From a post to Baen's Bar BuShips dated March 17, 2009:

More on the Keyhole platforms

    I really shouldn't be getting involved with this entire topic. For that matter, I don't have any business even skimming the Bar at the moment, with everything that's going on. Nonetheless...

    There are two varieties of Keyhole platforms. One of them, the first developed, is primarily a light-speed communications node and sensor platform designed to be gotten beyond the interference of the mounting ship's impeller wedge. It has some limited onboard power storage capability, and most ships fitted with it carried to a bit, in order to provide redundancy and also -- for the first time -- to give an impeller wedge-equipped warship an effective 360 degree coverage area for both communications and sensors.

    Keyhole-Two, on the other hand, is fitted with FTL telemetry and communications channels. Because the grav-pulse coms are a heck of a lot bigger than the light-speed coms, the platform had to get a lot bigger, as well. In addition, its power requirements rose pretty severely. And whereas the original Keyhole had only extremely limited anti-missiles self-defense capabilities, Keyhole-Two (in part because it's so much more valuable) has several point defense clusters added to the rest of its size and energy budget. As with the original Keyhole platform, ships equipped with Keyhole-Two are fitted with two platforms each, once again for combined redundancy and 360 degree coverage.

    By the time you get up to Keyhole-Two sizes, anything smaller than a capital ship is going to be giving up too much of its broadside weaponry -- offensive or defensive -- simply to carry the damned things (which are docked in hull recesses which are specifically designed and provided for the purpose) when they aren't deployed.

    Keyhole -- and Keyhole-2 -- are both towed systems, and they are not towed on any physical tether. They are towed on tractors, and they are primarily powered by transmission from the mothership. They do have some onboard propulsive capability, using the same impeller hardware which was developed for the Ghost Rider recon drones, but that capability is purely secondary. In theory, they could maintain the station on their onboard drives while remaining in the basket to be hit by power transmissions from the mothership and to continue to perform their relay functions. In fact, it's simpler and less complicated to operate them in what amounts to full-time towed mode. There are less things to go wrong, and if the ship takes battle damage sufficient to cut it off from a still functional Keyhole, the ship in question is probably so far up the creek already that it's not going to worry about bells and whistles.

    Superdreadnoughts and ships-of-the-wall generally can fairly readily be equipped with multiple Keyhole-One platforms -- that is, the platforms themselves are small enough, with sufficiently low energy requirements, but there's no real reason a ship the size of a waller couldn't be equipped with four or even six of them. Doing that would cut into volume (and broadside area) available for other purposes, however, and the RMN more or less decided that giving every ship in a battle squadron two of them and allowing for weapons to be handed off between one ship in another provided enough redundancy through simple dispersal of the system.

    One interesting thing the RMN has observed now that Keyhole-Two has actually been deployed in combat is that the platforms' "self-defense" capability has proved a very valuable adjunct to be Navy's starships' antimissile defenses. Indeed, our good friend Sonja Hemphill is currently tinkering around with a considerably smaller, simpler platform whose primary function would be missile defense and which could probably be fitted to smaller combatants.

    The main limiting factors which have so far restricted Keyhole and Keyhole-Two to capital ships are (1) the simple physical size of the platforms; (2) the amount of shipboard power generation and transmission designed into the system; (3) the fact that the system is most useful in long-range missile duels and that nothing smaller than a battle cruiser was likely to be engaging in extremelylong-range combat. Even the Saganami-C and the Roland are equipped with only dual-drive missiles, and BuShips and BuWeaps were thinking in terms of all-up MDMs. Keyhole-Two, in addition, there's no real point to providing the system to somebody who isn't also capable of firing the Apollo control missiles. It's entirely possible that a Keyhole-Two for battlecruisers, possibly with somewhat downsized capabilities, will eventually be produced for the Agamemnons and their Grayson and Andermani counterparts, but that's definitely been a secondary or even tertiary priority in light of other, more immediately critical demands.

    That's probably not everything about the system, but it's the best I can do without digging out my detailed technical notes (and spending a lot longer on this than I have any business doing). I hope it's enough to deal with most of the questions raised in this thread.