A Layered Defense: The SDI Architecture
The architecture, or basic plan, drawn up by the Fletcher Committee envisions a four-layered missile-defense system (see Figure 5). Each layer would be designed to intercept as much of the attack as possible. The attack goes through four sequential phases: (1) the boost phase , during which the great booster rockets bring the payload up to the speed required for intercontinental ballistic flight; (2) the post-boost phase , during which a "bus," or post-boost vehicle (PBV), sets each of several warheads (MIRVs) onto trajectories leading to specific targets; (3) the midcourse phase , during which the RVs coast along inertial paths far above the earth's sensible atmosphere; and (4) the terminal phase , during which the RVs reenter the earth's atmosphere and ultimately explode at or near their targets. Those who believe there is promise in this scheme base their opinion on two major factors.
First, each defensive layer compounds the effectiveness of the others. As the proponents point out, if each layer intercepts 90 percent of the warheads reaching it, altogether they would intercept 99.99 percent of those launched. Thus, if ten thousand warheads were launched, nine thousand would be destroyed in Phase I, nine hundred of the
Fig. 5. Major SDI Sensors and Weapons
remaining thousand in Phase II, ninety of the remaining hundred in Phase III, and nine of the remaining ten in the final phase. Unlike the single-layer missile-defense systems previously considered, if the weapons systems constituting any single layer failed or were overcome by successful enemy countermeasures, the other three could, in principle, largely compensate.
Second, defensive systems have not undergone a thorough review since the early 1970s. And, of course, many of the pertinent technologies (especially lasers, infrared detectors, and computers) have evolved very substantially since that time; important new ones (notably the free-electron and X-ray lasers) have also emerged. SDI proponents claim that these new technical advances open new technological possibilities for intercepting offensive RVs during each of the four flight phases and thus lead to the very high levels of "compounding" that make a real defense against missiles truly feasible. To appreciate both the possibilities and the remaining obstacles, it is essential to examine the technical issues affecting each phase.
Phase I: Boost-Phase Intercept
The boost phase is the first stage of a missile's flight. It includes that part of the missile's trajectory during which the great booster engines of the rocket are under power—that is, from takeoff until the last engine is shut off, typically at an altitude of about 250 km, some three minutes after launch. SDI research has focused most attention on interception during this phase. There are three good reasons for doing so.
First, offensive missiles are in their most vulnerable, or "softest," state during the boost phase. Boosters consist mainly of fuel tanks that must be as light as possible. It is therefore far easier to puncture these tanks or otherwise damage the missiles while they are still in powered flight. Otherwise, either the bus or the RVs themselves must be destroyed during later phases. Second, typical modern strategic missiles carry many nuclear warheads—up to fourteen in one case. Successful interception in the boost phase stops them all and thus accomplishes much more than would the interception of a partially empty bus or a single RV in one of the later phases. Third, the offensive systems are especially easy to identify and track during this phase. Both the United States and the Soviet Union have long had satellites deployed in geosynchronous orbits that are capable of detecting and, in principle at least, tracking the brilliant rocket plumes so characteristic of the boost
phase. Current systems can provide early warning that an attack is being launched, but they cannot provide the kind of tracking information needed for an actual intercept. SDI planners are therefore seeking to develop the Boost Surveillance and Tracking Satellite (BSTS), which would have this added capability. Essentially, the aim is to achieve a very substantial upgrading of the systems now in place in order to make them adequate for this new, more demanding purpose.
After the enemy strategic missile has been detected and tracked, it must somehow be destroyed. There are two quite different approaches to this problem. One is based on kinetic-energy kill weapons (KKW) devices, and the other on "directed-energy weapons" (DEW). Each is further subdivided into a number of specific technical possibilities.
According to SDI advocates, the most promising near-term boost-phase interceptor is one based on the use of Space based kinetic kill verhicles (SBKKVs), designated in 1987 as space-based interceptors (SBIs). As currently conceived, these vehicles would be relatively small (approximately 100 kg) missiles capable of accelerating a small but destructive payload (a projectile) up to velocities of several kilometers per second. They would be equipped with homing devices, presumably relying on infrared sensing, capable of causing the projectile to collide directly with and thus destroy the offensive missile while it is still in its boost phase. Individual SBKKVs would be mounted in groups (perhaps of ten) on a "mother" satellite—sometimes called a garage, sometimes a battle station—that would release them at the proper time. The mother satellite itself would also play a role in overall command and control and perhaps in acquiring and tracking the target as well.
Given the finite velocity of the SBKKV (perhaps 6 km per second)[10] and the short duration of the boost phase (currently some hundreds of seconds), the battle stations must be within roughly 1,000 km of their targets at the moment they are needed. This in turn means that they must be in low earth orbits and hence moving at very high speeds, comparable to those of their ICBM targets and the SBKKVs themselves. In order to be ready to intercept an attack at any arbitrary time, the battle stations must be scattered at locations above most of the earth's surface. Only a relatively small fraction of them will be in range of any particular missile launch at the moment it takes place. Thus, many more SBKKVs must be in orbit than would be needed to destroy the total inventory of enemy ICBMs. The interceptors that cannot reach their targets in time are called "absentees."
The "absentee ratio" (the ratio of the total number of interceptors to
the number in position) was much debated during the first years of SDI. Some asserted one particular ratio, and others denounced the use of differing ratios as politically motivated. In fact, the actual absentee ratio is critically dependent on the detailed parameters of the systems involved, including especially the maximum speed of the SBKKVs and the duration of the boost phase of the offensive missile. There is thus no such thing as the absentee ratio but, rather, a very wide range of possibilities from as low as 7:1 for the most optimistic assumptions (implicit in the Marshall Institute report, for example)[11] up to values many times that figure.
In view of the obvious importance of the weight and the speed of the KKWs, the current R&D program focuses much attention on both parameters. Most analysts agree that the current weight (approximately 300 kg if the KKWs are outfitted with the current F-15 ASAT payload) must be reduced by ten- to thirtyfold in order for the system to become economically practicable, and that speeds closer to 10 km per second would be advantageous. No physical laws stand in the way of either goal, but the engineering problems are formidable. For instance, for the rockets now available, the final weight of a rocket warhead accelerated to 6 km per second is less than one-twentieth of its initial weight, and the final weight must include the homing system as well as its destructive component.
Other KKW systems under consideration are based on alternative means of producing very high velocity projectiles, such as electromagnetic railguns, but the acceleration and guidance systems for such projectiles are still at a highly speculative stage. For chemical rockets, the final velocity of the payload is limited by practical considerations to values a few times that of the rocket-exhaust gases, which in turn have a practical limit of 3 km per second. In the proposed electromagnetic railgun, no such limits apply, at least in theory, because the projectile is propelled to extremely high velocities by a combination of fields of electrical currents. But adequate means for controlling and steering such projectiles have not yet been invented.
SDI advocates also believe that a number of directed-energy weapons hold promise for use as boost phase interceptors in the long run. These include chemically and electrically driven optical lasers, the free-electron laser, and the new and potentially much more powerful X-ray laser. In principle, the properties of lasers are particularly well suited for such applications. These devices produce extremely intense beams of so-called coherent light or other forms of electromagnetic radiation that
can be focused down into an extremely tight beam—one that spreads out very much more slowly than any other known type of light ray—as it travels for great distances. Within the atmosphere, various scattering processes can cause the beams to disperse, but in empty space beams can travel indefinitely over great distances without much diffusion.
In some cases (the chemical laser, for example) the laser would be mounted on a battle station, which would aim the laser at a particular target and cause it to operate at just the right moment. In other cases (such as the FEL, discussed below), the laser would be mounted on the ground, and the beam produced would be passed to a series of relay mirrors in space, eventually focusing it on the target. Because the velocity of light is for these purposes practically infinite, these systems are not constrained by time-of-flight considerations, as is true of the SBKKV, which must be within perhaps 1,000 km of the target in order to reach it in time.
But even a laser beam spreads out after it travels very large distances. So huge mirrors must be used to focus the laser beam down to an adequately small spot, perhaps a meter or less. The larger the source-to-target distance, the larger the mirror has to be. For typical situations (involving infrared lasers operating in space at a power level even as high as 25 mw), the mirrors would have to be 10 meters or more in diameter. For a configuration in which the lasers are mounted on the ground and their beams routed to the target by relay mirrors, the primary mirror must be even larger. Mirrors up to a few meters in diameter have been used both in space and on the ground, but the giant mirrors apparently required for the purposes of SDI are still only theoretical possibilities. A currently favored approach is to break the single huge mirror up into a large group of smaller ones, making a so-called segmented mirror. Such mirrors must be aligned with each other to an unprecedented degree of precision, however, in order to get them to act in the sort of unison required to produce tightly focused beams. In technical parlance, their output beams must all be "coherent" in order to achieve the focusing that a single large mirror produces. In principle, this is possible, but it has never been accomplished on the scale or in the circumstances required for this application.
