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What I mean is that, when the fuel and oxidiser are mixed, at what state (solid - liquid - gaseous) is the oxygen and the fuel. To be more precise, I would like to know how it is for the Merlin engine of the Falcon 9 and the F-1 engine from the Saturn V.

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  • $\begingroup$ Can you edit your question to make your intent more clear? Are you asking about a specific engine or more in general? Propellants usually don't get "pumped into the nozzle" unless you are asking about regenerative cooling. $\endgroup$ Jun 23, 2017 at 12:20
  • $\begingroup$ This question would be easier to answer if it were about a specific model of engine, like the F1 used in the first stage of the Saturn V for example. $\endgroup$ Jun 23, 2017 at 13:11

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I haven't been able to find a complete set of properties for the propellants entering the F-1 combustion chamber. You can see from the schematic, however, that after leaving the pump, the fuel and oxidizer pass directly into the combustion chamber through a couple of valves. enter image description here

The fuel and lox pumps wouldn't work if the propellants were gaseous (they were designed to pump liquids), so I think it's safe to conclude the propellants were liquids on entry to the combustion chamber.

reference

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  • $\begingroup$ An injector would not be necessary for the propellants if they were gaseous. But if the fuel was used to cool the walls of the combustion chamber and the nozzle before injection, a part of it may be gaseous due to evaporation by the heat removed from the walls. $\endgroup$
    – Uwe
    Jun 24, 2017 at 21:08
  • $\begingroup$ Isn't the purpose of the injector to break apart the liquid into a fine mist of tiny droplets? $\endgroup$
    – Innovine
    Jun 25, 2017 at 6:42
  • $\begingroup$ The injector should break apart the liquid into a fine mist of tiny droplets, but if there is only gas instead of a liquid, the injector would be useless. $\endgroup$
    – Uwe
    Jun 25, 2017 at 10:18
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At the risk of damaging my reputation here, I'm going to answer this question without citing sources. I'll instead buttress my argument with a sequence of logical inferences.

Firstly, pintle injectors (as are used in SpaceX's Falcon 9 launch vehicles) depend on the kinetics of liquid-phase propellants to achieve adequate atomization and mixing. Secondly, regenerative cooling in all orbital-class rocket engines (to my knowledge) is entirely in the liquid phase. The only gaseous-phase fluid conduits that are visible in orbital rocket engines is the turbopump exhaust ducts (which are obviously much more voluminous than the cooling channels in the combustion chamber). Also, the least volatile/most thermally stable constituent of bi-propellant rocket propellants is almost always selected as the regenerative coolant.

Taken together, these clues strongly suggest that both propellants in bi-propellant rocket engines (full-flow staged combustion engines excluded) are in the liquid phase as they enter the injectors.

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  • $\begingroup$ I've read that the hydrogen in RL10 and other regen-cooled hydrolox engines enters the chamber as a gas, but haven't found a solid citation. $\endgroup$ Jun 29, 2017 at 0:46
  • $\begingroup$ For hydrolox engines, this wouldn't surprise me given the low boiling point of hydrogen, which seems to be the preferred regenerative coolant...perhaps because of its low density and viscosity. In fact, given the high chamber pressures of hydrolox engines, I wouldn't be surprised if the hydrogen is admitted to the injectors as a supercritical fluid. $\endgroup$ Jun 29, 2017 at 0:56
  • $\begingroup$ @ Joel Neatrour the critical temperature of hydrogen is 32.95 K, the critical pressure is 13 bar. The boiling point at 1.013 bar is 20.35 K. Therefore if the turbo pump pressure is greater than 13 bar, the hydrogen is supercritical and there is only one phase, no gas and liquid anymore. 13 bar would be a very low Chamber pressures for a hydrolox engine. But the critical pressure of oxygen is 50.4 bar, for a chamber pressure of 40 bar, liquid and gaseous oxygen may exist both before the injector. $\endgroup$
    – Uwe
    Jul 1, 2017 at 16:47

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