Nike-X

The Sprint missile was the main weapon in the Nike-X system, intercepting enemy ICBM warheads only seconds before they exploded.

Nike-X was an anti-ballistic missile (ABM) system designed by the US Army to protect major cities in the United States from attacks by the Soviet Union's intercontinental ballistic missile (ICBM) fleet during the Cold War. The X in the name referred to its experimental basis, and was supposed to be replaced by a more appropriate name when the system was put into production. This never came to pass; the Nike-X program was canceled and replaced by a much lighter defense system known as Sentinel.

The system was developed in response to the failure of the earlier Nike Zeus system. It was calculated that a salvo of only four ICBMs would have a 90% chance of hitting the Zeus base, whose radars could only track a few objects at the same time. This was fine in the late 1950s when the Soviets had only a few dozen missiles, but by the 1960s it was predicted they would have hundreds and could afford to overwhelm Zeus. The attacker could also use radar reflectors or high-altitude nuclear explosions to obscure the warheads until they were too close to attack, making a single warhead attack highly likely to succeed.

Nike-X addressed these concerns by basing its defense on a very fast, short-range missile known as Sprint. They would wait until the enemy warheads descended below the altitudes at which decoys or explosions had any effect, rapidly determine their trajectory, and attack them. The entire engagement lasted only a few seconds, and could take place as low as 25,000 feet (7,600 m). To provide the needed speed and accuracy, Nike-X used a new radar system and building-filling computers that could track hundreds of objects at once and controlled salvos of many Sprints. Dozens and dozens of ICBMs would need to arrive at the same time in order to overwhelm the system.

A complete deployment would be extremely expensive to build, on the order of the total yearly budget of the Department of Defense. Robert McNamara felt the cost could not be justified and worried it would lead to a further nuclear arms race. He directed the teams to consider deployments where a limited number of interceptors might still be militarily useful. Among these, the I-67 concept suggested building a lightweight defense against very limited attacks. When the People's Republic of China exploded their first H-bomb in 1967, I-67 was promoted as a defense against a Chinese attack, and this system became Sentinel in October. Nike-X development, in its original form, ended.

History

Nike Zeus

The Nike missile family included Ajax (front), Hercules (middle) and Zeus (rear).

In 1955 the U.S. Army began considering the possibility of further upgrading their Nike B surface-to-air missile (SAM) as an anti-ballistic missile (ABM) to intercept ICBMs. Bell Labs, the primary contractor for Nike, was asked to consider the issue. They returned a report stating that the missile could be upgraded to the required performance relatively easily. But the system would need extremely powerful radar systems in order to detect the warhead while it was still far enough away to give the missile time to launch. All of this appeared to be within the state of the art, and in early 1957 Bell was given the go-ahead to develop what was then known as Nike II. Considerable interservice rivalry between the Army and Air Force led to the Nike II being redefined and delayed several times. These barriers were swept aside in 1957 after the launch of the R-7 Semyorka, the first Soviet ICBM. The design was further upgraded, given the name Zeus, and assigned the highest development priority.

Zeus was similar to the two Nike SAM designs that preceded it; it used a long range search radar to pick up targets, separate radars to track the target and interceptor missiles in flight, and a computer to calculate intercept points. The missile itself was much larger than earlier designs, with a range of up to 200 miles (320 km), compared to Hercules' 75 miles (121 km). It flew so fast it burned the outer layer of its skin off while climbing through the lower atmosphere. To ensure a kill at 100,000 feet (30 km) altitude, where there was little atmosphere to carry a shock wave, it mounted a large 400 kilotons of TNT (1,700 TJ) warhead. The search radar was a triangle 120 feet (37 m) wide, able to pick out warheads while still over 600 nautical miles (1,100 km) away, an especially difficult problem given the small size of a typical warhead. A new transistorized digital computer offered the performance needed to calculate trajectories for intercepts against warheads traveling over 5 miles (8.0 km) per second.

Test firings of the missile started in 1959 at White Sands Missile Range (WSMR) and were generally successful. Longer range testing took place at Naval Air Station Point Mugu, firing out over the Pacific Ocean. For full-scale tests, the Army built an entire Zeus base on Kwajalein Island in the Pacific, where it could be tested against ICBMs launched from Vandenberg Air Force Base in California. Test firings at Kwajalein began in June 1962, and were generally very successful, passing within hundreds of yards of the warheads, and even low-flying satellites.

Zeus problems

The Zeus system required two separate radars for each missile it launched, with extras for redundancy.

Zeus had initially been proposed in an era when ICBMs were extremely expensive and the Soviet fleet contained a few dozen missiles. These presented a threat to Strategic Air Command's (SAC) bomber bases, at a time when the US deterrent fleet was based entirely on manned bombers. The primary Zeus deployment concepts were intended to protect against the ICBMs being fired at these bases, or a larger network to defend against attacks with two ICBMs being launched at the largest US cities.

Technological improvements in both warheads and missiles through the late 1950s greatly reduced the cost of ICBMs. During a visit to the US in 1959, Nikita Khrushchev claimed to be building them "like sausages". By the time Zeus could be deployed in the early-to-mid 1960s it was expected it would face hundreds of ICBMs. Zeus used mechanically steered radars, like the Nike SAMs before it, limiting the number of targets it could attack at once. A study by the Weapons Systems Evaluation Group (WSEG) calculated that the Soviets had a 90 percent chance of successfully hitting a Zeus base by firing only four warheads at it. These did not even have to land close to destroy the base; a near miss would destroy its radars and render it impotent.

If this were not enough, a number of technical problems arose that appeared to make the Zeus almost trivially easy to defeat. One problem, discovered in tests during 1958, was that nuclear fireballs expanded to very large sizes at high altitudes, rendering everything behind them invisible to radar. This was known as nuclear blackout. Exploding a single warhead just outside the Zeus' maximum range, or even the explosion of the Zeus' own warhead, would allow warheads following it to approach unseen. By the time the warheads passed through the fireball, about 60 kilometers (37 mi) above the base, it was only about 8 seconds from impact. That was not enough time for the radar to lock on and fire a Zeus before the warhead hit its target.

It was also possible to deploy radar decoys to confuse the defense. Decoys are made of lightweight materials, often strips of aluminum or mylar balloons, which can be packed in with the reentry vehicle (RV), for little additional cost in terms of throw weight. In space, these are ejected to create a threat tube a few kilometers across and tens of kilometers long. Zeus had to get within about 1,000 feet (300 m) to kill a warhead, which could be anywhere in the tube. Zeus' inability to distinguish warheads from high-quality decoys was considered to be a major problem and the WSEG suggested that a single ICBM with decoys would almost certainly defeat Zeus.

The Army calculated that as many as twenty Zeus missiles would have to be fired to ensure a warhead in a typical threat tube would be hit. This meant that every ICBM the Soviets added to their fleet would require at least 20 new Zeus missiles to be built to counter it. But this would only improve the defense at a single site. Since the Soviets could aim that new warhead anywhere, at least in theory, every Zeus base would have to add 20 missiles. The balance was so heavily skewed towards the attacker in this cost-exchange ratio that Zeus was seen as effectively useless.

Nike-X

File:Emblem of the Nike-X Project Office.png

The Nike-X Project Office took over from Nike Zeus in 1964. The office's emblem features the statue of Nike of Samothrace, the Greek goddess of victory.

The Advanced Research Projects Agency (ARPA, today known as DARPA) was formed in 1958 by President Dwight Eisenhower's Secretary of Defense, Neil McElroy, in reaction to Soviet rocketry advances. ARPA was formed to oversee all missile development across the forces, in order to avoid duplicated effort and the huge expenditures that were apparently accomplishing little in comparison to the Soviets. As the problems with Zeus became clear, McElroy asked ARPA to consider the antimissile problem and come up with other solutions. The resulting Project Defender was extremely broad in scope, considering everything from minor upgrades to the Zeus system, to far-out concepts like antigravity and the new laser.

Meanwhile, one improvement to Zeus was already being studied; a new phased-array radar replacing Zeus' mechanical ones would greatly increase the number of targets and interceptors that a single site could handle, although the computers would have to match this performance. Additionally, multiple phased-array antennas could scan the entire sky without moving, so the radar could be hardened to much greater strengths. Known as the Zeus Multi-function Array Radar, or ZMAR, initial studies at Bell Labs started in 1960. In June 1961, Western Electric and Sylvania were selected to build a prototype, with Sperry Rand Univac providing the control computer.

By 1962 a decision on whether or not to deploy Zeus was looming. President John F. Kennedy's Secretary of Defense, Robert McNamara, once again turned to ARPA to study the Zeus system and offer any suggestions they might have to improve its effectiveness. ARPA noted that the problems with Zeus' limited traffic handling capability was already being solved by ZMAR, and pointed out that the problems with blackout and decoys was cleared below about 60 kilometers (37 mi). They proposed combining ZMAR with a new missile with much shorter range and far higher speed, which would attack the warheads after they reappeared on radar. Due to the time needed to develop an accurate track and launch the missile, this meant the interception would take place as low as 20,000 feet (6,100 m) altitude.

The report outlined four possible deployments and what types of attacks they might be used against; the first was a study of the existing Zeus system, the next was Zeus with ZMAR, and finally two systems with the new missile. One was based around the expensive ZMAR, and the other used Zeus' existing long-range radar for initial detection with a less expensive, shorter range version of ZMAR for guidance which would lower total system costs. The system optionally retained Zeus, which could be used in areas away from cities.

In late 1962 the Zeus system was completing its testing, and it was time for a deployment decision. Considering the issues, in January 1963 McNamara announced that the construction funds allocated for Zeus would not be released, and the funding would instead be used for development of the new system. The name Nike-X was apparently an ad hoc suggestion by Jack Ruina, the director of ARPA, who was tasked with presenting the options to the President's Science Advisory Committee (PSAC).

System concept

File:Nike-X deployment concept.gif

This image shows the arrangement of a typical Nike-X deployment. In the foreground is a missile site with a number of Sprint launchers and a two-sided MAR radar. In the background, upper right, is a second base with additional missiles and an MSR radar.

Decoys are lighter than the RV, so they will suffer higher atmospheric drag as they begin to reenter the atmosphere. This will eventually cause the RV to move out in front of the decoys, but the RV can often be picked out earlier by examining the threat tube as a whole and watching for objects that have lower deceleration. This process, known as atmospheric filtering, or more generally, decluttering, will not provide accurate information until the threat tube begins to reenter the denser portions of the atmosphere, at altitudes around 60 kilometers (37 mi). Nike-X intended to wait until the decluttering was complete, meaning the interceptions would take place only seconds before the warheads hit their targets, between 5 and 30 miles (8.0–48.3 km) away from the base.

Low-altitude intercepts would also have the advantage of reducing the problem with nuclear blackout. The lower edge of the extended fireball is also at about 37 miles (60 km) altitude. Operating well below this meant that deliberate attempts to create a blackout would not affect the operation of the Sprint missile. Just as importantly, because the Sprint's own warheads would be going off far below this altitude, their fireballs would be much smaller and only black out a small portion of the sky. The radar would have to survive the electrical effects of blackout and EMP but this was not considered a difficult problem. It also meant that the threat tube trajectories would have to be developed rapidly, before or between blackout periods. This demanded a very high performance computer.

The centerpiece of the Nike-X system was the MAR, the Z having been dropped from the name with the ending of the Zeus program. MAR used the then-new active electronically scanned array (AESA) concept to allow it to generate multiple virtual radar beams, simulating any number of mechanical radars needed. While one beam scanned the sky for new targets, others were formed to examine the threat tubes and generate high-quality tracking information very early in the engagement, additional beams were formed to track the RVs once picked out and still more to track the Sprints on their way to the interceptions. To make all of this work, MAR also required data processing capabilities on an unprecedented level. In the era of individual transistors and small-scale integrated circuits, the computers were huge and expensive. For this reason, Nike-X centralized the battle control systems at their Defense Centers, consisting of a MAR and its associated Defense Center Data Processing System (DCDPS).

Because the Sprint was designed to operate at short range, a single base could not provide protection over a typical US city, given urban sprawl. This required the Sprint launchers to be distributed around the defended area. Because the Sprint might not be visible to the MAR during the initial stages of the launch, Bell proposed building a much simpler radar at most launch sites, the Missile Site Radar (MSR). MSR would have just enough power and logic to generate tracks for its outgoing Sprint missiles, and would hand that information off to the DCDPS over voice quality phone lines. Bell noted that the MSR could also provide a useful second-angle look at threat tubes, which might allow the decoys to be picked out earlier. Used as radio receivers, they could also triangulate any radio broadcasts coming from the threat tube, which the enemy might use as a radar jammer.

When the system was first being proposed it was not clear whether the phased-array systems could provide the accuracy needed to guide the missiles to a successful interception at very long ranges. Early concepts retained Zeus Missile Tracking Radars and Target Tracking Radars (MTRs and TTRs) for this purpose. In the end the new radars proved more than capable and these radars were dropped.

Problems

Calculations repeatedly showed that simple fallout shelters like this one would save many more civilians than an active defense like Nike-X, and for far less money.

Nike-X had been defined in the early 1960s as a system to defend US cities and industrial centers against a heavy Soviet attack during the 1970s. By 1965 the growing fleets of ICBMs in the inventories of both the US and USSR was making the cost of such a system very expensive, in spite of a reasonable cost-exchange ratio on the order of 1 to 1.

This led to further studies of the system to try to determine whether an ABM would be the proper way to save lives, or if there was some other plan that would do the same for less money. In the case of Zeus, for instance, it was clear that building more fallout shelters would both be less expensive and save more lives than Zeus. A major report on the topic by PSAC in October 1961 made this point, suggesting that Zeus without shelters was useless, and that having Zeus might lead the US to "introduce dangerously misleading assumptions concerning the ability of the U.S. to protect its cities".

This led to a series of increasingly sophisticated models to better predict the effectiveness of an ABM system and what the opposition would do to improve their performance against it. A key development was the Prim-Read theory, which provided an entirely mathematical solution to generating the ideal defensive layout. Using a Prim-Read layout for Nike-X, Air Force Brigadier General Glenn Kent began considering Soviet responses. His 1964 report produced a cost-exchange ratio that required $2 of defense for every $1 of offence if one wanted to limit US casualties to 30 percent of the population. The cost increased to 6-to-1 if the US wished to limit casualties to 10 percent. The ABM system would only be cheaper than the ICBMs if the US was willing to allow over half its population die in the exchange. When he realized he was using outdated exchange rates for the Soviet ruble, the exchange ratio for the 30 percent casualty rate jumped to 20-to-1.

As the cost of defeating Nike-X was less than the cost of building Nike-X, many reviewers concluded that the construction of an ABM system would simply prompt the Soviets to build more ICBMs. This led to serious concerns about a new arms race, which it was believed would increase the chance of an accidental war. When the numbers were presented to McNamara, according to Kent, he;

...observed that this was a race that we probably would not win and should avoid. He noted that it would be difficult indeed to stay the course with a strategy that aimed to limit damage. The detractors would proclaim that, with 70 percent surviving, there would be upwards of 60 million dead.

In spite of its technical capabilities, Nike-X still shared one seemingly intractable problem that had first been noticed with Zeus. Facing an ABM system, the Soviets would change their targeting priorities to maximize damage, by attacking smaller cities for instance. But another solution was to drop their warheads just outside the range of the defensive missiles, upwind of the target. Ground bursts would throw enormous amounts of radioactive dust into the air, causing fallout that would be almost as deadly as a direct attack. This would make the ABM system essentially useless unless the cities were also extensively protected from fallout. But those same shelters would save many lives on their own, to the point that the ABM seemed almost superfluous. While reporting to Congress on the issue in the spring of 1964, McNamara noted that:

It is estimated that a shelter system at a cost of $2 billion would save 48.5 million lives. The cost per life saved would be about $40.00. An active ballistic missile defense system would cost about $18 billion and would save an estimated 27.8 million lives. The cost per life saved in this case would be about $700. [He later added that] I personally will never recommend an anti-ICBM program unless a fallout program does accompany it. I believe that even if we do not have an anti-ICBM program, we nonetheless should proceed with the fallout shelter program.

From about 1965, the ABM became what one historian calls a "technology in search of a mission." As the only strategic system being developed by the US Army (as opposed to tactical systems like the Pershing missile), they were unwilling to concede defeat and allow the program to be cancelled. As the cost of deploying a complete Nike-X system grew, it became clear that it would never survive through Congress and be deployed. In early 1965, the Army launched a series of studies to find a mission concept that would lead to deployment.

