Sentry program
Sentry, known for most of its lifetime as LoADS for Low Altitude Defense System,[a] was a short-range anti-ballistic missile (ABM) design made by the US Army during the 1970s.
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.
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.
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, 50,000 feet (15 km) or less.
With President Jimmy Carter's decision to base the MX in a series of less-hardened horizontal silos in 1977, LoADS development was accelerated.
Through the 1960s this was progressively improved, and the US estimated that the Soviets would have "substantial" capability[3] of directly attacking US missile silos by the 1980 time frame.
[3] 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.
[7] 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.
[10] One early conclusion was that it was possible to build missile silos that would survive any conceivable warhead exploding at a distance of 1 mile (1.6 km).
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.
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.
[18] 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.
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 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.
With a total of 4,500 shelters and 200 missiles moving between them, the system could soak up a significant portion of the Soviet's 5,928 ICBM warheads and survive.
If the Soviets needed to fire 4,500 warheads to ensure the destruction of the MX force, expending another 100 to soak up an ABM system was of little consequence.
They proposed replacing the shotgun-like projectiles with swarms of small unguided rockets that would fire at a range of about 1 kilometre (0.62 mi).
The first, Project Quick-Shot, was basically identical to the Feld and Tsipis version, although they did consider some sort of low-cost guidance system as well.
[32] The same end could be achieved by simply building more shelters as well, but the size of the proposed MPS network already took up large portions of Nevada and Utah, and adding more would not be easy.
LoADS had a radar with each missile, but these were very short-range and could not be considered part of a wide-area system, although observers suggested the Soviets would claim otherwise.
[1] More importantly, the 1974 amendment to the treaty required those radars to be in the vicinity of Grand Forks, which would not help MPS, which was located well over 1,000 miles (1,600 km) to the southwest.
The Army thus developed LoADS strictly as a technology demonstration program, with the option of rapid construction in case the ABM treaty ended.
[36] Seeking to lower the spiralling cost of the MX system, which had reached at least $35 billion by the end of the 1970s,[37] Jimmy Carter announced a greatly simplified deployment using simpler silos and reduced number of false launchers.
The launchers changed from vertical to horizontal, which would reduce the cost of building them and make it simpler to rapidly move the missiles in and out of the silos without it being easily seen.
In October 1981 they announced MX deployment would be sped up, cancelling the MPS basing and recommending the missiles temporarily be placed in existing Titan II and Minuteman silos until a final solution could be decided on.
[32][41] Given warning of an attack, the LoADS would be pushed through the roof of the shelter (or raised in the case of a vertical silo) to reveal its radar and begin searching for incoming warheads.
When one was determined to be approaching the location of the MX, LoADS would fire, attacking the warhead at very low altitude, about 40,000 feet (12 km).
