Impulse drive
In the fictional "Star Trek" universe, the impulse drive is the method of propulsion that starships and other spacecraft use when they are travelling below the speed of light. Typically powered by deuterium fusion reactors, impulse engines let ships travel interplanetary distances readily. Unlike the warp engines, impulse engines work on principles used in today's rocketry, throwing mass out the back as fast as possible to drive the ship forward. Practical challenges. There are three practical challenges surrounding impulse drive design: acceleration, time dilation and conservation of energy. In the show, inertial dampers compensate for acceleration. These  hypothetical devices would have to be set so that the propellant retained its inertia after leaving the craft otherwise the drive would be ineffective. Time dilation would become noticeable at appreciable fractions of the speed of light. Regarding energy conservation, the television series and books offer two explanations: