Sidewalls & the Wedge. Between them, these constitute the third line of defense, although they are the primary passive defense. Sidewalls will resist burn-through by current generation laser heads at anything much above 20,000-30,000 km, which is why the critical engagement ranges are so short. The impeller wedge itself will resist penetration by any currently known weapon. Because of this, point defense ignores any incoming weapon which tracking predicts will be unable to achieve an effective firing angle against the sidewall from within 30,000 km. One thing you must bear in mind is that the geometry of the wedge/sidewall/warship equation actually makes this a relatively tiny target. Consider: an SD's impeller wedge is 300 km across, but the ship herself has a maximum beam of less than a kilometer. She therefore represents a very small (relatively speaking) aiming point at the middle of a very wide roof and floor of impenetrable "armor." The sidewall is normally generated at a range of less than 10,000 meters from the actual ship, which means that, in the case of our SD from the example above, the sidewall will be 143 kilometers inside the outer "edges" of the wedge. There is thus a very considerable "tunnel" effect, which requires an attacker (enemy warship or missile) to achieve an almost perfect firing angle before it can even engage the sidewall, much less the ship.
Against enemy warships, this means, in effect, that no one can shoot you (broadside to broadside, at any rate) with energy weapons unless you can shoot back at him. It also means that it is very difficult for missiles to attain a firing angle in the first place, which is why they tend to be fired in such large numbers.
The throat of the wedge for an SD is about 190 kilometers deep; the after aspect's opening is only about 40 kilometers deep. This is why the throat is the most vulnerable aspect, and why missiles are usually programmed to try to get around their targets in order to achieve a down-the-throat shot. Usually, however, the target of the missile attack will turn to present its heavier broadside point defense to the enemy fleet in order to engage missiles as they try to attain that sort of firing position. In addition, of course, an entire task force will be organized into one or more missile defense nets whose units protect one another.
In addition, a ship can shift position somewhat within the area of its wedge. One can predict exactly where a ship will be within the volume of its wedge if it is seeking to attain the maximum possible acceleration (assuming that one has solid, reliable numbers on its inertial compensator's performance envelope), but at lower accelerations, ships can move "off center" within their wedges.
"Rolling ship" is thus a maneuver which is intended to turn a very narrow zone of vulnerability away from threat. It is a complete defense against direct energy fire, and a major defense against missiles not simply because it intercepts their direct fire but because it prevents the missiles' sensors from attaining hard locks on the ships within the wedges. At the relative velocities resulting from most missile fire control solutions, the laser head is always going to have a snap shot, with very little time to compute and adjust firing angles, as it crosses the sidewall/throat/kilt of a wedge, and if it doesn't know exactly where to find its target in the instant that the shot is available, it will miss.
Note that most of the above applies to single-ship duels or engagements with relatively low numbers of ships and relatively unencumbered maneuverability on either side. In formal wall-of-battle engagements, the defensive fleet's situation is both less flexible and less vulnerable. Because contact between wedges is lethal, ships must maintain a safe separation (usually the width of the widest wedge involved) from one another. A really well drilled fleet, however, with a CO who's willing to (a) trust his people's abilities and (b) take a few risks, can cut that separation in half, which has two major effects in defending against an opponent's missile fire. (1) The relative positions of the units in a vertically-stacked wall which has turned "up on its side" relative to the enemy mean that those ships at the center of the wall are almost entirely protected from any missiles more than a couple of hundred kilometers from them by the interposed wedges/sidewalls/hulls of the ships "above" and "below" them; and (2) simply turning the wall's units on its side builds a "road block" of wedges which the missiles must penetrate before they can engage at all. If a wall of SDs stacked four high with an impeller-roof-to--floor separation of 500 kilometers, a broadside separation of 150 kilometers and an ahead-and-astern separation of 200 kilometers turns up on its side, any missile which wants to attack will have to achieve a down-the-throat/up-the-kilt attack position or thread its way through a suddenly much constricted series of "holes in the wall" to get at the sidewalls. The traditional formation for a wall which has rolled ship is for the ships of the wall to do so at slightly offset angles. This increases the number of firing aspects from which missiles could expect to hit something, but it also makes the broadside point defense capability of ships in other tiers of the wall available to protect the exposed units at its "top" and "bottom." In addition, light screening units hidden on the far side of the wall from the enemy but taking missile intercept data from ships in the wall and/or recon drones deployed to see around the wall can also supply indirect defensive fire (counter-missiles) from a position in which they are relatively invisible (and so invulnerable to) the seekers of incoming missiles.
Note: That white dot-like line in the center represents a superdreadnaught, bow (and acceleration vector) toward the left.
Image by Russ Isler.
From an email dated April 21, 2002:
Assuming the brown is sidewall (which is what I assumed when Russ showed it to me some time ago), this is, indeed, an accurate representation of the wedge geometry. The exact angle of the wedge changes somewhat as acceleration goes up -- the throat tends to get narrower and the kilt wider as acceleration climbs, although it certainly isn't a gross deformation until very high (like lethal) accelerations are reached. I think I've also mentioned that the exact position of the ship within the wedge/sidewalls can be adjusted somewhat if less than optimum acceleration for power is accepted, which further complicates targeting considerations.