A Comparison of Jet and Rocket Engines
Jet engines and rocket engines are both used throughout the aerospace and aviation industries for a variety of applications. Despite this, they operate in different ways and are suited to use in different situations. In this blog, we will discuss how jet and rocket engines work, as well as their advantages and disadvantages.
The most common type of jet engine is the turbojet engine. In a turbojet engine, air is drawn into the turbojet by a compressor, which is essentially a large fan. The compressor slows down the incoming air, compresses it, thereby raising its pressure, and delivers it to the combustion chamber. The high-pressure air in the combustion chamber is then injected with fuel and ignited. The hot gases resulting from this explosion rush through a turbine and subsequently a jet pipe and nozzle at the rear. The rearward momentum of the exhaust gases results in forward thrust that moves the aircraft forward.
Turbojet engines are inefficient at low speeds, limiting their usefulness in vehicles other than aircraft. Though turbojet engines were the first form of gas turbine powerplants used in aviation, they have largely been replaced by new developments of the original design. Turbojet engines remain efficient for very high speeds, turbofan engines are better at medium speeds and turboprop engines are ideal for low speed aircraft. Turbojets are still frequently used in medium range cruise missiles, where their high exhaust speed, small frontal area, and relative simplicity are ideal. Additionally, they are not uncommon on supersonic fighter jets such as Russia’s MiG-25 interceptor.
Adversely, a rocket engine does not need air to function. This is because the rocket carries its own fuel and oxygen supplies, while a jet engine carries only fuel and obtains its oxygen from the air. In a rocket, the combination of fuel and oxygen is known as the propellant. Because rocket engines carry their own oxygen supply, they can operate in high altitudes above 30 miles and other areas where jet engines don’t have enough air to function. As such, rockets are commonly used to carry payloads such as satellites, people, and other cargo to extremely high altitudes, into orbit around the earth, to the moon, or throughout interplanetary space.
There is a wide range of types of rockets, each characterized by the form of the propellant as well as the types of fuels and oxygen sources they use. Some rockets use liquid propellants, some use solid, and others that use a variant of these. The most common variants are called gell propellants and hybrid propellants. Considering the high speeds they create, rocket engines are surprisingly energy efficient. This is largely due to the high combustion temperature and high compression ratio. In addition to this, rockets also have the highest thrust to weight ratio, thanks to the small volume of pressure vessels (pumps, pipes, combustion chambers) that make up the engine. The lack of an inlet duct and the use of dense liquid propellant allows the pressurisation system to be compact and lightweight.
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