Impulse speed

Impulse Engine Table
Class of Engine	Velocity Sustainable/Max
Class 1	.25c/.5c
Class 2	.5c/.5c
Class 3	.5c/.7c
Class 3A	.5c/.75c
Class 4	.6c/.8c
Class 4A	.6c/.85c
Class 4B	.65c/.85c
Class 5	.7c/.9c
Class 5A	.72c/.9c
Class 6	.75c/.9c
Class 7	.75c/.92c
Class 8	.75c/.95c

Starships used impulse propulsion systems (IPSs) to move slower than the speed of light, which was required for travel within a star’s gravity well, through solar systems, in starship combat, and similar situations. Impulse drives used fusion reactors to generate standard thrust via a standard Newtonian reaction—the thrust “pushes” the ship forward through space. Starships usually dropped to impulse when they encountered another ship to facilitate contact (or combat). IPSs had four components: the impulse reaction chamber (IRC), or fusion reactor, of which there are three per impulse engine; the accelerator/generator (A/G); the driver coil assembly (DCA); and the vectored exhaust director (VED). Energy released in the IRC by the fusion reaction was channelled to the A/G, which raised the velocity of the plasma and fed it on to the DCA. Electroplasma system (EPS) taps ran from the A/G to divert some energy to power other systems on the ship. The DCA created a field effect which aided the impulse propulsion process, then passes the energy on to the VDA, which directed it to thrust the vehicle in the desired direction.

A main impulse engine (MIE) capable of propelling a starship consisted of four linked impulse engines. These engines might have been grouped together, or divided into two groups of two to provide balanced thrust for structures such as separated saucer sections. An impulse engine was rated for the amount of velocity it could provide, expressed as a percentage of c (the speed of light). Thus, an impulse engine might be said to provide .5c thrust, meaning it could propel a vehicle at half the speed of light. Regardless of an engine’s maximum sublight speed, Starfleet generally limited impulse travels to .25c (paradoxically referred to as “full impulse”) to minimize the time-dilation problems which occur as a ship approaches speed of light travel. Only in starship combat situations do starships commonly move at impulse velocities in excess of .25c.

Impulse engines were also equipped with impulse capacitance cells which could provide a tiny burst of power to them. By releasing the cells’ power directly into the driver coils, a ship could generate a one-second burst of impulse speed when the engines were otherwise not functioning due to lack of power.

Note: The accompanying table lists the classes of MIEs. The XX/YY are Sustainable and Maximum velocities the engine could attain. Sustainable velocity is the highest speed it can maintain without stressing the ship or its engines (all impulse engines’ Standard speed is, by definition, .25c). Maximum velocity is the highest speed it can attain. Operating at Maximum impulse speed for longer than 24 hours risks damage to the engines.