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In the SpaceX video Starship Update at about 57 minutes Tim Dodd the Everyday Astronaut asks about how the Starship rotates from reentry orientation to landing (vertical) and Musk explains that in the future designs the rotation will be done at the last moment by hot gas thrusters (methalox).

Dodd then follows up by asking “are those then pressure fed?” and Musk's reply includes:

High pressure CH4 bottle, high pressure O2 bottle, and the great thing is that they don’t care what attitude you’re at. You can be at any attitude, any gee’s, any attitude and it will still fire.

The image shown in Wikipedia's Pressure-fed engine looks like if you turn it upside down you'll just blow pressurant straight into the engine. How do pressure-fed engines or thrusters work at "any gee’s any attitude" and not require ullage?

For more on that see answer(s) to What is and what isn't ullage in rocket science?

enter image description here

Screenshot from Scott Manley's July 2021 video Why SpaceX Will Move To New Thrusters To Simplify Starship, sharpened and annotated to indicate a thruster

Screenshot from Scott Manley's July 2021 video "Why SpaceX Will Move To New Thrusters To Simplify Starship" https://www.youtube.com/watch?v=vKiHCaA7hCY, sharpened and annotated to indicate a thruster

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    $\begingroup$ The capillary structures worked at all axis / all gees the shuttle experienced. $\endgroup$
    – Organic Marble
    Commented Sep 30, 2019 at 16:11
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    $\begingroup$ In space there were no restrictions on which way you could maneuver with the jets. Ascent of course the major accel was -X, and entry, -Z, but in space it could be anything - at much lower values of course. Anyway, hopefully someone Space X smart will come along and answer this. $\endgroup$
    – Organic Marble
    Commented Sep 30, 2019 at 16:17
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    $\begingroup$ Although wikipedia uses the heading Methalox Thruster, it says: "In his announcement of the Interplanetary Transport System (ITS) at the 67th International Astronautical Congress on 27 September 2016, Elon Musk indicated that all of the reaction control system thrusters for the ITS vehicles—ITS booster, Interplanetary Spaceship, and ITS tanker—will operate from the gaseous methane and oxygen supply in each of those vehicles. New thrusters will be developed for the purpose." -- If gaseous propellants are used (acceptable bulk inefficiency) then pressure tanks with bladders would work. $\endgroup$
    – amI
    Commented Sep 30, 2019 at 18:14
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    $\begingroup$ @aml if the stored propellants are gaseous, you might not even need a bladder. Maybe they are self-pressurizing. $\endgroup$
    – Organic Marble
    Commented Sep 30, 2019 at 19:36
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    $\begingroup$ @uhoh Al/Li bellows work with LOX and LH2. $\endgroup$
    – Meatball Princess
    Commented Oct 1, 2019 at 1:50

2 Answers 2

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The propellants are gaseous methane and oxygen propellants, they provide their own pressure just as the nitrogen used in the cold gas thrusters does. No diaphragms, bellows, or bladders involved. A thruster of the same size will give around 5x the thrust for the same propellant flow rate as the nitrogen thrusters they'll start with, so there's really no need to involve cryogenic liquids.

With no liquids (at least, none that need be involved during maneuvering...some small boilers or liquid injectors and heaters in the tanks themselves would be an effective way of filling the tanks prior to maneuvers), slosh is eliminated. Due to the lower density of gas, there are also no issues with column pressure in long plumbing lines (never mind the other issues of filling such lines with cryogenic propellants), so the system is practically unaffected by vehicle motions.

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  • $\begingroup$ I think you are suggesting that there are tanks which contain each propellant as gas, and that they can be recharged/replenished with more gas from time to time from the rockets main liquid propellant tanks? That makes sense to me at least. Is there any way to support this by citing some source or reference, or inferring from some public statements, or have you simply ruled out all the other possibilities, and so "once you have eliminated the impossible, whatever remains, however improbable, must be the truth"? $\endgroup$
    – uhoh
    Commented Nov 2, 2019 at 16:09
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Most small pressure-fed engines have some sort of positive expulsion device, it most frequently takes the form of a very flexible diaphragm, or bellow, or plastic bladder trapping the entirety of the propellant that's still in the tank, completely separating them from the pressurizing gas.

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  • $\begingroup$ You've mentioned Al/Li bellows in your comment, would that work for LOX and LCH4? I can't be sure, but I think that Musk mentioned that gas bottles of oxygen and methane were used to pressurize the LOX and Methane liquid propellants. $\endgroup$
    – uhoh
    Commented Oct 1, 2019 at 9:16
  • $\begingroup$ A flexible diaphragm made from plastic would not be flexible at the temperature of LOX. Many materials would be brittle at those temperatures. $\endgroup$
    – Uwe
    Commented Oct 1, 2019 at 11:30
  • $\begingroup$ @Uwe The seal rings on both the LOX and LH2 ball valves used on the SSME are plastic. Just don't know what kind of plastic. $\endgroup$
    – Meatball Princess
    Commented Oct 3, 2019 at 19:37
  • $\begingroup$ The flexibility may have been sufficient for seal rings but not for diaphragms. Finding flexible materials for LH2 valves was a very difficult problem during the Saturn rocket construction for the Apollo mission. LOX valvles used more than one decade of experiences with LOX rockets. $\endgroup$
    – Uwe
    Commented Oct 3, 2019 at 19:52

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