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missile, with the re-entry vehicles highlighted in red.]] A multiple independently targetable reentry vehicle ('''MIRV''') is a collection of Nuclear Weapon s carried on a single Intercontinental Ballistic Missile (ICBM) or a Submarine-launched Ballistic Missile (SLBM). Using a MIRV warhead, a single launched missile can strike several targets, or fewer targets redundantly. By contrast a unitary warhead is a single warhead on a single missile. MIRV bus.]] The military purpose of a MIRV is fourfold:
MIRVed land-based ICBMs were considered destabilizing because they tended to put a premium on Striking First . MIRVs threatened to rapidly increase the US's deployable nuclear arsenal and thus the possibility that it would have enough bombs to destroy virtually all of the Soviet Union's nuclear weapons and negate any significant retaliation. Later on the US feared the Soviet's MIRVs because Soviet missiles had a greater Throw-weight and could thus put more warheads on each missile than the US could. For example the US MIRVs might have increased their warhead per missile count by a factor of 6 while the Soviets increased theirs by a factor of 10. Furthermore, the US had a much smaller proportion of its nuclear arsenal in ICBMs than the Soviets. Bombers could not be outfitted with MIRVs so their capacity would not be multiplied. Thus the US did not seem to have as much potential for MIRV usage as the Soviets. However, the US had a larger number of SLBMs, which could be outfitted with MIRVs, and helped offset the ICBM disadvantage. It is because of this that this type of weapon was banned under the START II agreement. However, START II was never ratified by the Russian Duma due to disagreements about the ABM Treaty . The Russian government claimed to have developed the most advanced MIRV system as of 2006 for use in the Bulava Missile . MODE OF OPERATION In a MIRV, the main rocket motor (or Booster ) pushes a "bus" (see illustration) into a freely-falling Suborbital ballistic flight path. After the boost phase the bus maneuvers using on-board small rocket motors and a computerised Inertial Guidance System . It takes up a ballistic trajectory that will deliver a Reentry vehicle containing a Warhead to a target, and then releases a warhead on that trajectory. It then maneuvers to a different trajectory, releasing another warhead, and repeats the process for all warheads. MIRV launch sequence: 1. The missile launches out of its silo by firing its first stage boost motor (''A''). 2. About 60 seconds after launch, the 1st stage drops off and the second stage motor (''B'') ignites. The missile shroud is ejected. 3. About 120 seconds after launch, the third stage motor (''C'') ignites and separates from the 2nd stage. 4. About 180 seconds after launch, third stage thrust terminates and the Post-Boost Vehicle (''D'') separates from the rocket. 5. The Post-Boost Vehicle maneuvers itself and prepares for re-entry vehicle (RV) deployment. 6. The RVs, as well as decoys and chaff, are deployed during backaway (unlike the figure suggests this occurs at the start of the midcourse phase, so during ascenthttp://www.airliners.net/discussions/military/read.main/68181/) 7. The RVs and chaff re-enter the atmosphere at high speeds and are armed in flight. 8. The nuclear warheads detonate, either as air bursts or ground bursts.]] Details are closely-held Military Secret s. The bus's on-board Propellant limits the distances between targets of individual warheads to perhaps a few hundred km. Some warheads may use small Hypersonic Airfoils during the descent to gain additional cross-range distance. It is possible the buses can release Decoy s to confuse interception devices and Radar s, such as aluminized balloons or electronic noisemakers. re-entry vehicles, all eight (ten capable) fired from only one missile. Each line represents the path of a warhead which, were it live, would detonate with the explosive power of twenty-five Hiroshima-style weapons.]] Accuracy is crucial, because doubling the accuracy decreases the needed warhead energy by a factor of four for radiation damage and by a factor of eight for blast damage. Navigation system accuracy and the available geophysical information limits the warhead target accuracy. Some writers believe that government-supported geophysical mapping initiatives and ocean satellite altitude systems such as Seasat may have a covert purpose to map mass concentrations and determine local Gravity Anomalies , in order to improve accuracies of ballistic missiles. Accuracy is expressed as Circular Error Probable (CEP). This is simply the radius of the circle that the warhead has a 50 percent chance of falling into when aimed at the center. CEP is about 90–100 m for the Trident II and Peacekeeper missiles. SEE ALSO
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