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A multiple independently targetable re-entry vehicle, or '''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.

The military purpose of a MIRV is fourfold:
  • Provides greater target damage for a given missile payload. Radiation from a nuclear warhead diminishes as the square of the distance (called the Inverse-square Law ), and blast pressure diminishes as the cube of the distance. For example at a distance of 4 km from ground zero, the blast pressure is only 1/64th that of 1 km. Due to these effects several small warheads cause much more target damage area than a single large one. This in turn reduces the number of missiles and launch facilities required for a given destruction level.

  • Reduces the number of missiles required to attack a given number of separate targets. With single warhead missiles, one missile must be launched for each target. By contrast with a MIRV warhead, the post-boost (or bus) stage can dispense the warheads against multiple targets across a broad area.

  • Reduces the impact of SALT Treaty Limitations . The treaty initially limited number of missiles, not number of warheads. Adding multiple warheads per missile provided more target destruction for a given number of missiles.

  • Reduces the effectiveness of an Anti-ballistic Missile system that relies on intercepting individual warheads. While a MIRVed attacking missile can have multiple (3-12 on various US missiles) warheads, interceptors can only have one warhead per missile. Thus, in both a military and economic sense, MIRVs render ABM systems less effective, as the costs of maintaining a workable defense against MIRVs would greatly increase, requiring multiple defensive missiles for each offensive one.



MODE OF OPERATION

MIRV bus.]]
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 Navigation 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.

Details are closely-held Military Secret s, the bus' on-board Propellant limits the distances between targets of individual warheads. Some warheads may use small Hypersonic Airfoils during the descent to gain additional cross-range distance. It's 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 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 Gravitic Anomalies , in order to improve accuracies of ballistic missiles. Accuracy is expressed as CEP ( Circular Error Probable or Circle Of Equal Probability ). This is simply the radius of the circle that the warhead has a 50% chance of falling into when aimed at the centre. CEP is about 90-100 meters for the Trident II and Peacekeeper Missiles .


SEE ALSO




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