Hardpoint, Hardsite, and VIRADE

File:HibexAction1.jpg

For even higher performance, the Hardsite concept replaced Sprint with HiBEX, which could accelerate at up to 400 g.

One of the original deployment plans for Zeus had been a defensive system for SAC, but the Air Force argued against such a system, in favor of building more ICBMs of their own. Their logic was that every Soviet missile launched in a counterforce strike could destroy a single US missile. If both forces had similar number of missiles, such an attack would leave both forces with few remaining missiles to launch a counterstrike. Adding Zeus would reduce the number of losses on the US side, and thereby retain a counterstrike force. However, the same was true if they built more ICBMs instead of Zeus, yet those ICBMs were themselves a strong deterrent. Most importantly, the Air Force was far more interested in building its own missiles than the Army's, especially in the case of Zeus which appeared to be easily outwitted.

Things had changed by the early 1960s, when McNamara placed limits on the Air Force fleet at 1000 Minuteman missiles and 54 Titan IIs. This meant that the Air Force could not respond to new Soviet missiles simply by building more of their own. An even greater existential threat to Minuteman than Soviet missiles was the US Navy's Polaris missile fleet, which was essentially invulnerable to attack, and led some to question the need for any ground-based ICBM. If the ICBM was to offer value, there had to be the expectation that it could survive a Soviet attack in enough numbers for a successful counterstrike, as it was certain that Polaris would do so. An ABM might provide that assurance.

A fresh look at this concept started at ARPA around 1963–64 under the name Hardpoint. This proved interesting enough for the Army and Air Force to collaborate on a follow-up study, Hardsite. The first Hardsite concept, HSD-I, considered defending bases within urban areas that would have Nike-X protection anyway. An example might be a SAC command and control center or an airfield on the outskirts of a city. This meant that the cost of adding the protection was close to nothing, because a base would already be built to protect the city. The extra cost might be justified even if the system was not highly effective. The second, HSD-II, considered the protection of isolated bases like missile fields. Most follow-up work focused on the HSD-II concept.

Hardsite proposed building small Sprint-only bases close to Minuteman fields. Incoming warheads would be tracked until the last possible moment, decluttering them completely and generating highly accurate tracks. Since the warheads had to land within a certain distance of a missile silo to damage it, any warheads that could be seen to be falling outside that area were simply ignored. At the time, Soviet inertial navigation systems (INS) were not particularly accurate, and it was expected over half would fall outside this range and would not have to be attacked. This acted as a force multiplier, allowing a small number of Sprints to defend against a large number of ICBMs.

Although initially supportive of the Hardsite concept, by 1966 the Air Force came to reject it largely for the same reasons it had rejected Zeus in the same role. If money was to be spent on protecting Minuteman, they felt that money would be better spent by the Air Force than the Army. As Morton Halperin noted:

In part this was a reflex reaction, a desire not to have Air Force missiles protected by 'Army' ABMs. [...] The Air Force clearly preferred that the funds for missile defense be used by the Air Force to develop new hard rock silos or mobile systems.

Small City Defense, PAR

See also: AN/FPQ-16 PARCS

PARCS was originally designed to offer radar coverage over a large area, reducing the cost of the radars at each site in an SCD network.

During the project's development phase, the siting and size of the Nike-X bases became a major complaint of smaller cities. Originally intended to protect only the largest urban areas, Nike-X was designed to be built at a very large size with many missiles controlled by an expensive computer. Smaller sites were to be left undefended in the original Nike-X concept, the system was simply too expensive to build with only a few interceptors. Smaller cities complained that they were not only being left open to attack, but that their lack of defenses might make them primary targets. This led to a series of studies on the Small City Defense (SCD) concept. By 1964 SCD had become part of the baseline Nike-X deployment, with every city with a population over 100,000 being provided some level of defensive system.

SCD would consist primarily of a single autonomous battery centered on a cut-down MAR called TACMAR (TACtical MAR), along with a simplified data processing system known as the Local Data Processor (LDP). This was essentially the DCDP with fewer modules installed, reducing the number of tracks it could compile and the amount of decluttering it could handle. To further reduce costs, Bell later replaced the cut-down MAR with an upgraded MSR, TACMSR. They studied a wide variety of potential deployments, starting with systems like the original Nike-X proposal with no SCDs, to deployments offering complete continental US protection with a large number of SCD modules of various types and sizes. The deployments were arranged to be able to be built in phases, working up to complete coverage.

One issue that emerged from these studies was the problem of providing early warning to the SCD sites. MAR had been carefully tuned to provide just enough warning for their systems to complete the interception, and did not offer any sort of very long range warning. The SCD's MSR radars provided detection at perhaps 100 miles (160 km), which meant targets would appear on their radars only seconds before launches would have to be carried out. In a sneak attack scenario there would not be enough time to receive command authority for the release of nuclear weapons, which meant the bases would require launch on warning authority, which was politically unacceptable.

This led to proposals for a new radar dedicated solely to the early warning role, developing tracks only accurately enough to determine which MAR or SCD would ultimately have to deal with the threat. Used primarily in the first minutes of the attack, and not responsible for the engagements, the system could be considered disposable and did not need anything like the sophistication of the MAR. This led to the Perimeter Acquisition Radar (PAR), which would operate at VHF frequencies in order to greatly lower the cost of the electronics.

Zeus EX

Zeus EX, later known as Spartan, was the ultimate development of the original Nike Zeus.

Through late 1964 Bell was considering the role of Zeus in the Nike-X system. A January 1965 report noted that new understanding of high-altitude nuclear explosions might significantly improve the value of the Zeus. When a nuclear warhead explodes it gives off a huge number of high-energy X-rays which normally react with any nearby matter, including air, causing the air to ionize and block further progress of the X-rays. In the highest layers of the atmosphere there is too little matter for this to occur, and the X-rays can travel long distances. Enough of these hitting a reentry vehicle (RV) can cause damage to its heat shields.

