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 kgf/cm2.
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 kgf/cm2.
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, steel/reinforced-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 (kgf/cm2) or pounds per square inch (psi) (1 kgf/cm2≈14.2 psi). In the USA, the Atlas, Titan I and Titan II silos were constructed with a hardness of 20 kgf/cm2 (300 psi), while the Minuteman I silos (mid-1960s) were built with a hardness of some 85 kgf/cm2 (1,200 psi). Finally, in the 1970s, Minuteman III/Peacekeeper silos were built with a hardness of 140 kgf/cm2 (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 kgf/cm2 (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 kgf/cm2 (100 psi), with the next generation built to 20 kgf/cm2 (300 psi). Those built in the early 1970s for the SS-18 could withstand 425 kgf/cm2 (6,000 psi), which was achieved using concrete reinforced by concentric steel rings.
Alternative Basing Schemes
Although most of their ICBMs were always sited in silos, both the USA and the USSR repeatedly examined alternatives, both to increase survivability and, perhaps of greater importance in the USA than in the USSR, to reduce costs. In the USA, environmental factors also became an increasingly important consideration.
One of the US schemes was called Multiple Protective Structures (MPS) and consisted of a number of ‘racetracks’, each about 45 km in circumference and equipped with twenty-three hardened shelters. One mobile ICBM, mounted on a large wheeled TEL, would have moved around each racetrack at night in a random fashion, with decoy TELs and missiles adding to the adversary’s uncertainties. Basic MPS involved 200 missiles moving between 4,600 shelters covering an area of some 12,800 km2, but a more grandiose version envisaged 300 missiles moving around 8,500 shelters.
An enhanced version of MPS was proposed in the early 1980s, in which a new Small ICBM (SICBM) would have been deployed in fixed, hardened silos distributed randomly among the 200 racetracks of the MPS system, thus adding to the aiming points for the Soviet ICBM force. It was intended that the SICBM would be 11.6 m long and weigh 9,980 kg, have a range of 12,000 km, and carry a single 500 kT warhead; it would have been launched by an airborne launch-control centre. SICBM would have been housed in a tight-fitting container placed in a vertical silo hardened to approximately 530 kgf/cm2, and it would have required an exceptionally accurate incoming warhead to destroy such a target. Various other launch methods were also considered for SICBM, including a road vehicle, normal silos, airborne launch from a transport aircraft, and (possibly the only time this was ever considered for an ICBM) from a helicopter.
Another scheme was based on the racetrack principle of MPS, but this time with the TELs running inside shallow tunnels, 4 m in diameter. The TELs would simply have kept moving, thus avoiding the need for shelters, and would have had large plugs fore and aft to protect against nuclear blast within the tunnel. If required to launch, the TEL would have halted and used hydraulic jacks to drive the armoured roof upwards, breaking through the surface until the missile was raised to the vertical.
Deep Basing (DB) involved placing the ICBMs either singly or in groups deep underground, where they would ride out an attack and then emerge to carry out a retaliatory strike. One of the major DB schemes was the ‘mesa concept’, in which the missiles, crews and equipment were to be placed in interconnecting tunnels some 760–915 m deep under a mesa or similar geological formation. Following an enemy nuclear strike, the crews would have used special machines to dig a tunnel to the surface and then brought the launcher to the open to initiate a retaliatory strike. This scheme’s disadvantage lay in its poor reaction time and the difficulty it posed for arms-control verification. From the practical point of view it would have been necessary to find rock which was both fault-free and sufficiently strong to resist a Soviet nuclear attack, but which could nevertheless be drilled through in an acceptable time and without the machinery becoming jammed by debris. On top of all that, a second incoming nuclear strike when the drilling machine was near to the surface would have caused irreparable damage. A related project (Project Brimstone) examined existing deep mines, but also proved unworkable.
A totally different approach, known as Closely Based Spacing or ‘Dense Pack’, was also considered. This suggested that, instead of spacing missile silos sufficiently far apart to ensure that not more than one could be destroyed by one incoming warhead, 100 MX missiles should be sited in superhardened silos placed deliberately close together. The idea was that this would take advantage of the ‘fratricide’ effect in which incoming warheads would be deflected or destroyed by the nuclear explosions of the previous warheads. A spacing of the order of 550 m was suggested, and it was claimed that in such a scheme between 50 and 70 per cent of the ICBMs would have survived.
All the basing methods discussed above were either static or involved limited movement in a closed circuit, but the question of mobile basing was often considered as well. As described earlier, the German A-4 was designed as a road-mobile system, but an alternative rail-based option was also considered, and a similar scheme was designed and tested during the development phase of the Minuteman I. The plan was to have fifty trains, each of some fourteen vehicles, which would have included up to five TEL cars, each carrying a single missile, together with command-and-control, living-accommodation, and power facilities. The scheme was examined in great detail, and a prototype ‘Mobile Minuteman’ train was tested on the public railway. Although the scheme proved feasible, it was dropped in favour of silo deployment.
A similar proposal was considered during the long development of the Peacekeeper (MX) system, and very nearly became operational. This version would have consisted of twenty-five missile trains, each carrying two missiles. Each train would have consisted of the locomotive and six cars: two missile launch cars; a launch-control car, a maintenance car, and two security cars. In peacetime the trains would have been located in a ‘rail garrison’ sited on an existing Strategic Air Command base, which would have contained four or five shelters (known as ‘igloos’), each housing one train. These garrisons would each have covered an area of some 18–20 hectares, with tracks leading to the USA’s 240,000 km national rail network. On receipt of strategic warning the trains would have deployed on to this national network, where they would have rapidly attained a high degree of survivability. This scheme was under active development from 1989 until its cancellation in 1991.
As we have seen, the Soviet SS-24 Mod 1 was actually fielded in the rail-mobile mode. There were three rail garrisons, all in Russia, with four trains at two sites and three trains at the third. The trains had one launcher each, with two further cars for launch control, maintenance, and power supply.
The Soviets also fielded a road-mobile ICBM, the SS-25, which was also the last Soviet ICBM to enter service during the Cold War. This single-warhead missile was carried on a fourteen-wheeled TEL, which was raised on jacks for stability during the launch. The TEL and its missile were normally housed in a garage with a sliding roof which would be opened for an emergency launch. Given the necessary warning, however, the TELs deployed to pre-surveyed sites in forests.
One US proposal was the ‘continuous patrol aircraft’, in which a packaged missile was carried inside a large, fuel-efficient aircraft. On receipt of verified launch instructions, the missile would have been extracted by a drogue parachute, and once it was descending vertically its engine would have fired automatically, enabling the missile to climb away on a normal trajectory. Tests were carried out using a Minuteman I missile transported by a C-5 Galaxy and were completely successful. Large numbers of aircraft would have been needed to maintain the number required on simultaneous patrol. It would have been very difficult for a potential enemy to track them and even more difficult to guarantee the destruction of every airborne aircraft in a pre-emptive strike, but the main weaknesses of the scheme were the vulnerability of the airfields, the enormous operating costs, and, to a lesser degree, the decreased accuracy of the missile.