Sentry, known for most of its lifetime as LoADS for Low Altitude Defense System, was a short-range anti-ballistic missile (ABM) design made by the US Army during the 1970s. It was proposed as a defensive weapon that would be used in concert with the MX missile, a US Air Force ICBM that was under development.
The LoADS concept was one of a number of proposals that were made as part of a larger and acrimonious debate over the best way to base the MX missile. It was believed that by about 1980 Soviet ICBMs would improve to the point where it was possible to attack US ICBMs while they were still in their silos. A number of warfighting scenarios suggested that a surprise attack would significantly reduce the US stockpile and greatly blunt any counterattack. In the case of the MX, a single successful strike on a silo would mean ten warheads would not reach the USSR, making these extremely valuable targets. To ensure such an attack would fail, at least to the point where the Mutual Assured Destruction (MAD) would be maintained, a wide variety of basing options were considered that would guarantee the survival of at least dozens of MX's.
One way to improve the survival of the ICBM force would be to actively defend it with an ABM system. However, the 1972 ABM treaty greatly limited the number and geographical deployment of any ABM, with the aim of preventing whole-country protection and thus ensuring MAD. LoADS addressed these limitations by being deployed along with the radars and engagement computers needed to make a successful attack at only very short ranges, or less. The entire system was packed into a cylinder that looked like an MX and would be shuffled between the silos. As the LoADS could be in any of the silos at a particular missile site, the enemy would have to expend two warheads on every silo to ensure a hit, as one would be assumed lost to LoADS. This would not stop a successful counterforce attack, but it would make it significantly more expensive in terms of the number of warheads used, potentially requiring more warheads than the Soviets had.
With President Jimmy Carter's decision to base the MX in a series of less-hardened horizontal silos in 1977, LoADS development was accelerated. The system as a whole was renamed Sentry, with the missile becoming the Baseline Terminal Defense System, or BTDS. Work on MX and Sentry was further accelerated by incoming President Ronald Reagan in 1981. However, after another review of the MX program, Reagan chose a different MX deployment concept and cancelled Sentry, stating that it would violate the ABM treaty. As the questionable security of the MX once again became an issue, Reagan briefly entertained an even shorter-range system known as Swarmjet, before the entire MX program was severely curtailed with the ending of the Cold War.
Background ICBM vulnerability Early ICBMs had accuracy on the order of , which made them useful against only the largest targets. Through the 1960s this was progressively improved, and the US estimated that the Soviets would have "substantial" capability of directly attacking US missile silos by the 1980 time frame.
This led to a worrying scenario coming to dominate US strategic thinking through the 1970s and 80s. If the Soviets launched a sneak attack on the US using only a portion of their missile fleet, about , aimed solely at US silos and airbases, they could hope to destroy as much as 90% of the US's fleet. This attack would not kill many civilians, and leave the US with only a few ICBMs and bombers, along with the US Navy's Submarine-launched ballistic missile (SLBM) fleet. However, the SLBMs of that era were not accurate enough to attack missile silos, the US would not be able to respond with a counterforce attack against the substantial numbers of remaining Soviet ICBMs. The only option left would be a general attack on Soviet cities, which would cause the Soviets to respond in kind with their remaining fleet. In this situation, the Soviets would be in an extremely advantageous position for a negotiated peace.
When faced with such an attack, it might not be immediately clear what the targets are. The only way to be sure would be to wait until well into the attack and then respond in kind. This took long enough that some portion of the US fleet would be caught on the ground, and this "retaliation after ride-out" concept was considered too risky. Instead, the US policy moved to launch on warning, where they would respond to an attack as soon as it was clear one was in the making. This was not ideal, as it required them to maintain a high alert level that increased the risk of an accidental launch.
Missile, Experimental In 1964 the Air Force issued a contract to TRW to consider the problem of ICBM survivability under the name "Golden Arrow", looking for ways to make their own missiles as invulnerable as the Navy's. Ideally, the US ICBM fleet should be able to ride out an attack and then launch a response, which would require as many as 1,000 warheads, one for each Soviet ICBM silo. Not only did they have to survive, but they had to have the ability to be quickly retargeted so the surviving US missiles could be aimed only at the remaining Soviet missiles, not empty silos.
One early conclusion was that it was possible to build missile silos that would survive any conceivable warhead exploding at a distance of . It was assumed that Soviet warheads during the late 1970s would have accuracy better than one mile, so the question became how to ensure the warheads would not approach within that distance. Among the many solutions studied, one quickly rose to the top, the idea of valley basing. This would rely on the physical barrier presented by surrounding hills or mountains to keep warheads away from the silos in the valleys between them. Warheads approach on a shallow trajectory, so mountains or mesas with steep sides would mean the warhead would impact the terrain before reaching the silo.
