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 of thousands of missiles, 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 detonated their first H-bomb in 1967, McNamara proposed building a limited deployment that would primarily be a system to defend to a limited Chinese attack. This eased the 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.
By the time Richard Nixon took office in January 1969, public opinion had swung strongly against ABMs. Residents of the cities to be protected protested that it simply made them targets for more Soviet bombs, and there were a number of well organized public demonstrations against the system. Nixon ordered a review that suggested sweeping changes to the system, and the Sentinel program was cancelled in March 1969 after only 18 months of existence. In its place, an even lighter system intended primarily to defend USAF missile bases was introduced, the Safeguard Program.
History
Nike Zeus
The US Army began ABM research in 1955, after a Bell Labs report concluded that modern computers, radars, and missile systems had improved to the point where attacks on ICBM reentry vehicles (RVs) were a possibility. The task is not trivial; RVs are travelling at about per second and have a small radar cross section, perhaps only . Bell concluded that the primary issue would be detecting the RVs early enough to leave enough time for the missile to climb to its altitude. The Army began work on a system under the name Nike II, but later changed the name to Nike Zeus.
When Nikita Khrushchev claimed to be building ICBMs "like sausages", the US feared that a missile gap would form. For a period, the Soviets would have enough missiles to attack the USAF's bomber bases, while the US's own missile force was not large enough, by itself, to be an effective deterrent. The Gaither Report suggested that preventing this sort of attack was a top priority, and suggested that Zeus be deployed as quickly as possible to protect the bombers. Zeus development was accelerated in January 1958, and granted the highest national development priority.
By 1961, when John F. Kennedy took office, the system was in the testing phase, and deployment was planned for 1963. By this time, a number of serious technical problems with Zeus had become clear. One was that the Zeus radars had no way to distinguish between the RVs and radar decoys travelling together in an extended threat tube. The Army predicted that as many as twenty Zeus missiles would have to be launched to hit the RV hiding among the decoys. However, Zeus was not actually capable of doing this; Zeus could only track one target per radar, and most sites would have only two to four radars. A salvo of four ICBMs, or a single ICBM with four credible decoys, was almost certain to destroy the Zeus base.
Nike-X
A number of technical reviews from the late 1950s and early 1960s all concluded that Zeus would offer little protection against a large attack. McNamara asked ARPA to study the system, and they outlined a number of potential development paths. McNamara cancelled development of Zeus on 5 January 1963, and announced that the money would be directed to the development of a new concept, Nike-X.
Nike-X addressed Zeus' problems in two ways. The first was to replace Zeus' mechanical radar systems with an active electronically scanned array, which would allow it to track hundreds of targets at once, both the incoming ICBMs as well as the outgoing interceptors. The second dealt with decoys; Nike-X would wait until the RVs began to reenter the lower atmosphere, at which point the lighter decoys would quickly slow down and reveal the RV hiding among them. A very high-speed missile would then attack them in the few seconds between this decluttering and the enemy warhead triggering, at altitudes as low as .
By 1965 the Nike-X system was itself well into its testing phase when the cost of implementing any sort of reasonable protection became a serious problem. The baseline system required 7,000 Sprint missiles and would cost $40 billion ($ billion in , about ½ the annual military budget). Yet even with this system, as much as 30% of the US population was expected to die in a full exchange. McNamara noted that protecting 70% of the population could also be achieved by building fallout shelters, and this would cost far less than Nike-X. He refused to deploy unless funds were also provided for shelters.
Another major problem for Nike-X was the Soviet's own ABM system. The Soviet system was generally similar to Zeus, so the Air Force responded by adding MIRV warheads to their Minuteman missiles to overwhelm it. The Air Force noted that the Soviets could do the same to Nike-X; it cost much more to defend against additional Soviet warheads than build the warheads. McNamara feared that deploying any ABM would lead to another arms race, one that would increase the odds of an accidental war.
