I can't seem to find an answer on this.

I thought that when oxygen and hydrogen get injected they collide and break bonds etc. to form water vapour which because of the pressure in the combustion chamber get propelled out through the nozzle.

However, I also read somewhere that the propellants explode in the combustion chamber. I don't understand the process on how they explode, for example if they collide or just mix or what causes them to explode.

Im overall just confused what actually goes on as there aren't really any sources that explain that, and just say that they get injected in and produce water vapour and pressure + heat

If someone could help that would be awesome!

  • $\begingroup$ It's always a challenge to answer questions that contain "I have read..." when there is no link or quote to see what it is you've read or in what context it was written. The more background information you give, the better the answers can be. Can you find a quote or link somewhere to the use of "explosion"? Thanks! $\endgroup$
    – uhoh
    Feb 21, 2019 at 3:01

1 Answer 1


Mixing oxygen and hydrogen at ambient temperature doesn't do anything exciting -- it just yields a mix of the two gases.

In the presence of sufficient heat, mixed oxygen and hydrogen will burn, combining into (mostly) water vapor and releasing still more heat, which maintains the reaction. This is technically called combustion and is much the same as what happens when you burn any flammable substance with oxygen.

If hydrogen and oxygen combust together in an enclosed chamber with no way to release the pressure, the increasing heat and pressure will eventually burst the chamber; that would be an explosion, but that's not what happens in a properly operating rocket engine.

If there's an opening at one end of the combustion chamber, then hot expanding gases will escape and produce some thrust. If the opening is a convergent-divergent nozzle of appropriate design, the gases go out at supersonic speeds and produce lots of thrust; that's a good rocket engine.

Because cold hydrogen and oxygen don't combust by themselves, a hydrogen-oxygen rocket engine needs some kind of igniter -- either an electrical spark system (like spark plugs in an automobile engine) or a chemical igniter of some kind; I believe spark igniters are more common for hydrogen engines.

Some rocket fuel/oxidizer combinations do ignite spontaneously without a source of heat; these are called hypergolic propellants.


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