Im reading about rocket engines and I have a question about is it possible to use fuels in the form of gases than a liquid in a rocket engine?
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9$\begingroup$ Many designs keep the propellants supercritical, so they're not really gases, but they're not really liquids either. They can be pumped like liquids but they fill the container like a gas. $\endgroup$– J...Commented Nov 8, 2019 at 16:23
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6 Answers
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Possible: yes.
Feasable: not really (at least not for power applications).
The main trick is energy density (per volume) - gases tend to be quite significantly less dense than liquids - and thus the tanks would need to be much larger and heavier - so they are commonly used in their condensed liquid form.
For small engines gases have been used - both as cold gas thrusters and in amateur experiments.
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$\begingroup$ A cold-gas thruster isn't really a "fuel," but your point is well taken. $\endgroup$ Commented Nov 8, 2019 at 14:25
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2$\begingroup$ @CarlWitthoft many would disagree, but it is semantics. Cold gas is considered in the same class as mono-propellant. It is a fluid that is expanded with a nozzle to accelerate it in a particular direction. Catalyst based mono-prop is expanded chemically, and cold gas is expanded by dropping the pressure. $\endgroup$– AaronCommented Nov 8, 2019 at 15:40
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2$\begingroup$ "Fuel" typically means a substance that is combined with an oxidizer. However, if one generalizes "fuel" to mean "propellant" (fuel and oxidizers qualify as propellants) then the gas used in cold gas (and warm gas and hot gas) thrusters qualifies as "fuel". $\endgroup$ Commented Nov 9, 2019 at 0:58
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Yes, and it is currently being done on a few engines, notably SpaceX's Raptor engines. They run on liquid oxygen and liquid methane. These are run through turbopumps in two different mixture ratios, burning a small part of the fuel which spins the pumps and vaporizes the rest of the fuel. When they enter the combustion chamber they are both in gaseous form. This improves fuel mixing and therefore combustion efficiently, as gases mix faster than liquids. As such it is referred to as a "gas-gas" engine. Note however that the fuel is stored in liquid form, as it is much denser when liquid.
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6$\begingroup$ But wait, isn't it always gas which is burning, even if the original fuel is solid (e.g. Wood) or liquid (e.g. Kerosene)? $\endgroup$ Commented Nov 9, 2019 at 14:18
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1$\begingroup$ @EricDuminil Not always (unless you count the visible flame as the only thing "burning"). But gases do have the valuable property of being easy to mix - a gas fuel with a gas oxidizer mixture can be very well mixed indeed. Solid coal and wood (though wood is very hard to ignite without the volatiles) do burn. Iron-magnesium powder burns. Gunpowder burns. Aluminium-oxide rockets burn. It's more blurry with liquids - liquids evaporate readily, and the gas will burn better than the liquid, so... $\endgroup$– LuaanCommented Nov 11, 2019 at 9:22
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To complement the answer:
1 L contains 1141 g of liquid oxygen
1 L contains 1 g of gaseous oxygen (at 1ATM)
Of course you can compress it but the container will add more weight (highly undesirable).
But that's not over!
Rockets consume not only a lot of propellant, but they consume it fast.
A Saturn V consumes 18000 kg every second.
That would be 18143 m³ per second, about 20 olympic swimming pools. You can't force that much air in an engine.
For reference, a scuba diving tank is heavy and can hold a pressure of about 200 atm – one order of magnitude less than liquid oxygen.
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$\begingroup$ The density of gaseous oxygen is 1.429 and not 1 g/l $\endgroup$– UweCommented Nov 8, 2019 at 10:16
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$\begingroup$ Using gaseous oxygen from a container with 1 bar inside and outside is impossible anyway. You have to replace the used volume of oxygen with another gas without mixing them. But then the container does not loose weight. $\endgroup$– UweCommented Nov 8, 2019 at 10:25
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1$\begingroup$ There is no such pressure at normal temperatures. $\endgroup$– fraxinusCommented Nov 8, 2019 at 18:43
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1$\begingroup$ @NibblyPig, to expand on fraxinus's point, oxygen's critical temperature is -118˚C. Above that temperature, compression simply produces a supercritical fluid. $\endgroup$– MarkCommented Nov 8, 2019 at 23:48
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1$\begingroup$ @Uwe I rounded it, the idea was to give a sense of scale $\endgroup$– AntziCommented Nov 9, 2019 at 3:03
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You need a liquid to cool a large and hot combustion chamber efficiently. Ablative cooling has been used for smaller engines only. Heat transfer from the solid chamber walls to a gas is too small for cooling.
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$\begingroup$ The closed-loop cooling would need a big and hevy heat exchanger to transfer the heat to the atmosphere, But the heat exchanger would not work in the upper atmosphere and above. $\endgroup$– UweCommented Nov 9, 2019 at 21:21
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$\begingroup$ @Uwe: Radiative cooling is a thing, you know. $\endgroup$– VikkiCommented Nov 10, 2019 at 1:03
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2$\begingroup$ @Sean If you post comments like that you should expect comments like this :-). Of course radiative cooling is "a thing"/ And the fact that it is used in a relatively small number of special cases is a clue that it's not usually the best solution. A look at the effort and mass that goes into a channel cooled rocket nozzle gives a very good indication that they would have used radiation cooling if they possible could have. see eg Titan 1 closeup photo collection - I took these in 2003 in the NASA Ames carpark. $\endgroup$ Commented Nov 11, 2019 at 2:38
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Other answers have focused on the fact that you wouldn't want to run a rocket engine on gases because the tanks would be too large. One thing that hasn't been mentioned yet is the turbopumps.
Turbopumps consist of a turbine which runs on burned fuel and oxidiser. This runs a pump which pumps the liquid fuel from a low pressure tank to the high pressure needed for the combustion chamber. If the fuel were pumped in gaseous state, the pumping power requirements would be much, much higher. So the turbopumps would be much heavier, as well as using power which could otherwise be used in the main combustion chamber / exhaust nozzle.
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Burning gases is an aim.
Carrying fuel of maximum density is another.
Propellant density and thus increased propellan payload was (and is) of such interest that in the 1990s the use of "Slush Hydrogen" was propoesed - ex mix of Solid (!) and liquid Hydrogen which increased Hydrogen payload by about 20%. The problems proved to be considerable. A summary of the slush hydrogen technology program for the National Aero-Space Plane - NASA, 1995
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Turning liquids into well mixed gaseous products is a major aim in rocket engines.
These photos illustrate the effort taken to do this:
Google Photos version here
Small sample:
The "injector" is the many many small holes lining the surface of the back wall of the combustion chamber.
As a bonus the channels for nozzle fuel cooling can be clearly seen.
Click image for higher resolution view:
