Low-observable trimaran — general arrangement

The main gun is a rail gun: two parallel rails 8 m long with a channel 70 mm across between them, and a current of about 3 MA driven up one rail, across an armature and back down the other. The force on the armature is half the rails' inductance gradient times the current squared, some 2.3 MN, and in 1.4 ms it carries a 100 g projectile in its sabot, about 280 g in all, to roughly 11 km/s.

What such a current does to a rail is the reason for most of what stands round the barrel. The face the armature rides on is not the rail itself: it is a copper tape 102 mm wide laid along it and renewed from a cassette at the breech. The arc sits on the tape; the tape is drawn on between shots and rolled to its profile afresh on the way in, so the surface the next round meets is always new. The tape survives because it carries only about a fifth of the current, the rest diffusing into the rail under it.

The round does not start from rest under full current. A pneumatic pusher on the bore's own axis, behind the breech, throws it onto the rails at about 300 m/s: dry nitrogen at 4.3 MPa, 53 kN over a stroke of 508 mm. That speed adds a thousandth to the muzzle energy and is not there for ballistics. It is there so that the arc never stands in one spot at the breech.

The barrel is pumped down for the shot and closed at the muzzle by a foil membrane the projectile breaks. Eight metres of air in the channel weigh half as much as the projectile, and the scatter of muzzle velocity that air would cause is the largest term in the accuracy budget: velocity matters more than aim. Between shots the same volume is purged with dry gas from the breech.

The pulse comes from two compulsators, pulsed alternators whose rotors stand side by side in the stabilised cap under the gun and turn in opposite senses, so that discharging them gives the mount no reaction torque. They are spun up from the ship through the mount's joints. The pulse itself never leaves the cap and the gun above it.

The mount took its shape by elimination: each line below is the survivor of several arrangements that were drawn, measured and dropped. The design journal keeps the ones that were dropped, with the reasons.

1. The round, a bolt in its sabot, is seated in the cradle cut in the pusher's piston, which sets its clocking in the channel.
2. The barrel and the breech flask, between 130 and 230 litres, are pumped down. A fresh frame of foil closes the muzzle.
3. The equatorial brake clamps the cap to its seat. From here to the end of the shot the mount is part of the ship.
4. The pusher fires. A coil at the breech reads the piston's speed, and only a piston moving fast enough closes the firing circuit: the threshold is a velocity gate, so a slow start cannot be followed by a pulse.
5. The contactors close and the compulsators discharge through the coaxial joint, the elevation telescopes, the bus over the barrel and the bridge onto the rails: about 3 MA for 1.4 ms.
6. At the muzzle the sabot's petals part and the foil bursts. The recoil, 3.2 kN.s, goes through the breech and the trunnions into the clamped cap and the hull.
7. Within a few milliseconds of the round leaving, the purge valve opens. Dry gas from the breech sweeps the channel and keeps the sea air out of it, and its pressure on the piston's face re-cocks the pusher: one throttle washer in place of a spring, a brake and a cocking drive.
8. The tape is drawn on, the foil advances one frame, the brake lets go and the cap hangs free again while the compulsators are brought back up to speed.

The tempo is set by energy, not by mechanism: about two rounds a minute. The reaction wheel would saturate against the seat's friction in 6 s if nothing unloaded it, which the brake does at every shot.

Figures given as 'about' are estimates from the design journal and are not held by the model. The rest are read from the model.

The model is the mount as it would be seen through its own structure: everything in it is connected to something. These parts of the design were worked out in the journal and are not drawn.

• The ammunition magazine and its exchange. The round is taken as arriving in the pusher's cradle. Where rounds are stowed, how a magazine reaches the breech and how an empty one is changed are not modelled; the tape cassette, its hatch and the arm that carries it are.
• The thunder generator. The design's prime store of energy is propellant, not a flywheel hall or a capacitor bank: charges of battleship size fired inside the hull against captive pistons that compress magnetic flux, about 320 MJ of chemical energy for a round. It is what sets two rounds a minute, and its name is literal: the most modern gun on the ship fires only after the hull has thundered the old way. The model stops at the compulsators in the cap.
• The pulse interrupter. A current built up in an inductance has to be thrown into the rails by breaking a circuit that carries megaamperes. The journal's answer is a sacrificial fuse, a fourth consumable after tape, foil and powder. It is not modelled.
• The drive of the counterweight. The position of its halves is a function of elevation alone, a cam or a winch off the elevating drive; the mechanism is not drawn.