Test launch of a Peacekeeper ICBM by the 1st Strategic Aerospace Division(1 STRAD).
The original German A-4 missile employed a brilliantly simple road-mobile system, in which the missile was carried on a four-wheeled trailer known as a Meillerwagen. When the missile was to be launched, the Meillerwagen raised it to the vertical and then lowered it on to a small launch platform. Each site had a crew of 136 men, with many more men and vehicles in the logistics chain.
The Germans also gave active consideration to launching the A-4 missile from a train. According to a 1944 plan, each train would carry six ready-to-use missiles, and include an erector–launcher car, seven fuel-tanker cars, a generator car, a workshop, a spares car and several cars for the crew. On top of this, however, the train would also carry all the vehicles normally associated with a missile battery, in order that the unit could dismount from the train and operate independently of it, which brought the whole battery up to the unwieldy total of seventy to eighty freight cars, probably requiring at least two separate trains. Separate logistic trains were planned to bring further supplies of fuel and missiles. Prototype trains were running before the end of the war, but the system was not a practicable proposition in view of the air supremacy of the Allies, for whom all trains were a high-priority target.
ICBM forces were originally built to threaten the opponent’s civil population, which in itself was not a difficult task: the warheads were relatively inaccurate, but the cities were large and the warheads powerful. It was obviously highly desirable, from both political and military viewpoints, to defend the population from this threat, in the same way that bombers had been opposed by a mixture of fighters and anti-aircraft guns during the recent war. It was not feasible at the time to intercept incoming ICBMs, so the only defence was to attack the ICBMs at their source, which could be done only by conducting a pre-emptive strike with other ICBMs. Thus the position was rapidly reached where the ICBMs’ principal target was the other side’s ICBMs, moving on to other missions only when that first battle had been decided. It was therefore necessary to optimize the attacking potential of one’s own missiles while ensuring their survivability in the face of an opponent’s first strike. There were four possibilities:
• superhardened silos, which would withstand even the most powerful incoming warhead;
• using a greater number of silos than missiles, so that the opponent would waste warheads on empty silos;
• making the missiles mobile, as the Germans did, so that the enemy could not locate them;
• using anti-ballistic-missile (ABM) defences.
The essence of the problem can be illustrated by a simplified example in which the aggressor (A) has 100 ICBMs, each with ten warheads, while the other side (B) has 500 ICBMs, each with three warheads. (For the purpose of this example, all missiles and warheads are perfectly available and reliable, and each warhead will kill one silo.) Thus A is capable of destroying 1,000 silos, and if he carries out a pre-emptive strike he requires to use only fifty missiles, leaving B with no missiles. A still has fifty missiles and is clearly the winner. If, however, B builds another 500 silos, but no more missiles, and spreads his 500 ICBMs randomly among the 1,000 silos, A, not knowing which silos are occupied, must attack all 1,000. Both sides then end up with zero ICBMs, which is a better outcome for B than the first, but is unsatisfactory from a military point of view. But if B now builds a total of 2,000 silos, half his missiles (i.e. 250) must survive the attack.
The first missiles, such as the early Atlas and Thor, were located in a shed, primarily for protection from the weather, and were taken out to enable them to be raised to the vertical for fuelling and launch. The missiles were also located close to each other. Both factors together made the missiles extremely vulnerable to incoming missiles, which did not need to be too accurate to achieve a kill.
The next step was to place the missiles in semi-hardened shelters and to separate these shelters so that one incoming warhead could not destroy more than one missile. In addition, the shelters had split roofs, so that the missile could be raised, fuelled and launched without wasting time moving it out on to a launch pad. As the perception of the threat increased, the spacing between individual missiles increased yet further and the shelters became bunkers, recessed into the ground.
The next step was to mount the missile vertically rather than horizontally, and to put it in a hole in the ground. The USAF, however, adopted a ‘halfway’ system with the Atlas and Titan I missiles, in which the missile stood upright in a silo which, in the case of Atlas, was some 53 m deep and 16 m in diameter, resting on the launch platform, which was counterbalanced by a 150 tonne weight. The launch procedure involved fuelling the missile in the silo and then using hydraulic rams to raise the entire launch platform and missile to the surface, where the missile was then launched. Titan I had a super-fast fuelling system and a high-speed elevator which reduced reaction time to approximately twenty minutes, while the silo and all associated facilities were hardened to withstand an overpressure of 20 kgfcm2.