To take full advantage of this effect, the Zeus would have to have a much larger warhead dedicated to the production of X-rays, and would have to operate at higher altitudes. A major advantage was that accuracy needs were much reduced, from a minimum of about 800 feet (240 m) for the original Zeus' neutron based attack, to something on the order of a few miles. This meant that the range limits of the original Zeus, which were defined by the accuracy of the radars to about 75 miles (121 km), were greatly eased and attacks could take place at much greater range. This Extended Range Nike Zeus, or Zeus EX for short, would be able to provide protection over a wider area, reducing the number of bases needed to provide full-country defense. These missiles would also be expensive.

Nth country, DEPEX, I-67

In February 1965 the Army asked Bell to consider different deployment concepts under the Nth country study. This examined what sort of system would be needed to provide protection against an unsophisticated attack with a limited number of warheads. Using the Zeus EX, a small number of bases could provide coverage for the entire US. The system would be unable to deal with large numbers of warheads, but that was not a concern for a system that would only be tasked with beating off small attacks.

With only small numbers of targets, the full MAR was not needed and Bell initially proposed TACMAR to fill this need. This would have shorter detection range, so a long range radar like PAR would be needed for early detection. The missile sites would consist of a single TACMAR along with about 20 Zeus EX missiles. In October 1965 the TACMAR was replaced by the TACMSR from the SCD studies. Since this radar had even shorter range than TACMAR, it could not be expected to generate tracking information in time for a Zeus launch. PAR would thus have to be upgraded to have higher accuracy and the processing power to generate tracks that would be handed off to the TACMSRs. During this same time, Bell had noted problems with long wavelength radars in the presence of radar blackout. Both of these issues argued for a change from VHF to UHF frequencies for the PAR.

Further work along these lines led to the Nike-X Deployment Study, or DEPEX. DEPEX described a system similar to that initially considered under Nth Country, but was designed to grow as the nature of the threat changed. They imagined a four-phase deployment sequence that added more and more terminal defenses as the sophistication of the Nth country missiles increased over time. In December 1966, the Army asked Bell to prepare a detailed deployment concept combining the light defense of Nth country with the point defense of Hardsite. On 17 January 1967 this became the I-67 project, which delivered its results on 5 July. I-67 was essentially Nth country but with additional bases near Minuteman fields, armed primarily with Sprint. The wide-area Zeus and short-range Sprint bases would both be supported by the PAR network.

Continued pressure to deploy

Robert McNamara had resisted pressure to deploy Zeus knowing it would have little real-world effect, and faced the same problem with Nike-X four years later.

The basic outlines of these various studies were becoming clear by 1966. The heavy defense from the original Nike-X proposals would cost about $40 billion ($292 billion today) and offer limited protection and damage prevention in an all-out attack, but could be expected to blunt or completely defeat any smaller attack. The thin defense of Nth country would be much less expensive, around $5 billion ($36 billion today), but could only have any effect at all under certain limited scenarios. Finally, the Hardsite concepts would cost about the same as the thin defense, and provide some protection against a certain class of counterforce attacks.

None of these concepts appeared to be worth deploying, but there was considerable pressure from Congressional groups dominated by hawks who continued to force development of the ABM even when McNamara and President Johnson had not asked for it. The debate spilled over into public and led to comments about an "ABM gap", especially by Republican Governor George W. Romney. Further support came from the Joint Chiefs of Staff (JCS), who used the Soviet construction of A-35 ABM systems around Tallinn and Moscow as an argument to demand their own. This was the first strong vote of support from the JCS for ABM; the Air Force had previously been dead-set against any Army system and had publicly criticized their earlier efforts in the press. According to one historian, this was likely due to the rapid improvement of the US Navy's missile fleet, which could survive any conceivable attack, and led the Air Force to support any way to improve the survivability of their own defenses.

McNamara attempted to short-circuit deployment in early 1966 by stating that the only program that had any reasonable cost-effectiveness was the thin defense against the Chinese, and then noted there was no rush to build such a system as it would be some time before they had an ICBM. Overruling him, Congress provided $167.9 million ($1 billion today) for immediate production of the original Nike-X concept. McNamara and Johnson met on the issue on 3 November 1966, and McNamara once again convinced Johnson that the system could not justify the cost of deployment. McNamara headed off the expected counterattack from Romney by calling a press conference on the topic of Soviet ABMs and stating that the new Minuteman III and Poseidon SLBM would ensure the Soviet system would be overwhelmed.

Another meeting on the issue was called on 6 December 1966, attended by Johnson, McNamara, the deputy Secretary of Defense Cyrus Vance, Walt Rostow of the National Security Agency (NSA) and the Joint Chiefs. Rostow took the side of the JCS and it appeared that development would start. However, McNamara once again outlined the problems and stated that the simplest way to close the ABM gap was to simply build more ICBMs, rendering the Soviet system impotent and a great waste of money. He then proposed that the money sidelined by Congress for deployment be used for initial deployment studies while the US attempted to negotiate an arms limitation treaty. Johnson agreed with this compromise, and ordered Secretary of State Dean Rusk to open negotiations with the Soviets.

Nike-X becomes Sentinel

By 1967 the debate over ABM systems had become a major public policy issue, with almost continual debate on the topic in newspapers and magazines. It was in the midst of these debates, on 17 June 1967, that the Chinese tested their first H-bomb in Test No. 6. Suddenly the Nth country concept was no longer simply theoretical. McNamara seized on this event as a way to deflect criticism over the lack of deployment while still keeping costs under control. On 18 September 1967 he announced that Nike-X would now be known as Sentinel and outlined deployment plans broadly following the I-67 concept.

