4 ms·

Silly question, maybe, but what is filling the "empty" volume of that cylinder as the oxygen bubbles float around? Clearly it can't be oxygen or nitrogen gas, s

Silly question, maybe, but what is filling the "empty" volume of that cylinder as the oxygen bubbles float around? Clearly it can't be oxygen or nitrogen gas, so is it just vacuum? In a total vacuum, wouldn't the liquid oxygen atomize? (And wouldn't pulling a vacuum in the tank on the way up make it a lot harder to pump the liquid out of it in the atmosphere?)

argiopetech 3y ago
I also don't know, but wouldn't it be gaseous oxygen? Edit: The tank isn't at vacuum. It started full of liquid oxygen. I assume (without researching) that the oxygen would change phase and fill the void with gas as the liquid is pumped out.
- pfdietz 3y ago
  That would just cool the LOX and ultimately cause it to freeze. Such a system would require a heater, and even then there could be problems from sloshing causing the pressurizing gas to condense. Overall, it's less of a problem in pump fed rockets, since the gas only has to pressurize the propellant up to the inlet pressure of the pumps, not above the pressure of the rocket thrust chamber as it would in tank pressure fed rockets.
  - noduerme 3y ago
    What happens if the liquid oxygen is boiling as it's fed to the pump....?
    - pfdietz 3y ago
      Pumps require a certain inlet pressure or they will experience cavitation. I think a liquid close to boiling would make this worse.
  - argiopetech 3y ago
    Ah, I hadn't considered that it might be pressurized with an inert gas. Thanks!
    - pfdietz 3y ago
      Also, Falcon 9 now uses subcooled LOX (at 66K) to increase its density.
krisoft 3y ago
Not silly question at all. To my knowledge the Falcon uses helium to pressurise the tanks. So the gas you can see is helium. It has several benefits: does not burn, and does not freeze at the temperature of the liquid oxygen. It is also a relatively simple system. They “just”[1] have liquid helium in a composite overwrapped pressure vessel inside the tank and most likely they have a regulator on it to release gas as the oxygen/fuel is used up. 1: this is not easy at all, they lost a rocket due to the helium bottle breaking free once: https://sma.nasa.gov/SignificantIncidents/assets/spacex-falcon-9-failure-investigation-focuses-on-copv-struts--nasaspaceflight.com.pdf https://sma.nasa.gov/SignificantIncidents/assets/spacex-falc... > wouldn't pulling a vacuum in the tank on the way up make it a lot harder to pump the liquid It would make it impossible to pump. But that is not the only reason you need a pressurant. The tank wall is relatively thin. It would implode in the lower atmosphere if you would have vacuum in it.

argiopetech 3y ago

I also don't know, but wouldn't it be gaseous oxygen? Edit: The tank isn't at vacuum. It started full of liquid oxygen. I assume (without researching) that the oxygen would change phase and fill the void with gas as the liquid is pumped out.

pfdietz 3y ago
That would just cool the LOX and ultimately cause it to freeze. Such a system would require a heater, and even then there could be problems from sloshing causing the pressurizing gas to condense. Overall, it's less of a problem in pump fed rockets, since the gas only has to pressurize the propellant up to the inlet pressure of the pumps, not above the pressure of the rocket thrust chamber as it would in tank pressure fed rockets.
- noduerme 3y ago
  What happens if the liquid oxygen is boiling as it's fed to the pump....?
  - pfdietz 3y ago
    Pumps require a certain inlet pressure or they will experience cavitation. I think a liquid close to boiling would make this worse.
- argiopetech 3y ago
  Ah, I hadn't considered that it might be pressurized with an inert gas. Thanks!
  - pfdietz 3y ago
    Also, Falcon 9 now uses subcooled LOX (at 66K) to increase its density.

krisoft 3y ago

Not silly question at all. To my knowledge the Falcon uses helium to pressurise the tanks. So the gas you can see is helium. It has several benefits: does not burn, and does not freeze at the temperature of the liquid oxygen. It is also a relatively simple system. They “just”[1] have liquid helium in a composite overwrapped pressure vessel inside the tank and most likely they have a regulator on it to release gas as the oxygen/fuel is used up. 1: this is not easy at all, they lost a rocket due to the helium bottle breaking free once: https://sma.nasa.gov/SignificantIncidents/assets/spacex-falcon-9-failure-investigation-focuses-on-copv-struts--nasaspaceflight.com.pdf https://sma.nasa.gov/SignificantIncidents/assets/spacex-falc... > wouldn't pulling a vacuum in the tank on the way up make it a lot harder to pump the liquid It would make it impossible to pump. But that is not the only reason you need a pressurant. The tank wall is relatively thin. It would implode in the lower atmosphere if you would have vacuum in it.