In 1960 the US created a Single Integrated Operational Plan [SIOP] for nuclear targeting. The plan listed 3,729 targets in the Soviet Union, North Korea, China and Eastern Europe. These targets would not only be attacked by US land based missiles, sea launched missiles and bombers but also by Britain’s Royal Air Force. The RAF was scheduled to destroy three air bases, six air defence targets and forty eight cities.
It was anticipated that if the SIOP was put into full effect it would result in the death of about 54% of the population of the Soviet Union and about 16% of the Chinese population; a total of 220 million people. That was in the first three days. Many tens of millions would die later from injuries, fallout and social collapse.
Later [late 1960s?] vulnerability numbers [VN] were calculated for SIOP targets. For example, the 1969 Defense Intelligence Agency Physical Vulnerability Handbook for Nuclear Weapons assigned a VN of 11Q9 to road-mobile strategic missiles and a VN of 46P8 to a nuclear weapon storage bunker.
The first two digits of the vulnerability number indicates the blast required to give a 50 percent probability of achieving the required level of damage. The higher the first two digits the greater the targets resistance to blast damage. The third character (P or Q) of the VN specifies whether the target should be best attacked using overpressure [crushing pressure] or dynamic pressure [horizontal pressure]. PVN targets are destroyed by crushing, QVN targets are destroyed by being knocked over.
Overpressure is created either by a surface burst or by the downwards blast wave generated by an accurate air burst. Dynamic pressure acts horizontally to blown things over. Dynamic pressure is usually generated by air bursts. These not only generate a spreading blast wave but a ‘precursor wave’ when highly compressed air from the fireball hits the ground and spreads outward, travelling faster than the blast wave.
Some targets are relatively resistant to overpressure [e.g. locomotives or armoured fighting vehicles] and more susceptible to dynamic pressure.It is easier to overturn a tank with dynamic pressure than crush it with overpressure. The Russians recognised this and designed a tank which was hard to tip over. Heavy tank “Object 279” was unique and had no NATO equivalent. It had an unusual ellipsoidal shape protecting the tank from overturning if hit by a shock wave of a nuclear explosion.
Attacks against QVN dynamic pressure-sensitive targets threaten an area and are particularly appropriate for mobile targets such as road or rail mounted missiles. Targets such as missile silos are highly resistant to dynamic pressure and must be attacked with overpressure.
The last character, known as the K-factor, accounts for the increase in the duration of the blast wave with increasing yield. A higher yield weapon will have a greater probability of destroying a target at a given pressure than a lower-yield weapon because the blast wave from the higher-yield weapon lasts longer.
Note that in Table 4.5 roadbed and track targets have a VN of 45Z0. I assume that Z numbers were given to targets which are resistant to overpressure and dynamic pressure and require cratering to produce sufficient damage.
In addition to a VN number a required level of damage was specified for each target.
The Soviet Union employed both road and rail mounted missiles. Road-mobile strategic missiles were given a VN of 11Q9. The Q in the VN indicates the target is best attacked by dynamic pressure, by being blown over rather than crushed. In this case the target is best attacked as a Q target because dynamic pressure affects a greater area and is therefore a better choice for hard to find mobile targets.
The damage level for this target type is defined as transporter overturned and missile damaged. In other words the aim of an attack on such a target would be to generate a blast wave which tipped the launch vehicle on its side before it could launch its missile.
For a 100 kiloton weapon [e.g. the W76 warhead], the optimum height of burst to attack a target with a vulnerability number of 11Q9 is approximately 1,250m [no local fallout would be expected with that burst height] and the corresponding damage radius is 2,875 m. Thus dispersed SS-25 vehicles can be threatened over an area of approximately 26 square kilometres by a single W76 air burst. Twenty six square kilometres sounds a lot but a box ten miles square would have a area of 259 square kilometres.
Mobile missiles are targets that are easy to damage but hard to find. However, the missiles operate out of a number of bases so targeting did not involve searching the entire Soviet Union.