The rocket science is a very vast topic, and one of the most important parts of it is the fuel used. When it comes to space shuttles, there are two basic options for fuel, liquid and solid. Solid fuel is a bulky mixture oxidizer and fuel which is poured into the rocket booster, cooked to the consistency of a pencil eraser, and finally set fire during the launch. The energy produced is let out through a nozzle, creating enough thrust to lift the rocket. This technology is used by submarines and nuclear weapons.
The liquid rocket engines, on the other hand, have fuel tanks inside the boosters, one for oxidizer and one for fuel. The fuel and oxidizer are chilled to very low temperatures so that they are not converted into gas and at the time of launch are mixed inside the engine, ignited and then let out through the nozzle. The output is thrust and a tongue of the very hot exhaust. SpaceX rockets use this system to power u their rockets.
When discussion about rockets, the term ISP (specific impulse) is an important one. The thrust generated by a specified amount of fuel (solid/liquid) is measured by this, and liquid engines are found to have higher ISP than solids. Then there is a different metric called the ISP density, which is of more focus for the engineers of Orbital ATK.
The latest and efficient approach is the use of liquid engines in the 1st and 2nd stages of a rocket. This has gained support from some of the high-class personals associated with commercial space renaissances like the Blue Origin’s Jeff Bezos and SpaceX’s Elon Musk. Their space vehicles make use of the extra thrust as well as the other advantages of liquid engines. The liquid fuel engines operate in such a way that their fuel tank’s entire lining is not charred and hence the turnaround time between these flights are very short comparatively. Simply refill and be ready to take off again.
The liquid engines are very crucial for the last phase of a rocket’s flight. It needs a heavy push to deliver the payload in place which is achieved by liquid engines. The New Gen Launch rocket of the Orbital ATK will be using upper stages of liquid to deliver the satellites of the Air Force to the orbit. The advantage of using both can be seen in NGL, where we have solid lower stages for high thrust and liquids for higher ISP and capabilities to start/stop.