Testing

Although the original Nike-X concept was cancelled, a number of its components were built and tested both as part of Nike-X and the follow-on Sentinel. The following section discusses the main developments during the Nike-X period.

MAR

File:MAR-I radar.jpg

MAR-I at White Sands, seen looking towards the south-south-west. The transmitter is on the small dome on the right, with its associated receiver on the main dome above it. The elements fill only a small area of the original antenna outlines.

Work on the ZMAR radar was already progressing by the time McNamara cancelled Zeus in 1963. Two experimental systems had been built consisting of a single row of elements, essentially a slice from a larger array. One, built by Sylvania, used MOSAR phase-shifting using time delays, while the other, by General Electric, used a "novel modulation scanning system". Sylvania's system won a contract for a test system, MAR-I.

To save money, the prototype MAR-I would only install antenna elements for the inner section of the original 40 foot (12 m) diameter antenna, populating the central 25 feet (7.6 m). This had the side-effect of reducing the number of antenna elements from 6,405 to 2,245 but would not change the basic control logic. A full sized, four sided MAR would require 25,620 parametric amplifiers to be individually wired by hand, so building the smaller MAR-I greatly reduced cost and construction time. The transmitter face was similarly reduced. Both antennas were built full sized and could be expanded out to full MAR performance at any time. In spite of these cost reduction methods, MAR-I cost an estimated $100 million to build ($763 million today).

A test site for MAR-I had already been selected at WSMR, about a mile off of US Route 70, and some 25 miles (40 km) north of the Army's main missile launch sites along WSMR Route 2 (Nike Avenue). A new road, WSMR Route 15, was built to connect the MAR-I to Launch Complex 38 (LC38), the Zeus launch site. MAR-I's northern location meant that the MAR would see the many rocket launches taking place at the Army sites to the south, as well as the target missiles that were launched towards them from the north from the Green River Launch Complex in Utah. This provided the test program with numerous free targets.

Since MAR was central to the entire Nike-X system, it had to survive attacks directed at the radar itself. At the time, the response of hardened buildings to nuclear shock was not well understood, and the MAR-I building was dramatically over-designed. It consisted of a large central hemispherical dome of 10 foot (3.0 m) thick reinforced concrete with similar but smaller domes arranged on the corners of a square bounding the central dome. The central dome held the receiver arrays, and the smaller domes the transmitters. The concept was designed to allow a transmitter/receiver pair to be built into any of the faces to provide wide coverage around the radar site. As a test site, MAR-I only installed the equipment on the north-west facing side, although provisions were made for a second set on the north-east side that was never used. A tall metal clutter fence surrounded the building, preventing reflections from nearby mountains.

Groundbreaking on the MAR-I site started in March 1963 and construction proceeded rapidly. The radar was powered up for the first time in June 1964 and achieved its first successful tracking on 11 September 1964, repeatedly tracking and breaking lock on a balloon target over a 50-minute period. However, the system demonstrated very low reliability in the transmitter's travelling wave tube (TWT) amplifiers, which led to an extremely expensive re-design and re-installation. Once upgraded, MAR-I demonstrated the system would work as expected; it could generate multiple virtual radar beams, could simultaneously generate different types of beams for detection, tracking and discrimination at the same time, and had the accuracy and speed needed to generate many tracks.

By this time work had already begun on MAR-II on Kwajalein, which differed in form and in its beam steering system. The prototype MAR-II was built on reclaimed land just west of the original Zeus site. Having learned more about nuclear hardening, this version was built of thinner concrete and had provisions for antennas on only two faces, built into a horizontally truncated pyramid. Like MAR-I, in order to save money MAR-II would be equipped with only one set of transmitter/receiver elements installed, but with all the wiring in place in case it had to be upgraded in the future. Nike-X was cancelled before MAR-II was complete, and the semi-completed building was instead used as a climate-controlled storage facility.

Testing on MAR-I lasted until 30 September 1967. It continued to be used at a lower level as part of the Sentinel developments. This work ended in May 1969, when the facility was mothballed. In November, the building was re-purposed as the main fallout shelter for everyone at Holloman Air Force Base, about 25 miles (40 km) to the east. To hold the 5,800 staff and their dependents, starting in 1970 the radar and its underground equipment areas were completely emptied.

Stirling Colgate wrote a letter to Science proposing MAR's salvaging as he felt it would make an excellent radio astronomy instrument. With minor re-tuning it could be used to observe the hydrogen line. This did not come to be, but over 2000 of the Western Electric parametric amplifiers driving the system ended up being salvaged by Colgate's New Mexico Tech. A number found their way into the astronomy field, including Colgate's supernova detector, SNORT. About 2,000 remained in storage at New Mexico Tech until 1980. An assay at that time discovered that there was well over one ounce of gold in each one, and the remaining stocks were melted down to produce $941,966 for the university ($3 million today). The money was used to build a new wing on the university's Workman Center, known unofficially as the "Gold Building".

MSR

File:Stanley R Mickelsen Safeguard Complex Missile Site Control (cropped).jpg

The TACMSR at Mickelsen was the only complete MSR built. Note that the antenna elements only fill the center of the circular areas; the larger area was intended for possible future expansion.

Bell ran a number of studies to identify the sweet spot for the MSR that would allow it to have enough functionality to be useful at different stages of the attack, as well as being inexpensive enough to justify its existence in a system dominated by MAR. This led to an initial proposal for an S band system using passive scanning (PESA) that was sent out in October 1963. Of the seven proposals received, Raytheon won the development contract in December 1963, with Varian providing the high-power klystrons (twystrons) for the transmitter.

