4 ms·

Wouldn't the pressurant/backfill gas dilute the propellant as it was added?

CraftThatBlock 4y ago
Not an expert, but fuel and oxidizer is stored as liquid (at cryogenic temperatures), so adding gas creates pressure at the "top" of the tank, pushing the liquid down into the engine (in addition to gravity).
bumlebi 4y ago
Only if the pressurant gas dissolves into the liquid propellant. Even then, it doesn't even dilute it per se, it just makes something akin to rocket-fuel-soda which causes all sorts of flow problems in your feed lines. Naturally, if your propellant is stored as a gas, adding more gas to pressurize will of course dilute it as you say, but I've never heard of something like this. So the short answer here is: no, it won't.
Torkel 4y ago
I think it’s liquid oxygen/methane that goes into the engine. So backfilling with some other gas in the gas part of that should be ok. No expert though :)
simne 4y ago
Sure, if the pressurant gas dissolves into the liquid propellant. But mostly chosen pressurant which is not dissolves, or used some type of separating membrane (or piston), for example, in Soviet space stations used https://en.wikipedia.org/wiki/Metal_bellows https://en.wikipedia.org/wiki/Metal_bellows
showerst 4y ago
In some cases they use a bladder or a push plate.
hydrogen7800 4y ago
Expert here. I'm a longtime lurker on HN, but registered just for this comment. I design aerospace pressure vessels for a living. As showerst alluded to, there are often things called "propellant management devices" or PMDs. There are a few reasons these might be used. In zero G, you need the propellant sump to remain wetted with propellant; it would be bad to ingest the pressurant/ullage gas into your engines. Some PMDs allow the pressurant and propellant to occupy the same volume, and will use surface tension devices inside the tank to direct liquid to the sump. These can be screens, vanes, channels, etc. In other cases, the pressurant and propellant are kept separate by a bladder. These bladders can be rubber, or even metal. The propellant mass is usually a large proportion of a space vehicle's mass, and you can't have that much sloshing around when you need dynamic control. A metal diaphragm keeps the propellant more or less static, and its center of mass in a predictable location. As for the pressurant gas dissolving into the propellant in non-PMD tanks, I don't know enough about that. I imagine the solubility of He (it is usually helium) in these propellants is either accounted for, or negligible. Edit: P.S. Software engineering is mostly foreign to me and much of HN content is over my head, but I like he level of discourse here. So, when a topic came up that I could actually contribute to, I jumped.
- mLuby 4y ago
  Love how much expertise there is here; thanks for adding yours! When you say "metal bladder/diaphragm" is that a sliding wall with propellant on one side and pressurant on the other? (I can't imagine how the seals in that would work.) Or do you mean the metal actually deforms in place?
  - hydrogen7800 4y ago
    Yeah, the metal deforms. They are single use devices. Imagine a spherical metal tank, with a thin metal hemisphere welded inside. The hemisphere would invert as the propellant is expelled. Hard engineering and manufacturing problems here. I think there have been piston tanks (which would be reusable with sliding elastomer seals. Not many elastomers play well with hypergolics), but they would be very weight inefficient.
- kortex 4y ago
  Nice writeup! Former chemist here. Helium is indeed poorly soluble in most liquids. It's actually used to sparge (purge by bubbling through) solvents to drive out other dissolved gasses. This is because Helium has very weak intermolecular forces due to its electronic symmetry. For that same reason, it's also as close to an ideal gas, giving you the most pressure/volume bang for your mass buck (only hydrogen is better, and that's bad for oxidizer tanks for obvious reasons). But this also limits the ability to cryogenically condense helium, which would improve storage density. But you really don't need much in turbopump fed engines.
- jacksonkmarley 4y ago
  Awesome detailed answer! Much appreciated.
- MertsA 4y ago
  Neat, that's one subject that's always bugged me as every article or video out there just handwaves away settling of propellant tanks in orbit with "ullage thrusters". No one ever mentions how you get a nice continuous feed to your ullage thrusters though.

