Starship: Methane and Oxygen can't both be liquid, except in a narrow temperature range. Is this a problem for interplanetary voyages?

On an extended spaceflight in Starship, the liquid propellant and oxidizer will tend towards the same temperature. The tanks can be refrigerated by boiling off contents, but your mileage may vary doing that. The tanks might be warmed by rotating the ship. But you are still stuck with both tanks being the same temperature inside.

Attached is a phase diagram of oxygen and methane. There is a narrow temperature gap between solidified Methane and tank-rupturing Oxygen pressure. Of course, the boiled O2 could just be vented. But where's the fun in launching liquid O2 just to boil it off?

I can see Starship running out of Helium, cold cranking those turbines as Mars looms ahead.

These diagram graphics are from Engineering Toolbox. I’ve edited them so the horizontal axes (temperature) line up. It appears there is only a 12 °C gap between the freezing point of methane and the temperature where the vapor pressure of oxygen exceeds the safe working pressure of the Starship tank wall (6 bar).

It would be tricky to maintain tank temperature within this band using only passive means. The shiny side of Starship is uninsulated and low emissivity. The heat shield side is heavily insulated and high emissivity. I suspect that cooling the tanks would be easy since the shaded side would try equilibrate with the cosmic background radiation temperature (2.7 K). But it would be difficult to acquire heat from the sunny side due to the low emissivity of stainless steel on the shiny part of the hull, and the high insulation of the heat shield on the other side of the hull.

• It looks like you have something severely wrong with your oxygen plot. Oxygen's critical point is at about 50 bar, but you have it somewhere in the neighborhood of 10^5 bar. You have its triple point far too high as well. Oxygen's boiling point and methane's melting point are nearly the same at 1 bar. Oct 22, 2021 at 0:09
• Your basic premise is true, but to a vastly lesser point than your diagram seems to think. At 1 bar, Oxygen boils at -183C, and Methane freezes at -182C. That's not very horridly incompatible!! The diagram makes a LOT more sense if you use a phase diagram for Oxygen that uses the same vertical scale as the Scale of the Methane diagram! Oct 22, 2021 at 1:17
• Thanks for pointing out the error. Very sloppy of me. I have edited the original posting, including Engineering Toolbox data. I was wrong that there was "no overlap" in liquid state between oxygen and methane. I corrected this to a "narrow overlap". I can't figure out how to edit the original question. It should be, "There is a narrow overlap in temperature/pressure in which oxygen and methane are both liquid. Is this a problem for using this fuel duo for interplanetary travel, particularly in the Starship as presently conceived?" Oct 23, 2021 at 2:51
• "The shiny side of Starship is uninsulated and very low emissivity".. Ok, as what did you take the emissivity of the 304L stainless that StarShip is made of? Asking because the emissivity of stainless can vary from 0,004 to 0.93, depending on the metal specific formulation, the surface roughness, surface contaminants, and the exact level of polishing applied. Frankly, I have no idea of the emissivity of Starship, but as its reflectivity is only mediocre(it's best described as somewhat shiny grey), the emissivity cannot be very far from 0.5 either. Oct 23, 2021 at 5:11
• I can just see Starship running out of Helium Hard to run out of something you don't launch with! Operational starships will use autogenous pressurization for their tanks and RCS, heating the propellants if necessary to produce boil-off and increase pressure rather than relying on helium or any other pressurant. Mars' atmosphere does have some He, but it's better not to need to re-tank an additional fluid (also, it's getting very expensive even here on earth) Oct 24, 2021 at 23:25