|Manufacturer||Western Electric & McDonnell Douglas|
|Weight||29,000 lb (13,100 kg)|
|Length||55 ft 2 in (16.8 m)|
|Diameter||3 ft 7 in (1.08 m)|
|Warhead||W71 nuclear; 5 Mt|
|Engine||1st Stage: Thiokol TX-500 (2200 kN);
2nd Stage: Thiokol TX-454;
3rd Stage: Thiokol TX-239
|Wingspan||9 ft 9.6 in (2.98 m)|
|460 mi (740 km)|
|Flight altitude||350 mi (560 km)|
The LIM-49A Spartan was a United States Army anti-ballistic missile, designed to intercept attacking nuclear warheads from ICBMs at long range and while still outside the atmosphere. For deployment, a defensive five-megaton atomic warhead was planned to destroy the incoming ICBM. It was part of the Safeguard Program.
Spartan was the ultimate development in a long series of missile designs from the team of Bell Laboratories and Douglas Aircraft Company that started in the 1940s with the Nike. Spartan was developed directly from the preceding LIM-49 Nike Zeus, retaining the same tri-service identifier, but growing larger and longer ranged, from the Zeus' 250 miles (400 km) to about 450 miles (720 km).
Spartan was initially developed as part of the "Nike X" project, later becoming the Sentinel Program. This was eventually cancelled and replaced with the much smaller Safeguard Program, which was eventually deployed from October 1975 to early 1976.
The US Army started their first serious efforts in the anti-ballistic missile arena when they asked the Bell Labs missile team to prepare a report on the topic in February 1955. The Nike team had already designed the Nike Ajax system that was in widespread use around the US, as well as the Nike Hercules that was in the late stages of development as the Ajax's replacement. They returned an initial study on Nike II in January 1956, concluding that the basic concept was workable using a slightly upgraded version of the Hercules missile, but requiring dramatically upgraded radars and computers to handle interceptions that took place at thousands of miles an hour.
Work began on the resulting LIM-49 Nike Zeus system in January 1957, initially at a low priority. However, several developments that year, including the development of the first Soviet ICBMs and the launch of Sputnik I, caused the schedule to be pushed up several times. In January 1958 Zeus was given "S-Priority", the highest national priority, with aims to deploy the first operational sites in 1963.
To fully test the system, the Army took control of Kwajalein Island from the US Navy, and began building an entire Zeus site on the island. By 1962 the system was ready for testing, and after some initial problems, demonstrated its ability to intercept warheads launched from California. Eventually fourteen "all up" tests were carried out over the next two years, with ten of them bringing the missile within the lethal radius of its warhead, sometimes within a few hundred meters.
In spite of Zeus' smooth testing program and successful interceptions, it was becoming increasingly clear that the system would not be effective in a real war scenario. This was due primarily to two problems; decoys would shield the warhead from detection until it was too late to intercept it, and the rapid increase in the number of ICBMs threatened to overwhelm the system
The former problem was becoming increasingly obvious from about 1957. Missiles designed to carry a specific warhead found themselves with excess throw-weight as warhead physics improved and they became smaller and lighter. Even a small amount of excess capacity could be used to throw radar decoys or chaff, which are very light weight, and would provide additional radar returns that would make it difficult to pick out the warhead. As long as the decoys spread out or blocked an area larger than the lethal radius of the interceptor, several interceptors would have to be launched to guarantee the warhead would be hit. Adding more decoys was extremely inexpensive, requiring very expensive ABMs to be added in response.
At the same time, both the US and USSR were in the midst of introducing their first truly mass produced ICBMs, and their numbers were clearly going to grow dramatically during the early 1960s. Zeus, like Hercules and Ajax before it, used mechanically directed radar dishes that could track only one target and one interceptor at once. It was planned that Zeus bases would actually consist of several launcher sites connected to a central control, but even in this case the site might be able to guide perhaps four to six missiles at once. With the ICBM fleet reaching hundreds even before Zeus could become operational, it would be easy to simply overwhelm the defense by flying enough warheads over it that it couldn't guide interceptions rapidly enough.
The solution to both of these problems is to improve speed, both of the defending missiles and the defense system as a whole.
Decoys are less dense than warheads, and not aerodynamic. Therefore they are subject to more deceleration when they begin the re-enter the upper atmosphere. The warhead, which is dense and streamlined, experiences less deceleration from air resistance, eventually flying out in front of the decoys. The rate at which this happens depends on the types of decoys used, but the warhead will have pulled past even advanced types by the time it is between 250,000–100,000 feet (76,000–30,000 m). At that point the warhead is open to attack, but leaves only 5 to 10 seconds before impact. To handle these scenarios, a very high speed missile was required. Zeus was simply not fast enough to perform such an attack, it was designed for interceptions lasting about two minutes.
