Defending an area against incoming missiles is a difficult task. Missiles are incredibly fast and present a small target. Assuming you know they’re coming, you have to be able to track them accurately if you’re to have any hope of stopping them. Then, you need some kind of wonderous missile of your own that’s fast enough and maneuverable enough to take them out.

It’s a task that at times can seem overwhelmingly impossible. And yet, the devastating consequences of a potential nuclear attack are so great that the US military had a red hot go anyway. In the 1970s, America’s best attempt to thwart incoming Soviet ICBMs led to the development of the Sprint ABM—a missile made up entirely of improbable numbers.

Mach 10? You Betcha

The Sprint anti-ballistic missile was an engineering effort in response to the nuclear threat posed by the Cold War. This missile, with its astonishing performance and parameters, was designed to intercept incoming ballistic missiles during their terminal phase in the moments before impact.

Despite its crucial task, and its impressive capabilities, the Sprint missile had a relatively short operational life, a reflection of the rapidly evolving strategic landscape of the time.

Developed in the late 1960s as part of the United States Army’s Safeguard Program, the Sprint was a key component of a layered missile defense system intended to protect against Soviet intercontinental ballistic missiles (ICBMs). It was intended to operate in tandem with the longer-range LIM-49 Spartan missile. The Spartan was designed to engage threats outside the atmosphere, with Sprint missiles addressing those that penetrated further inward.

The technical specifications of the Sprint missile were nothing short of remarkable. Powered by a solid-fuel rocket, it could accelerate to a speed of Mach 10 within 5 seconds of launch, covering an altitude of 30 kilometers in roughly 15 seconds. That’s 12,000 km/h for those of you playing along at home. This incredible acceleration of roughly 100 G was necessary to intercept ICBM warheads re-entering the Earth’s atmosphere at high velocities.

Sprint had to launch so quickly that there was no time to open hatches or silo doors. Instead, Sprint was designed to be ejected from its launch bay via an explosively-driven piston which punched the conical missile straight through the fiberglass cover on its silo. The first stage solid rocket fired for just 1.2 seconds, disintegrating shortly after due to the intense aerodynamic forces on the airframe. The second stage would fire shortly afterwards, boosting Sprint to an interception at altitudes between 1.5 to 30 km. Total flight and interception time was intended to be on the order of 15 seconds.

The immense speed of Sprint posed multiple engineering challenges. The missile’s skin was designed to withstand temperatures of up to 6,200 degrees Fahrenheit due to air friction at these speeds, with a special ablative coating to prevent it from burning up in flight.

One of the most notable features of the Sprint was its guidance system. It used a unique ground-based phased array radar system that could track incoming warheads and guide the missile to its target with phenomenal precision. This system allowed for mid-course corrections in the missile’s trajectory, a critical capability given the high speeds and short reaction times involved. However, the need for communication with the ground was a challenge, given Sprint’s intense speed. The friction with the air and the resulting intense heat tended to create a plasma around the missile, which made radio communication difficult. Incredibly powerful radio signals were required to penetrate the plasma and exhaust plume of the missile.

Sprint was not a hit-to-kill vehicle. With ICBMs incoming at even higher Mach numbers than Sprint itself, just getting close to an incoming missile was an engineering feat at the very edge of possibility with the prevailing technology. Sprint made up for this with the warhead it used to destroy incoming missiles—a nuclear one, in fact. Each Sprint missile mounted a 1-kiloton W-66 “enhanced radiation” warhead. These warheads were specially designed to not just destroy incoming missiles with blast effects, but with intense neutron flux from the nuclear fission reaction.

Detonating nuclear warheads over your own soil might seem reckless in the extreme, but it was the Cold War. It was deemed highly favorable to use small warheads high in the atmosphere for defence, versus having enemy weapons in the megaton-range destroying entire cities on the ground.

Despite its advanced capabilities, the Sprint missile was in service for a relatively short period, from 1975 to 1976, as part of the Safeguard Program. There were several reasons for this brief operational life. First, the strategic arms limitation talks (SALT) between the United States and the Soviet Union led to treaties that limited the development and deployment of anti-ballistic missile systems, including the Safeguard Program. These defensive systems were considered a threat to the delicate balance between the two superpowers. Without viable defences against ICBM attacks, each power could be reasonably assured of its own destruction if it chose to fire its own missiles. Having a working ABM system would allow one side the ability to strike without fearing retaliation, ruining the “safe” concept of mutually assured destruction (commonly referred to as MAD).

Additionally, the high cost of deployment and maintenance of such a complex system, combined with rapid technological advancements in offensive missile technology, made the Sprint system appear less cost-effective and strategically viable over time. Questions arose around whether a defence system based around Sprint could reasonably expect to counter Soviet missiles deploying multiple independent reentry vehicles, which could allow one missile to deliver up to 10 warheads on independent trajectories.

The end of the Sprint missile’s service did not necessarily signify a failure, unless one considers huge expenditure for little end product a failure. Ultimately, it demonstrated the challenges of developing defensive systems in the nuclear age and the dynamic nature of military technology and strategy.

The Sprint missile demonstrated the technical feasibility of intercepting ICBMs during re-entry, a concept that has continued to influence missile defense strategies to this day. However, it couldn’t get around the ultimate concept that doomed many anti-ballistic missile defence schemes. While one can hope to intercept one ICBM, or even a handful, an attacker only needs to increase their number of missiles by a small amount to rapidly increase the numbers of intercepters required by a defender.

In retrospect, the Sprint anti-ballistic missile represents a fascinating moment in Cold War history; Fears around national security drove the development of a missile of truly wild performance. And yet, at the same time, it proved largely useless for its intended mission. The sheer scale of the potential conflict it was built for overwhelmed its very purpose.

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• Source: https://hackaday.com/2024/02/13/sprint-the-mach-10-magic-missile-that-wasnt-magic-enough/