Nike-X



Nike-X was a proposed US Army anti-ballistic missile (ABM) system designed to protect major cities in the United States from attacks by the Soviet Union's Intercontinental ballistic missile fleet. The name referred to its experimental basis, and it was intended it would be replaced by a more appropriate name when the system was put into production. This never came to pass; the original Nike-X concept was canceled and replaced by a much thinner defense system known as the Sentinel Program that used some of the same equipment.

Nike-X was a response to the failure of the earlier Nike Zeus system. Zeus had been designed in the 1950s to face a few dozen Soviet ICBMs, and its design would mean it was largely useless by mid-1960s when it would be facing hundreds. 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 warheads at the same time. Worse, the attacker could 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. Large numbers would be clustered near potential targets, allowing successful attack right up to the few last seconds of the warhead's re-entry. They would operate below the altitude where decoys or explosions had any effect. Nike-X also used a new radar system that could track hundreds of objects at once, allowing salvos of many Sprints. Dozens of ICBMs would need to arrive at the same time in order to overwhelm the system. Nike-X considered retaining the longer range Zeus missile, and later developed an extended range version known as Zeus EX. It played a secondary role in the Nike-X system, intended primarily for use in areas outside the Sprint protected regions.

Nike-X required at least one interceptor missile to attack each incoming warhead. As the USSR's missile fleet grew, the cost of implementing Nike-X began to grow as well. Looking for lower-cost options, a number of studies carried out between 1965 and 1967 examined scenarios 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 Chinese 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.

Nike Zeus


As early as 1955 the US Army began considering the possibility of further upgrading their Nike B surface-to-air missile (SAM) system to intercept ICBMs. Bell was asked to consider the issue, and returned a report noting that the missile could be upgraded to the required performance relatively easily. However, in order to detect the warhead while it was still far enough away to give the missile time to launch would require extremely powerful radar systems. 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. Lingering inter-service rivalries between the Army and Air Force led to the Nike II being re-defined several times. When these 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 generally similar to the two Nike designs that preceded it, using 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. While similar in concept, Zeus was different in form. The missile itself was much larger, with a range of up to 200 miles, compared to Hercules' 75 miles. It flew so fast it burned the outer layer of its skin off while climbing through the lower atmosphere. To ensure a kill at 100000 feet altitude, where there was little atmosphere to carry a shock wave, it mounted a large 400 kt warhead. The search radar was a 120 foot wide triangle able to pick out warheads while still over 600 nmi away (an especially difficult problem given the small size of a typical warhead), and a new digital computer was used to be able to calculate trajectories for intercepts taking place at relative velocities over 5 miles 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. For full-scale tests, the Army built a new 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


Zeus had initially been proposed to defend widely-dispersed Strategic Air Command (SAC) bases against attacks by a few dozen missiles, or as a wider defense involving attacks with two ICBMs being launched at each major US city. But by the time Zeus could be deployed in the early-to-mid 1960s it was expected a nuclear war would consist of ICBMs fired in the hundreds.

Zeus used mechanically steered radars, like the Nike SAMs before it. A typical Zeus site would have between two and six Target Tracking Radars, limiting the number of launches it could carry out at one time. A study by the Weapons Systems Evaluation Group (WSEG) calculated that the Soviets had a 90% 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, due to the difficulty of hardening the mechanical radars in any reasonable fashion. This meant that 4 Soviet ICBMs could eliminate 100 Zeus missiles, a superb exchange ratio.

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 one 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 km above the base, it would be too late 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 RV for little additional cost in terms of throw weight. In space, these are ejected to create radar returns that are indistinguishable from the RV. The result is a large number of radar objects stretched out in a threat tube approaching a given target, a few kilometers across and tens of kilometers long. Zeus had to get within about 1000 feet 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 good quality decoys would almost certainly be able to hit a Zeus base.

Nike-X
ARPA, today known as DARPA, was initially formed in 1958 by President 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 ABM 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 then-new laser. One improvement to Zeus had already been suggested; a new phased-array radar replacing Zeus' mechanical ones would greatly increase the number of targets and interceptors that a single site could handle, as well as allowing the radar to 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 this time a decision on whether or not to deploy Zeus was looming. President 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 returned a report outlining four basic concepts. First was a study of the existing Zeus system considering various scenarios where it might be used effectively. The next replaced Zeus with a shorter-range but higher-speed missile to allow it to attack warheads that had approached closer to the ground, which would help with both decoys and nuclear blackout. The next used a new short-range phased-array radar that allowed for greatly increased salvo rates, while still using Zeus' long-range radar for early detection.

The fourth concept, NX, combined the new missile and radar. NX was based around the ZMAR radar, used for tracking everything from the incoming warheads to outgoing interceptors. The interceptors would be a short-range missile for point defense, known as Sprint. New computers would track hundreds of incoming targets and outgoing interceptors, and communicate that information between widely distributed missile batteries. A single NX Defense Center would provide protection over large metropolitan areas. The system optionally retained Zeus, which could be used in areas away from cities. The name Nike-X was apparently an ad hoc suggestion by Jack Ruina, who was tasked with presenting the options to the President's Science Advisory Committee (PSAC).

