The simplest of all jet engines is the ramjet. The ramjet consists of a long cylindrical tube made of metal, open at both ends. The tube bulges in the middle and tapers off at both ends. This shape causes air entering the front of the engine to expand and develop a higher pressure in the center of the engine. Within the engine, the compressed air is used to burn a fuel, usually a kerosene-like material. The hot gases produced during combustion within the engine are then expelled out the back of the engine. These exiting gases can be compared to the air escaping from a rubber balloon. As the gases leave the back of the jet engine (the nozzle exit), they propel the engine itself in a forward direction.
When the ramjet engine is at rest, no air enters the front of the engine, and the engine provides no thrust. Once the engine is moving through the air, however, it begins to operate more efficiently. For this reason, the use of ramjet engines is usually reserved for aircraft that travel at very high speeds.
A typical ramjet engine today has a length of about 13 ft (4 m), a diameter of about 39 in (1 m), and a weight of about 1,000 lb (450 kg). A ramjet engine of this design is capable of producing a thrust of 9,000 Newtons (N) (about 2,000 lb), giving a maximum velocity of about Mach 4 at higher altitudes.