Starship On-orbit refueling - Options and Discussion

[TheRadicalModerate]

Ot may be OK to let methane freeze while it’s being stored in the depot. But you’ll definitely need to condition the prop to be liquid before it’s transferred, to say nothing of using it in an engine start.

[OTV Booster]

Ive got a rich suite of background processes working on orbital refueling and two concerns have popped out of the que. The current discussion on propellant temperature needs is one. QD sealing is another.


Cryogenic seals are a tough problem. Adding in multiple sealing cycles can only make it tougher. Our discussion has focused on using either the current QD layout or something similar. This puts the oxidizer and fuel seals in close proximity. If both start leaking liquid, local conditions determine the possibility of vaporization. If vapor is present only the lack of an ignition source prevents ignition.


I suspect the current state of material science is inadequate for perfecting the seals. I will gladly defer to wiser minds on this.


AIUI, the flow of the propellants themselves will build up triboelectric potential differences. This isn't insurmountable but it has to be dealt with.


Up until now I've been a strong advocate of working with current QD layout but I'm having second thoughts. Might it be best to keep the LOX connection where it is and having a second QD plate up by the methane tank? This would keep leakage separated and make an ignition source a non problem. It's a PITA but so is unintended consequences. Could this arrangement be worked to ease some of the temperature concerns?


Or am I just pissing in a tea pot and creating a tempest? (Did I get that right?  )

[Robotbeat]

Ignition is basically impossible in a non-pressurized volume. Any gas dissipates at approximately the speed of sound into the vacuum of space, so you’ll not have enough gas around. Additionally, you need BOTH fuel and oxidizer to get combustion in space. This is one way where refueling in space is actually easier than on the ground where there’s ambient pressure and the atmosphere already has oxygen in it, so oxygen alone is sufficient.

The materials science of sealing isn’t really any harder than on the ground (vacuum welding is a thing, but it’s actually rare for this to happen). But there’s additional advantages in that you don’t have to worry about ice (or even liquefied air) condensation, which at cryogenic temperatures is as strong as like epoxy. On the ground, condensation and frost can build up which interferes with a seal or makes it difficult to disengage. Not so in vacuum.

[OTV Booster]

Ignition is basically impossible in a non-pressurized volume. Any gas dissipates at approximately the speed of sound into the vacuum of space, so you’ll not have enough gas around. Additionally, you need BOTH fuel and oxidizer to get combustion in space. This is one way where refueling in space is actually easier than on the ground where there’s ambient pressure and the atmosphere already has oxygen in it, so oxygen alone is sufficient.

The materials science of sealing isn’t really any harder than on the ground (vacuum welding is a thing, but it’s actually rare for this to happen). But there’s additional advantages in that you don’t have to worry about ice (or even liquefied air) condensation, which at cryogenic temperatures is as strong as like epoxy. On the ground, condensation and frost can build up which interferes with a seal or makes it difficult to disengage. Not so in vacuum.

What you say about leaks into vacuum supporting combustion is comforting. I will now go and clean out that teapot.


I agree with what you say about in atmosphere freezing but have to disagree on the materials science not being any harder in space than on the ground. On the ground seals can be changed out after every use if necessary and it's no big thing. Depot would be another story.


Extrapolating from seals I've worked with, rubber or rubber like seals need to be warm to be pliable enough to seal. At low pressure and starting warm they will allow a good cryo seal. How high can the pressure go? IDK. Teflon works fine on a cryo ball valve but I don't know how well this translates to a face seal.


Thinking it through, maybe seal conditioning (heat) would be the last step before coupling.

[Paul451]

Ignition is basically impossible in a non-pressurized volume. Any gas dissipates at approximately the speed of sound into the vacuum of space, so you’ll not have enough gas around.

Not quite. The initial leak will carry away heat, lowering the temperature of the leak-area and leaking fluid until it reaches a new equilibrium. If the leak is very minor, it's not a drama. But if it's enough that the equilibrium is below the freezing point of the leaking fluid/gas, it will start to freeze-boil, building up a slush around the leak. (See the LOx leak on the last F9 launch.) If that slush is a mix of oxygen and fuel, it could form an impact explosive... A boiling/fracturing impact explosive. (The ignition source is free.)

[TheRadicalModerate]

Do we know how the terrestrial QD seals are loaded after contact?  Will the same mechanism work in microgravity?