'Insulation balls' for minimising autogenous ullage gas heating

[edzieba]

Autogenous pressurisation is an elegant way to use heated propellant gasses to provide ullage pressure and avoid a separate inert ullage gas system. The downside is that direct contact between the liquid propellant and the hot gas form of that propellant will tend to condense that gas and result in ullage pressure continuously decreasing unless replenished rapidly enough to counteract that decrease. More ullage gas bled from the engines (or generated by resistive heating) means more energy that is not available to be expended propelling the vehicle instead, so minimising condensation is desirable.

In addition to nozzle design stirring of the propellant surface, and vehicle control to minimise sloshing, reducing direct contact between propellant and ullage gas would be desirable to insulate the ullage gas from the liquid propellant. Propellant bellows are already flight proven for hypergolic propellants, but are comparable in mass to the tanks themselves and may be prone to failure in cryogenic conditions. Propellant pistons IIRC have not been flown on space launch vehicles, but are used in short range missiles, but also have the issue of high mass and require tank walls that cannot deform in order to both provide an effective sliding seal (also difficult at cryogenic temperatures) and prevent the piston cocking over and jamming within the tank.

'Shade balls' are used to minimise boiloff of water from terrestrial lakes. They are plastic spheres that float on the water's surface, one or two layers deep.
A similar setup could be used within propellant tanks to minimise contact between ullage gas and liquid - spheres that float halfway immersed and settle to a perfect hexagonal packing would reduce exposed fluid surface to ~10% of normal area, and this could be reduced further with multiple sphere sizes - and to minimise mixing of propellant and ullage gas (ullage gas jets would need to physically displace the higher mass spheres before they could impinge on the propellant). Rather than plastic spheres to block radiation impingement, for cryogenic use stainless steel balloons of just sufficient wall thickness to be lightly pressurised (with inert cold gas of a lower boiling point than the propellant) against casual 'popping' from normal loading during flight conditions would be preferable, as those would be cryogen compatible and of minimal mass. This imposes no additional requirements on tank construction beyond a mesh screen on the inlet (if not already present) of hole size small enough to prevent ingestion, and can be retrofitted to existing tanks through the tank access port. They would not appreciably modify propellant loading and unloading procedures as they do not form a solid barrier within the tank.
The major issues would be:
- The potential for impact damage to the tank wall under high jerks with empty tanks (worse case would be the balls clustered at the front of an empty tank, allowing the vehicle to accelerate at full thrust the entire tank length before impacting)
- The potential for small debris generation if balls are burst and cannot sink to the bottom of the tank before 'grinding' action from adjacent balls produces particles small enough to pass the propellant inlet screen
- Permanent occupation of tank volume (impact may be reduced by occupying what would otherwise be 'dead' ullage space needed to avoid inlet jet interaction)
- Non-zero additional dry mass

[octavo]

I'm not qualified to give any kind of worthwhile opinion, but this sounds like a really clever idea to me.

[InterestedEngineer]

Cool idea!

Instead of SS, I would pick a material that can deform to handle the 'jerk' problems, and also of soft enough material  that ingesting bits of it into impellers won't damage anything.

I would also try not to depend on their inflation to maintain any structural integrity.

Additional requirement:  Handle 1-10 bar pressure changes in the tank.

How about an open cell foam ball with material that can handle cryogenic temps?   

Or maybe just a plastic ball that can handle cryogenic temperatures that has holes to deal with pressure changes.  My dog has a toy like that, it bounces back after he munches on it.

[DanClemmensen]

"Shade balls" only work because ponds are in a gravity field. I thought the ullage gas problem also occurred in microgravity?

Here on Earth you can buy the cheaper spherical type or the more expensive rhomboids. The rhomboids self-align with smaller gaps than the spheres.

[HMXHMX]

I'm not qualified to give any kind of worthwhile opinion, but this sounds like a really clever idea to me.

I agree, it is clever.  But not to be snarky, it was also clever back in the 1950s when first proposed at Redstone Arsenal and was known as the "beer can solution" to boiloff.  In that implementation, instead of spheres, closed end tubes (i.e., beer cans, evacuated or helium filled) are used to form a mat of floating insulation on the surface of a cryo tank.

I can't say with certainty if this was just proposed or actually tried, but a couple of the commercial space ventures of the 1980s planned to use this method of insulation as well.

