LIM-49 Nike Zeus

XLIM-49 Nike Zeus was an anti-ballistic missile (ABM) system developed during the late 1950s and early 1960s. It was developed by Bell's Nike team, and was initially based on the earlier Nike Hercules anti-aircraft missile. The original Zeus A was designed to intercept warheads in the upper atmosphere, and mounted a 25 kiloton W31 nuclear warhead to guarantee a "kill". During development it was greatly enlarged and extended into a totally new design, Zeus B, intended to intercept warheads at ranges of hundreds of miles using the 400 kiloton W50 warhead. This model proved itself able to intercept warheads, and even satellites, in several successful tests.

Throughout its development critics raised serious concerns about its real-world capabilities and cost effectiveness. From the start it was noted that the price of the system would be more than the ICBMs it would attack. More worrying was the inability of the radar systems to distinguish warheads from decoys, in spite of considerable effort to develop solutions to this problem. Moreover, it was discovered during development that a nuclear explosion in space would blind radars from detecting objects at high altitudes, rendering the entire concept of exo-atmospheric interceptions questionable.

Attention turned to the Nike X concept, a layered system with more than one type of missile. The Zeus missile underwent further upgrades to the Zeus EX, and was later renamed as the LIM-49 Spartan (keeping the same tri-service identifier). The Nike X also added a shorter-range missile, the Sprint, and greatly improved radars and computer systems that provided defence over a wide area. Nike X eventually emerged as the Sentinel Program and then the Safeguard Program, which was operational only for a few months.

Nike Ajax
During World War II the US Army Air Force (USAAF) concluded that existing anti-aircraft guns, only marginally effective against existing generations of propeller-driven aircraft, would not be effective at all against the emerging jet-powered designs. Like the Germans and British before them, they concluded the only successful defence would be to use guided weapons.

As early as 1944 the US Army started exploring anti-aircraft missiles, examining a variety of concepts. They split development between the Ordnance department and the Army Air Force based on whether or not the design "depend[ed] for sustenance primarily on the lift of aerodynamic forces" or "primary on the momentum of the missile". Official requirements were published in 1945; Bell Laboratories won the Ordnance contract for a short-range line-of-sight weapon under Project Nike, while a team of players won the contract for a long-range design known as "Ground-to-Air Pilotless Aircraft" in Project Thumper. Thumper moved to the US Air Force when that branch was formed in 1948. In 1946 the USAAF also started two early research projects into anti-missile systems in Project Thumper and Project Wizard.

In 1953, Project Nike delivered the world's first operational anti-aircraft missile system, the Nike I. Nike tracked both the target and the missile using separate radars, commanding the later into a collision with the former. Nike was initially deployed at military bases starting in 1953, especially Strategic Air Command bomber airfields, and general deployment then followed at US cities, important industrial sites, and then overseas bases. Similar systems quickly emerged from other nations, including the S-75 Dvina (SA-2) from the USSR, and the English Electric Thunderbird in the UK.

Nike Hercules
By the mid-1950s aircraft performance had improved so much that the Nike had limited capability to successfully attack the aircraft before they would be within weapons launching range. The interception could be made even more difficult, or impossible, through the use of stand-off weapons that allowed the aircraft to drop its bombload while still outside of the missile's ~30 mi range. There were also concerns that the system's relatively low resolution radars would make it difficult to pick out targets flying in formation, leading to an outright miss.

An obvious solution to the accuracy problem was to use a nuclear warhead, and Bell proposed two updated versions of the missile to carry one. The "Ajax" version used the existing missile with a gun-type warhead, while the "Hercules" was enlarged to carry a more efficient but rounder implosion-type device. The Army chose the Hercules option, which was initially known as Nike B. Soon after starting work, the decision was made to eliminate liquid fuels, and the missile underwent a major redesign into a much larger solid fuel form. The new missile used four of the Ajax's solid rocket boosters to get it into the air quickly, and flew at higher speeds to interceptions as far away as 75 mi and altitudes over 100000 ft. In 1956 both missiles were renamed using Bell's code names, Nike I becoming the MIM-3 Nike Ajax and Nike B the MIM-14 Nike Hercules.