An initial prototype design was developed between January and May 1964. When used with MAR, the MSR needed only short range, enough to hand off the Sprint missiles. This led to a design with limited radiated power. For Small City Defense, this would not offer enough power to acquire the warheads at reasonable range. This led to an upgraded design with five times the transmitter power, which was sent to Raytheon in May 1965. A further upgrade in May 1966 included the battle control computers and other features of the TACMSR system.

As it was expected that the Sprint and Zeus missiles would be ready in time for the MSR to be used with them, the decision was made to skip construction of an MSR at White Sands and build the first example at Kwajalein. As the earlier Zeus system had taken up most of the available land on Kwajalein Island itself, the missile launchers and MSR were to be built on Meck Island, about 20 miles (32 km) north. This site would host a complete TACMSR, allowing the Army to test both MAR-hosted (using MAR-II) and autonomous MSR deployments. A second launcher site was built on Illeginni Island, 17.5 miles (28.2 km) northwest of Meck, with two Sprint and two Spartan launchers. Three camera stations built to record the Illeginni launches were installed, and used for tracking long after the program shut down.

Construction of the launch site on Meck began in late 1967. As the island is only a few feet over sea level, it was decided not to build the MSR in the form it would have in a deployment system, where the computers and operations would be underground. Instead, the majority of the system was built above ground in a single-floor rectangular building. The MSR was built in a boxy extension on the north-western corner of the roof, with two sides angled back to form a half-pyramid shape where the antennas were mounted. Small clutter fences were built to the north and northwest, the western side faced out over the water which was only a few tens of meters from the building. Illeginni did not have a radar site, it was operated remotely from Meck.

Sprint

File:Squirt missile leaving the launcher.jpg

The sub-scale Squirt was used to test Sprint concepts.

Main article: Sprint (missile)

On 1 October 1962, Bell's Nike office sent specifications for a high-speed missile to three contractors. The responses were received on 1 February 1963, and Martin Marietta was selected as the winning bid on 18 March.

Sprint ultimately proved to be the most difficult technical challenge of the Nike-X system. Designed to intercept incoming warheads at an altitude of about 45,000 feet (14,000 m), it had to fly so quickly that its outer layer became hotter than an oxy-acetylene welding torch. This caused enormous problems in materials, controls, and even receiving radio signals through the ionized air around the missile. The development program was referred to as "pure agony".

In the original Nike-X plans, Sprint was the primary weapon, and thus was considered to be an extremely high-priority development. To speed development, a sub-scale version of Sprint known as Squirt was tested from Launch Complex 37 at White Sands, the former Nike Ajax/Hercules test area. A total of five Squirts were fired between 6 November 1964 and 1965. The first Sprint Propulsion Test Vehicle (PTV) was launched from another area at the same complex on 17 November 1965, only 25 months after the final design was signed off. Sprint testing pre-dated construction of an MSR, and the missiles were initially guided by Zeus TTR and MTR radars. Testing continued under Safeguard, with a total of 42 test flights at White Sands and another 34 at Kwajalein.

Zeus EX/Spartan

Main article: LIM-49 Spartan

Zeus B had been test fired at both White Sands and the Zeus base on Kwajalein. For Nike-X, the extended range EX model was planned, replacing Zeus' second stage with a larger model that provided more thrust through the midsection of the boost phase. Also known as the DM-15X2, the EX was renamed Spartan in January 1967. The Spartan never flew as part of the original Nike-X, and its first flight in March 1968 took place under Sentinel.

Reentry testing

One of the reasons for the move from Zeus to Nike-X was concern that the Zeus radars would not be able to tell the difference between the warhead and a decoy until it was too late to launch. One solution to this problem was the Sprint missile, which had the performance required to wait until decluttering was complete. Another potential solution was to look for some sort of signature of the reentry through the highest levels of the atmosphere that might differ between a warhead and decoy; specifically, it appeared that the ablation of the heat shield might produce a clear signature pointing out the warhead.

The reentry phenomenology was of interest both to the Army, as it might allow long-range decluttering to be carried out, as well as to the Air Force, whose own ICBMs might be at risk of long-range interception if the Soviets exploited a similar concept. A program to test these concepts was a major part of ARPA's Project Defender, especially Project PRESS, which started in 1960. This led to the construction of a number of high-power radar systems on Roi-Namur, the northernmost point of the Kwajalein atoll. Although the results remain classified, a number of sources mention the failure to find a reliable signature of this sort.

In 1964, Bell Labs formulated their own set of requirements for radar work in relation to Nike-X. Working with the Army, Air Force, Lincoln Labs and ARPA, Nike-X ran a long series of reentry measurements with the PRESS radars, especially TRADEX. By the late 1960s it was clear that discrimination of decoys was an unsolved problem, but that the techniques might still be useful against less sophisticated decoys. This work appears to be one of the main reasons that the thin defense of I-67 was considered worthwhile. At that time, in 1967, ARPA passed the PRESS radars to the Army.

Description

A typical Nike-X deployment around a major city would consist of a number of missile batteries. One of these would be equipped with the MAR and its associated DCDP computers, while the others would optionally have an MSR. The sites were all networked together using communications equipment working at normal voice bandwidths. A number of the smaller bases would be built north of the MAR to provide protection to this central station.

Almost every aspect of the battle would be managed by the DCDPS at the MAR base. The reason for this centralization was two-fold; one was that the radar system was extremely complex and expensive and could not be built in large numbers, the second was that the transistor-based computers needed to process the data were likewise very expensive. Nike-X thus relied on a small number of very expensive sites, and a large number of greatly simplified batteries.

