Fission/fusion power reactors
First, let me admit that I did not express myself with sufficient clarity when my characters in Echoes of Honor reflected on the advantages of fission over fusion power. Without getting into all of the specifics--
(a) There is a typo in the section addressing the longevity of the Shrikes' fuel cores. It should have read 18 months of active operations, not simply 18 years. I have not figured out where the change came from. It was not in my original draft; it did appear in the ms I actually submitted to Baen, and I missed it in the proofreading. The only thing I can think of is that in a moment of sleep-deprived, late-night dementia, I myself made the change on disk, although why I would have "corrected" it to such a clear contradiction of my own tech bible evades me. Sigh.
(b) The advantages of the fission reactors in the LACs are several. First, they dispense with the need for reactor mass, which is very bulky and uses up a large amount of internal volume in conventional LACs. (Note: I understand someone has suggested moving the bunkerage outside the core hull. Naval designers have concluded--for reasons I find reasonable, at any rate--that putting their fuel outside the armor is a Bad Idea.) By removing the requirement for reactor mass, the Shrikes achieve two very important things: one, they cut less deeply into their internal volume, and, two, they free up their still not inconsiderable bunkerage for reaction mass, which is particularly important given the much higher than usual use they make of reaction thrusters during their attack maneuvers. A second, very important reason to go to a fission reactor is that fusion reactors in the Harrington universe, while far smaller than anything we could build today, are still big suckers. Wrapping a LAC around one explains the reason that one simply cannot put any really useful amount of energy weapons--or internal launchers of any sort--into a conventionally powered LAC. A third reason is that fusion reactors come with a far higher energy cost. In larger units, this is more than offset by their output; a LAC, however, has much lower overall power requirements than any other warship type. They don't need as much efficiency, but they would be required to make the same startup and maintenance energy payments as a considerably larger vessel. For all intents and purposes, a Shrike would have to squeeze in essentially the same power plant as a destroyer, which was another reason pre-Shrike LACs simply could not mount a useful energy armament or achieve much endurance. Another factor to bear in mind, although a minor one, is that LACs don't have Warshawski sails. Recall that starships' sails are also an energy source whenever they are deployed within a grav wave, but a LAC doesn't have that advantage in extending its endurance.
From a post to ALT.BOOKS.DAVID-WEBER dated September 20, 1998:
There are fusion plants, and here are fusion plants. If you will recall, Honor, in The Honor of the Queen, mentioned "super dense" mag bottle technology in connection with the FTL-capable recon drones.
Essentially, there are two types of fusion plants in use in the Harrington universe. One is the type mounted in virtually all starships, which uses gravity generators to induce fusion. These are very big (relative to, say, a Shrike fusion [fission -Ed.] plant), produce lots of power, and require a right smart amount of power for startup and maintenance. The efficiency and scale of fusion works to make these plants useful/desirable on the larger scale. (And note that a courier boat is actually a bit larger than a LAC and is basically nothing more than a hull wrapped around a fusion plant, a hyper generator, and a set of warshawski sails. I never said a LAC could not be built around a fusion plant; I simply said that it put a massive squeeze on the internal volume of the LAC which the fission plant does not.)
The second type of fusion reactor is that used in pinnaces, assault shuttles, recon drones, etc. It relies on less esoteric technology, using lasers to intiate fusion, but is also less efficient and produces much less power on a per-ton basis. It's input energy costs are lower, and its output and fuel supply are sufficient for small craft for a limited period of operation.
From a reposted email to Baen's Bar BuShips dated September 14, 2004:
I think your memory is correct, although right off the top of my head, I don't remember where I made the statement. I would assume it was in an informational post at some point, but exactly when or where eludes me.
In answer to your question, Grayson-style fission piles aren't really very practical for anything bigger than a LAC. The problem is that both the fission pile and the LAC fall between two stools, as it were. A LAC's energy requirements are much lower than those of any hyper-capable ship, but much higher than those of, say, a pinnace. And as I know I stated elsewhere, there are essentially two different types of fusion reactors -- one, which is for all intents and purposes a highly evolved descendent of current-day thinking about fusion, and one which uses the Honorverse's ability to manipulate gravity. The former, enormously miniaturized from anything we could consider building today, is what provides the power for small craft, like pinnaces. Its output, however, is grossly inadequate to meet the energy requirements of Honorverse hyper- capable vessels. Remember, these ships are routinely playing with power levels suitable for frying half of Chicago as the result of a fairly minor oopsie.
The second type of fusion plant is much larger than anything which could be crammed into a pinnace, and it also has much higher input energy requirements of its own. It's really bigger and more powerful than most destroyers require, but anything else would fall short of the destroyer's energy requirements. The problem before the Graysons came along with their advanced fission technology was that the only real choice a LAC designer had was to go ahead and figure out how to cram a destroyer-sized fusion plant into a hull which was maybe 25% the size of the larger vessel. Even without the Warshawski sails, that put an enormous squeeze on the internal volume of the LAC, which helps to explain why it was impossible to squeeze in a really effective armament while simultaneously producing a "light attack craft" which was ridiculously over- powered.
The fission plant is actually much less efficient, on a ton-for-ton basis, than the standard starship fusion plant. It's output is just barely adequate to meet a LAC's energy budget requirements, but only with very careful management of the capacitor rings built into the vessel. Basically, the Manticoran designers were able to exchange the sheer size of the fusion plant for both the fission plant and a sizable increase in the previous standard of LAC capacitor-stored power. This leaves them with a few tactical problems, including the fact that unlike larger vessels, they simply do not have the current power generation capability to keep the capacitors which feed their spinal grasers topped off. Their energy armament is good for a limited number of full-power shots out of its own dedicated capacitors. A few additional shots can be "borrowed" from other systems' capacitors, but there's a limit to how often you can steal from Peter to pay Paul, and managing a Shrike-class' energy budget is almost as much an art as a science.
A destroyer is over-powered with a standard fusion reactor, true, but it would require not simply one, but several, fission plants to provide the energy it does require. The trade-off between fusion plant and bunkerage requirements and number of fission plants (and increased capacitors) falls firmly on the side of the fusion plant. In other words, to replace a destroyer's fusion plant and bunkerage capacity with fission plants would actually end up requiring you to use more of the destroyer's internal volume rather than less. And, obviously, if that's going to be true for a destroyer, it's even more true for larger starships.
The possibility of using fission plants for courier boats probably does exist, although I very much doubt that the savings in operating expenses would be sufficient to interest anyone in designing a completely new class of vessel for this service.
By the way, I've been thinking about that "typo" about the endurance of the Shrike between reactor recorings. At the time it appeared in the book, I couldn't understand -- or reconstruct -- how it had gotten there. I think I concluded that I'd probably made the change inadvertently, when I was so tired from pushing the deadline on the book that it just slipped past me. The more I thought about that 18-month endurance, though, the more I've come to wonder if perhaps I made the change from the value stipulated in the tech bible deliberately, and I was simply so tired when I did it that I don't now remember doing so. Given the endurance on present-day the fission reactors, 18 years really isn't particularly out of the question for technology as advanced as this. I don't think I've indicated the shorter endurance (the 18-month value, that is) anywhere else in the series, so I'm inclining towards allowing the 18-year endurance to stand. I haven't totally made up my mind in that direction, but I think it's likely that I will.