АҚШ-тың "Sprint" зымырандық қорғаныс жүйесі

The Sprint was a two stage, solid fuel anti ballistic missile (ABM), armed with a W66 enhanced radiation thermonuclear warhead used by the United States Army during 1975–76. It was designed to intercept incoming reentry vehicles (RV) after they had descended below an altitude of about , where the thickening air stripped away any decoys or radar reflectors and exposed the RV to observation by radar. As the RV would be traveling at about , Sprint needed to have phenomenal performance to achieve an interception in the few seconds before the RV reached its target. Sprint accelerated at 100g, reaching a speed of in 5 seconds. Such a high velocity at relatively low altitudes created skin temperatures up to , requiring an ablative shield to dissipate the heat. The high temperature caused a plasma to form around the missile, requiring extremely powerful radio signals to reach it for guidance. The missile glowed bright white as it flew. Sprint was the centerpiece of the Nike X system, which concentrated on placing bases around large cities to intercept Soviet warheads. The cost of such a system quickly became untenable as the Soviets added more ICBMs to their fleet, and Nike X was abandoned. In its place came the Sentinel program, which used Sprint as a last ditch defense against RVs that evaded the much longer ranged LIM 49 Spartan. Sentinel was itself changed to become the Safeguard Program, which was operational only for a few months from October 1975 to early 1976. Congressional opposition and high costs linked to its questionable economics and efficacy against the then emerging MIRV warheads of the Soviet Union, resulted in a very short operational period. During the early 1970s, some work was carried out on an improved Sprint II, which was mostly concerned with the guidance systems. These were to be dedicated to the task of protecting the Minuteman missile fields. Further work was canceled as US ABM policy changed.

The US Army had considered the issue of shooting down theater ballistic missiles of the V 2 missile type as early as the mid 1940s. Early studies suggested their short flight times, on the order of 5 minutes, would make it difficult to detect, track, and shoot at these weapons. In theory, it would be easier to attack intercontinental ballistic missiles, with their longer flight times and higher trajectories. In 1955, the Army gave Bell Labs, who had developed the earlier Nike missiles, a contract to study the ABM issue. They returned a report saying the concept was within the state of the art and could be built using modest upgrades to the latest Army surface to air missile, the Nike Hercules. The main technological issues would be the need for extremely powerful radars that could detect the incoming ICBM warheads long enough in advance to fire on them, and computers with enough speed to develop tracks for the targets in engagements that lasted seconds. Bell began development of what became Nike Zeus in 1956, working out of the Nike development center at Redstone Arsenal. The program went fairly smoothly, and the first tests were carried out in the summer of 1959. By 1962, a complete Zeus base had been built on Kwajalein Island and proved very successful over the following year, successfully intercepting test warheads and even low flying satellites.

During the period Zeus was being developed, several problems arose that appeared to make it trivially easy to defeat. The simplest was that its 1950s era mechanical radars could track a limited number of targets, and it could be easily overwhelmed by numbers; a report by the Gaither Committee suggested a salvo of four warheads would have a 90% chance of destroying a Zeus base. This was of little concern during early development when ICBMs were enormously expensive, but as their cost fell and the Soviets claimed to be turning them out "like sausages", this became a serious problem. However, other issues also became obvious in the late 1950s. One issue was that nuclear explosions in space had been tested in 1958 and found that they blanketed a huge area with radiation that blocked radar signals above about altitude. By exploding a single warhead above the Zeus sites, the Soviets could block observation of following warheads until they were too close to attack. Another simple measure would be to pack radar reflectors in with the warhead, presenting many false targets on the radar screens that cluttered the displays. As the problems piled up, the Secretary of Defense Neil H. McElroy asked ARPA to study the anti missile concept. ARPA noted that both the radar decoys and high altitude explosions stopped working in the thickening lower atmosphere. If one simply waited until the warheads descended below about 60  km, they could be easily picked out on radar again. However, as the warheads would be moving at about at this point, they were only seconds from their targets. An extremely high speed missile would be needed to attack them during this period.

The result of the ARPA study came at the height of the debate over the Zeus system in the early 1960s. The new Secretary of Defense, Robert McNamara, convinced President Kennedy that Zeus was simply not worth deploying. He suggested using the funds allocated to its deployment to develop the ARPA system, which became known as Nike X, a name given by engineering professor Jack Ruina when he was reporting on the concept. Nike X required great improvements in radars, computers, and especially the missile. Zeus had an attack profile lasting about a minute; Nike X's interceptions would last about five seconds.

Work on initial investigations into "Follow On Sprint" was underway in the second quarter of 1968. Los Alamos were examining two warheads for the Upstage II design variation. By third quarter 1971, Sprint II was incorporated into a new module for Safeguard called Hardsite Defense (HSD) and a joint Atomic Energy Commission/DoD working group was examining new warheads that would require less tritium. HSD was described as:

text= [consisting] of an autonomous module for close in, low altitude intercept (≈10,000 to 30,000 ft) and is based upon three radar/data processor units located about 10 nautical miles apart. The module will have six or seven firing sites containing about 100 modified Sprint interceptors to defend approximately 21 silos. By the first quarter of 1972, the system was renamed Site Defense and its purpose was to defend Minuteman silos. Over the original Sprint missile, the Sprint II interceptor had slightly reduced launch dispersion, increased hardness to the effect of nuclear weapons, and decreased miss distance. Los Alamos staff expected a request for warhead development sometime in FY 1972 1974. A Phase 2 feasibility study report was completed by Los Alamos in third quarter 1972 and investigations into warhead design continued into first quarter 1973. It is unclear when Sprint II was canceled; however, a report on Sprint II electrical connectors was published in April 1977.