|Images of the Impulse Drive exhaust vents on various models of the Enterprise. Images (c) & (TM) Paramount Pictures.|
Impulse drives come from the Star Trek Universe, and like most Star Trek tech it is based on fairly exotic physics concepts. Source material from Star Trek, which now spans over 35 years, is sometimes notoriously inconsistent, and the exact nature of the Impulse Drive often changes on a scriptwriter’s whim. However, I’ve run across two "official" (i.e., approved by owning company Paramount) explanations, listed below.
GRAVITY WAVE IMPULSE DRIVE
Tech Level: 19
The gravity wave scheme is probably the neatest (in every sense of the word) explanation for how the Impulse Drive works. It seems likely that it was created by the show’s science advisors and then promptly ignored and/or dumbed down by numerous scriptwriters. It is, however, the best fit in describing the capabilities of the drive as seen on the various shows through the years.
First of all, an impulse drive is NOT a conventional fusion or ion reaction drive as many people (and even some older source material) assume; those are reserved for a Federation ship’s "maneuvering thrusters." In fact, its proper name is the Inertial Magnetronic Pulse Drive--or more simply the I.M.Pulse, or impulse, Drive.
The drive works as follows: a pellet of deuterium-deuterium fusion fuel is fused in a high-energy reaction (perhaps by a Daedalus-like system with crossed high-energy particle beams) that is contained and modulated in a "magnetronic" field. What exactly a magnetronic field is, and how it differs from a plain ol’ magnetic field, is not explained. Judging from how its used, though, it may be a magnetic containment field merged with a strong nuclear force or gravitic force field. (Not a "force field" in the Star Trek sense, but a small region of space where the strong nuclear force or the gravitic force is enhanced on a quantum level.)
Basically, the magnetronic field contains and focuses the fusion implosion to such a degree that it generates a substantial amount of gravitic as well as electromagnetic energy. These powerful but short-lived gravity waves are used to push or pull the ship in various directions. By "pulsing" the drive thousands or even millions of times a second, a Federation ship can achieve the insane accelerations we often see on the show.
The "focusing" of a high-energy fusion reaction to produce gravity waves may sound odd, but it is actually based on solid theory. Certain types of black holes called kugelblitzes (German for "ball lightning") can be created solely by extreme energy densities, just as conventional singularities can be created by extremes of matter density. Kugelblitzes are thought to have been formed in the wake of the Big Bang. The impulse drive may, in fact, be constantly creating extremely short-lived, or "virtual," microscopic kugelblitzes that evaporate after a few microseconds, living just long enough for the ship to use their gravitational influence.
According to Star Trek’s canon, Impulse Drives operating all-out at peak efficiency ("full impulse") can achieve 25% of lightspeed, often within a few seconds. It is possible to go even faster, but at an ever-increasing cost to engine efficiency. Ninety percent lightspeed or so is supposedly the theoretical maximum, and the ship would have to be pouring all its power for hours into the drive to sustain ever-tinier increases in acceleration.
Needless to say, a gravity wave Impulse Drive is an extremely powerful and versatile form of sublight propulsion. Since the gravity waves produced can be either attractive or repulsive (like anti-matter, anti-gravity --theoretically-- exists, but is not generally found in nature) a ship equipped with an impulse drive can accelerate or decelerate without changing it orientation.
The Impulse Drive exhaust ports shown on the Enterprise are not used in the same way conventional rockets exhausts are; they are used solely to vent the plasma the fusion pulses generate, and only add relatively small amounts of acceleration to the ship compared to that generated by the Impulse Drive proper. It should be noted, however, that even this ‘waste’ exhaust would still be dozens of times more powerful than any conventional rocket that exists today.
SPACE/TIME DRIVER COIL IMPULSE DRIVE
Tech Level: 21
An alternative explanation for the Impulse Drive (also "canon") is that the fusion reactors power "space/time driver coils" which "generates a sub-warp cochrane field around the vessel, reducing its effective mass to boost acceleration." So, basically, it uses a ‘low-power’ warp bubble (aka cochrane field) to alter its mass relative to the rest of the universe.
From what I understand of the physics involved, this feat would actually be much more difficult than warp speed. How exactly could you fine-tune a warp bubble so precisely that it just alters mass? And if you could, why not reduce the mass to near zero and hit near-lightspeed every time? And could you use such a system to increase mass, creating an anti-Impulse missile weapon that could slow a ship down to a relative sublight crawl?
These properties would actually make the driver coil Impulse Drive a derivative technology of the Warp Drive, accounting for its higher tech level than its gravity-wave cousin.
Short articles on the theoretical workings of the Impulse Drive as gravity wave generator:
An article on the Impulse Drive as Space/Time Driver Coils:http://www.ditl.org/index.htm?daybody=/whatsnew/datnewfeb2000.htm
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