Hercules was designed from the start to operate from Ajax bases. However, it protected a much greater area, so not as many sites were needed to provide coverage of potential targets. Early deployments starting in 1958 were on new sites, but Ajax units started converting as well. Conversions were largely complete by 1960, leaving only a few Ajax sites in use. The last active Nike Ajax batteries were relieved of their mission in December 1961, followed by the last Army National Guard unit in May 1964.

Nike II


At first considered invulnerable, improvements in computer tracking systems and radars appeared to make an ICBM interception possible, albeit difficult. As early as February 1955, the Army concluded that missile systems had advanced enough to attack ICBMs, and in March they contracted Bell's Nike team to begin a detailed 18-month study of the problem under the name Nike II.

Bell returned an initial study in January 1956 that demonstrated the need to intercept the incoming warheads at 100 mi altitude, and suggested that this was within the abilities of an upgraded version of Hercules. Given the 5 mi-per-second approach speed, the system would have to react very quickly in order to produce the required accuracy, so new radars and computers with a very high data processing rate would be needed. Moreover, the missile's time-of-flight to that altitude, combined with the warhead's approach speed, demanded that the warhead be initially detected at about 1,000 mi range. This too was considered possible, although it would require extremely powerful radars - warheads are quite small and have minimal radar returns. After running 50,000 simulated intercepts on analog computers, Bell returned a final report on the concept in October 1956.

While the Army was carrying out its ABM work with the Nike team, the Air Force and Navy were also involved in ABM research. The Navy was proposing developments of its Talos missile system and some consideration was given to a land-based Talos in this role. The Air Force had cancelled Thumper in 1948, but Wizard had continued and was still trying to get its CIM-10 Bomarc missile into service in the anti-aircraft role. Work on anti-missile defence continued within the Wizard project throughout.

Wilson Memo
At this point a fight broke out between the Air Force and Army over who had precedence to develop land-based ABM systems. After the war, the Army used its large stocks of anti-aircraft artillery to set up defensive rings around US cities and other sites, and then supplanted these with Ajax missiles as they became available. The Air Force concentrated primarily on long-range fighters and missiles, including Bomarc. This separation of duties eventually became semi-formalized, with the Army given the task of defending "point" targets at short ranges while the Air Force was tasked with providing "area" defence.

Arguments over this arrangement started with Hercules, whose longer range threatened the Bomarc. The Air Force began a policy of publicly denigrating the Hercules and agitating for the missile defense program to be handed to them, going so far as to claim the Army was "unfit to guard [the] nation" in the New York Times. With the introduction of the Zeus these arguments grew more strident, with the Air Force noting that the long range of the system meant that ABMs should be considered area weapons, and therefore part of the Air Force's mission. The Army noted that the definition of "point" specifically mentioned weapons being sited near the targets they were defending. This was the case with Zeus, who's long range was intended to improve reaction time, not defensive range. They also pointed out that the defense role had been theirs since the War.

The argument was eventually decided by the US Secretary of Defense, Charles Erwin Wilson. Wilson considered a wide range of problems between the Army and Air Force and sided with the Air Force on most of them. Among the many issues considered in his 26 November 1956 memorandum, he limited the Army to weapons with 200 mi range, and those involved in ground-to-air defense to only 100 mi. This forced the Army to turn over its PGM-19 Jupiter IRBM systems to the Air Force, and to limit the range of their ABM and advanced anti-aircraft developments.

Wilson's decision was highly controversial, and led Army Colonel John Nickerson Jr., to publicly denounce Wilson in the press, while leaking details of their latest missile design, the Pershing. The resulting flap led to calls for Nickerson to be court-martialed and was compared to the Billy Mitchell court-martial in the 1920s.

Nike Zeus
In 1957 the Bell team was given the go-ahead to develop Nike II, now known as Nike Zeus. Zeus was based on the Douglas DM-15 missile, which was essentially a slightly scaled-up Hercules with an improved, more powerful single-piece booster replacing Hercules' cluster of four smaller boosters. Intercepts would take place at the limits of the Wilson requirements, at ranges and altitudes of about 100 mi. Prototype launches were planned for 1959. For more rapid service entry there had been some consideration given to an interim system based on the original Hercules missile, but these efforts were dropped. Likewise, early requirements for a secondary anti-aircraft role were also eventually dropped.