[Craigles]

Wow, but ... Here are more issues to think about:
What should fill these balls? Helium and hydrogen leak. GN2, too, would need to be kept at a range of pressures. Catalytic converter beads - - would they even float? Might aerogels be too flimsy and require pressurization also? What size should these balls be, to avoid FOD in the turbo pumps?

[Asteroza]

I'm not qualified to give any kind of worthwhile opinion, but this sounds like a really clever idea to me.

I agree, it is clever.  But not to be snarky, it was also clever back in the 1950s when first proposed at Redstone Arsenal and was known as the "beer can solution" to boiloff.  In that implementation, instead of spheres, closed end tubes (i.e., beer cans, evacuated or helium filled) are used to form a mat of floating insulation on the surface of a cryo tank.

I can't say with certainty if this was just proposed or actually tried, but a couple of the commercial space ventures of the 1980s planned to use this method of insulation as well.

Would it be correct that the beercan method was expected to be applied while under gravity only?

The edge case for this seems to be a single ball, in the form of a sliding diaphram in a tank (sorta like a piston), correct?

I wonder if there is a gas selective cryogenic liquid/gas membrane available? If ullage gas exists, it could infiltrate such a membrane balloon/ball perhaps?

Or would lining the tank walls with a hydrophilic analog of a surface treatment to selectively attract liquid to the walls, thus pushing gas towards the center, might work better in microgravity?

[InterestedEngineer]

PTFE and its related Gore-Tex (expanded-PTFE) might be a useful material to consider.  PTFE is already used in highly oxidative environments including liquid oxygen.

Include in requirements "not reactive with LOX".

I"m not sure if there's widespread production of PTFE foams but here's an article on it.

https://link.springer.com/article/10.1007/s10934-021-01090-4

The other thing to consider would be blankets like those used to insulate swimming pools at night.  A Gore-Tex blanket is something that is widely available.

Another advantage of PTFE is it's a lubricant, so it won't harm impellers if fragments are ingested.

[HMXHMX]

I'm not qualified to give any kind of worthwhile opinion, but this sounds like a really clever idea to me.

I agree, it is clever.  But not to be snarky, it was also clever back in the 1950s when first proposed at Redstone Arsenal and was known as the "beer can solution" to boiloff.  In that implementation, instead of spheres, closed end tubes (i.e., beer cans, evacuated or helium filled) are used to form a mat of floating insulation on the surface of a cryo tank.

I can't say with certainty if this was just proposed or actually tried, but a couple of the commercial space ventures of the 1980s planned to use this method of insulation as well.

Would it be correct that the beercan method was expected to be applied while under gravity only?

The edge case for this seems to be a single ball, in the form of a sliding diaphram in a tank (sorta like a piston), correct?

I wonder if there is a gas selective cryogenic liquid/gas membrane available? If ullage gas exists, it could infiltrate such a membrane balloon/ball perhaps?

Or would lining the tank walls with a hydrophilic analog of a surface treatment to selectively attract liquid to the walls, thus pushing gas towards the center, might work better in microgravity?

Yes, this will only work under acceleration.  Other techniques apply to on-orbit storage, MLI, etc.

[edzieba]

Would indeed only work in a G-field or under acceleration, but that is a problem for regular propellant feed too. You either perform a settling burn first (which would settle the balls as well as the propellants, with the worst case just being increased ullage requirements at the start of the burn as the balls float to the top) or you burn to depletion and use separate header tanks. The big ullage-condensation losses occur during the initial long launch burns, where the balls would work fine.
Reducing boiloff during coast is a separate issue.

I'm not qualified to give any kind of worthwhile opinion, but this sounds like a really clever idea to me.

I agree, it is clever.  But not to be snarky, it was also clever back in the 1950s when first proposed at Redstone Arsenal and was known as the "beer can solution" to boiloff.  In that implementation, instead of spheres, closed end tubes (i.e., beer cans, evacuated or helium filled) are used to form a mat of floating insulation on the surface of a cryo tank.

I can't say with certainty if this was just proposed or actually tried, but a couple of the commercial space ventures of the 1980s planned to use this method of insulation as well.

Haha, whenever you think you have an original good idea, turns out someone decades ago also though they were clever!

[Twark_Main]

I'm not qualified to give any kind of worthwhile opinion, but this sounds like a really clever idea to me.