Likewise, the solution to dealing with massive numbers of warheads was to use faster computers and radars, allowing many interceptors to be in flight at once. Zeus was being developed just as digital computers were starting a massive improvement in performance through parallel processing, and radar systems were likewise introducing the first phased array radar (Passive electronically scanned array) systems. Combining the two would allow hundreds of warheads and interceptors to be tracked and controlled at once. As long as the interceptor missile wasn't significantly more expensive than the ICBM, which was likely given to their relative sizes, overwhelming such a system would be a losing proposition.
Studying all of this, ARPA outlined four potential approaches to an ABM system. The first was Nike Zeus in its current form. The second was Zeus combined with a new radar system, the third included new radars and computers. Finally, the "X" plan called for all of these changes, as well as a new short-range missile. As the shorter range missile would overlap with Zeus, X also called for Zeus to be modified for even greater range as Zeus EX. After considerable debate, the decision was made to cancel the existing Zeus deployment and move ahead with the X plan.
The original Zeus had been designed to attack warheads in the upper atmosphere, at altitudes of about 25,000 feet (7,600 m). At this altitude the blast effect of the warhead is greatly attenuated, simply because there isn't enough air to carry the shock wave. Zeus relied instead of the action of neutron heating, which in the same thin atmosphere would work over distances on the order of a few kilometres. In this system, the warhead is designed to give off large numbers of neutrons, which strike the enemy warhead and cause the nuclear fuel inside it to undergo induced fission events. If this occurs rapidly enough the warhead will "fizzle", and no longer detonate properly.
When Zeus was modified to Zeus EX, its range was greatly increased and the expected interception altitudes raised by an equal amount. Intercepts would now take place as high as 450 miles (720 km), well outside the atmosphere. At this altitude the x-rays generated by the warhead are free to travel long distances, as there are not enough air molecules to block their progress. When the x-rays strike the incoming warhead, they heat it so rapidly that shock waves are formed that can break it up. These effects operate over very long range, depending on the size of the warhead.
To take advantage of this effect, the Lawrence Livermore National Laboratory developed the W71 warhead. This was lined with gold instead of the traditional lead or depleted uranium, maximizing the x-ray production. Under good conditions, this warhead had a lethal radius as much as 30 miles (48 km), although it was later stated to be 12 miles (19 km) against "soft" targets, and as little as 4 miles (6.4 km) miles against hardened warheads.
However, it was also realized that this same effect would cause an electromagnetic pulse that might make it difficult for the radar systems to see other warheads until that effect faded. The warhead's own fireball also presented an opaque mask to radar, allowing following warheads to hide behind it.
Zeus had been cancelled in 1962 when as it became clear the system offered very little in the way of protection against the sorts of large-scale attacks being envisioned by the 1970s. Nike X had been designed specifically to handle these cases, with a single site expected to be able to track 20 warheads and attack 10 of them at once. However, Nike X soon found itself with similar problems with the introduction of MIRV.
As missile performance grew and warhead size shrank, by the mid-1960s it became possible for a single missile to throw several warheads. This produced the "MRV" system, short for multiple reentry vehicle, which allowed the damage to be spread over a greater area, particularly effective against cities. Against such a system, ABMs like Spartan remained effective; under ideal conditions, a single Spartan could destroy all the warheads launched by a single missile. However, with the addition of a small rocket motor and a suitable guidance system, MRV became "MIRV", the multiple independently targetable reentry vehicle. In MIRV, each warhead could be directed against a different target, separated by great distances. This meant that several Spartans would have to be launched to shoot down a single missile's payload. As the cost of the interceptor and ICBM were roughly the same, one could afford to simply overwhelm the ABM system by adding a few more ICBMs with multiple warheads.
To illustrate this problem, consider an ABM system tasked with the defence of the entire USA, and an enemy that wishes to attack New York, Chicago, Washington, Dallas and Los Angeles. Using single-warhead missiles or MRV, one missile would be needed for each target. The ABM system could then add a single interceptor missile to counter it. However, with MIRV, a single ICBM with five warheads could attack all of these targets at once, while the ABM system would still need five interceptors. But the problem is actually much worse; as the MIRV could choose to launch all five of its warheads at a single target, each ABM site would need to have five missiles, not one. Adding a single additional ICBM would demand another 25 interceptors in response.
The Spartan missile was in operational service for only a few months, from October 1975 to early 1976. A combination of high costs and the SALT I treaties made the missiles unpopular politically.
- Aircraft of comparable role, configuration and era
- Related lists
- M. Todd Bennett (ed), "National Security Policy, 1969–1972", 2011, p. 41.
- Bennett 2011, p. 54.
|Wikimedia Commons has media related to Spartan missiles.|
- Directory of U.S. Military Rockets and Missiles
- a further development of the Nike Zeus B missile
- index of pictures
- Mickelsen Safeguard Complex
- W71 nuclear warhead for the Spartan