Sentinel program

Sentinel was a proposed US Army anti-ballistic missile (ABM) system designed to provide a light layer of protection over the entire United States, able to defend against small ICBM strikes like those expected from China, or accidental launches from the USSR or other states. The system would have seventeen bases, each centered on its Missile Site Radar (MSR) and a computerized command center buried below it. The system was supported by a string of five long-range Perimeter Acquisition Radars (PAR) spread across the US/Canada border area and another in Alaska. The primary weapon was the long range Spartan missile, with short range Sprint missiles providing additional protection near US ICBM fields and PAR sites. The system would initially have a total of 480 Spartan and 192 Sprint missiles.

Sentinel was a response to the rapidly rising costs of the earlier Nike-X concept. Nike-X was designed to handle full out attacks by the Soviet ICBM force, stockpiling more interceptors than the Soviets had ICBMs. As the number of Soviet ICBMs grew, the number of interceptor missiles required to maintain the defense soared. Calculations suggested it would cost twenty times as much to defend against the Soviet missiles as it cost the Soviets to build them. Robert McNamara felt that deploying Nike-X would prompt the Soviets to produce more missiles, and thereby increase the odds of an accidental war.

Although these problems were well known, the Johnson administration was under intense political pressure to deploy an ABM system, especially as the Soviets were known to be building one of their own. McNamara spoke in public several times to explain why Nike-X was not worth deploying, but the pressure continued to build and Congress voted to provide deployment funding over his wishes. When the Chinese exploded their first H-bomb in 1967, McNamara proposed a building a limited deployment that would primarily be a system to defend to a limited Chinese attack. This eased pressure to deploy a larger system, while also keeping costs under control. Sentinel was announced on 18 September 1967, and construction on the first Sentinel base outside Boston started in 1968.

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