I agree, it is clever.  But not to be snarky, it was also clever back in the 1950s when first proposed at Redstone Arsenal and was known as the "beer can solution" to boiloff.  In that implementation, instead of spheres, closed end tubes (i.e., beer cans, evacuated or helium filled) are used to form a mat of floating insulation on the surface of a cryo tank.

I can't say with certainty if this was just proposed or actually tried, but a couple of the commercial space ventures of the 1980s planned to use this method of insulation as well.

Haha, whenever you think you have an original good idea, turns out someone decades ago also though they were clever!

I just wish they had been clever enough to document their work.

Can anyone find any reference to this effort, beyond the oral history conveyed by HMXHMX?

[john smith 19]

Haha, whenever you think you have an original good idea, turns out someone decades ago also though they were clever!

Yes.

If you dig hard enough in the files you find a lot has been tried, or at least considered, at some time.

An actual original idea is extremely rare.

plastic balls in beer cans were a thing in some countries a few years ago so I did think of this also as a way to surpress heat transfer from the warm (hot) ullage gas to the propellant. But I wasn't sure you could find a LOX compatible plastic.

[john smith 19]

I just wish they had been clever enough to document their work.

Can anyone find any reference to this effort, beyond the oral history conveyed by HMXHMX?

A lot of these things were invented (and discarded) under the pressure of the USSR launching Sputnik, and the US realizing that if they could put up a satellite that big then even with their heavy nuclear weapons they had a viable ICBM as well. 

If something failed it was not investigated, just dumped. Why document why you didn't use something?  No (perceived) time to find out more. Hence the creation of "folklore" and old-wives-tales like "It's impossible to cool a rocket chamber with LOX" which was proved nonsense in the 90's at NASA (and at Orbital IIRC) but remains a pervassive belief (despite other combinations using the oxidizer in preference to the fuel for chamber cooling).

In software development it's called "Rationale capture" and it remains a very hard thing to do unobtrusively although very valuable if a design hits some sort of limitation and the team has to "back up" and consider what new direction to go in (and why didn't they go in that direction originally?) 

[Barley]

I just wish they had been clever enough to document their work.

Can anyone find any reference to this effort, beyond the oral history conveyed by HMXHMX?

The credit for a discovery should not go to the first discoverer but to the last.  The last discoverer did the work to make sure it stays discovered.

[Jim]

The mass of the balls and the loss of propellant volume likely outweighs the additional propellant/gas needed to prevent ullage collapse.

[Jim]

I'm not qualified to give any kind of worthwhile opinion, but this sounds like a really clever idea to me.

I agree, it is clever.  But not to be snarky, it was also clever back in the 1950s when first proposed at Redstone Arsenal and was known as the "beer can solution" to boiloff.  In that implementation, instead of spheres, closed end tubes (i.e., beer cans, evacuated or helium filled) are used to form a mat of floating insulation on the surface of a cryo tank.

I can't say with certainty if this was just proposed or actually tried, but a couple of the commercial space ventures of the 1980s planned to use this method of insulation as well.

It was done to prevent slosh until slosh baffles were install in tanks.

https://blog.aerospacenerd.com/p/lessons-learnt-rocket-fuel-sloshing-jupiter-missile

[Proponent]

... "folklore" and old-wives-tales like "It's impossible to cool a rocket chamber with LOX" which was proved nonsense in the 90's at NASA (and at Orbital IIRC) but remains a pervassive belief (despite other combinations using the oxidizer in preference to the fuel for chamber cooling).

Wasn't it the Soviets who proved lox could be used? I'm thinking of the RD-170 family of engines using lox-rich staged combustion. Weather lox is used for chamber cooling in that case I do not know, but I'd have thought that would be an easier problem than a lox-rich turbine.

[Hog]

ORSC=Oxidizer Rich Staged Combustion.  NK-15 for N-1 and the follow on NK-33, of which Orbital bought a bunch for conversion to AJ-26.

Are we getting the ability for chamber cooling confused with the ability to manufacture components which would withstand use in oxidizer-rich applications (ORSC AND FFSC)? Are they one-in-the-same?

[edzieba]

Worth bumping, now we know that not only is ullage collapse a potential issue, but ullage freezeout when using mixed exhaust gasses.
Floating insulators would not only slow chilling of the ullage components prone to freezeout, but they could also act as nucleation points to keep the frozen reaction products floating at the top of the tank rather than sinking down to the inlet. Could be mass-competitive with inlet filtration or engine-side HX.