A cryogenic rocket engine is a rocket engine that uses a cryogenic fuel or oxidizer, that is, its fuel or oxidizer (or both) is/are gases liquefied and stored at very low temperatures.
Cryogenic engines are fundamentally different from electric motors because there isn't anything rotating in them. They're essentially reaction engines, working according to Newton's law: "to every action there is an equal and opposite reaction."
The cryogenic (or rocket) engine throws mass in one direction, and the reaction to this is a thrust in the opposite direction. This thrust lifts the rocket, and helps in the launching and movement of the rocket.
Cryogenic rocket engines are more efficient than solid propellant based engines. They can deliver more thrust for a given mass of propellant. This is particularly useful for raising heavier payloads to higher orbits. When compared to conventional liquid rocket engines using fuel storable at room temperature, cryogenic engines develop 1.5 times more thrust.
For the propellant to be efficient, it should develop greater thrust per unit mass of propellant consumed per unit time, a factor which we call Specific Impulse (Isp). High values of specific impulse are obtained from high exhaust gas temperature and from exhaust gas having very low (molecular) weight.
To be efficient, therefore, a propellant should have a large heat of combustion to yield high temperatures, and the combustion products should contain light molecules made of elements such as hydrogen (the lightest), carbon and oxygen.
Another important factor is the density of a propellant. A given weight of
A dense propellant can be carried in a smaller, lighter tank than a low-density propellant of the same weight.
The advantage of cryogenic propellants is that they are the most energetic and, therefore, have the highest specific impulse. The (liquid hydrogen + liquid oxygen) combustion yields the highest amount of total energy and the product of combustion is water vapour, with a low molecular weight.