Chevaline () was a system to improve the penetrability of the warheads used by the British Polaris nuclear weapons system. Devised as an answer to the improved Soviet anti-ballistic missile defences around Moscow, the system increased the probability that at least one warhead would penetrate Moscow's anti-ballistic missile (ABM) defences, something which the Royal Navy's earlier UGM-27 Polaris re-entry vehicles (RVs) were thought to be unlikely to do.
Chevaline used a variety of penetration aids and decoys to offer so many indistinguishable targets that an opposing ABM system would be overwhelmed attempting to deal with them all, ensuring that enough warheads would get through an ABM defence to be a reasonable deterrent to a first strike. The project was highly secret, and survived in secrecy through four different governments before being revealed in 1980.
The system was in service from 1982 to 1996, when the Polaris A3T missiles it was fitted to were replaced with the Trident D5.
Military and political requirements
The origins of the Chevaline requirement grew from the conclusion of several British governments that in the event of a Soviet nuclear attack on the UK alone, as had been threatened in late 1950s by Soviet Premier Nikita Khrushchev and Prime Minister Nikolai BulganinA.J.R.Groom, "British Thinking About Nuclear Weapons", Frances Pinter, 1974, pp. 190–191. it was unrealistic to expect that the US would retaliate against the Soviet Union and risk an attack on major American cities. That conclusion by successive British governments was the basis of their justification given to the British people for an independent nuclear retaliatory capability.
For some time this deterrent force had been based on the Royal Air Force's V bomber force. This looked increasingly vulnerable in the face of ever-increasing Soviet Air Defence Forces, and various reports by the RAF suggested their bombers would be incapable of successfully delivering free-fall ("gravity") bombs by 1960. This had been considered from the early 1950s and the planned solution was a move to the Blue Streak medium range ballistic missile, but this suffered extensive delays for a variety of reasons.
As it became clear Blue Streak would not be available by 1960, interim solutions were considered. Among many possibilities, the RAF eventually selected the Blue Steel standoff missile to allow its bombers to fire their weapons while still (hopefully) outside the range of the defensive fighters. This system provided marginal capability and a number of projected developments were considered as Blue Steel II, which increased both its range to improve the survivability of the bomber and speed to improve the survivability of the missile.
A better solution came along in the form of the US's AGM-48 Skybolt missile. The US bomber force was facing the same sorts of problems as the British V bombers, and were attempting to solve it in a similar fashion, with a long-range standoff missile. In this case the missile had a projected range of just under . The distance from London to Moscow is about , so Skybolt would allow the V-bomber force to attack Russia from sites not far off the British coast, with complete impunity. Skybolt seemed like such a good solution that work on both Blue Steel II and Blue Streak were cancelled.
Skybolt development was cancelled in early 1962 and led to a major political row. Finally a compromise was reached later that year; the Nassau Agreement had the Royal Navy taking over the deterrent role with newly acquired Polaris missiles. This arrangement was formally outlined in the Polaris Sales Agreement.
One key part of the agreement was that the UK would develop their own warheads for the missiles, as the UK military and political establishments were rather worried about losing their own nuclear production and design capability. Having already put some effort into the Skybolt warhead, it was decided to adapt this design, based on the thermonuclear secondary of the US W59, matched to a primary trigger based on a wholly UK design developed from the Cleo device tested at the Pampas instead of the W58 used on US Navy missiles.
The ABM problem
Throughout this period both the US and USSR were working on anti-ballistic missile systems. Developing an ABM system was largely an engineering issue, requiring radars with high resolution and targeting computers that could adjust trajectories fast enough given an approach speed in thousands of miles an hour. Given that these items would be expensive, the ABM interceptor itself was likely to cost about the same as an ICBM (or less), so any increase in the enemy's stockpile could be countered for an additional expenditure of about the same amount. This was a game the US was clearly willing to play, although it would seem the USSR was considerably less capable of doing so given the size of the two countries' respective economies.
The Soviets were confirmed as working on an ABM system in 1961 when they made their first successful exo-atmospheric interception of an ICBM. An American response was to develop "Antelope", a system designed to overwhelm an ABM defence with decoys, or penaids (penetration aids). In the end the US abandoned Antelope because they could achieve the same ends simply by throwing additional warheads, which was becoming increasingly inexpensive with the introduction of MIRVs. With MIRV, a single missile could launch multiple warheads that separated in space, forcing the defender to use multiple ABMs to attack them. A single new ballistic missile would demand multiple ABMs to counter it, this cost-exchange ratio ultimately convincing the US that ABMs were infeasible.
