# Are there examples of a spacecraft designed for transitions from supercritical to liquid+gas mixtures?

The question What happens when supercritical fuel tanks deplete below critical point? suggests some possible scenarios when the pressure of a tank is in danger of dropping below the supercritical point in temperature or pressure:

• Do rockets not use the remaining contents?
• Do they just use up the remaining contents without halting given the acceleration of the vehicle would make location of liquid and gaseous sections predictable?
• Heat the contents so it stay supercritical until mostly empty?

The currently accepted answer thoroughly documents how the Space Shuttle procedures invoke a redline so that the H2 and O2 reactant tanks for electrical power always remained supercritical and never became sloshable liquid+gas mixture.

The O2 and H2 tanks had quantity "redlines" defined - operation of the tank heaters below those quantities could cause the heaters to overheat. The redlines were 2.5 % in the H2 tanks, 6.5 % in the O2 tanks.

So I'd like to ask if there are any examples where a tank of something in a spacecraft was allowed to drop below supercritical and change into a mixture of liquid plus gas that then had the potential to slosh, and if it had some mechanism for handling said slosh potential.

Question: Are there examples of a spacecraft designed for transitions from supercritical to liquid+gas mixtures?

This could happen in tanks where contents are cryogenic and then warm up over time to become supercritical then depleted to drop below supercritical, or tanks initially charged as supercritical then depleted, or any other imaginable scenario.

This answer discusses transitions a bit.

• The temperature of supercritical fluid oxygen may be too high for a transition to liquid plus gas. Above the critical point temperature only a transition to gas is possible.
– Uwe
Oct 10 '19 at 10:01
• That's a good point to remember, but during a mission temperatures can change as well.
– uhoh
Oct 10 '19 at 10:40
• @OrganicMarble this is what I was trying to ask when I was asking about the graphs! However this gentleman phrased it much better! And I thank him for doing so :). Oct 12 '19 at 3:23
• The Apollo mission rules section 11 say manual heater ops will be used if necessary to maintain supercriticality and they give a pressure below which the tank is considered lost. Apollo was a little different than shuttle because they had batteries. But I know of no spacecraft that put all the  into designing a supercritical storage system and then allowed it to be operated subcritical. But that doesn't mean there isn't one! Which is why this is a comment instead of an answer. Oct 12 '19 at 4:42