The Indian Space Research Organisation (ISRO) successfully conducted the hot test on a semi-cryogenic(CRYO) engine (SE2000). With the test now it is  a step closer to finalising the crucial cryogenic stage where powering the booster stages of launch vehicles is realised.This test is known as   the Power Head Test Article (PHTA). This is the first hardware test for the development of semi-cryo engines. Unlike the cryogenic engine a semi-cryogenic engine uses a liquid oxygen – kerosene combination. In this system  kerosene is readily storable. This combination offers advantages like high-density impulse (with respect to cryogenic), less toxic (with respect to storage) and cost-effectiveness.

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Cryogenics

It is the study of the production of materials at extremely cold temperatures. The temperatures studied in cryogenics are those below -243.67 degrees Fahrenheit (120 Kelvin). Ssuch low temperatures do not occur in nature. A cryogenic rocket engine is a rocket engine that uses a cryogenic fuel or oxidizer. Its fuel or oxidizer (or both) is gases liquefied and stored at very low temperatures.

Thus, all cryo rocket engines are liquid engines. But they should be distinguished from rocket engines that use earth-storable liquid propellants that are liquids at ordinary temperatures and can, therefore, be stored as liquids easily.

                The most common cryogenic propellants used in rocket engines are liquid hydrogen (LH2). LH2 w liquefies at −253° C, as the fuel. And liquid oxygen (LOX), which liquefies at −183° C, as the oxidizer.

The most important reason to prefer a cryogenic engine is its efficiency. The cryogenic engine provides more thrust with each kg of propellant it uses compared to other propellant.

Cryo propellants are preferred as rocket propellants when rockets have to carry payloads of high mass because they have the greatest efficiency in terms of thrust generated. The advantage of cryo propellants is that they are the most energetic and, therefore, have the highest specific impulse. The LOX+LH2 combustion yields the highest amount of total energy and the product of combustion is water vapor, with a low molecular weight.