This development left the British in a particularly bad position. Although the Polaris was fairly immune to direct attack in the submerged ballistic missile submarines, this meant little if the warheads could not make it through the Soviet defences – especially if the Soviets were convinced of this. Although Polaris did have multiple warheads, they were not MIRV - all three were released on the same trajectory and remained fairly close together during atmospheric entry. This meant that all three could be attacked by a single ABM with a large warhead. A single with 16 missiles would throw a total of 48 warheads, making it conceivable that many of them might be destroyed by even a small number of ABMs. This meant the fleet could no longer guarantee the success of the deterrent's objective – to threaten Soviet state power.
Something had to be done in order to maintain the relevance of the UK's nuclear deterrent. Although developments in the ABM field had been watched throughout the 1960s, exactly what to do about the problem apparently remained a fairly low priority. In 1967 the US offered a newer version of the Polaris, the A3T design, which featured a "hardened" missile airframe intended to better protect it against ABMs. As the UK had not yet received their missiles, they agreed to use the A3T version as it required relatively few other changes.
KH.793
In 1970 serious efforts to explore the ABM problem started. By this point the US and USSR had agreed in the ABM Treaty to deploy up to 100 ABMs at only two sites. MIRVs had so seriously upset the balance between ABM and ICBM that both parties agreed to limit ABM deployment largely as a way of avoiding a massive buildup of new ICBMs. The only good news for the UK in this development was that it clearly defined the problem they were facing; their attacks had to be able to credibly defeat a 100-interceptor ABM defence around Moscow. Thus started project KH.793, a one-year project to identify potential solutions.
One option would be to build additional Polaris platforms and keep more of them at sea. Two Resolutions would throw 96 warheads and almost guarantee penetration, while three would make it a certainty. This would require a fleet of at least five submarines to keep two on station at all times, as well as larger crew complements, training and logistics support, and appeared to be the most expensive option.
Several "low cost" options were also explored. Among these were "Topsy", the A3T missile the UK had already agreed to use. Another option was Antelope, which used hardened warheads along with penaids, although it reduced the payload to two warheads in order to save weight for the penaids. They also explored a "superhardened" version known as Super Antelope, a further improvement on the warhead that also used a manoeuvrable warhead "bus" to deploy the penaids further apart in space.
The Royal Navy preferred to upgrade to the Poseidon missile, increasing the number of warheads from three per missile to between ten and thirteen of a newer and lighter design. In this case a single Resolution could launch up to 208 warheads, guaranteeing that some would get through. This option also had the advantage of maintaining commonality with the US Navy, as well as offering greater range, and thus increasing the safety of the launch submarines.
The US also favoured the Poseidon, although for more technical reasons. They felt that while the decoy approach was useful against near-term ABMs deployed by the USSR, that it was considerably less useful against "point defence" type interceptors. This is because the decoys are so much lighter than warheads; when they started to hit the upper atmosphere the decoys would slow more than the warheads and thus "declutter", allowing the warheads to be attacked. This would require a much faster-reacting radar and computer system than an ABM using a long-range interceptor, one that could safely wait until only a few moments before detonation, but this was by no means impossible and was part of the US's own ABM systems.
As the UK had decided to produce its own warheads and bus, this option would require considerable development effort in order to produce a new MIRV bus of their own. Although in theory the UK might be able to use the US-designed bus, these options were being explored in the midst of the ABM Treaty, and it was not clear whether the treaty might forbid this technology being transferred. Some related options were explored, including "Option M" which used a simple "de-MIRVed" bus, "Hybrid" (or "Stag") which put the newer Poseidon warheads on the existing Polaris A3T missiles, and "Mini Poseidon", a similar adaptation with a smaller six-Poseidon-warhead payload on the A3T.
In the end the higher levels of the British political system decided against the urgings of their own Chiefs of Staff and went with the penaid approach on the existing A3T missile. This decision was made official late in 1973 by the Edward Heath administration, which changed the name from Super Antelope to Chevaline. The name 'Chevaline' was the result of a telephone call to London Zoo from an official at the Ministry of Defence. Prompted by a request for a name-change from his boss, the Secretary of State for Defence, Lord Carrington, the official asked the zoo to 'imagine an animal like a large antelope' and inquired whether there was such a beast. The zoo told him there was a South African creature called a Chevaline, and the official 'thought that sounded rather good'.Peter Hennessy, Cabinets And The Bomb p.278. Oxford University Press, 2007 (The creature to which the zoo was probably referring was the Roan Antelope, Hippotragus equinus which is known in French as 'Antilope chevaline'.)
Initially it was planned to make the project public, but a change of government in the 1974 elections ended these plans. A new review again concluded that the project should continue, even though the naval staff continued to be opposed. Full development started in January 1975.
Development