In August 1957 the Soviets successfully launched their R-7 Semyorka (SS-6) ICBM, and followed this up with the successful launch of Sputnik 1 in October. Wilson's concerns about inter-service rivalry were swept aside, and a fresh look at the Army and Air Force projects followed. By this point the Army was already planning for tests and deployment, while the Air Force had failed to deliver Bomarc even in the anti-aircraft role. In January 1958, Secretary of Defense Neil McElroy re-directed the AF's Wizard solely to radar research, overturned the Wilson Memo, and gave the Army free hand to develop the Zeus system as they saw fit. The National Security Council gave Zeus "S-Priority", the highest national priority. Additional funds were requested to the Zeus program to ensure an initial service date in the 4th quarter of 1962, but these were denied, delaying service entry until some time in 1963.

Freed of constraints, the Army re-designed the system to be the system they wanted instead of the one they were limited to. A new missile design emerged with a much enlarged upper fuselage and three stages, more than doubling the launch weight. This version greatly extended the range of the system, with interceptions taking place at over 200 mi range and over 100 mi in altitude. An even larger booster took the missile to hypersonic speeds while still in the lower atmosphere, so the missile fuselage was covered over completely with a phenolic ablative heat shield to protect the airframe from melting. The new DM-15S Nike Zeus B (the earlier model retroactively becoming the A) received a go-ahead for development on 16 January 1958. The entire system, including the new 120 foot wide radar systems, required 200 acres to deploy.

Testing


Test firings of the original A models of the missile began in 1959, mostly at White Sands Missile Range. The first attempt on 26 August 1959 was of a live booster stage and dummy sustainer, and broke up shortly before booster/sustainer separation. A similar test on 14 October was a success, followed by the first two-stage attempt on 16 December. The first complete test of both stages with active guidance and thrust vectoring was successfully carried out on 3 February 1960. Data collected from these tests led to changes to the design to improve speed during the ascent. The first test of the Zeus B took place in May 1961, and on 14 December a Zeus passed within 100 ft of a Nike Hercules being used as a test target.

Many test firings followed over the next three years, but White Sands was too close to its own launch sites to truly test an ICBM flight profile. Consideration was given to using Point Mugu in California, which would launch against missiles flying from Cape Canaveral, but range safety requirements placed limits on the potential tests. The Atlantic Test Range, to the south-east of Canaveral, had a high population density and little land available for accurate downrange tracking stations. Eventually Kwajalein Island was selected, as it was 4,800 miles from California, perfect for ICBMs, and already had a US Navy base with considerable housing and an airstrip.

The Zeus site, known as the Kwajalein Test Site, was officially established on 1 October 1960. As it grew in size, it eventually led to the entire island complex being handed over to the Army from the Navy on 1 July 1964. The site took up a considerable amount of the empty land to the north side of the airfield. On 26 June 1962 the system attempted to intercept the warhead from an SM-65 Atlas missile fired from Vandenberg AFB, but the radar system malfunctioned and the test failed. A second test on 19 July was a success, with the Zeus passing within 2 km of the target, close enough that its 400 ktonTNT warhead would have destroyed it. Another test on 12 December 1962 was a complete success, with the Zeus passing only 200 m from its target. Of the fourteen tests carried out over the two-year test cycle, ten of them were successful in bringing the Zeus within its lethal range.

Problems
In spite of tremendous capabilities, and successful tests, Nike Zeus was becoming outdated even faster than it could be developed. By the early 1960s the ICBM stockpiles of both the US and USSR were growing at a rapid rate, and attacks would now consist of hundreds of warheads. The Zeus system could only attack one target at a time, so unless hundreds of bases were set up, some warheads would simply "leak through" the system while it was busy with other targets. Attacking the Zeus site itself would be simply a matter of arranging two warheads to follow each other by less than the Zeus' interception time.

Another serious issue was system cost, a problem that was shared by any ground-based interceptor. While an ICBM could target any location in the US, the limited radar sight range of the ABMs meant they could only defend a small area. Even with a longer-range missile, they would not be able to reach distant targets in the time between detection and impact. That meant that each ABM site would, theoretically, have to have enough interceptors to shoot down every Soviet ICBM, so any widespread system would cost much more than the ICBM force. Deploying an even marginally effective system initially called for about 60 Zeus bases with 50 missiles each, at a price of $10 billion ($77 billion 2010 dollars). This would protect only major Strategic Air Command and military communication sites to ensure the US ability to counterattack. A system of 120 sites would be required to extend some protection to cities and industry as well. At $1 million per missile, Zeus was more expensive than the ICBMs it faced.

