Space Cryogenics is hard.
NASA's fill-'er-up Moon rocket 'confidence' test sees mixed results
NASA engineers spent the weekend studying the data after another attempt to fill the agency's monster Space Launch System (SLS) produced mixed results. Dubbed a "confidence test" by NASA, the core stage of the SLS was partially filled with liquid oxygen to check out the newly replaced seals around where the propellant leaked …
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Monday 16th February 2026 16:30 GMT Gary Stewart
Cryogenics is hard
Depends. SpaceX and Blue Origin both use cryogenic oxygen (as does NASA) and cryogenic methane with almost no leak problems. However, cryogenic hydrogen is very hard. With all the years that NASA has been using it, since Apollo at least up to the Space Shuttle which is the basis for the modified SLS rocket engines you'd think that they would have a better handle on it. Especially since the first launch also had liquid hydrogen leak problems. I am not a fan of the SLS program because they use outdated rocket technology (thanks congress!) but I do hope they get this problem fixed fairly soon so they can complete their part of the Artemus program.
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Monday 16th February 2026 16:46 GMT MyffyW
Re: Cryogenics is hard
Methane boiling point is 100 K, whilst for Hydrogen it's 20 K.
Explosion limits for Methane are 5% to 15%, whilst for Hydrogen it's 4% to 75%
Engineering tolerances are markedly different for the two fuels.
The real reason for Hydrogen's use is high energy per unit of mass and high specific impulse. Which sort of matters when you're heading to the moon.
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Monday 16th February 2026 22:31 GMT Gary Stewart
Re: Cryogenics is hard
The main problem with hydrogen is that it is the smallest atom and can find places to escape through in just about any gasket or flexible material. Thus preventing hydrogen from leaking especially in break away connections used to fuel a rocket and keep it topped off until launch presents extremely difficult engineering problems. That is why more modern reusable rockets like "Starship" and New Glen have moved to super chilled methane. Not quite the same bang for the buck but improved rocket engine design (the SLS engines are simply modified shuttle engines) and being much easier to handle makes it the fuel of choice for the next generation of heavy lift vehicles. But like I previously wrote, NASA has been handling liquid hydrogen for over 50 years so it makes the current SLS leak problems somewhat perplexing.
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Monday 16th February 2026 22:58 GMT Doctor Syntax
Re: Cryogenics is hard
"The main problem with hydrogen is that it is the smallest atom and can find places to escape through in just about any gasket or flexible material."
Yup. We used to locate leaking joints in the carbon dating system pipework by dribbling a little hydrogen out of a rubber tube over each joint in turn until we found the one that changed the sound of the vac, pump.
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Monday 16th February 2026 23:57 GMT MachDiamond
Re: Cryogenics is hard
"The main problem with hydrogen is that it is the smallest atom and can find places to escape through in just about any gasket or flexible material. "
It's the smallest atom, but He is the smallest molecule (one atom) as H prefers to pair up with another H (H2) and becomes larger than a single He atom.
Hair splitting, for sure. They are both bastards to seal in since they slip out like a mouse under a door that you swear they shouldn't be able to get under.
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Tuesday 17th February 2026 00:05 GMT MachDiamond
Re: Cryogenics is hard
"That is why more modern reusable rockets like "Starship" and New Glen have moved to super chilled methane. Not quite the same bang for the buck but improved rocket engine design (the SLS engines are simply modified shuttle engines) and being much easier to handle makes it the fuel of choice for the next generation of heavy lift vehicles."
If there's margin. Elon has a rocket that's bigger than the Saturn V that's going to need a couple of dozen more to refill it to get to the moon.
When I was working on rockets, we used 99.9% IPA as the fuel and LOx as the oxidizer as the fuel was safer to work with. As I was leaving, they were developing a cryo-methane motor and I did some of the electronics hardware for it, but wasn't there when they got it into testing. I don't know if it's just me, but methane fired motors seem like they radiate much more heat than the alcohol ones. A lot more. It's a factor as it's radiated heat so it can mean more attention to fireproof sleeving on wires and feed lines down around the engine(s).
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Monday 16th February 2026 22:15 GMT grndkntrl
Re: Coal dust
Whilst you jest, the hydrogen used is most likely created from steam reformation of methane, as currently ~99% of the global hydrogen supply is produced in this way. Only ~1% is produced using green technologies: https://en.wikipedia.org/wiki/Hydrogen_production.
Whilst it's not as dirty as coal, it's ultimately still nowhere near as clean as some proponents like to claim it to be.
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Tuesday 17th February 2026 09:49 GMT LogicGate
What the current situation proves is that while amazing things can be done when enough funding and effort is spent, spending the same funding on pork over as many election districts as possible will not necessarily lead to technological progress. Who would have thought.
With regards to doubting the moon landings: This load of BS has been debunked so many times that it is not worth my time.
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Saturday 21st February 2026 02:20 GMT Hurn
Do people just expect typos these days?
"the core stage of the SLS was partially filled with liquid oxygen to check out the newly replaced seals around where the propellant leaked in a recent Wet Dress Rehearsal (WDR)"
Err.. shurley that should read "liquid hydrogen" ? I doubt they ran liquid oxygen into the liquid hydrogen tank. Even if they did, the temp difference would invalidate the test.
