What exactly is Impulse Power and how does it work?
There's been a lot of conflicting stuff over the years, as misinterpretations of the original show crept in more and more. The modern notion is that it's a simple Newtonian force-counterforce fusion rocket. Rick came up with the overly complex and clunky "force field thrust diverters" to account for reverse impulse.
But going back to the speculative-fiction underpinnings of TOS, the stuff they were getting from Isaac Asimov, the RAND Corporation, and what NASA and the US Air Force said they were working on that would be coming over the next decades, and going by design cues Matt Jefferies put on his ships and spread out tidbits he dropped in interviews... One thing he was clear on, from early pre-production, was that this ship wasn't going to be rocket-propelled. No Flash Gordon flames coming out of any of the engines. He knew whatever the warp engines did to
warp space/time, it took and generated a lot of energy that was likely not too healthy for organic beings, so he put those pods way out away from the ship. And for the sublight drive, he came up with a lower-intensity version of the same principle, that's in the name. Every engine cycle, it provides an
impulse in the desired direction.
Given that he put a reactionless drive assembly on his 1990s-era
Botany Bay, I'm comfortable saying he presumed we were going to come up with artificial gravity and gravity manipulation within the next quarter century, at the rate we were progressing. I also like that impulse and warp are related, but very different in scale of effect. As one of the characters in Diane Carey's novel "Final Frontier" put it, explaining the differences to a confused security officer, "Warp is as far above impulse as impulse is above walking."
There's a bit in "The Hitchhiker's Guide to the Galaxy" where the creation of the infinite improbability drive was described. They had been able to make
finite improbability drives for ages, but infinite improbability kept eluding them, and they wrote it off as a virtual impossibility. Then, one evening, a graduate student was sweeping up the lab and found himself reasoning thusly: "If it's a virtual impossibility, then it stands to reason that it's a finite
improbability. All I need to do is work out exactly how improbable it is." He did this, fed the number into a finite improbability generator, turned it on, and materialized the infinite improbability drive out of thin air.
That's almost how I feel about Cochrane's invention of the warp drive. It wasn't some totally unique technology, never before seen in all of Human existence. He took an impulse coil, said to himself, "Hmm... What happens if I take a bunch of these, stack 'em, and run a whole lot of power through them?" One coil distorted the continuum enough for a ship to slide "down" the gravitational incline at fractional-
c speeds, even getting fast enough relativistic effects became a factor. His thing was making a coil stack powerful enough that it could distort space/time past the apparent local threshold of the speed of light. We had impulse pwer in the 1990s, and he broke the light barrier seventy years later. That's roughly the same span as Kitty Hawk to Apollo, so it must've seemed plausible at the time.
Another sidebar into the no glow in TOS versus the glow from TMP on... The
Phoenix almost certainly didn't have antimatter or dilithium. Early drive systems were almost certainly nuclear powered. Fission and, later, fusion. Even early warp ships. Part of the speed improvements of the previous cornerstone class before the
Constitution was probably introducing antimatter power. But it would have been rare and valuable, and only used for the warp drive. In TOS, there seem to still be fusion generators in impulse engineering, and they're powered separate from the warp drive. In TMP, a big feature of the new ship is the vertical intermix shaft running up through the dorsal to an "impulse deflection crystal", and dialogue that indicates at least some ship's systems that previously hadn't been are now powered directly from the warp engines. Andy pointed out "backup" fusion generators in the impulse deck, but it seems that was the advent of antimatter-powered impulse engines as the new standard. Even if starships are speed-governed to prevent strong relativistic effects, that'll let them accelerate a lot faster.
All of the impulse decks we see seem to be rigged with aft-facing heat radiators that probably also serve double duty venting waste gas from the fusion reactors (or, later, spent warp plasma) back along the typical direction of flight, so they don't plow into their own wake.