Through the late 1950s a number of new effects related to high altitude nuclear explosions also demonstrated serious concerns with the system. Nuclear fireballs are opaque to radar, and ones at very high atmospheres could be anywhere from 100 to 1000 km across, the exact size being area of considerable uncertainty. Anywhere within this range would be equally bad; a few explosions, including those of the Zeus' own warheads, would leave large patches of the sky where the radar could not see approaching warheads for minutes. Other effects included electromagnetic pulse (EMP) which might disturb communications and radio, and the Christofilos Effect, which appeared to allow large swaths of the sky to be blanketed with a semi-opaque layer that might persist for long periods. Additionally, the huge areas of heated and shocked air filled with charged particles acted as radio frequency lenses, which refracted the radar signals too much to be useful for aiming. It was known that higher frequencies mitigated some of these effects to a degree, but the uncertainties surrounding them were enormous, and radars of the era were already stretching the state of the art at the S-band.

These effects presented an almost trivial way to attack the Zeus bases, using two warheads following the same rough trajectory but a few moments apart in time. Given the 5 mps speeds and the approximate 2 mile lethal radius of the Zeus warhead, the warheads could be as little as one second apart. The first warhead would either allow itself to be attacked by Zeus, or deliberately explode just outside its maximum range. This would produce a large area where the radars could not see the second warhead as it approached. When it finally cleared the fireball only a few moments later, it would be so close to the base that it would be impossible for the Zeus to reach it while it was still in the upper atmosphere. This effect can be magnified using a "lofted" trajectory which allows the warheads to descend almost vertically onto the bases, further limiting their reaction time, or by using a low-altitude "fractional orbital" trajectory that makes the warhead appear on radar only a few hundred kilometres from the base.

As if all this were not enough, the issue of decoys and other radar countermeasures became a serious issue. This problem was first eluded to in 1958 in a number of public talks that mentioned Zeus' inability to discriminate targets. Several test programs, notably DAMP and PRESS, were expected to demonstrate how to tell warheads from decoys at long ranges, but apparently did the exact opposite (the results were classified).

The "whole idea" of decoys is that they are very light, so they can be launched in large numbers along with a warhead. In space these present credible radar returns that look, to some degree or another, like the return of the warhead. For a system like Zeus, as long as the decoys spread out over several miles during their flight, several missiles will be required to attack them and guarantee that the warhead hiding among the decoys will be destroyed. This leads to the possibility of an all-decoy attack which uses up the stock of interceptor missiles for practically no cost to the attacker. The only reliable way to "declutter" the decoys is to wait until the hit the upper atmosphere and begin to slow down due to drag, while the heavy re-entry vehicle continues on essentially unsolved. On a typical trajectory the warhead would have been travelling through the upper atmosphere for some distance before the Zeus reached it, and a special radar was used to declutter the warheads during this period. This could be reduced using a lofted trajectory, which reduced the time in the atmosphere to the point where decluttering was too late for the Zeus to respond.

One could make heavier decoys that slow further into the atmosphere, but only at the cost of being able to carry fewer of them for any given launch weight. But a particularly worrying type of "decoy" could be created by attaching a small explosive device to the missile's upper stage, causing it to fragment after reaching space. These fragments had a higher density than something like a metalized balloon or chaff, and would not declutter until well within the atmosphere. Commenters felt that a much shorter-range and faster reacting system would be needed to effectively deal with these sorts of decoy objects, one that operated within the lower atmosphere where the decluttering would be complete.

Cancellation
Throughout development, Zeus was the focus of "fierce controversy" in both the press and military circles. Even as the testing was being carried out, it was unclear if development would continue. President Eisenhower’s defense secretaries, Neil H. McElroy (1957-59) and Thomas S. Gates, Jr. (1959-61), were unconvinced that the system was worth the cost. Eisenhower remained skeptical throughout, questioning whether an effective ABM system could even be developed in the 1960s. Another harsh critic on cost grounds was Edward Teller. He pointed out that while a system may be technically feasible, the increased cost to an enemy of penetrating an ABM screen would be less than the cost of strengthening the screen to prevent penetration.

