They were solid-fuel rockets, though solid fuels had been long abandoned for long-range missiles.
It was also the only way to stop acceleration by solid-fuel rockets.
Solid-fuel launchers exploded and blasted the lifeboat clear of the parent ship.
One solid-fuel stage had imparted to the ship a velocity which would carry it to the altitude of the missile it was to intercept.
But solid-fuel rockets simply can't be made to burn with absolute evenness as a team.