Sorry Kron1 you lost me.
These single megaton warhead missiles are city killers, to replace them with MIRVs is just using lower yield warheads in a more dispersed pattern.
What megaton warheads are we talking about?
Do you mean the old single warhead SS-18 25 megaton warhead for Cheyanne mountain (ie the Stargate Base in the TV series).
AFAIK they are not in service any more and the standard in service SS-18s and SS-19s have 150-200KT warheads in loads of 10 and 6 respectively.
Against super hardened targets then accuracy and high yield are very important, but as you mention three relatively low yield warheads can do much more damage to an area target like a city than even a very big warhead can. As an example 3 x 150KT warhead missiles evenly distributed around a large city like London would do rather more damage than a single warhead of considerably more power like a 5MT bomb because the smaller bombs deliver damage more efficiently.
If you think about it it makes perfect sense... a 50 gramme block of HE is easily enough to kill a human yet to kill the 6 billion people on this planet would require 300 million tons of HE or 300MT. With SS-18 missiles having 25 Megaton warheads that means 12 missiles with 12 warheads to wipe out all of humanity... it clearly is not enough. Even though they are big warheads.
Obviously the 12 missiles is more practical and delivery is much easier. The concept is the same as with a cluster bomb... when you just want to kill people then a single 500kg HE charge is not as effective as 450 x 0.75kg HE fragmentation charges that shower over a wide area when released.
A missile isn't going to carry its max number of MIRVs as half of them will be decoys.
TOPOL-M only carries one warhead. The number of warheads a missile carries includes space for decoys and jammers and other bits and pieces. When they talk about the RS-24 carrying 6 warheads they mean that is the normal number of warheads it will carry. That is how many it is assumed to carry regarding arms limitations agreements. If you further remove warheads from that figure you are reducing the number of real warheads you have... which doesn't make sense. If you are only allowed 550 warheads from ICBMs then why would you drop 100 to carry decoys? Clearly you include the decoys with the normal load of the missile. The RS-24 carries 6 warheads plus decoys. The SS-18 carries 10 warheads in the most widely deployed model plus decoys. etc etc. TOPOL-M was limited by the START2 treaty to only be allowed one warhead. That is why they needed the RS-24 design to allow 6 warheads plus decoys and jammers etc.
Also, the trajectory of MIRVs will only be able to engage targets in vicinity to the original target within a 100km radius.
Over the entire flight path of the missile... and considering the warheads of the TOPOL-M and RS-24 can manouver to evade ABMs I would suggest the figure is much much bigger than 100kms.
The point of MIRV is to get warheads through ABM defence where a single warhead would fail.
No the point of MIRVs is to enable attacks from multiple directions to ensure that if one missile fails that all targets get good coverage. It also means you can increase exponentially the number of warheads you have without greatly increasing cost.
[quote]Then they disperse for maximum kills over population centers. Russian MIRVs are INS set for city killing dispersal patterns, not PGMs like American MIRVs can be. You might be able to replace two missiles for one with the MIRVs, but not a missile for each MIRV.[quote]
If Russian MIRVs were only used for city busting then they wouldn't bother with MIRVs, they would simply have stayed with MRVs which merely fall in a spread pattern over one large area target like a city.
The missile warhead design intended to evade ABMs is called MaRV and is pretty much what the TOPOL-M and Bulava and RS-24 warheads are.
Most planners for nuclear war will cross target so for instance if you are talking about a large city like London you would have one warhead from a Delta III sub missile aimed at London and another warhead from that same missile aimed at Paris and the rest would be aimed at other targets on the way. A different sub, say it was a Typhoon when they were in service might have had one missile with a warhead aimed at London and one a Paris and other targets in Europe etc etc so that every target was covered with lots of different sources so if one sub is sunk before it can launch all its missiles that all targets still get a good thrashing.