Turbine propulsion is the propulsion of a vehicle through the use of a turbine. Due to the turbine's low torque, land vehicles are rarely propelled by a turbine. Marine vessels, however, have employed turbines for propulsion since their invention in the late 19th Century. The most common types of turbines used for propulsion are steam and gas turbines, both of which have their own unique advantages and disadvantages. Today, many modern military ships employ steam turbines for their high speeds and efficiency.
The first practical steam turbine used for propulsion and energy generation was invented by Sir Charles Parsons in 1884. The Parsons turbine improved over time, and was eventually installed in a specially-built ship. This ship, known as the "Tubrinia," was the first watercraft ever to be powered by a turbine. Launched in 1894, the ship could reach speeds of 34.5 knots (39.7 mph, 63.9 km/h), and was used to demonstrate the potential for steam turbine propulsion in marine vessels.
Since then, the use of steam turbine propulsion became increasingly popular for marine vessels. Turbines were favored over the large reciprocating steam engines of the period, due to their higher thermal efficiencies, speed and power-to-weight ratio. Cruise liners, such as the Cunarder "Lusitania," employed steam turbines as their prime movers, allowing them to travel significantly faster than their piston-driven counterparts. The steam turbine-driven SS "United States" still holds the Blue Riband for the fastest West-bound transatlantic crossing, with a maximum speed of about 38 knots (43.8 mph, 71 km/h).
With the development of nuclear power, steam turbine propulsion continues to be a widely-used power source. Nuclear-powered vessels, including submarines, employ steam turbines as their prime movers. Nuclear reactors replace boilers to heat boiling water into super-heated steam. The steam is then run through the turbine, where it is then exhausted and condensed for future use.
Despite their low torque, steam turbines have seen some use in land vehicles. The Ford Nucleon was a proposed nuclear-powered car, wherein a small nuclear reactor powered a turbine. In 2009, a British team of engineers broke the world land speed record for steam-powered automobiles using steam turbines. There have also been some locomotives run by steam turbines.
One of the downfalls of steam turbines is their low torque. To overcome this problem, however, reduction gears are used to increase torque at the expense of revolutions per minute (RPMs). While torque demands vary by application, nearly all steam turbine-driven vehicles require a certain amount of torque that cannot be attained through a direct drive. Most vehicles driven by steam turbines, therefore, employ reduction gears.
An alternative to reduction gears is a turbo-electric drive, wherein the turbine spins a generator that powers an electric motor. The electric motor then servers as the prime mover for the vehicle, with the steam turbine providing its power. Due to their expense, however, turbo-electric systems have only been used in a small number of ships and locomotives.
Gas turbines also play a very important role in modern turbine propulsion. Unlike a steam turbine, whose high-velocity steam turns multiple rotors, gas turbines use combustion to produce the thermal expansion necessary for rotation. Gas turbines are not only far more powerful than steam turbines, but they can also rotate at far higher speeds. Due to this, gas turbines have been used for aircraft, locomotives, ships, and even heavier military vehicles.