The Repulsor engines of a CCS-Class Battlecruiser.

The repulsor engine is a propulsion system used by Covenant starships. Little is known about its mechanisms, other than that it produces quantum fluctuations that can affect personnel within its range, and leaves a wake that can affect other starships.

It is possible that repulsor engines are a form of "reactionless drive" that make use of negative matter. Negative matter would have negative mass and would have a "repulsive" effect on the normal matter of the ship if placed behind it.