Archives

Filters

Pearls of Weber

A collection of posts by David Weber containing background information for his stories, collected and generously made available Joe Buckley.

FTL fire control communication lag

What would be considered a generally accepted practical command-and-control loop lag-time maximum?

In simplest terms, the shortest one you can get with whatever your current technical capabilities are. (G)

I assume that this is being asked in reference to the advantages Apollo offers over standard, light-speed missile telemetry. Given the nature of the Apollo platforms, the answer becomes "the shortest range at which the attacker's command-and-control loop is shorter than the effective command-and-control loop for the opposing active defenses and the attack missile has time to utilize corrections." I realize that that reply is a bit cumbersome, but the problem is that the geometry of engagements will be such as to preclude a hard and fast answer. Basically, the advantage of Apollo is that it puts the attacker inside the defender's control loop, and please note that the higher the terminal attack velocity of the missiles, the shorter both the defender's and the attacker's effective response time (not necessarily the same thing as a command-and-control loop) becomes, because of the speed with which the attacking missiles will cross the defensive zone. I have someone -- I think it was McKeon -- make this point in a staff discussion of Apollo capabilities.

This particular calculation was never really a factor before, because it was literally impossible to get inside the defender's loop with light-speed limited telemetry to the attack missiles except at extremely shorter ranges. At that time, the trade-off was between basic target resolution and light-speed lag. The attack missiles' sensors and on board AI were at a sufficient disadvantage versus the defender's EW that the computational ability of the launching ship's computers and fire control was necessary to obtain a hard lock on the target. Even though the telemetry links became increasingly "arthritic," the ability to see and positively identify a missile's target at all required their use up to and well beyond a point at which the defender's command-and-control loop was significantly shorter than the attacker's. It was, if you will, a question of clumsy control that could at least see the target versus whatever disadvantages controlling the missiles at that range brought with it.

Even then, however, the attacker reached a point at which the command-and-control lag was simply unacceptable because it began degrading, rather than improving, target solutions. (Note that there is a difference between target identification/resolution and the ability to generate a targeting solutions which brings the missile and the target close enough together for a successful attack.) At the time of On Basilisk Station, the maximum powered missile envelope from rest was just under 6.8 million kilometers, or about 22.5 light-seconds. At that range, a light-speed control loop would be the next best thing to 46 seconds; a one-way telemetry link would be only 22.5 light-seconds, but the data upon which any targeting commands were based would still be 46 seconds old because the firing ship would depend upon light-speed sensors for any fine targeting adjustments. (It would be able to see the gravitic signature of the impeller wedge on its Warshawskis FTL, with only a .36-second delay, but a wedge's gravitic signature isn't really sufficient for fire control purposes.) Given the acceleration capabilities and maneuverability of Honorverse ships, just the evasive movements of the target if given 3/4 of a minute to work with at, say, 500 gravities, would create significant problems for the missile, regardless of any active defenses. A ship at rest relative to the attacker at the time a maximum-range missile of this era was launched would be up to a velocity of 225 KPS and would have traveled only 5,188 kilometers by the time the missile reached burnout. But if the target ship were traveling at 32,000 KPS, it could change its position by over 1.4 million kilometers before the missile reached its predicted position.

With the advent of Apollo, at that same range, the command-and-control loop would be 1/62nd of 46 seconds, or only .75 seconds. Obviously, that's going to put the attacker far, far inside the target's defensive command-and-control loop, and our 32,000 KPS relative velocity from above would allow the target to shift its position by only about 24,000 kilometers, well within the targeting capability of a laser head (whose. There is still going to come a time, however, largely dependent upon the vectors of the attacker and defender, at which even Apollo's FTL capability is going to fall behind the defender's combined ability to generate a miss through evasive maneuvers and kill incoming missiles with active defenses. Precisely where that falls, however, is pretty much a case-by-case calculation.

The higher terminal velocity of a MDM also comes into play, obviously, since at extended ranges, when it's had an opportunity to crank its own base velocity up to as much as 80% light-speed, it's very velocity will severely reduce the possibility of effective evasive maneuvers specifically directed at it. (That is, if a ship is individually maneuvering to evade a specific incoming missile, its response time will be sufficiently limited to make that maneuver ineffectual. If, on the other hand, the fleet is using a preplanned evasion pattern for all of its units, then the question of what you might think of as directed evasion maneuvers -- that is, an evasion attempt intended to dodge a specific incoming missile -- won't be much affected. Under a preplanned pattern, the ship is going to go wherever the ship was going to go anyway, after all.)

Let's say that our Apollo missile has a closing velocity of .8 cee, that the target is roughly 148,000,000 km away (496 light-seconds), and is traveling at 19,000 kilometers per second and is capable of an acceleration of about 400 gravities. The telemetry transmission lag to the missile (that is, the one-way light) is about eight seconds. The two-way command-and-control loop is thus about 16 seconds. In 16 seconds, a ship traveling at 19,000 KPS and accelerating at 400 gravities can change its position by over 304,000 km; by the time the missile travels 16 seconds at .8 cee, however, [allow] it will have moved 12.8 light-seconds (or roughly 3.8 million kilometers), which means that any command, to be effective, would have to be given when the missiles were at least that far from the target, or they couldn't respond within their own maneuver parameters.

Taking the same maneuvering parameters for the defender and the attack missiles, and reducing the range to 74,000,000 km (248 light-seconds, or roughly four light-minutes), the two-way command-and-control loop drops to only eight seconds. The target's maximum position change in 8 seconds is only 152,000 kilometers, and the attack missiles will have traveled only 6.4 light-seconds (or around around 1.9 million kilometers). The situation is far better from the attacker's perspective, although the target is still about 8.5 million kilometers outside current generation MDM endurance envelopes, which would require a ballistic phase somewhere in the approach. "Better" isn't necessarily the same thing as "good," however. At the Battle of Manticore, Fifth Fleet got hammered because it didn't realize what was about to happen to it for several reasons. It was, in many respects, a sitting target. Had it been maneuvering hard, and had it been able to give its active defenses good targeting solutions, it would have taken much less damage. That's one reason Honor went for such massive overkill in her first strike; she couldn't be certain how "on its toes" Fifth Fleet's missile defense actually was at the moment she committed the attack birds.

