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The title more or less sums it up.

A rocket engine works pretty much by combining fuel and oxidizer, and expelling some propellant resulting in foward motion as described by Newton's third law of motion (colloquially, to each action there is an equal and opposite reaction).

Combining oxygen and hydrogen gives water in a highly energetic reaction: $$ 2\text{H}_{2} + \text{O}_{2} \rightarrow \text{H}_2\text{O} + \text{energy} $$

The water (vapor) has to go somewhere, and as such is ideal to throw out the back of the rocket.

Consequently, is it correct to say that a LH2/LOX rocket engine uses water as propellant? If not, then where is the flaw in my reasoning?

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  • $\begingroup$ This question was inspired by Do any realistic rocket systems use different substances for propellant and fuel? $\endgroup$ – a CVn Nov 26 '16 at 18:41
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    $\begingroup$ LH2/LOX engines are run fuel-rich to make sure the O2 doesn't eat away at everything, as well as increase the specific impulse due to the lowered molecular weight of the exhaust. So the exhaust is actually water and H2. Whether you can call the exhaust the propellant is an uninteresting matter of semantics. $\endgroup$ – Mark Adler Nov 27 '16 at 5:08
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    $\begingroup$ Analogy: saying a LH2/LOX runs on water is like saying a coal plant runs on CO2. $\endgroup$ – radex Nov 27 '16 at 15:56
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    $\begingroup$ Much of the reaction mass is water, but that's not the propellant. $\endgroup$ – Russell Borogove Nov 27 '16 at 18:38
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    $\begingroup$ @Russell Borogove: Depends on how you look at things, doesn't it? You could think of water as the propellant, H2 and O2 as the energy source needed to heat/accelerate that propellant. You could get the same result by heating water in a nuclear reactor, no? (Assuming you could build a lightweight reactor that could withstand those temperatures, of course.) $\endgroup$ – jamesqf Dec 1 '16 at 5:52
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No, it is not.

The exhaust is water, not the propellant. The propellant is a bi-propellant consisting of liquid H2 and liquid O2.

The bi-propellant is injected into the combustion chamber, where it reacts and produces H2O (water) as exhaust.

Just because the exhaust is used to push the rocket forward that does not make it the propellant. The propellant is what is used in the production of energy or exhaust gas - which is H2 and O2.

In rockets and aircraft, propellants are used to produce a gas that can be directed through a nozzle, thereby producing thrust. In rockets, rocket propellant produces an exhaust, and the exhausted material is usually expelled under pressure through a nozzle. The pressure may be from a compressed gas, or a gas produced by a chemical reaction. The exhaust material may be a gas, liquid, plasma, or, before the chemical reaction, a solid, liquid, or gel. In aircraft, the propellant is usually a fuel and is combusted with the air.

(Source, Highlights mine)

So water is the exhaust, not the propellant.

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