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.

[2][3] The high temperature caused a plasma to form around the missile, requiring extremely powerful radio signals to reach it for guidance.

Congressional opposition[4] and high costs linked to its questionable economics[5] and efficacy against the then emerging MIRV warheads of the Soviet Union, resulted in a very short operational period.

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.

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.

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.

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 60 kilometers (37 mi) altitude.

[1] 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.

[7] 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.

[8] HSD was described as:[8] ... [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.

[9] A Phase 2 feasibility study report[10] was completed by Los Alamos in third-quarter 1972 and investigations into warhead design continued into first-quarter 1973.

[12][13] The Sprint was controlled by ground-based radio command guidance, which tracked the incoming reentry vehicles with phased array radar and guided the missile to its target.