The absentee problem also applies to DEWs. Each available laser must therefore destroy many targets sequentially and quickly. To do so, the "dwell time" on any one target, combined with the time needed to "slew," or move, to another one, must be kept very small—seconds or less. This in turn means that the power of the laser—the energy it can
deliver in unit time—must be correspondingly large when used in a BMD mode.
The study produced for the American Physical Society noted that chemical-laser technology is not yet adequate for the demands of boost-phase interception: "We estimate that chemical laser output powers at acceptable beam quality must be increased by at least two orders of magnitude for HF/DF [hydrogen fluoride/deuterium fluoride] lasers for use as an effective kill weapon in the boost phase."[12]
The free-electron laser (FEL) is a very powerful device that, in its current versions, is large and heavy and requires great amounts of electric power. In the first place, it involves the construction and operation of an extremely powerful electron-beam accelerator. For typical SDI applications, the energy of the individual electrons must be at least 100,000,000 electron volts, the currents must be from 10,000 to 100,000 amperes (for a total peak power of 1 to 10 trillion watts), and the quality of the beam must be very finely controlled. The beam is passed through a magnetic "wiggler" that produces certain crosswise oscillations in the beam's motion. Under the right circumstances, this "wiggling" beam can be used to generate (or amplify) light to form an extremely powerful and highly focused laser beam. By varying either the beam energy or the spacing of the wiggler, it is possible to vary the wavelength of the light emitted. Thus, in principle, the FEL has considerably more flexibility than most other lasers, including the chemical type described above. The idea is to deploy FELs on the ground and to deliver the laser energy to the targets by means of a series (at least two) of huge relay mirrors orbiting in space. Problems include the construction, maintenance, and operation of these mirrors (including fast slewing). In addition, because the laser is ground-based, the output optics must cope with distortion caused by the atmosphere—the very distortion that causes stars to "twinkle." For the application envisioned here, such distortions, if uncorrected, would cause the beam to spread out to such a degree that it would become totally ineffective.
In brief, the distortion of FEL beams could be corrected by, first, determining the nature and cause of the instantaneous distortions by means of a "reference" beam. The laser's main beam-forming mirror would then have to be distorted so as to compensate for them. Corrections are feasible, in principle—but, again, they have never been accomplished on the scale and in the circumstances involved in SDI applications. In addition to all these other problems, ground-based lasers cannot penetrate cloud layers. Therefore, enough FELs must be deployed in many meteorologically independent locations, each with good enough
average weather so that there are virtually no periods when all might be blocked by clouds. The APS report concluded that for strategic-defense applications, FELs "require validation of several physical concepts"[13] thus far only theoretically developed.
The nuclear-pumped X-ray laser is a novel concept dating from the late 1970s. The enormous energy of a nuclear-weapon explosion is used to energize the lasing medium, which creates a laser beam consisting of relatively soft X-rays. The X-rays produced by such devices are readily absorbed by air, however, and are therefore useful only when operated in outer space and fired against objects that are themselves located at extremely high altitudes, generally above 80 km. Research on these unusual devices is said to be making progress, but nearly all authorities believe they are still a very long way from any military application. The total energy that could be produced by such a system is huge and can be divided, in theory, among many independently directed beams. Precisely because the X-ray laser is potentially so powerful and destructive, it would constitute an especially rich and attractive target for any counterdefensive action that might precede a large-scale offensive strike. For that reason, its advocates usually propose that it not be placed permanently in orbit, but be employed in a "pop-up" mode instead. That is, it would be deployed on the surface or in submarines and be lofted into space only when needed. But the pop-up mode requires the system to be forward-based, very near to Soviet territory; the rockets would have to be ready to be launched on seconds' notice.
