Physics

The Difference between a Rocket and a Jet



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Most of us will be familiar with rockets and jets. When we watch a Space Shuttle launch or an Air Force fighter take off we are seeing both types of engine in action. They certainly look similar - with all that heat and noise and fury - but there is a fundamental difference between them and that is in where they get the oxygen to generate the combustion needed to produce the thrust that propels them along.

Rocket and jet engines are both 'reaction' engines. This means that they obey Isaac Newton's Third Law of Motion: For every action there is an equal and opposite reaction. In both engines, fuel and an oxidizer (any substance containing oxygen) are combined and ignited and the resulting high-pressure gases are then exhausted from the rear of the engine. This is the 'action'. The 'equal and opposite reaction' is the force that pushes the rocket or jet forwards.

A rocket carries both the fuel and the oxidizer for the duration of its flight. This is why rockets can operate in the oxygen-free vacuum of space; they have on board all the oxygen they need for combustion. Some rockets use solid fuels while others use liquid fuels (rocket fuel is known as propellant). A solid rocket fuel is a substance in which the fuel and the oxidizer are mixed together into a solid block that is ignited to produce thrust. Liquid-propellant rockets carry the fuel and the oxidizer in different containers and they are only mixed when they enter the combustion chamber.

The three engines of the Space Shuttle orbiter and the two rocket boosters that help launch it are good examples of both types of rocket engine. The two solid rocket boosters on either side of the huge external fuel tank each use a solid propellant called ammonium perchlorate composite. The ammonium perchlorate is the oxidizer and it is mixed with fuel (powdered aluminum), a catalyst consisting of iron oxide, a binding polymer (that acts as a secondary fuel) and an epoxy curing agent. The Shuttle's main engines use liquid propellant - liquid hydrogen as the fuel and liquid oxygen as the oxidizer - that is fed to them from the external tank.

Unlike rocket engines, a jet engine doesn't carry an oxidizer and must get its oxygen from the atmosphere. A fan sucks air into the engine and a compressor squeezes it to increase its pressure. This compressed air is then forced into a combustion chamber where it is mixed with fuel and ignited. The resulting expanding gases exiting the combustion chamber then spin a turbine - which is linked to the fan and the compressor, ensuring a continuous airflow - before exhausting from the rear of the engine.

The basic jet engine design hasn't really changed much since the first jet engines (called turbojets) were perfected during WWII. The main design improvements over the years have concentrated on increasing engine power and fuel efficiency. Turbofans are now usually the jet engine of choice both in the civil and military fields. A turbofan only allows some of the incoming air to go through the combustion stage; the rest - slightly speeded-up - goes around the outside of the central core 'cold' and exits with the hot gases that are leaving the combustion stage.

This means that turbofans get maximum thrust with minimum fuel consumption, making them fuel efficient and ideally suited to power commercial airliners. The large round engines you see on airliners are that shape because of the large-diameter fan used to suck in the air. These commercial engines are called high bypass-ratio turbofans because they are designed to allow a significant amount of the incoming air to bypass the combustion stage, making them extremely efficient at subsonic speeds. Turbofans on supersonic military jets have a lower bypass-ratio in order to increase thrust, and they also make use of afterburners - where fuel is added to the hot exhaust gases - when a surge of thrust is required, such as for take-offs and during air-to-air combat maneuvers.

While we see jet engines all around us, rockets are not such a common sight. Solid fuel rockets are used mainly as boosters - such as the Space Shuttle boosters described above - and as military ballistic missiles. These rockets, once lit, cannot be stopped and their thrust is controlled by the mixture of the solid fuel and by varying the surface shape of the burn area. This is a channel drilled through the center of the fuel block which burns outwards.

On the Shuttle boosters this channel is in the shape of an eleven-pointed star. As these points burn off, the burn surface-area decreases, and so, therefore, does the thrust of the rocket. Liquid-propellant rockets are more efficient than their solid-fuel cousins and, more importantly, by varying the rate of fuel combustion the thrust can be controlled far more accurately.

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