Dec 142013
LLNL artwork depicting the hit-to-kill terminal stage of the Brilliant Pebbles, sans the booster drop-stages. It does not seem to have a go-forward thruster; that would seem to have been provided by the boosters. What it does have is a fair amount of divert capability provided by the thrusters mounted at what should be the center of gravity.
While dimensions are unknown with any certainty, this would be a fairly small system. The kill vehicle – which in this case is somewhat difficult to distinguish from the propulsion system – had a mass goal of only 2.5 kilograms (according to THIS), while the booster was to mass about 100 kilograms. THIS source said the vehicles were about one meter long.
The 2.5 kg goal I don’t think was really the goal, I think that’s just Wood’s estimate of the smallest possible weight that would still guarantee destruction of even an armored ICBM fuselage. I seem to recall a Boeing presentation that suggested all-up weight with the life jacket was 1800 kg, and I think it’s safe to estimate the interceptor and its fuel would be more than half of that. Note that there were four fuel tanks that wrapped around the interceptor that are not shown in this diagram, they acted as a “zero stage” and were ejected when empty.