Because such constraints and requirements greatly limit the utility of such a device in a strictly defensive mode, the X-ray laser is now usually thought of as a potential counterdefensive system—that is, a weapon for destroying an enemy's space-based defenses, including its own counterpart. Thus, the nuclear-pumped X-ray laser might be used in the first phase of an attack to destroy an enemy's space-based strategic defense. Such a strike would assure successful penetration of the follow-on offensive forces. In this role, the X-ray laser would be used essentially as an ASAT, a role that is always less demanding than BMD. The APS study reserved judgment on the military utility of the X-ray laser pending further demonstration of the technical potential, concluding that these devices "require validation of many of the physical concepts before their application to strategic defense can be evaluated."[14]
Countermeasures and Counter-Countermeasures Countermeasures designed to thwart all of the boost-phase systems described above have been proposed and were reviewed at the outset by one of the panels of
the Fletcher Committee. Decoys and certain tactical ploys can be used to confuse the boost-phase acquisition and tracking system. In the case of the interceptors themselves, the easiest countermeasure to institute and the hardest for the defense to overcome is the so-called fastburn booster. Indeed, existing U.S. ICBMs and the generation of Soviet ICBMs (in particular the SS-24 and SS-25) now being developed have much shorter burn times than the aging Soviet SS-18, against which the first-generation SBKKVs are oriented. Moreover, by pushing currently available solid-rocket technology to its limits, the offense can, at only a modest penalty in performance, cause the rocket boosters to complete the task of accelerating payloads in less than 60 seconds while still at an altitude of only about 80 km—greatly aggravating the "absentee" problem. In the view of most analysts, the number of battle stations that would then be required for SBKKVs is so high as to become impossible in practical terms. The lower altitude of booster burnout also makes the X-ray laser much less promising: X-rays can penetrate the atmosphere down to only about 80 km. And to counter the type of directed-energy weapons that can easily penetrate the atmosphere, the booster may also be spun about its long axis so as to spread out the incoming laser energy; lightweight protective coatings may also be added for further protection.
It is possible, in addition to deceiving and foiling the defenses, to attack them directly as well. All the space-based components are vulnerable to attack by similar systems deployed by the other side or by so-called space mines; they can be attacked by ground-based ASAT systems as well. As already noted, the pop-up version of the X-ray laser is one example of such a counterdefensive system.
But the story does not end there. All these countermeasures have, conceptually at least, counter-countermeasures. The deployment of more, lighter, and especially faster SBKKVs might mitigate the problems created by the fast-burn booster. Lasers with still more power might come along to defeat the boosters' defenses against earlier DEWs. And the space-based defensive assets could deploy a variety of self-defenses—including proliferation, maneuvering, decoys, and deception—to foil or confound a counterdefensive attack.[15]
The basic point is that this process is endless. In a "man-againstnature" contest, such as getting a man to the moon, once the problem is solved it stays solved. In a "man-against-man" contest, once a particular problem is solved, once we have learned to deal with a particular countermeasure, another arises to take its place—and so it goes, on and on,
generally without ever coming to an end. There is no "last move." It is misleading to suggest there might be. Strategic defenses are not something that can be developed and installed at a certain time; they must be viewed as a process that, once begun, never reaches a permanent end.
The forty-year contest between the strategic arm of the U.S. Air Force and the Soviet National Air Defense Force (PVO Strany), reviewed in chapter 2, is a case in point. Throughout that long period, the Soviets have been creating and modernizing a defense in depth against an airborne attack. They have tried to build, in effect, a multilayered air-defense system analogous to that promoted by Reagan and the SDIO for defense against missiles. Over the same forty years, however, the U.S. Air Force has been constantly improving and updating its procedures and technologies for assuring bomber penetration. These have included electronic countermeasures, greater speed, the ability to fly very close to the ground, and a variety of standoff weapons, from the Hound Dog of the 1950s to the SRAM and ALCM of today. At no time during this long period has the total multilayer Soviet defense ever even approached the 90 percent kill-capability that SDI advocates project for each individual layer of the proposed missile-defense system. The U.S. Air Force remains confident of its ability to penetrate Soviet defenses.
There is at present no reason to believe that the final result of an effort to build strategic defenses will be any different in the case of SDI than in the case of the Soviet air Soviet air-defense system. The problem for both offenses and defenses will be far more complex, but the results will not necessarily favor the defense. Michael M. May has hinted at the possibilities in a way that makes the use of the Star Wars label especially apt:
If space-based weapons were to be developed and deployed … all space assets, whether needed for defense or offense, for warning or other purpose, would have to be operated in a hostile environment. They would have to be hardened beyond anything now contemplated, at commensurate cost, or alternatively, be mobile, defended, proliferated or hidden. Space hardware would replicate the characteristics of earthbound military hardware. We would have the space analog of tank corps, carrier battle groups and stealth bombers.[16]
Phase II: Post-Boost-Phase Intercept
After the boosters burn out, a post vehicle, commonly called a "bus," launches each of its RVs onto trajectories leading to particular targets. This procedure takes several minutes, requiring the bus to fire small, on-board rocket engines more or less continuously as its course
is adjusted from one trajectory to the next. It is therefore possible, in theory, to detect and track the bus by means similar to those used for the main booster. But the engines of the post-boost vehicle produce much less thrust and energy, so the detection systems must be much more sensitive. Similarly, once acquired and tracked, the bus may be attacked by systems similar to those used for the boost-phase intercept. But, again, the problem of attacking the bus is harder because it is smaller, less visible, and generally tougher than the booster. As the bus deploys its RVs and decoys, interception gets progressively harder, and the third phase begins. Moreover, a bus is not an absolute requirement if independent guidance systems were to be incorporated into each warhead. All of the defensive weapons described above as interceptors for Phase I can be used for interception in Phase II, including some of those SBKKVs on platforms that were out of attack range during Phase I.
