GS-XS AM-A-19 Project PLATO
In January 1949, the Army established a formal requirement for ballistic missile defense that early in 1951 spawned the PLATO Project that was to provide antiballistic missile (ABM) protection for the field army against short and medium range ballistic missiles. Project PLATO was directed primarily toward the defense of tactical areas against present day missiles and toward missiles which may be expected in the near future. Defense of the zone of the interior [aka CONUS] would also be a consideration.
The Army increased the requirement in 1954 to defend against intercontinental ballistic missiles (ICBM) in the 1960–70 time frame. Primary consideration was given to the performance and design requirements of a guided anti-missile missile system that may be effectively utilized as a countermeasure to ballistic type missiles adapted to carry warheads of extremely high destructive properties.
The objective of Phase I of Project PLATO was to estimate the characteristics of ballistic missiles likely to be used by an enemy in the event of war and to propose a guided anti-missile missile system or other countermeasure, making full use of the experience of previous efforts in this direction. If a defense radius of 50 miles is used, nine such defenses will protect all the large cities in England. If a defense radius of 25 miles is used, 13 such defenses are required to protect all the large cities. If the defense radius is 12.5 miles, 28 such deferses are required.
Although it appeared that it would be possible to design the system to obtain a single shot kill probability close to unity, it may not be possible to obtain such a high probbhility of reliable operation. It may, therefore, be necessary to fire more than one interceptor missile to obtain a near unity engagement kill probability. If these multiple defensive missiles are used in ripple fire, it will be necessary to have a spacing which will prevent the detonation of the first warhead from incapacitating the later missiles. It is estimated that this spacing between interceptors would have to be at least 5,000 ft.
A number of studies emerged, with one in 1956 suggesting a Nike-Zeus variant. PLATO was shut down in 1959, not for technical reasons, but because of funding problems. The follow-on to PLATO was the Field Army Ballistic Missile Defense System (FABMDS) program that began in 1959. However, as this had a long lead time, with an expected operational date of 1967, the Army sought other equipment.
A survey of transmitter tubes suitable for use in the acquisition radar indicated that suitable klystrons can be developed and that a suitable beam power tetrode would be available. The "state of the art" was essentially that there qs no tube available as a standard item which would meet the requirements of the PLATO acquisition radar transmitter. There was, however, a great deal of development activity at all of the manufacturers contacted, namely EitelMcCullough, Inc., General Electric Co., RCA, and Sperry-Rand, Inc. The tube types under investigation included triodes, tetrodes, klystrons, and traveling wave tubes, with power output goals of up to 10 megawatts peak.
Consideration was given to the possibility of a low-level phasing system which would drive power amplifiers associated with antenna elements or groups of elements. In addition to the fact that this method makes possible early realization of an experimental system, it is of more than immediate interest because it potentially provides a number of operational advantages.
Pencil Beam Tracker
The pencil beam tracker operated at S band with a 20-foot diameter dish. This large dish size was necessary in order to obrain satisfactory range with realizable powers. An overall skin-tracking absolute accuracy of 0.5 mil was the objective and had been shown to be approximately attainable. The problem, therefore, was one of designing a tracker about three times the Nike size while maintaining or improving its accuracy, eliminating as much as possible the many sources of absolute accuracy drift and at the same time considerably increasing the dynamic operating rates.
The mechanical studies made it clear that the difficulties associated with unbalanced systems are so great that it is desirable to consider balanced systems only. A study by Steel Prcducts Engineering Company of the mechanical problems associated with a pencil beam precision tracking system indicates thai the weight of the mount and antenna system to meet the PLATO requirements will be of the order of 25 tons. The mechanical tolerances specified for this study were substantially those used for the Nike system. Based on the Nike I field tests, these tolerances should result in an overall system angular tracking accuracy which would match the PLATO system requirements.
Use of a solid propellant rocket engine resulted in some important advantages in the PLATO system. However, there were some disadvantages as compared with the use of liquid propellants. In general, liquid propellants offered higher performance, both in terms of propellant specific impulse and total impulse/weight ratio. In addition, liquid propellants are less sensitive in terms of variation of thrust with engine temperature.
