize and go for the landing pad it will leave fuel and oxygen splashing around during the flip making this much harder, yes you can add baffles to migrate this but it don't look like its done. 

I also don't think its relevant for an return from orbit. Here the main tanks would be hot so leftover liquid methane and LOX will slosh over the hot plates behind the heat tiles and will burn off fast, better just to vent it or make more pressurized gas for later. After reentry the tanks start to cool so you need to add more gas to keep pressure up, however unlike the header tanks who need an set pressure to feed the turbo pumps the main tank pressure is just for structural integrity so you have more margins the ship is empty after all. 

 

 

[RCgothic]

New video from Everyday Astronaut and some Elon comments:

 

 

 

 

 

 

 

[sevenperforce]

I spammed a few questions at Elon since he's active right now; let's see if he bites.

Spoiler

Asking a few different ways should improve the odds that he understands the question and responds. Feel free to retweet like @RCgothicdid, if you wanna.

 

3 minutes ago, magnemoe said:

You can keep the pressure equal even with main tank to header valves closed simply by letting the gas flow into both tanks. 

That works if you have a separate press line already. Also, where is the gas coming from? If it's from Raptor's autogen press line then it is going to die when Raptor stops firing.

4 minutes ago, magnemoe said:

Now once you have good ullage as in braking burn open the valves to let exec methane to enter the header tank. Here you might want the main tank pressure to be slightly higher than the header tank to push the methane in.

The problem is gas flow velocity. Liquid methane is essentially incompressible, but gaseous methane is extremely compressible. If you're relying on the autogen press line entering the main tank to pressurize the header tank via valves, it will take a moment for the gas entering from the press line to expand enough to reach the valves.

[sevenperforce]

If the header tank is pressed via valves from the main tank, this is what I'm thinking probably happened:

[mikegarrison]

The whole "header tank" idea is very problematical. They clearly need to work this out, perhaps with a different solution. They may need a more active scavenging system that makes certain that enough fuel/oxidixer is available for a complete landing burn.

[tater]

Closures this week canceled, moved until after Christmas:

 

[RealKerbal3x]

10 minutes ago, mikegarrison said:

The whole "header tank" idea is very problematical. They clearly need to work this out, perhaps with a different solution. They may need a more active scavenging system that makes certain that enough fuel/oxidixer is available for a complete landing burn.

I'm not an engineer, but I can't think of an alternative solution that would solve as many problems as the header tanks do. They provide propellant for the landing burn, minimise propellant sloshing as they're always full, and balance the COM for stable EDL. Sure, they had the fuel header pressurisation issue on SN8, but the fact that they're not immediately scrapping newer SNs probably implies that that issue can either be fixed by software or minor hardware alterations. Starship's EDL sequence is incredibly dynamic and successfully drawing sloshing propellant from the huge main tanks during the violent flip maneuver sounds like it'd be harder than simply altering the header tank system slightly.

9 minutes ago, tater said:

Closures this week canceled, moved until after Christmas:

As far as I can tell, this means one of two things:

1. They need more time to fix SN9.
2. They need more time to scrap SN9 and bring SN10 up to speed.

[RCgothic]

3 minutes ago, RealKerbal3x said:

I'm not an engineer, but I can't think of an alternative solution that would solve as many problems as the header tanks do. They provide propellant for the landing burn, minimise propellant sloshing as they're always full, and balance the COM for stable EDL. Sure, they had the fuel header pressurisation issue on SN8, but the fact that they're not immediately scrapping newer SNs probably implies that that issue can either be fixed by software or minor hardware alterations. Starship's EDL sequence is incredibly dynamic and successfully drawing sloshing propellant from the huge main tanks during the violent flip maneuver sounds like it'd be harder than simply altering the header tank system slightly.

As far as I can tell, this means one of two things:

1. They need more time to fix SN9.
2. They need more time to scrap SN9 and bring SN10 up to speed.

Header tanks also insulate the propellant from direct sunlight via the vacuum in the main tanks.

[tater]

4 minutes ago, RealKerbal3x said:

As far as I can tell, this means one of two things:

1. They need more time to fix SN9.
2. They need more time to scrap SN9 and bring SN10 up to speed.

Or they want to have a Christmas holiday for their staff.

[RealKerbal3x]

2 minutes ago, tater said:

Or they want to have a Christmas holiday for their staff.

Yeah, that's a possibility too. It'd be well deserved if that was the case.

[mikegarrison]

11 minutes ago, RealKerbal3x said:

I'm not an engineer, but I can't think of an alternative solution that would solve as many problems as the header tanks do. They provide propellant for the landing burn, minimise propellant sloshing as they're always full, [...]

Well they weren't "always full" this time, were they?

[RealKerbal3x]

Just now, mikegarrison said:

Well they weren't "always full" this time, were they?

As far as I can tell the reason why they weren't full was the pressurisation issue, but as I said I'm not at all qualified to comment on this  Hopefully it's an easy fix.