Phase III: Midcourse Intercept
After phases I and II are completed, the midcourse phase, lasting some twenty minutes, begins. During this phase the individual RVs (perhaps originally amounting to ten thousand, but perhaps greatly reduced by the interceptions effected during the previous phases) ineluctably follow ballistic trajectories toward the targets. They still emit infrared radiation, but at longer wavelengths and at far lower power levels than during the previous phases. Indeed, the individual RVs are about the same size and temperature as a human being. One may therefore understand the problem by comparing it to the task of detecting a human being just by the heat the body radiates using a device located thousands of miles away.
As in phases II and IV, perhaps the most effective countermeasure in Phase III is the deployment of decoys. As long as the RVs remain in outer space, the decoys may be of very low density—in the form, for example, of metallic balloons. There is no drag, and only the surface characteristics of the decoys—shape, reflectivity, emissivity—need to mimic the real thing. The decoys are usually thought of as weighing one one-hundredth as much as the RV itself, so there could be about a hundred times as many of them.
Those who are optimistic about SDI's prospects generally believe that detection and analysis of the emissions from the RVs and the decoys will enable discrimination. To distinguish the RVs from the decoys, they propose the use of various means of "active" or "interactive" discrimination. One such technique involves the use of laser beams that would
transfer enough momentum to a lightweight balloon to change its velocity by an amount that could be detected by a Doppler laser radar. Another involves the use of a beam of high-energy, neutral hydrogen atoms that penetrate deeply into even the heavy RVs and interact very differently with balloons and decoys. It is also assumed that interceptors similar to those employed in the initial phases would also be used here. For both detectors and interceptors, however, the task is much harder in Phase III. As a result, less attention has been devoted to interactive discrimination, and none of the near-term deployment schemes focuses on it. An elaborate experiment, code-named "Delta 181," was conducted in February 1988, at a cost of $250 million, to collect information on how objects look in space, including potential decoys. Some important elements of this experiment failed, but large amounts of data were produced, which have yet to be fully analyzed.
Phase IV: Terminal Defense
Finally, the offensive RVs, hidden among a cloud of decoys, come to a point where they must reenter the atmosphere. Atmospheric drag then effectively slows down the balloon decoys, and the RVs must proceed on their own, unprotected during the last minutes of the flight—save, perhaps, for a few, much heavier, decoys.
The means proposed for intercepting the warheads that remain at this point are similar to those first proposed in the 1950s. In brief, ground-based radars, perhaps housed in movable vehicles and aided by an airborne optical system (AOS), would detect and track the incoming warheads. The radars would pass the information on to the computers, which would interpret it and then instruct interceptor rockets to lift off and proceed on a course toward the incoming RVs. When in range, the interceptor missiles would release a KKV, which would use a passive or active homing device to intercept and destroy the incoming RVs by impact or explosion nearby. This intercept system might be extended into the late midcourse regime if there were no cloud of decoys at that point. The current plan for accomplishing terminal defense includes two contemplated missile systems, the High Endoatmospheric Defense Interceptor (HEDI) and the Exoatmospheric Reentry Vehicle Interceptor System (ERIS). HEDI is designed as a high-acceleration weapon that would intercept any remaining warheads. The objective would be to destroy the warheads as high in the atmosphere as possible because they could be salvage-fused to explode on contact; an explosion at a low altitude,
even if not yet on its target, would still do severe damage. ERIS would also be ground-based but with slower acceleration and longer range; it could intercept warheads in late midcourse, just before they reenter the atmosphere.
Countermeasures against this type of defense have been known for a long time. They include exhausting and saturating the defenses, using heavy decoys, countersimulation, precursor nuclear bursts, and the like. If a terminal defense were the only layer deployed, such countermeasures would probably work. But if it were deployed as the last of a four-layer system, it could probably destroy the remnants of even a large attack, depending on how the other layers had functioned, of course.