In 1961, the incoming administration of President Kennedy and Secretary of Defense Robert McNamara agreed to continue development funding through FY62, but declined to provide funds for production. McNamara summed up both the positives and the concerns this way:

"Successful development [of Zeus] may force an aggressor to expend additional resources to increase his ICBM force. It would also make accurate estimates of our defensive capabilities more difficult for a potential enemy and complicate the achievement of a successful attack. Furthermore, the protection that it would provide, even if for only a portion of our population, would be better than none at all... There is still considerable uncertainty as to its technical feasibility and, even if successfully developed, there are many serious operating problems yet to be solved. The system, itself, is vulnerable to ballistic missile attack, and its effectiveness could be degraded by the use of more sophisticated ICBMs screened by multiple decoys. Saturation of the target is another possibility as ICBMs become easier and cheaper to produce in coming years. Finally, it is a very expensive system in relation to the degree of protection that it can furnish."

These earlier concerns about cost and effectiveness, as well as new difficulties in terms of attack size and decoy problems, led McNamara to cancel the Zeus project in January 1963. In its place they decided to continue work on a greatly improved system, Nike-X.

While reporting to the Senate Armed Services Committee in February, McNamara noted that they expected the Soviets to have an initial ABM system deployed in 1966, and then later stated that the Nike-X would not be ready for use until 1970. Noting a "defensive gap", Strom Thurmond began an effort to deploy the existing Zeus as an interim system. On 11 April 1963, Thurmond led the Congress in an effort to fund deployment of Zeus. In the first closed session of the Senate in twenty years, Zeus was debated and the decision was made to continue with the planned development of Nike-X with no Zeus deployment. The Army continued the testing program until December 1964 at White Sands Missile Range, and May 1966 at Kwajalein Missile Range.

Nike-X, Sentinel, Safeguard
Through the late 1950s, the newly formed ARPA had been working on a major study of ABM systems under "Project Defender". As part of these studies they examined Zeus in depth. They concluded their studies in 1961, outlining four development paths for the Zeus system. The most basic was to continue deployment of the existing Zeus. Two plans outlined modifications featuring newer radars, or newer radars and computers. The final plan, NX, called for an updated Zeus, new radars and computers, and a new high-velocity missile for lower-altitude interception. The Department of Defence chose the NX plan, and started work on what they called Nike-X.

Nike-X featured a new and enormously powerful phased array radar able to locate targets simultaneously throughout a huge volume of space, working at higher frequencies that would help see though the blanking caused by high-altitude explosions. New computers with hundreds of times the performance of the Zeus models would interpret this data and develop tracking information for hundreds of targets, which included incoming warheads, decoys and outgoing interceptors. Each site would now be able to attack several targets at once. To address the problem of decoys that could not be discriminated, and the potential blinding of long-range radar even at higher frequency, the new Sprint missile was used for in-atmosphere interceptions, accelerating at up to 90 gee. To increase range, Zeus B was further upgraded to the much larger Zeus EX, and later renamed Spartan.

In the end, even these developments would prove to be too little for the task. The introduction of MIRV technology in the late 1960s meant that the USSR could overwhelm even the Nike-X system by placing more warheads on existing missiles. Warheads were far less expensive than the missiles needed to shoot them down, so the simplest solution to the presence of ABM systems was to greatly increase the number of warheads in everyone's inventory. Doing so would have numerous spin-off effects in terms of security concerns. This worrying development led to the ABM Treaty of the early 1970s.

Anti-satellite role
After the project's cancellation some work with the Zeus continued as an experimental anti-satellite system. There were concerns that the Soviets might develop satellites that were armed with nuclear weapons, which it could drop without the tell-tale signs of a rocket launch. McNamara formed Project Mudflap in April 1962 to address this, using the Zeus missile as an interceptor. The first test firing of the Mudflap concept was carried out at White Sands in December 1962, with the Zeus reaching an altitude of over 150 km.Testing was then moved to Kwajalein, and on 24 May 1963 the system was successful against an instrumented Agena D target. The Kwajalein site was officially active between 1964 and 1967, where it was known as Program 505. In 1967 it was replaced by a Thor-based system, Program 437.