MAR

File:MAR-I radar with protective domes.jpg

MAR-I had protective covers that slid up over the antenna elements, riding upward on the rails from their underground storage.

MAR was an L band active electronically scanned array phased-array radar. The original MAR-I had been built into a strongly reinforced dome, but the later designs consisted of two half-pyramid shapes, with the transmitters in a smaller pyramid in front of the receivers. The reduction in size and complexity was the result of a number of studies on nuclear hardening, especially those carried out as part of Operation Prairie Flat in Alberta, where a 500-short-ton (450,000 kg) ball of TNT was constructed to simulate a nuclear explosion.

MAR used separate transmitter and receivers, a necessity at the time due to the size of the individual transmit and receive units and the switching systems that would be required. Each transmitter antenna was fed by its own power amplifier using travelling wave tubes with switching diodes and striplines performing the delays. The broadcast signal had three parts in sequence and the receivers had three channels, one tuned to each part of the pulse chain. This allowed the receiver to send each part of the signal to different processing equipment, allowing search, track and discrimination in a single pulse.

MAR operated in two modes, surveillance and engagement. In surveillance mode the range was maximized, and each face performed a scan in about 5 seconds. Returns were fed into systems that automatically extracted the range and velocity, and if the return was deemed interesting, the system automatically began a track for threat verification. During the threat verification phase, the radar spent more time examining the returns in an effort to accurately determine the trajectory, and then ignored any objects that would fall outside its area.

Those targets that did pose a threat automatically triggered the switch to engagement mode. This created a new beam constantly aimed at the target, sweeping its focus point through the threat tube to pick out individual objects within it. Data from these beams extracted velocity data to a separate computer to attempt to pick out the warhead as the decoys slowed in the atmosphere. Only one Coherent Signal Processing System (CSPS) was ever built, and for testing it was connected to the Zeus Discrimination Radar on Kwajalein.

Nike-X also considered a cut down version of MAR known as TACMAR. This was essentially a MAR with half of the elements hooked up, reducing its price at the cost of shorter detection range. The processing equipment was likewise reduced in complexity, lacking some of the more sophisticated discrimination processing. TACMAR was designed from the start to be able to be upgraded to full MAR performance if needed, especially as the sophistication of the threat grew. MAR-II is sometimes described as the prototype TACMAR, but there is considerable confusion on this point in existing sources.

MSR

As initially conceived, MSR was a short-range system for tracking Sprint missiles before they appeared in the MAR's view, as well as offering a secondary target and jammer tracking role. In this initial concept, the MSR would have limited processing power, just enough to create tracks to feed back to the MAR. In the anti-jamming role, each MAR and MSR would measure the angle to the jammer, and the MAR would perform triangulation.

The MSR was an S band passively scanned phased-array radar (PESA), unlike the actively scanned MAR. A PESA system cannot generate multiple signals like AESA, but is much less expensive to build because a single transmitter and receiver is used for the entire system. Additionally, the same antenna array can easily be used for both transmit and receive, as the area behind the array is much less cluttered and has ample room for switching in spite of the large radio frequency switches needed at this level of power.

Unlike the MAR, which would be tracking targets primarily from the north, the MSR would be tracking its interceptors in all directions. MSR was thus built into a four-faced truncated pyramid, with any or all of the faces carrying radar arrays. Isolated sites, like the one considered for Hawaii, would normally have arrays on all four faces. Those that were networked into denser systems could reduce the number of faces and get the same information by sending tracking data from site to site.

Sprint

Sprint was the centrepiece of the original Nike-X concept, but relegated to a secondary role in Sentinel.

Sprint was the primary weapon of Nike-X as originally conceived, and would be placed in clusters around the targets being defended by the MAR system. Each missile was housed in an underground silo and was driven into the air before launch by a gas-powered piston. The missile was initially tracked by the local MSR, which would hand off tracking to the MAR as soon as it became visible. A transponder in the missile could respond to signals from either the MAR or MSR for accurate tracking.

Although a primary concern of the Sprint missile was high speed, the design is actually not optimized for maximum energy, but instead relies on the first stage (booster) to provide as much thrust as possible. This leaves the second stage (sustainer) lighter than optimal, in order to improve its maneuverability. Staging is under ground control, with the booster being cut away from the missile body by explosives. The sustainer is not necessarily ignited immediately, depending on the flight profile. For control, the first stage used a system that injected Freon into the exhaust to cause thrust vectoring to control the flight. The second stage used small air vanes for control.

The required acceleration was such that the solid fuel had to burn ten times as fast as contemporary designs like the Pershing or Minuteman. Both the burning fuel and skin friction created so much heat that radio signals were strongly attenuated through the resulting ionized plasma around the missile body. It was expected that the average interception would take place at about 40,000 feet (12,000 m) at a range of 10 nautical miles (19 km; 12 mi) after 10 seconds of flight time.

Two warheads were designed for Sprint starting in 1963, the W65 at Livermore, and the W66 at Los Alamos. The W65 was entering Phase 3 testing in October 1965 with a design yield of around 5 kilotons of TNT (21 TJ), but this was cancelled in January 1968 in favor of the W66. The W66's explosive yield is reported as being in the "low kiloton" range, with various references claiming it is anywhere from 1 to 20 kilotons of TNT (4.2 to 83.7 TJ). The W66 was the first enhanced radiation, or neutron bomb, to be fully developed, tested in the late 1960s and entering production in June 1974.

See also

• Project Nike, the technical office that ran Nike-X.
• The A-135 anti-ballistic missile system was the Soviet equivalent to Nike-X.