Star Trek Starship Propulsion Systems
Impulse Drive
Starships use impulse propulsion systems (IPSs) to move slower than the speed of light, which is required for travel within a star's gravity well, through solar systems, in starship combat, and similar situations. Impulse drives use fusion reactors to generate standard thrust via a standard Newtonian reaction - the thrust "pushes" the ship forward through space. Starships usually drop to impulse when they encounter anther ship to facilitate contact (or combat).
IPSs have 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 is channeled to the A/G, which raises the velocity of the plasma and feeds it on to the DCA. Electroplasma system (EPS) taps run from the A/G to divert some energy to power other systems on the ship. The DCA creates a field effect which aids the impulse propulsion process, then passes the energy on to the VDA, which directs it to thrust the vehicle in the desired direction.
A main impulse engine (MIE) capable of propelling a starship consists of four linked impulse engines. These engines may be grouped together, or divided into two groups of two to provide balanced thrust for structures such as separated saucer sections.
An impulse engine is rated for the amount of velocity it can provide. Regardless of an engine's maximum sublight speed, Starfleet generally limits impulse travels to 25 (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 excess of 25.
The accompanying table lists the classes of MIEs available.
The Game Master may allow a crewmember to make a Moderate (16) Capital Ship Repair Test to increase the output slightly (+.10c if the Test succeeds exactly, +.20c if it succeeds by 4 points or more; a ship's impulse speed cannot exceed .95c). Any additional impulse speed obtained in this way lasts 1D+1 rounds. Impulse engines are also equipped with impulse capacitance cells which can provide a tiny burst of power to them. By releasing the cells' power directly into the driver coils, a ship can generate a one-second burst of impulse speed when the engines are otherwise not functioning due to lack of power.
| Class of Engine | SU | Velocity | Space | Atmosphere | Base Maneuverability |
| Class 1 | 5 | .25c / .5c | 12.5 | 460; 1,325 km/h | +0D+1 |
| Class 2 | 10 | .5c / .5c | 12.5 | 460; 1,325 km/h | +0D+2 |
| Class 3 | 15 | .5c / .7c | 17.5 | 548; 1,575 km/h | +0D+2 |
| Class 3A | 18 | .5c / .75c | 18.75 | 566; 1,638 km/h | +0D+2 |
| Class 4 | 20 | .6c / .8c | 20 | 590; 1,700 km/h | +1D |
| Class 4A | 22 | .6c / .85c | 21.25 | 614; 1,763 km/h | +1D |
| Class 4B | 23 | .65c / .85c | 21.25 | 614; 1,763 km/h | +1D |
| Class 5 | 25 | .7c / .9c | 22.5 | 645; 1,825 km/h | +1D |
| Class 5A | 28 | .72c / .9c | 22.5 | 645; 1,825 km/h | +1D |
| Class 6 | 30 | .75c / .9c | 22.5 | 645; 1,825 km/h | +1D+1 |
| Class 7 | 35 | .75c / .92c | 23 | 655; 1,850 km/h | +1D+1 |
| Class 8 | 40 | .75c / .95c | 23.75 | 666; 1,888 km/h | +1D+1 |
- SU: The SU cost for each main impulse engine (regardless of whether the MIE is one unit, or divided into two smaller units).
- Velocity: The Sustainable and Maximum velocities the engine can 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. The Game Master should roll 2D every 10 minutes of travel beyond 24 hours, on a result of 2, 3 or 4, the impulse drive suffers damage. Repairs take one hour to one day per point of damage as if the engine had taken 25% damage.
- Space: The base Space rating, calculated as one quarter of its Maximum Velocity.
- Atmosphere: This is the corresponding Atmospheric speed of the Impulse Drive. This rating is only used if the ship is atmospheric capable.
- Base Maneuverability: This rating is added to the vessels maneuverability rating from its Starship Size. If the ship has no sublight drive, then disregard the maneuverability ratings altogether. If the ship has multiple impulse engines, the ratings are added together in the same manner as firelinking weapons. i.e. the first added engine adds +1D, the second through the fourth added provide 2 pips each, the fifth through the tenth engines added provide 1 pip each. Beyond ten additional impulse engines it will take an exponential increase in drives for every one pip of maneuverability gain.
Acceleration Upratings
Standard impulse engines accelerate at half their maximum impulse speed per round. Some vessels are equipped with uprated impulse systems which can accelerate more quickly than this. The accompanying table lists packages, SU costs, and effects.
The SU cost for the uprating includes modifications to the structural integrity field and inertial dampening field so that the ship can handle the increased stress on the hull. If a ship has multiple impulse engines, it must buy an acceleration package for each one, and the packages must all be the same one.
| Package | SU | Rate of Acceleration |
| Class Alpha | 2 | 66% (two-thrids) of maximum impulse speed per round |
| Class Beta | 4 | 75% (three-quarters) of maximum impulse speed per round |
| Class Gamma | 8 | 100% of maximum impulse speed per round |
Impulse Thrusters
SU Cost: 2 x Size
Ships of Size 5 and smaller can also install impulse thrusters, a special type of thruster which improves a ship's speed and maneuverability. They add +.10c to the ship's impulse speed (maximum of .95c) and provide +1D maneuverability.
Notes
| Impulse Speeds | C | Space | Atmosphere |
| 1/4 Impulse | .0625c | 6.25 | 335; 963 km/h |
| 1/2 Impulse | .125c | 12.5 | 460; 1,325 km/h |
| 3/4 Impulse | .1875c | 18.75 | 566; 1,638 km/h |
| Full Impulse | .25c | 25 | 690; 1,950 km/h |
Notes
- Atmosphere: This speed does NOT correspond with the actual percent of the speed of light but corresponds with the Star Wars D6 Space ratings. This was done for the ease of game play. If you wish to use the actual percent of the speed of light in km/h, please go here.
Source
- Stardock Starship and Construction Manual (pages 38-40)
- thedemonapostle