Impulse engine

Exterior of the impulse engine on a Galaxy-class starship

Impulse engines are the primary means of sublight propulsion for space vessels in Star Trek.

Operating Principles

An impulse engine consists of one or more nuclear fusion reactors connected to exhaust vents that generate thrust to accelerate the ship. The thrust generated by an impulse engine is typically tiny compared to the actual mass of the ship, but the use of "mass lightening" technology (involving subspace fields) reduces the effective mass of the ship enough for the thrust to provide considerable acceleration.

Because impulse engines include fusion reactors, they are typically the ship's secondary power source, outpowered only by the warp core.

Trivia

Purposefully overloading and detonating the impulse engines of a Constitution-class starship will produce an explosion equivalent to approximately 98 megatons.