A completely new launch system was introduced with Titan II, in which the missile was launched direct from the silo. There was, however, considerable concern about the effects of the rocket efflux on the missile during the few seconds that the missile was still inside the silo, so the missile rested on a large flame deflector, which directed the efflux into two large ducts exhausting to the atmosphere a short distance from the silo. Each missile complex was 45 m deep and 17 m wide and occupied nine levels, which housed electrical power, air conditioning, ventilation, and environmental protection, as well as hazard sensors and the associated corrective devices. At the centre was the launch duct, in which the missile was suspended in an environmentally controlled atmosphere. A walkway extended from the missile silo to a blast lock which provided controlled access between the silo and the tunnels leading upward to the above-ground access and laterally to the launch-control centre (LCC). The LCC was a three-level, shock-isolated cage suspended from a reinforced-concrete dome and housed two officers and two enlisted men. As with the Titan I silo, the Titan II silo was hardened to 20 kgfcm2.
When it learned that the Soviets were launching direct from the silo, the USAF followed suit and the Minuteman I missile became the first US missile to use the ‘hot launch’, in which the missile rose from the silo surrounded by the flames and smoke from the rocket motor. The next Soviet innovation was the ‘cold launch’, in which a gas generator within the silo produced a pressure sufficient to propel the missile some 20–30 m clear of the silo before its first-stage motor fired. This protected the silo from damage, enabling it to be reused within a fairly short space of time. It was used by the Soviets from the SS-17 onwards, and by the USAF in Peacekeeper (MX).
Following their introduction in the mid-1960s, underground silos became increasingly complicated and expensive structures. Ideally they were located at a relatively high altitude, to improve the missiles’ range, and in springy ground, to absorb as much as possible of the shock waves from incoming warheads. The silo was a vertical, steelreinforced-concrete tube, housing an elaborate suspension and shock-isolation system which supported the missile as well as providing further insulation to minimize the transfer of shock motion from the walls and floor of the silo to the missile. The top third of the silo housed maintenance and launch facilities, which were known as the ‘head works’ in USAF parlance. Finally, the missile tube was capped by a massive sliding door, which provided protection against overpressure by transmitting the shock caused by the explosion of an incoming warhead to the cover supports rather than to the vertical tube containing the missile; it also provided protection against radiation and EMP effects. The door was designed to sweep the area as it opened, to prevent debris falling into the silo tube and possibly interfering with the launch process.
Individual silos were grouped together for control purposes, but were sited sufficiently far apart to ensure that one incoming warhead could not destroy more than one missile. Control was exercised by an underground command centre, manned by a small crew of watchkeepers, whose functions included operating the dual-key safety system in which launch could be authorized only by two officers acting independently. This command centre was linked to its superior headquarters and to the individual silos under its control by telecommunications and by systems-monitoring links. This introduced a further problem: the vulnerability of these links to blast and, in particular, to electromagnetic pulses (EMP). Making these links survivable against the perceived threats (known as ‘nuclear hardening’) became an increasingly complex and expensive undertaking as the Cold War progressed.
The protection factor (‘hardness’) of a silo was measured by its ability to withstand the overpressure resulting from the blast effects of a nuclear explosion, and was expressed in kilograms-force per square centimetre (kgfcm2) or pounds per square inch (psi) (1 kgfcm2≈14.2 psi). In the USA, the Atlas, Titan I and Titan II silos were constructed with a hardness of 20 kgfcm2 (300 psi), while the Minuteman I silos (mid-1960s) were built with a hardness of some 85 kgfcm2 (1,200 psi). Finally, in the 1970s, Minuteman IIIPeacekeeper silos were built with a hardness of 140 kgfcm2 (2,000 psi). By this time, however, the silos were so expensive that, despite reports that the Soviets were ‘superhardening’ their silos to resist overpressures of 425 kgfcm2 (6,000 psi), Congress repeatedly refused to authorize any further hardening of US silos.
The Soviet programme of silo building, refurbishment and hardening was more successful. The earliest silos, built before 1969, were hardened to withstand an overpressure of some 7 kgfcm2 (100 psi), with the next generation built to 20 kgfcm2 (300 psi). Those built in the early 1970s for the SS-18 could withstand 425 kgfcm2 (6,000 psi), which was achieved using concrete reinforced by concentric steel rings.