The time for a decision on Zeus came in late 1962. Considering the issues, in January 1963 McNamara announced that the construction funds allocated for Zeus would not be released, and the Zeus development funding would instead be used for development of the new system.

System concept
Decoys are lighter than the reentry vehicle (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, opening it to attack. But the RV can often be picked out before this by examining the threat tube as a whole and watching for portions of it that have higher speeds. 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. Nike-X intended to wait until this point, and then launch a high-speed missile at the RV, meaning the interceptions would take place only seconds before the warheads hit their targets, between 5 and(-) away from the base.

Low-altitude intercepts would also have the advantage of reducing the problem with nuclear radar blackout. This effect occurs at similar altitudes as decluttering, about 60 km. Operating well below this altitude meant that deliberate attempts to create nuclear blackout would not effect the operation of the Sprint. Just as importantly, because the Sprint's own warheads would be going off well 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, including 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.

The upside to this approach was that Nike-X did not have to launch multiple missiles in order to ensure the warhead would be hit, although in practice two would be launched at every target for redundancy reasons. This had been the concept with Zeus as well, but the introduction of decoys upset this, with one Army study suggesting that every ICBM would require as many as twenty Zeus missiles to be launched at it to ensure the warhead was hit. This meant that every missile the Soviets added to their fleet would require twenty new Zeus'. A 20-to-1 exchange rate may sound bad enough, but because the Soviets can target that single ICBM anywhere in the US, it actually means that every Zeus base would have to add twenty new missiles. This terrible cost-exchange ratio was one of the primary reasons Zeus was abandoned.

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, and then additional beams were formed to track the RVs once picked out, and 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 required 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 missile 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, as well as offering a way to triangulate jammers within the tube.

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. However, this capability proved useful during testing; while the new radars were still being built, early launches used the MTRs built during the Zeus test program.

Problems
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 blunt, 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". They concluded that there was no way to justify the large scale deployment of Zeus, which at that time called for 70 Zeus bases under the control of NORAD.

This led to a series of increasingly sophisticated models to better predict the effectiveness of an ABM system and what the offence 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% of the population, which increased to 6-to1 if the US wished to limit that to 10%. 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% 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."

McNamara was convinced of the validity of cost-benefit analysis, which suggested the ABM was simply a bad deal. 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 Pershing), 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


One of the main deployment plans for Zeus had been a defensive system for SAC, but the Air Force argued against such a system. They noted that adding a Zeus to a missile field required the Soviets to use another missile to attack that field, but the same was true if you added another ICBM. The Air Force was far more interested in building its own missiles than the Army's, especially for a system that appeared likely to be of little practical effect.

Things had changed by the early 1960s. McNamara had already placed limits on the Air Force fleet, 1000 Minuteman missiles and 54 Titan II's. 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 than Soviet missiles was the US Navy's Polaris missile fleet, which was considered to be largely 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. 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. 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 as falling outside that area were simply ignored. This was expected to be true for well over half the Soviet warheads of that era. This acted as a force multiplier, allowing a small number of Sprints defend against a large number of ICBMs; one might need to launch only 30 interceptors to counter a force of 100 ICBMs.

To counter this system, the attacker would have to assign additional missiles to each silo to use up the supply of Sprints, which would require several missiles in order to place enough inside the area that would cause a Sprint launch. Although there was no expectation that the system would actually stop a major attack if attempted, the idea was simply to force any counterforce attack to use many more warheads than an undefended site, and thereby eliminate a number of low-cost attack scenarios.

Unfortunately, this also leads to the possibility of defeating the system by attacking the radar. In this case it is still possible for the Hardsite to ignore any warheads that will fall outside its own lethal area, but as radars are difficult to protect to the same level as a silo, a smaller number of warheads would be needed to ensure they fell within their larger lethal range. As the various Hardsite studies progressed, the MSR was progressively hardened, but it was never enough. This problem led to the Virtual Radar Defense system (VIRADE), which included radars that would be moved between sites on railways, forcing additional warheads to be expended to attack each potential site. This would be extremely expensive to deploy.

Another problem identified during the Hardpoint studies was the data processing requirements were beyond even the large machines envisioned for Nike-X. This was also becoming a problem even for a baseline city defense, as the number of ICBMs grew. This led to further studies on units able to handle much higher processing loads, and resulted in the Parallel Element Processing Ensemble computer, or PEPE, one of the earlier experiments in parallel processing.

Although initially supportive of the concept, by 1966 the Air Force came to reject Hardpoint 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
During the project's development phase, fighting broke out over the siting of the Nike-X bases. Originally intended to protect only the largest urban areas, smaller cities complained that they were not only being left open to attack, but that their lack of defences 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, 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, 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 have to have 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
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 simply isn't enough matter for this to occur, and the x-rays can travel long distances. Enough of these hitting a re-entry vehicle 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 operate at higher altitudes. A major advantage was that accuracy needs were much reduced, from a minimum of about 800 feet 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, 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 not be tasked with beating a deliberate Soviet attack.

