Propulsion system

A propulsion system is a machine or system of machines that produces thrust to push or pull a vehicle from a position of relative rest into motion or to provide an acceleration or deceleration for a vehicle already in motion. The objective of a propulsion system is to maintain the vehicle’s ability to propel itself and maneuver.
Ground propulsion
'Ground propulsion' is a different term than transport, because it refers to solid bodies being propelled. Those bodies may be mounted on vans or using wheels, the latter dominates for standard applications.
The primary and most natural type of propulsion is the use of muscle power. Vehicles drawn by an animal have nearly disappeared, although there are some communities left, where one can make a living solely relying on the techniques employing muscle energy in a very efficient form like bicycles, wheelchairs etc.
Civilizations employ other types of methods to transport bodies with higher velocities. In the last two centuries up into the 1970s, most use steam engines. Now the main focus is on
* internal-combustion engines,
* electric motors (which include linear motors being part of the track),
* or a combination.
Turbines are usually not used because of the small part load efficiency. Also, other types with external combustion like the Stirling engine or without combustion like the fuel cell are used in planes.
Marine propulsion
Propulsion systems for ships fall into three categories: human propulsion, sailing, and mechanical propulsion.
Mechanical propulsion systems generally consist of a motor or engine turning a propeller, or less frequently, an impeller or wave propulsion fins. Steam engines were first used for this purpose, but have mostly been replaced by two-stroke or four-stroke diesel engines, outboard motors, and gas turbine engines on faster ships. Nuclear reactors producing steam are used to propel warships and icebreakers, and there have been attempts to utilize them to power commercial vessels (see ).
For ships with independent propulsion systems for each side, such as manual oars or some paddles, steering systems may not be necessary. In most designs, such as boats propelled by engines or sails, a steering system becomes necessary. The most common is a rudder, a submerged plane located at the rear of the hull. Rudders are rotated to generate a lateral force which turns the boat. Rudders can be rotated by a tiller, manual wheels, or electro-hydraulic systems. Autopilot systems combine mechanical rudders with navigation systems. Ducted propellers are sometimes used for steering. Some propulsion systems are inherently steering systems.
Spacecraft propulsion
'Spacecraft propulsion' is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the back/rear of the vehicle at very high speed through a supersonic de Laval nozzle. This sort of engine is called a rocket engine.
All current spacecraft use chemical rockets (bipropellant or solid-fuel) for launch, though some (such as the Pegasus rocket and SpaceShipOne) have used air-breathing engines on their first stage. Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resistojet rockets for orbital station-keeping and some use momentum wheels for attitude control. Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north-south stationkeeping. Interplanetary vehicles mostly use chemical rockets as well, although a few have used ion thrusters and Hall effect thrusters (two different types of electric propulsion) to great success.
Vehicle propulsion
'Vehicle propulsion' refers to the act of moving an artificial carrier of people or goods over any distance. The power plant used to drive the vehicles can vary widely. Originally, humans or animals would have provided the means of propulsion, later being supplemented by wind power (e.g. sailing ship). Since the Industrial Revolution, mechanical propulsion has been possible, initially using steam engines. More recently, most vehicles use some form of internal-combustion engine, with various energy sources to power them.
Air propulsion
'Air propulsion' is the act of moving an object through the air. The most common types are propeller, jet engine, turboprop, ramjet, rocket propulsion, and, experimentally, scramjet, pulse jet, and pulse detonation engine. Animals such as birds and insects obtain propulsion by flapping their wings.

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