Not an expert, but fuel and oxidizer is stored as liquid (at cryogenic temperatures), so adding gas creates pressure at the "top" of the tank, pushing the liquid down into the engine (in addition to gravity).

bumlebi 4y ago

Only if the pressurant gas dissolves into the liquid propellant. Even then, it doesn't even dilute it per se, it just makes something akin to rocket-fuel-soda which causes all sorts of flow problems in your feed lines. Naturally, if your propellant is stored as a gas, adding more gas to pressurize will of course dilute it as you say, but I've never heard of something like this. So the short answer here is: no, it won't.

Torkel 4y ago

I think it’s liquid oxygen/methane that goes into the engine. So backfilling with some other gas in the gas part of that should be ok. No expert though :)

simne 4y ago

Sure, if the pressurant gas dissolves into the liquid propellant. But mostly chosen pressurant which is not dissolves, or used some type of separating membrane (or piston), for example, in Soviet space stations used https://en.wikipedia.org/wiki/Metal_bellows https://en.wikipedia.org/wiki/Metal_bellows

showerst 4y ago

In some cases they use a bladder or a push plate.

hydrogen7800 4y ago

Expert here. I'm a longtime lurker on HN, but registered just for this comment. I design aerospace pressure vessels for a living. As showerst alluded to, there are often things called "propellant management devices" or PMDs. There are a few reasons these might be used. In zero G, you need the propellant sump to remain wetted with propellant; it would be bad to ingest the pressurant/ullage gas into your engines. Some PMDs allow the pressurant and propellant to occupy the same volume, and will use surface tension devices inside the tank to direct liquid to the sump. These can be screens, vanes, channels, etc. In other cases, the pressurant and propellant are kept separate by a bladder. These bladders can be rubber, or even metal. The propellant mass is usually a large proportion of a space vehicle's mass, and you can't have that much sloshing around when you need dynamic control. A metal diaphragm keeps the propellant more or less static, and its center of mass in a predictable location. As for the pressurant gas dissolving into the propellant in non-PMD tanks, I don't know enough about that. I imagine the solubility of He (it is usually helium) in these propellants is either accounted for, or negligible. Edit: P.S. Software engineering is mostly foreign to me and much of HN content is over my head, but I like he level of discourse here. So, when a topic came up that I could actually contribute to, I jumped.

mLuby 4y ago
Love how much expertise there is here; thanks for adding yours! When you say "metal bladder/diaphragm" is that a sliding wall with propellant on one side and pressurant on the other? (I can't imagine how the seals in that would work.) Or do you mean the metal actually deforms in place?
- hydrogen7800 4y ago
  Yeah, the metal deforms. They are single use devices. Imagine a spherical metal tank, with a thin metal hemisphere welded inside. The hemisphere would invert as the propellant is expelled. Hard engineering and manufacturing problems here. I think there have been piston tanks (which would be reusable with sliding elastomer seals. Not many elastomers play well with hypergolics), but they would be very weight inefficient.
kortex 4y ago
Nice writeup! Former chemist here. Helium is indeed poorly soluble in most liquids. It's actually used to sparge (purge by bubbling through) solvents to drive out other dissolved gasses. This is because Helium has very weak intermolecular forces due to its electronic symmetry. For that same reason, it's also as close to an ideal gas, giving you the most pressure/volume bang for your mass buck (only hydrogen is better, and that's bad for oxidizer tanks for obvious reasons). But this also limits the ability to cryogenically condense helium, which would improve storage density. But you really don't need much in turbopump fed engines.
jacksonkmarley 4y ago
Awesome detailed answer! Much appreciated.
MertsA 4y ago
Neat, that's one subject that's always bugged me as every article or video out there just handwaves away settling of propellant tanks in orbit with "ullage thrusters". No one ever mentions how you get a nice continuous feed to your ullage thrusters though.