Ever since the first humans looked up into the sky and saw birds, we have been enthralled with flight. In the last hundred years, not only have we achieved it, we have come to excel at it, going to speeds and heights that birds could never dream of. The keys to our success have been the jet engine and the rocket engine. These two types of motors operate on similar principles, but they use different methods, and have different strengths and weaknesses.
The rocket engine is, in fact, the oldest type of engine used by humanity. The Chinese discovered gunpowder and were using it to make simple rocket engines by the 10th century. Rocket engines can be subdivided into two types: solid rockets and liquid rockets. Solid rockets have a simple tube filled with a solid fuel such as gunpowder. These have the advantage of being extremely cheap and simple, and are often used for missiles or fireworks, where the engine is expected to be destroyed. Liquid fuel rockets are more complicated. In these, a liquid fuel such as kerosene or liquid hydrogen is held in one tank, while liquid oxygen is held in a separate tank. These are then mixed in the engine itself and ignited. These rockets have the advantage of being controllable, whereas solid rockets will simply burn until their fuel is exhausted Both types of rocket engines have the advantage of not requiring air to work.
Jet engines are more complicated than rocket engines, and required the discovery of the propeller before they themselves could be designed. Unlike rocket engines, jet engines require air to work. The most common type of jet engine, the turbojet, operates by pulling in air with a powerful fan. This air is compressed in the body of the engine, and fuel is added. It is then ignited, expelling it out the back of the engine. The turbojet is effective up to speeds of about Mach 2, or twice the speed of sound. After that, it becomes unstable, making it unsuitable for reaching space. To achieve higher speeds, another type of jet engine, called a ramjet, must be used. This type of engine has no fan. It relies on the high speed of the aircraft to compress the air. The disadvantage of this is that the ramjet cannon operate from a standstill, and so another type of engine must be used to get the plane to high enough speeds. However, the ramjet is capable of propelling an aircraft to enormous speeds, theoretically enough to reach orbital velocity, although like all jets, it is unable to operate in the vacuum of space.
When measuring the efficiency of an engine, two values must be considered: Thrust to weight ratios and specific impulse. Thrust to weight ratio is simply the amount of force an engine can produce in relation to its mass. Specific impulse is the amount of thrust produced per unit of fuel burned. The highest thrust to weight ratios belong to rockets, with their extremely simple design. Ramjets also have a high thrust to weight ratio due to their simplicity, but this is tampered by their need for an addition engine to reach the speeds needed to function. Turbojets have a low thrust to weight ratio due to their relative complexity. However, rockets have by far the lowest specific impulse. Enormous amounts of fuel are needed in rockets, offsetting their efficiency. The best specific impulse belongs to ramjets, which have no need to carry an oxidizer like a rocket, and no need to drive a fan to take in air like a turbojet.
As we continue to advance, new types of jets will be discovered. Perhaps in the future, ships will be propelled to other planets by engines powered with fission or fusion. Ion jets, which magnetically accelerate charged particles, are already used in interplanetary probes. These engines are incapable of producing more than a tiny amount of thrust, but have a specific impulse that dwarfs any kind of conventional engine, and so are ideal for the years long voyages throughout our solar system. Or perhaps some entirely new method of propulsion will be revealed to us.