Description
Nike Zeus was originally intended to be a straightforward development of the earlier Hercules system giving it the ability to hit ICBM warheads at about the same range and altitude as the maximum performance of the Hercules. In theory, hitting a warhead is no more difficult than an aircraft; the interceptor does not have to travel any further or faster, the computers that guide it simply have to select an intercept point farther in front of the target. In practice, the difficulty is detecting the target early enough that the intercept point is still within range of the missile. This demands much larger and more powerful radar systems, and faster computers.

The key development for the new system was the Zeus Acquisition Radar, or ZAR, which provided wide-area early warning and initial tracking information. This enormously powerful radar was driven by multiple 1.8 MW klystrons and broadcast through three 80 ft wide antennas arranged as the outside edges of a rotating equilateral triangle. The signal was so powerful that it was considered dangerous to people within 330 ft, so the entire system was surrounded by a 65 ft high fence located 350 ft away from the antenna. The signal was received on separate set of three antennas, situated at the centre of a 80 foot diameter Luneburg lens, which rotated synchronously with the broadcaster under a 120 ft diameter dome. Around the receiver dome was a large field of wire mesh, forming a reflector.

Targets picked out by the ZAR were then illuminated by the Zeus Discrimination Radar (ZDR). The name describes the purpose of this radar, to discriminate the warhead from nearby decoys. The ZDR watched for differences in velocity as the warheads and decoys decelerated in the upper atmosphere, which is a function of the decoy weight, which implies that you would face larger numbers of lighter decoys or a small number of heavier ones. ZDR was designed to discriminate lightweight decoys, 2% of warhead weight, at altitudes up to 200000 ft. Once the warhead had been picked out, information was passed to the Target Tracking Radar (TTR), the missile launched and tracked by the Missile Tracking Radar (MTR) and then guided to an interception using radio commands. Running all of this was the Target Intercept Computer (TIC), which used twistor memory for ROM and core memory for RAM.

A single Zeus installation would normally consist of six launcher sites, each with sixteen missiles, two TTRs and one MTR. All of these would be fed initial information from a single ZAR and ZDR shared across the entire installation. This would allow an installation to salvo six missiles at a time at different targets, although two missiles would normally be fired at each target. It was expected that the ZAR would take 20 seconds to develop a track and hand off a target to one of the TTRs, and 25 seconds for the missile to reach the target. With these sorts of salvo rates, Zeus was expected to be able to successfully attack 14 "bare" warheads per minute. Its salvo rate against warheads with decoys is not recorded, but would depend on the ZDR's processing rate more than any physical limit.

The original D-15 Zeus A was similar to the original Hercules, but featured a revised control layout and gas "puffers" for maneuvering at high altitudes where the atmosphere was too thin for the aerodynamic surfaces to be effective. The Zeus B interceptor itself was 14.7 m long, 2.44 m wide, and 0.91 m in diameter. This was so much larger than the earlier Hercules that no attempt was made to have them fit into the existing Hercules/Ajax launchers. Instead, the B models were launched from silos, thus the change of numbering from MIM (surface launched) to LIM (silo launched). Since the missile was designed to intercept its targets in space, it did not need large maneuvering fins of the A model. Rather, it featured a third rocket stage with small control jets to maneuver in space. The missile had a maximum range of 250 mi and altitude of 200 mi.

The Zeus was designed to attack its targets through the action of neutron heating. This relied on the interceptor's warhead releasing a huge number of high-energy neutrons (similar to the neutron bomb), some of which would hit the enemy warhead. These would cause fission to occur in some of the warhead's own nuclear fuel, rapidly heating the "primary", hopefully enough to cause it to melt. For this to work, the Zeus required a 400 kt enhanced radiation warhead, and had to manoeuvre within 2 km of the target warhead. The earlier 25 kiloton model from the Hercules was too small for this role. When Zeus B evolved into Zeus EX that worked at even higher altitudes, the W50 also proved too small and a much larger warhead was used.

Specifications

 * Different sources appear to confuse measures between the Zeus A, B and Spartan. These are taken from US Strategic and Defensive Missile Systems 1950-2004 unless otherwise noted: