Rocket engine needs enormous amount of thrust to escape Earth’s gravitational pull. However the chemicals used for engines are Hydrogen and Oxygen (Hydrogen used as a fuel, while Oxygen as a oxidiser) that produces a good thrust, found in Earth in the form of gas. Carrying hydrogen and oxygen in their gaseous form will require a bigger fuel chamber, which not only increase the size but also weight of the rockets and this will mean undertaking of impossible task to send a launch vehicle into space. So the solution is to use hydrogen and oxygen in their liquid form or in a cryogenic form which is easier to transport, as the volume of propellent decrease. As density increases in liquid form, more thrust can be produce in less burning time. Such engines are called Vryogenic engine.
A Cryogenic rocket stage is more efficient and provides more thrust for every kilogram of propellant it burns compared to solid and earth-storable liquid propellant rocket stages. Specific impulse (a measure of the efficiency) achievable with cryogenic propellants (liquid Hydrogen and liquid Oxygen) is much higher compared to earth storable liquid and solid propellants, giving it a substantial payload advantage.
Unlike a Cryogenic engine, a Semi Cryogenic engine use Refined kerosene instead of liquid hydrogen. The liquid oxygen used as a Oxidiser. That’s the advantage of using a Semi Cryogenic engine as it require Refined Kerosene which is lighter than liquid fuel and can be stored in a normal temperature. Kerosene combines with liquid oxygen provide a higher thrust to the rocket. Refined Kerosene occupies lesser space, making it possible to carry more propellent in a Semi Cryogenic engines fuel compartment. A semi cryogenic engine is more powerful, environment friendly and cost effective as compare to cryogenic engine.