interface NozzleAllocation
A layout, prepared for nozzleFiring: every valve's wrench, once.
The valves never see the drive. The demand keeps the two engines apart —
linear is the thrusters and drive is the throttle — so a stern pod
whose exhaust leans a little aft is lit by a push ahead on the thrusters,
which is right, and not by a burn on the drive, which would glow it at a
quarter through every transit.