The reactionless drive technology used by the Dominion is a side effect of combining the FTL drive technology and the Gravitic plate technology. Partially initiating an FTL jump field inside phased Gravitic plate fields allows hyper-mass to be accessed as an infinite supply of high-density reaction mass, as soon as it is brought into this universe the mass conveniently proceeds to a fusion reaction.
Hyper-Mass thrusters interfere with FTL drives. Ships that activate FTL with the Hyper-Mass thrusters running are not usually seen again.
Hyper-mass Thrusters are not truly a reactionless drive technology, but a reaction drive with an infinite fuel supply and no fuel carriage requirements. [It’s similar in concept to filling your bag of holding with a supply of nuclear plasma…]
Although capable of effectively infinite acceleration as long as they are supplied with electrical power, Hyper Mass Thrusters have a practical upper limit on speed of about 0.10c to 0.15c (where c = the speed of light), depending on the individual design of the thruster and the ship to which it is attached. Below the 0.1c limit, Newtonian dynamics calculations suffice to model the ship’s performance in space. However once speed rises above that threshold, relativistic factors come into play that effectively cause the acceleration to change from a constant to a reducing variable value, typically falling to 0 somewhere between 10% – 15% of the speed of light. In general use, only the fastest ships will need to consider the speed limit if crossing a whole star system. For game purposes, it’s sufficient to model this as linear acceleration to the upper limit then cruise at a constant velocity before starting the – also linearly modelled – deceleration phase. For inter-system travel, the acceleration phase can mostly be ignored (because acceleration phase is in days vs cruising phase is in decades or centuries) and the trip time calculated as if conducted at constant-speed.