I suppose I really ought to point out that I did mention that the Apollo control missiles have extraordinarily capable AIs compared to anything carried previously by shipkiller missiles. This is an important caveat which I deliberately did not stress in At All Costs (mostly because I wanted to emphasize it in a later book, and also to avoid "infodump overload"). Basically, in addition to being communications platforms, the Apollo control missiles are data processing platforms. They incorporate software specifically designed to analyze command and control data in relationship to target position, speed, and acceleration at the moment a command is received. That is, the control bird can effectively override a command or (more often) choose from a cascade of commands based on the controlling ship's best estimate of the target's potential maneuvers at the time those commands were sent. This allows a considerably greater degree of flex in terms of target assignment, at least. It doesn't restore the ability of the Apollo link to anticipate or provide real-time counters to the target's EW and counter-missile patterns at such extended range.

Now, having said all of this, I imagine that my faithful readers can begin to understand why I didn't go into a complete analysis of Apollo's potentials (and potential weaknesses) in the book. There is that little matter of the "infodump" of which I have occasionally been accused, and there were already enormous amounts of numbers, accelerations, velocities, etc., etc.. Under the circumstances, I rather doubted that five or six pages of arcane analysis by a team of Manticoran tactical officers would really have gone down that well with the average reader. I think, however, that the fact that I didn't -- that I settled for the staff discussion in which McKeon and Yanakov throw around just a few numbers -- may have prevented a lot of readers from really considering just how radically the targeting environment has changed or how difficult that makes it for people who have never seen Apollo operate from the inside to really extrapolate the system's capabilities. At first glance, because the defender in the initial engagements simply doesn't know Apollo is coming, or have any grasp of its potential capabilities and so is back into that "sitting target" category, Apollo's advantages appear even greater than they are.

In addition, while Haven may know what the maximum transmission speed for Apollo is there's simply no way for it to know what the maximum transmission range might be. Don't forget that the Manties had to do yet another of their marvelous miniaturization tricks to squeeze all of this into a platform the size of roughly four standard MDMs. This is an extraordinarily small platform compared to, say, a Ghost Rider recon drone, and Apollo's maximum reception range is much, much shorter than that of the recon drone.

Thomas Theisman, Lester Tourville, and Genevieve Chin had never had the opportunity to examine an Apollo command missile, but they clearly appreciated that the command birds were being fired from the same pods as the shipkillers. That defined a maximum set of dimensions for the missile. Without knowing exactly what the Manticorans had done, which would have required a direct examination of the hardware, they couldn't know what the effective maximum FTL communications range was, and they underestimated it. In part (again, something I didn't go into because of the infodump issue), the underestimation of the maximum range related to the question of what the maximum acceptable command-and-control loop length was. By this what I mean is that in assessing the maximum communications range Manticore would have felt necessary to design into the new system, they were influenced by their own analysis of how much range (defined by the communications lag) would have any practical value. Given the difficulties inherent in building extended range FTL capability into something as small as an MDM, the Havenite assumption was that Manticore would have begun deploying the system as soon as BuShips was able to squeeze a useful range into it, rather than delaying introduction while R&D engineered the system to permit transmission ranges greater than would be useful anyway.

The ranges at which Honor fired in the Battle of Manticore -- up to eight light-minutes against Tourville and 75,000,000 km against Chin -- were both outside the maximum reception range Haven's intelligence types had assumed the control birds would possess. In fact, in Tourville's case, the range was outside Apollo's effective reception range; Honor used a Hermes buoy, with its much greater reception range, and the inner-system FTL recon platforms, to coordinate the "demonstration attack" on Second Fleet. In many respects, that attack was essentially a bluff, although I deliberately didn't say so in the book, since at the moment I'm rather looking forward to allowing the reader to fully appreciate Tourville's reaction when he finds out that it was in a later scene. Honor didn't begin to have the available bandwidth to coordinate full-scale missile attacks at that range, so she deliberately restricted her "demonstration" to a number of missiles she did have the bandwidth to control through her jury rigged telemetry set up.

The tactical considerations discussed above also help to explain why Honor was intent, in her pre-Battle of Manticore tactical thinking, upon closing to the very edge of the Havenites' effective MDM range. In the case of a sustained engagement against an opponent who had accurately projected the system's strengths and weaknesses, she needed to reduce the FTL lag (if that doesn't look like an oxymoron) to a minimum to make best use of the system. Which, in turn, is why she would have followed much the same flight profile Third Fleet actually adopted if Eighth Fleet had made transit first. That's what she was thinking about in the book when she reflected that had she been in Kuzak's place, the Havenite ambush would have worked against her, as well. She probably would have shot Fifth Fleet to pieces with the advantage of Apollo, but Fifth Fleet would probably have come pretty close to returning the favor, since it would have been within its own effective engagement range of Eighth Fleet. The result of that exchange would probably have been that Second Fleet would have been able to continue onward to take at least the near-Sphinx system infrastructure, that both Second Fleet and Fifth Fleet would have been effectively destroyed, but that the Manticoran Alliance would have been left without Eighth Fleet's solid core of Apollo-equipped SD(P)s.

I hope all of the above helps. And for any sharp-eyed souls out there who noted that I somehow inexplicably failed to provide you with Apollo's actual reception range, I can only say tum-te-tum-te-tum.