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This entry concerns the different types of jet engine, such as turbine jets, ramjets and scramjets.
Most jet engines feature some arrangement of turbines - machines which convert the kinetic energy of a moving fluid into mechanical energy by causing a bladed rotor to rotate. Air is drawn in at the front, usually by the action of some sort of fan. It is compressed by the action of the rotating turbines. Fuel is added and combustion takes place. The combustion products, which have a much larger volume than the air taken in, are exhausted at the rear.
The advantage of the turbine jet is that it can accelerate from standstill to a very high speed in a short time. The disadvantage is that the fan, turbines and other moving parts are complex, expensive, and must be manufactured and maintained to extremely fine tolerances1.
There are a number of sub-types of turbine jet, described below.
Turbojet - the 'pure' jet engine. It consists of an air intake at the front, a compression section, one or more fuel injection points, a combustion chamber and an exhaust point. In a turbojet, all of the air taken in is fed to the compression chamber.
Turbofan - a turbojet with an enlarged or separate primary compression turbine. This acts essentially as a jet-powered propeller. The gas accelerated by it is not fed into the engine but through ducting at the sides. The adoption of increasingly high-bypass turbofans (where a higher proportion of the ingested air is not pumped through combustion) makes each new generation of airliners appear to have chunkier engines.
Turboprop - similar to a turbofan, but the propeller is not enclosed in a casing. To an observer this looks like a conventional propeller aircraft engine with a large, complicated looking lump behind the propeller. This type of engine has efficiency advantages over pure propeller engines, but because of the relatively exposed position of the prop is not capable of the high speeds associated with 'proper' jet engines.
Turboshaft, or Gas Turbine - uses the design of the turbofan to generate rotary motion rather than actual jet propulsion. In fact, jet engines are really a development of this earlier invention. The design has been used for power generation, as well as in transport vehicles. Swiss Federal Railways first ran a gas turbine locomotive in 1941, and Rover built the world's first gas turbine car in 19502. These engines are also found in vehicles including fire engines, ships, helicopters, and incredibly, the American M1 tank.
A ramjet is elegant in its simplicity. It consists of little more than a tube which varies in cross section along its length. A narrow inlet widens to a compression zone where the air slows down and where fuel is injected. The tube narrows again to take the combustion products to the exhaust point. All four points of the engine cycle - induction, compression, combustion, exhaust - take place at different points along its length.
The advantage of the ramjet is its very simplicity. Because it has no moving parts it is incredibly reliable and can achieve enormous speeds without recourse to the exotic materials and engineering required for turbofan manufacture.
The immediately obvious disadvantage of the ramjet is that it depends on the velocity of the incoming air to generate the compression. It cannot, therefore, accelerate from a standing start. Instead, some other means must be used to achieve the minimum speed for its operation. In most cases this means either firing the vehicle out of a catapult, which can be somewhat hard on the pilot (if there is one), or using another type of engine as a booster to achieve the minimum speed. For this reason its main use to date has been in military applications.
The air inside a ramjet is moving more slowly than the air outside. This is important, because it gives the fuel and the air time to mix and burn properly. However, as the air inside the ramjet approaches supersonic speed, mixing ceases to be as efficient. Also, the enormous compression and air friction increases the temperature to a point where the fuel starts to decompose rather than burn. The actual speed at which this starts to happen varies according to design, but is typically around Mach 6 (a little over 4,200mph).
Scramjets work like ramjets except the air and combustion products all move at supersonic speeds. In a scramjet, the inlet is much wider, the compression is less and the air remains at supersonic speeds. This keeps the air cooler, but as the fuel must burn in a few milliseconds the design of the jet is much more critical. Of course, all this means that the vehicle has to be moving supersonically before the engine will even start. On the other hand, theoretical maximum speeds are of the order of Mach 25 (about 18,000mph), which is getting to the sort of speed you need to achieve orbit!
Turbine Jets: Very common, speeds from zero to Mach 3+
Ramjets: Rare, speeds from Mach 0.3 to Mach 6
Scramjets: Experimental only, theoretical speeds from Mach 2 to a potential Mach 25 or more