Given the heat from combustion and increased temperature, would the LOx still be in liquid form after emanating from the injector? Or will it instantly transform into gas?

Are injectors cooled using the flow of LOx or is there a separate cooling system for the injector? If cooled using LOx, then this additional heating would make the LOx's temperature to rise even further. This might cause the phase transformation even before entering the combustion chamber.

  • $\begingroup$ There will be no instantly evaporation of liquid oxygen. This will take some time, but a very short one. I guess some milliseconds or even microseconds. But it is difficult to avoid oxidation of the injector itself. $\endgroup$
    – Uwe
    Feb 6, 2018 at 10:05
  • $\begingroup$ Is the phase diagram or table for Oxygen available? I am able to find diagram for very high pressure range(it's 1GPa after 0 along x-axis). $\endgroup$ Feb 6, 2018 at 13:10
  • 1
    $\begingroup$ I've seen videos of fuel mixture diffusing and igniting in a car engine "combustion chamber", wow wouldn't it be great if there was a way to do this with even a small rocket engine? Diamond or sapphire window might work for a short time? $\endgroup$
    – uhoh
    Feb 6, 2018 at 15:39
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    $\begingroup$ A diamond window should not be exposed to a high concentration of oxygen. Diamond's ignition point is 720–800 °C in oxygen and 850–1000 °C in air. Sapphire does not burn and its melting point is at 2030 to 2050 °C. $\endgroup$
    – Uwe
    Feb 6, 2018 at 16:28
  • $\begingroup$ The LOX (and fuel) are sprayed as liquid through the injector plate, as impinging streams (typ LOX on LOX and fuel on fuel) to break into small droplets for mixing. Good info at:heroicrelics.org/info/f-1/f-1-injector.html $\endgroup$
    – amI
    Feb 6, 2018 at 21:59

1 Answer 1


For most rocket engines the fuel is used as nozzle and chamber coolant, which heats it up. It may enter the combustion chamber as gas or liquid, depending on design. But the LOX is merely pumped to the engine, and enters at storage temperature of 90K, and very much a liquid.

Example: the Space Shuttle SSME: enter image description here

This LOX is sprayed using various techniques to form very fine droplets, which then mix with the fuel and ignite, promptly evaporate to gas, and burn some more.

Unless your engine is a SpaceX Raptor or other(what other?) full-cycle engine where the all LOX is partially preburned and enters the combustion chamber as a supercritical fluid, which is neither gas not liquid, having both temperature and pressure above its critical point. For Oxygen, that is 155K and 50 Bar pressure.

In the case of Raptor it injects Oxygen at something like 750K and 380 Bar.

enter image description here

  • $\begingroup$ What about oxygen-rich staged combustion engines such as the RD-180 and the BE-4? IIUC, for these engines, the LOX all goes through the preburner (along with a small fraction of the fuel) and is hence heated (presumably to a gas) before it reaches the combustion chamber. $\endgroup$ Jul 12, 2023 at 16:41

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