Complex sentence: In air combat, the gun had to be pre-rotated so as to fire at a top rate as soon as the trigger released the cartridge feed; otherwise too much time would be lost accelerating the mechanism.
Split into simpler sentences: In air combat, the gun had to be pre-rotated so as to fire at a top rate as soon as the trigger released the cartridge feed. This was necessary since otherwise too much time would be lost accelerating the mechanism.