The impulse drive may actually be a misnomer. Some sources hold it is a contraction of "I.M.Pulse Drive", standing for Inertial Magnetronic Pulse drive. It operates not through expulsion of energy or mass but through less-controlled distortions of local space-time. Star Trek starships do, in fact, have a propulsion system based on a more conventional reaction drive; usually, they are referred to (on the series) as 'thrusters,' occasionally with an adjective such as 'docking thrusters,' 'maneuvering thrusters' or 'maneuvering jets'.

The impulse drive works (supposedly) through the following sequence of events. A fusion fuel is fused to produce energy. However, the actual fusion of the fuel pellet takes place inside a magnetronic field. The field is used to amplify or modulate the burst of energy into the gravitic range; as a result, each miniature fusion explosion creates a very small gravity wave. Since the impulse drive units are offset from the ship's center of mass, these waves serve to 'push' or 'pull' the vessel through space.

Impulse fusion reactions are likely to be D-D, or deuterium-deuterium reactions. Although D-T, or deuterium-tritium reactions are easier to start, tritium is radioactive and has a very short half-life as well. Technical sources for Star Trek: The Next Generation finally admit to fuel tanks aboard the Enterprise-D: they contain 'slush deuterium' for use both as a fusion fuel and to serve as the matter for the dilithium-focussed matter/antimatter reaction that powers the warp drive and the rest of the ship. Oh, and as 'base mass' for the replicators aboard.

Impulse engines, in Trek, do have an 'exhaust' where the energy and plasma byproducts of the fusion reactions are vented. On the Enterprise, these vents are at the aft of the Primary Hull, just to either side of the connecting dorsal, and glow orange when in use.

In a way, the impulse drive is just a super-high-tech version of Project Daedalus, without the mess.

My sources for this are both my own hazy memory of the original Star Trek Technical Manual, as well as the (slightly better) source of Diane Carey's Trek Novel 'The Final Frontier.' Since Diane Carey has been claimed to be a pen name for D.C. Fontana, one of the original Star Trek scriptwriters, this would offer some confirmation of my own whole-cloth reconstruction above. Note that in 'The Final Frontier' the I.M. Pulse drive is in fact the 'Internally Metered Pulse' drive, not Inertial Magnetronic; I specifically recall "Magnetronic' being in there, but bow to her as the Authority on par with the Great Bird of the Galaxy on this one.

According to Star Trek Captain's Chair (a game that lets you tour the bridges of Star Trek's many ships) impulse engines are a secondary propulsion system for the vessel, employing nuclear fusion reactors to accelerate helium plasma products to near lightspeed, producing Newtonian thrust for space-normal velocities. I also remember a note on it saying: "Rocket Engines!" So I assume that impulse engines are a clean burning type of propulsion engines.

The impulse drive has a low level static warp bubble and it works at sub light speeds. It is what allows the impulse drive to go faster than a normal fusion rocket. It is a kind of space distortion, the use of a low level warp field helps the impulse drive avoid inertia. Without kinetic or mass energy build up there is no momentum or any inertia. It is a space distortion that produces a gravity shield. You do not have to go light speed for this to work.

A low level warp field allows the impulse drive to go faster with less energy output and can go from a stop to max impulse speed with in a very short time with no g-force effects and no time delay from relativity. The propulsion is high energy plasma going through the impulse manifold. Unlike the warp drive propulsion is not from space time distortions. The low level warp field stops the relativity and inertia effects, but takes a lot less energy than the warp drive. At first this was done with subspace emitters then later the plasma went through a drive coil before exiting the impulse manifold. The drive coil produced a low level warp field from the plasma.

I have been told that in the star fleet manual it is suggested for ships not to go past 0.5C at impulse. I look at the shows than Theoretical physics in the fields described on the shows. I very seldom look at the star fleet manuals. The writers consult the scientific community on how to make the shows look real, but do not always get it correct. According to general relativity all matter moves in time space and all space time and gravity is connected. To a point this has been proven using an atomic clock, time shows down in a jet to a very small degree. The ship that went to the moon time had slowed down by a few minutes. The GPS everyone uses today has a program that accounts for the differed rate in time of the satellites and the GPS. Time in space moves faster than on earth to a very small degree, but it is enough to throw the GPS off. Now time slows to movement on an exponential curve but at 0.5C it would be large enough to produce noticeable effects. Gravity affects Relativity, mass produce gravity or a mass less gravitational field. Without this you cannot go to half light speed in less than a second. The storylines shows the ships doing this.

On star trek a sub space bubble alters the effects of Relativity like the branes in M- theory. In Real warp theory using a false vacuum state is 1 warp metric that is believed to produce the effects of the branes which allows light or energy to move faster than light in normal space and the lowering of inertia or faster energy drop-offs. This is all connected it changes the effects of relativity.

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