Fission Pulse Engine

The fission pulse engine was the first reaction engine for space flight, following rocket propulsion. It was most commonly used between 2056 and 2130, and was eventually succeeded by the fusion pulse engine.
The engine worked by conducting a series of small nuclear detonations at the rear of the ship, behind a pusher plate. Early forms of this used a flat plate with a feeder to the side, but this gradually moved to a cone-shaped plate with the feeder as an aperture in the apex of the cone itself.
While the engine produced prodigious thrust when compared to rocket propulsion, it still suffered from a number of inefficiencies, most particularly the conversion of considerable portions of the mass into radiation, which was not only unsafe for anyone vaguely behind the engine, but was massless and therefore reactionless. Nonetheless throughout the second half of the lifetime of this technology, it was very cheap compared to the fusion pulse engine, and so remained widely used, until the latter became more economical.