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


The basic outlines of these various studies were becoming clear by 1966. The heavy defense from the original Nike-X proposals would cost about $40 billion ($ billion today) and offer limited protection and damage prevention. The thin defense of Nth country would be much less expensive, around $5 billion ($ 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 defence, and provide some protection against a counterforce attack.

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 didn't ask for it. Further support for deployment came from the Joint Chiefs of Staff (JCS), who used the Soviet construction of 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; previously the Air Force had been dead-set against any Army system, and had publicly blasted 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. The debate spilled over into public and led to comments about an "ABM gap", especially by Republican Governor George W. Romney.

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 noting there was no rush to build such a system as it would be some time before they had an ICBM. Overruling him, Congress provided $167.9 million ($ 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 simply wasn't worth deploying. He then 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 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 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 solution to the problem of a Nike-X's lack of mission. 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


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 diameter antenna, populating the central 25 feet. 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.

A test site for MAR-I had already been selected at WSMR, about a mile off of U.S. Route 70, and some 25 miles 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 unrelated 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. 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 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 proceeded rapidly. The radar was powered up for the first time in June 1964. However, this 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 the Holloman Air Force Base, about 25 miles to the east. To hold the 5,800 staff and their dependents, the radar and its underground equipment areas had to be completely emptied. Starting in 1970, the radar began to be dismantled.

Stirling Colgate wrote a letter to Science bemoaning 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 of these 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 $941,966 for the university ($ million today). The money was used to build a new wing on the university's Workman Center, known unofficially as the "Gold Building".

MSR


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 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 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 build 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


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 45000 feet, 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 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
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.

Re-entry 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 re-entry 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 re-entry 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
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 ton 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 required switching systems. Both systems worked in concert to be able to generate multiple steerable beams. Each transmitter antenna was fed by its own power amplifier using travelling wave tubes with switching diodes and strip lines performing the delays. The signal generally consisted of a single pulse chain modulated at different frequencies so the single pulse could be used for search, track and discrimination. The receivers had three channels, one tuned to each part of the pulse chain. After reception and conversion to intermediate frequency, the signals were sent to two units, the Search Signal Processor (SSP) and Video Pulse Converter (VPC). The SSP examined the long range detection signal to extract rough range, direction and speed through doppler shift. The VPC received the tracking signal and digitized it for processing in the accurate tracking and discrimination systems.

MAR operated in two modes, surveillance and engagement. In surveillance mode the range of the radar was maximized, and the system scanned the entire sky every 20 seconds. Returns were fed into systems that automatically extracted the range and velocity of the object, 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 more accurately determine the trajectory, and then eliminated any objects that would not be falling into the area defended by the MAR.

Those targets that did pose a threat to the Defense Center's area automatically triggered the switch to engagement mode. In this mode the radar's range was reduced to allow more accurate tracking of the target. As the return strength grew, a sub-beam was generated and left staring at the target. By rapidly changing the tuning of the receiver delays, the system could sweep through the threat tube in range while keeping the width constant, thereby maximizing the energy being sent into the tube. In contrast, a conventional radar antenna with a fixed angle would put less energy onto more distant targets as a side-effect of the inverse square law. Data from those elements being used in the monopulse precision tracking mode was sent to the Coherent Signal Processing System (CSPS). The CSPS extracted velocity data to attempt to pick out the warhead as the decoys slowed in the atmosphere. One CSPS was built but not installed on MAR-I, it was instead connected to the Zeus Discrimination Radar on Kwajalein for testing.

Nike-X originally planned to alternately use a cut down version of MAR known as TACMAR. This was essentially a MAR with half of the elements hooked up, reducing its price considerably 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 following instructions from the MAR and create tracks to feed back to the MAR.

The MSR was an S band PESA phased-array radar, unlike the actively scanned MAR. In the PESA system, a single signal is sent and received to the entire radar face, and delay systems in the antenna elements achieve steering. This means a PESA system cannot generate different waveforms in different directions, but this level of sophistication was not needed in the MSR role. The upside is that this eliminates the need for a separate oscillator and amplifier for each antenna element; instead one, or more commonly a small number, of active elements feeds the entire array. The delays were based on diode shifters with 16 possible shifts held in a 4 bit register in the control computer. 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.

After some consideration, a solution to feeding the microwave energy to the antenna elements was found in a concept known as a space array. This consisted of a large empty chamber behind the antenna face with separate feed horns from the klystron amplifier and receivers at the back of the chamber. The feed horns were aimed at the back of the face where the delay units were positioned, feeding the signal through to the transmit/receive antennas on the outside face of the array.

An advantage to this design is that the signal is supplied through the air, meaning that the individual elements only need to be supplied with power, the RF does not have to come through a cable. This allowed the elements to be packaged into long tubular containers that could be individually removed from the exterior of the building. Replacement did not take place until a number of units had failed, at which point all the failed units were replaced at once, a task that took about two minutes per unit. Failures were tracked both electronically for basic faults, as well as the periodic sending and receiving a test signal to and from a nearby antenna at each of the sixteen different shift values.

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 of 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 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 40000 feet at a range of 10 nmi after 10 seconds of flight time.