New concept
In 1962 the US carried out a series of high-altitude nuclear tests, notably Starfish Prime, which demonstrated that the burst of x-rays released by a warhead can travel long distances. In the lower atmosphere they travel only a few meters before interacting with the air, which is one of the major mechanisms creating the fireball that forms around the explosion. In space, their mean free path was on the order of tens of kilometers. When the x-rays strike solid material they cause it to heat so rapidly that shock waves form. These can be powerful enough to break up an RV's heat shield or cause it to separate from the airframe. In March 1965, Bell was given the go-ahead to develop a new Zeus design based on this concept, and entered detailed planning in October.
The original Zeus had to maneuver within about for its neutron heating mechanism to safely guarantee the enemy warhead would be disabled. Due to the limitations of the angular resolution of the radars, this limited the range of the engagement to about , although the missile itself was capable of much greater range, as much as .
With the new warhead concept, the lethal range was greatly extended, potentially as great as several kilometers. Since the threat tube of the RV and decoys was perhaps a kilometer across, that meant that a single missile could kill the warhead even if it was completely surrounded by decoys and chaff, although more than one might be needed to cover the length of the tube. And since the required accuracy was reduced by an order of magnitude, the radars could provide the guidance over a much greater range, potentially out to the radar horizon.
In keeping with the desire to develop the new design as rapidly as possible, Zeus EX was designed to provide about range, about the limit that could be provided by adapting the existing Zeus design. Much of the extra range was accomplished with slight improvements in the fuels, slightly larger motors, and different flight paths that flew out of the atmosphere earlier to reduce drag. Additional range was provided by using the third stage motors, originally intended for last-second maneuvering, as an additional booster. With the accuracy requirements lowered, these last second adjustments were no longer needed.
Smaller systems Throughout the development of the Nike-X system, the Army, Air Force and ARPA repeatedly examined smaller deployments with more limited goals.
One concept, initially proposed by ARPA as Hardpoint, was a small Nike-X system placed close to USAF missile bases. The idea was to protect them from any sort of limited attack; the Soviets could overwhelm the system, but only by using up a large portion of their forces. The Air Force and Army were initially interested and collaborated on follow-up studies under the name Hardsite. Over time the Air Force soured on the idea of giving the Army any strategic role, and proposed that the funding instead be used to build missile silos in hard rock, which would have the same effect in terms of protecting the missiles but for far less money.
Another problem that arose was that Nike-X ignored the defence of smaller cities, whose politicians then complained that they would become prime targets for the Soviets. ARPA responded by introducing the Small City Defense concept (SCD). Unlike Nike-X, the SCD radars did not have the ability to detect the warheads at long range, so some other early warning system would be needed. This led to the development of the low-cost Perimeter Acquisition Radar (PAR), whose function was to measure the threat tubes while they were still minutes away, determine their targets, and warn the appropriate SCDs.
In February 1965 the Army asked Bell to consider a very light version of Nike-X intended to provide protection only under the limited attacks by smaller nuclear-armed countries, the so-called Nth Country concept. Bell combined the radars and computers from the SCD concept with Zeus EX, which pushed the engagement range out to hundreds of miles. This allowed a single base to provide coverage over multi-state areas, although it lost the advantages of Sprint in terms of nuclear blackout avoidance. For a system limited to the most basic attacks, the new design could provide all-country defense for a reasonable cost.
Continued debate on Nike-X
By 1966 ABM development in the US had been a high priority project for a decade, and had constantly found itself at the disadvantage compared to improving ICBMs. In spite of its tremendous technical advances, Nike-X was no more able to protect the US population than the Zeus had been in the 1950s. These problems were widely known, but there was intense political pressure to deploy a system anyway. McNamara had opposed deployment throughout. Things came to a head in 1966, when McNamara once again refused to begin construction but the Senate Armed Services Committee provided $167.9 million ($ million today) anyway.