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In the fictional Star Trek universe, the impulse drive is the method of propulsion that starships and other spacecraft use when they are traveling below the speed of light. Typically powered by nuclear fusion reactions, impulse engines let ships travel interplanetary distances readily. For example, Starfleet Academy cadets use impulse engines when flying from Earth to Saturn and back.

There are three practical challenges surrounding impulse drive design: acceleration, time dilation and energy conservation. In the show, inertial dampers compensate for acceleration. Time dilation would become noticeable at appreciable fractions of the speed of light. Regarding energy conservation, the television series and books offer two explanations:

• A popular explanation identifies it as a rocket or a reactive thruster. However, in Star Trek: Voyager, "thruster" and "impulse" refer to separate propulsion systems, implying that Intrepid class starships have both systems.

• A less-common yet more plausible explanation calls for a gravitational distortion, a wave, through space, which the ship essentially rides. As such, "one half impulse" and "full impulse" measure speed and not acceleration.

Some Star Trek fans claim that impulse drives are actually ion propulsion drives. Impulse engines may also be a type of nuclear pulse system such as suggested for the real-life projects

• Star Trek: The Next Generation Technical Manual indicates that the impulse engines are nuclear fusion engines whereas the plasma from the fusion reactor powers a massive magnetic coil to propel the ship. It is a form of magnetoplasmadynamic thruster. This is used in conjunction with the ship's warp drive's alteration of the ships relativistic mass, to achieve mid-to-high sub-light speeds. Thrusters, on the other hand, are closer to the designs of a high-efficiency reactant propellant (i.e. a sophisticated rocket engine) and are usually used for high-precision maneuvers. Ion propulsion drives are explicitly detailed to be used in Star Trek by Dominion and Iconian Starships and facilities.

• Since a ship traveling at impulse velocities (slower than, but approaching, the speed of light) is still traveling in the normal space-time continuum, concerns of time dilation apply, so high relativistic speeds are avoided unless absolutely necessary; impulse power is therefore customarily limited to a maximum of ¼ lightspeed. (Warp travel, on the other hand, does not involve time dilation effects.)

• Project Orion, involving detonating nuclear weapons behind a spacecraft to accelerate it, or,
• The British Interplanetary Society's Project Daedalus, fusing deuterium and helium-3 with a beam (either lasers or neutrons) to create exhaust thrust.

Ion propulsion engines use electricity to ionise fuel like mercury or xenon and propel it with a high electric field to generate a low but continuous thrust.

Impulse Drives are also found in the online space simulation MMORPG, Ogame

• Physics and Star Trek
• Physics and Star Wars
• Warp drive
• MemoryAlpha:Impulse drive