|Quoting RedFlyer (Reply 3):|
If the "Bus" has a small rocket engine, then does its motor burn continuously during its trajectory after booster separation and it releases the RVs at points along that trajectory?
The burn happens (relatively) briefly near the start of the ballistic flight. The idea is to nudge each RV
enough so that the extra velocity accumulated over the entire flight time adjust the impact point as desired. That has to be done at the beginning of the flight. Very roughly, let's say you impart a 300mph nudge to an RV
at the beginning of a 20 minute coast. That'll move the impact point roughly* 100mi. The same nudge just before reentry might move the impact point 15-20mi.
While some RV
do use aerodynamics to further adjust their impact point, the problem that poses for an ICBM RV
is that there is very little time when that's effective (only the last two or three minutes of the flight), which severely limits the amount of "spread" you can get. Some early MIRV
'd ICBMs used that technique to scatter several smaller warheads around one large target, the idea being that five 100kt warheads spread a few miles apart will do much more damage to a city than a single MT
class warhead right in the center. But that's not quite hitting separate targets.
As a counter example, the Indian Agni III is a bit different in that it uses a rather shallow reentry angle, which gives the RV
a lot more aerodynamic flight time.
Remember that in any case, the MIRV
'd RVs are going to hit in an ellipse no more than a few (3-4) hundred miles across.
*It's rather more complicated than that - the possible impact points from a given delta-V will form something of an ellipse.