TRW suggested building three bases with thirty missiles each, and estimated that one of the three bases would survive an all-out counterforce attack. To ensure that 600 warheads would survive, each missile would have to carry about 20 warheads. These studies led to the Missile, Experimental, or MX, project, which started in 1971. The design rapidly firmed up, emerging with ten-warheads on more missiles in order to raise the survivors to 1,000 warheads. But the basing became not so much a specific concept as a catch-all program for every possible concept, from railway deployments, to underground racetracks, to mobile systems. A review of the options written in the mid-1970s ran into hundreds of pages.
Hardpoint and Hardsite
Through the 1950s and 60s, the US Army had been developing ABM systems as a defense against the ICBM. At first these were tasked with countering a force of dozens or maybe 100 warheads, targeted against US cities and large industrial sites. This was seen as a relatively simple task not unlike the Army's existing anti-aircraft role. However, as the numbers of ICBMs in the Soviet forces rapidly grew, the price of implementing such a system quickly ran out of control, reaching $40 billion ($ billion in ), a significant amount of the total budget of the US Department of Defense.
ARPA was asked to consider the problem of using the technology in a cost-effective way, and noticed one possible application. As Soviet warheads were accurate to within only a few kilometers, and they had to fall within less than a kilometer to kill a missile silo, that meant the Soviets would have to launch several warheads at every US silo in a counterforce attack. The defender had a significant advantage here; they could watch the incoming warheads, see if any were going to come within the lethal range of a silo, and then shoot just those ones. This meant a small number of ABMs would be effective against a huge number of Soviet warheads.
ARPA began researching such a system under the name Hardpoint. To ensure they shot only at valid targets, it had to generate very accurate tracks for the enemy warheads. They also had only seconds to develop that track and launch a missile, assuming that the air would be filled with nuclear explosions that would render higher altitudes opaque to radar, an effect known as nuclear blackout. They began development of a new radar known as the Hardpoint Demonstration Array Radar, or HAPDAR, and a new extremely high-speed missile known as HIBEX. The entire engagement would last just over two seconds, with the intercept taking place at about altitude. They later added a second stage to HIBEX known as UPSTAGE, demonstrating over 300 g of lateral acceleration, allowing it to counter maneuvering warheads.
ARPA released its Hardpoint study in 1965. It presented a way for the Army to offer an effective ABM system at a reasonable cost. At the same time, it would ensure that the Air Force's missiles would survive any sort of small-scale attack envisioned by the military planners. This led to a series of Army-Air Force studies under the name Hardsite. Hardsite initially considered the idea of using the existing Nike-X hardware being proposed around large cities to deliberately offer additional protection to Air Force and other military bases in the area. A second series of studies considered cut-down deployments to protect remote bases, like the missile fields. This later concept quickly captured the majority of follow-up work.
ABM treaty Robert McNamara felt that deploying an ABM system would simply prompt the Soviets to build more ICBMs to overwhelm it, leading to a new arms race. He also felt that more missiles meant more chances for an accidental launch, which would actually increase the chance of a war and lower overall security. McNamara constantly delaying deployment of Nike-X, but faced increasing criticism for doing so. When the Chinese tested their first H-bomb in 1967, McNamara announced a much smaller deployment known as Sentinel as a "Chinese oriented" system.
The President also offered to hold off construction on Sentinel if the Soviets did the same. As the Soviets continued construction of their ABM at Moscow, the President ordered the construction of the first Sentinel site outside Boston. This was met by a firestorm of protest on the part of local citizens who were not happy about a nuclear missile base being built literally in their back yard.
The incoming President Nixon was forced to realign the system to something more similar to Hardsite, using the Nike-X systems for the defense of the Minuteman fields in a new system known as Safeguard. This placed the launch sites in distant locations that were less controversial.
In the midst of construction of the first two Safeguard sites, the Soviets returned to the negotiations. This led to the 1972 Anti-Ballistic Missile Treaty, limiting each side to a total of 100 interceptor missiles located in up to two ABM bases, and amending that to only one base in 1974. The Soviets chose to complete their system around Moscow while the US continued with construction of one of the Safeguard sites outside Grand Forks, North Dakota. The argument for and against the Safeguard system continued both in the military and public arenas, and the system was ultimately shut down in 1975 after only one day of official operation.
LoADS Continuing work