On the other hand, the solid propellant is a simple device with an established reliability in the neighborhood of 99% as compared with the 90% to 95% of liquid propellants. The higher cost of a solid engine was somewhat offset by the need of auxiliary equipment for the liquid. This would inclide propellant handling equipment, provision for inspection of tank linings exposed to corrosive fluids, and check-out for the various moving parts, i.e. valves, pumps, etc. Another advantage of the solid propellant rocket engine was that its casing made of an alloy itself to withstand the high internal pressure can be used as the load carrying structure of the missile. Starting with this basic structure, the wings, nose, boat-tail may be attached to the casing by the use of fittings incorporated in the casing design.
Because the comparison shows advantages for both methods of propulsion, a single design was not selected. The S-3a configuration utilized liquid propellants; the S-7 was designed around a solid engine.
In the optimization of the PLATO vehicle, it was found that in some cases the vehicle would require a means of retaining its structural strength at high rates of heat input. This can be accomplished by several means but only with an increase in takeoff weight which is, of course, undesirable. An optimizing process was, therefore, undertaken with the view to reducing this additional weight to a minimum.
It was determined that the high elevation angle enemy trajectories placed the severest restrictions on the distance which can be defended behind the launch site and that the low angle trajectories limit the defense in the forward direction.
The obvious enemy tactic to penetrate the defense is to exceed the defense capacity of any of the individual components in the PLATO system. This can be accomplished by having more attacking target missiles than can be handled simultaneously by the prediction system, by exceeding the rate fire of the interceptor launchers, or by continuous fire which exhauuts tne supply of interceptors. Because the duration of engagement for a single track is of the order of 90 seconds, it appears that the enemy could well have the capability of mounting a simultaneous attack. Since the PLATO system is being planned to achieve sirmaltaneity with several interceptor missiles, it would have been folly not to give the enemy credit for achieving a similar capability.
Another method by which the enemy can hope to attack the defended area without being intercepted is tc fire more missiles than the defense is capable of intercepting with its available supply of interceptors. This is actually a very costly procedure and would not be resorted to unless all other methods fail. However, it could occur during a sustained attack during which the catastrophic "super saturation" fails to materialize.
A potent countermeasure against the precision tracking radar would be decoy targets released by the target missile near the terminal phase of the interception. It is assumed that these decoy targets would be released in the form of cylindrical rods resonant at the radar wavelength. In the upper atmosphere such rods would presumably travel with the target and not be resolved by the radar. As the air density increases, however, the rods would fall behind the target due to drag differences, and would have the effect of pulling automatic range tracking gates off the true target. Thus the effectiveness of these decoy rods depends on their behaving like the target at high altitude and then separating from the target just before interception.
Study of the feasibility of the use of rods as countermeasure decoys indicated that a few pounds of metal in the form of quarter-inch rods could produce an echo equal to that of a ballistic missile. Round steel rods would lag a streamlined missile by about half a mile at 175,000 ft. and would therefore be resolved from the target by the radar, but might seriously confuse the tracking.
The attainment by the PLATO systea of a high level defensive capability against ballistic missiles may cause the enemy to use other means of attacking the installations in the defended area (such as airplanes), or alternatively, may cause the enemy to attack the defensive system itself ir order to permit effective attack on installations in the defended area. Thus, if the survival probability of the defended area against ballistic attack were 0.99, say, while the survival probability against an air attack of comparable magnitude were significantly lower, say 0.75, it would be Squite likely that the enemy would resort to attack by air rather than by missile. As a consequence, if a high level defense against ballistic missiles is attained, the requirements as to the supply of interceptors and defensive warheads during a war may be greatly diminished since the enemy would be forced to use weapons other than ballistic missiles to achieve success.
The most vulnerable part in the system was undoubtedly the acquisition radar which is continuously radiating and is physically lerge and difficult to disguise. In addition, it is a part of the system which is not functionally required to be redundant. Other parts of the system were vulnerable to a varying but lesser degree. The large radar antennas will suffer moderate damage (sufficient to prevent use until extensive repairs are effected) from atomic blast overpressures of the order 3 psi and that radios, radar, and other electronic equipment are modertely damaged by 5 psi. The blast effect is considered to be the major cause of damage to equipment of this type.
In order to prevent saturation of the system, several launchers are required at each launch site. The number of launchers required will be small if the missiles at all three sites have sufficient range capability to cover the entire defended area. If the missile coverage from the three sites does not overlap, more missiles and warheads will be required at each site.
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