The primary requirement for the MMW program was to provide prime power to space based weapon and sensor systems, as well as providing power for the Orbital Transfer Vehicles needed to deploy these components.
From the outset, the SDI program focused on the technologies needed to develop a layered defense that would initially attempt to intercept ballistic missiles during the boost phase, the first few minutes of their flight when the rocket motor is still burning. Additional layers would attempt interception in the post-boost phase when the missile's warheads are released into space, the mid-course phase, the approximately 20 minutes that it takes the warheads traverse intercontinental distances, and the terminal phase as the warheads reenter the Earth's atmosphere.
By adding additional layers, the SDI hopes to improve the overall effectiveness of the defense. Thus, while two layers that are each capable of intercepting 50% of incoming warheads would intercept only 75% of the warheads, four such layers would intercept almost 95% of the warheads. Additional layers could further reduce the leakage of warheads through the defense. And since each layer would use different types of sensors and interceptors, the defense as a whole could be less vulnerable to countermeasures.
Several factors render the boost-phase intercept the most highly leveraged layer of the defense:
+ The number of targets to be intercepted is relatively small (a few thousand missiles) compared to the large number of objects that would be present in later phases (tens of thousands of warheads, hundreds of thousands of decoys, and billions of bits of chaff and aerosols).
+ The exhaust plume of ballistic missiles during the powered portion of their flight produces a large thermal signature that is relatively easy to detect and difficult to simulate. This is in contrast to the much greater difficulty of detecting the much smaller signature of warheads during midcourse, and the greater ease of implementing anti-simulation decoys during this phase.
+ Boost phase kill assessment is relatively simple, since lethal damage to ballistic missiles results in their catastrophic destruction, which is quite visible, while lethal damage to warheads may be much more difficult to observer unambiguously. In the absence of positive kill assessment, defense resources may be wasted servicing targets which have already been killed.
Despite the high leverage of boost-phase engagement, there are several major challenges that must be overcome if these advantages are to be realized:
- In practice, gaining line-of-sight access to the boost of long-range ballistic missiles using ground-based systems is extremely difficult, and is effectively impossible for most classes of weapons.(1) Thus boost-phase engagement of requires that weapons be based in space.
- Space-based systems in low (less than 2,000 km altitude orbits) will spend much of their time passing over portions of the globe that are not of military interest. The portion of time a battle station is over an area of interest is known as its "absentee ratio." This figure varies according to the range of the weapon system and the altitude at which it is deployed. In principle, a weapon deployed in geostationary orbit over an area of interest would have an absentee ratio of 1, if it had an effective engagement range of over 36,000 kilometers. In practice, such long ranges are very difficult to achieve, and deployments in lower orbits (less than 2,000 km) will have absentee ratios that may range from 10 (for high-performance long-range directed energy weapons) to 100 for low-performance short-range kinetic energy weapons). The total number of weapon platforms that must be deployed in space is the number that will be required to be present in the battlespace, multiplied by the absentee ratio.
- In addition to absentee ratio considerations, total constellation size is a function of the duration of the boost-phase, as well as the number of targets that each weapons platform can service during the boost-phase. Current liquid fueled missiles typically have a five minute boost-phase, while solid fueled missiles typically have a three minute boost phase. During the first minute or so of the boost phase of a missile's flight it is inside the Earth's atmosphere, which shields it from engagement by most space-based weapons. Higher acceleration solid fueled missiles, so-called "fast burn boosters" can have shorter boost phases which reduce the amount of boost-phase time the missile spends above the atmosphere. In addition, missiles can be hardened or rotated to reduce their vulnerability to directed energy weapons, increasing the amount of time required to destroy them. All of these countermeasures can increase the number of space-based weapons that must be within the battlespace, an increase in constellation size that is compounded by the absentee ratio problem.
- Space-based systems are vulnerable to hostile defense suppression tactics, using various types of ground-based directed and kinetic energy and nuclear armed anti-satellite weapons, as well as pre-deployed space mines. Although survivability measures, such as hardening, maneuverability, and shoot-back tactics can reduce these vulnerabilities, they add to system complexity, mass and cost, and can reduce the leverage of boost-phase systems.
Thus the military and strategic utility of space-based weapon systems is highly contingent on the scope and character of the threat.
1 - Space-Based Directed Energy Weapons
Space-based directed energy weapons have a number of attractive features that make them a leading candidate for boost-phase engagement weapons. Beams travelling at the speed of light (300,000 km/sec) from lasers, or at near light speed from particle beam weapons, can rapidly deposit lethal fluences of energy on ballistic missile targets at ranges of thousands of kilometers. Since laser system brightness varies as the inverse of the square of the wavelength, substantial improvements in brightness can be realized for a given power and aperture, if the wavelength is decreased and the decrease is accompanied by commensurate improvements in optics figure quality, wavefront control/beam quality and pointing accuracy. There are at least two concepts for producing such short wavelength laser beams.
Excimer lasers utilize Excited Dimers, such as Xenon and Chlorine (which emit at 0.30 microns), Xenon and Fluorine (which emit at 0.35 microns), or Krypton and Fluorine (which emit at 0.25 microns), that are electrically stimulated to emit coherent light at wavelengths in the near ultraviolet region of the spectrum. This provides a higher lethality against hardened targets than is possible using infra-red lasers. There are two candidate operating modes - single pulse (SP) or repetitively pulsed (RP). However, because these lasers have very high power requirements (several GWe) they have not been a leading candidate for space-based systems.
Free electron lasers (FELs) operate by interacting high energy electron beams with magnetic wiggler fields to convert the electron beam kinetic energy into optical radiation. Compared to Excimer lasers, FELs offer simpler power conditioning requirements and a relatively mature technology base derived from work on electron beam accelerators. Because of their wavelength selectability and relatively high electrical efficiency, FEL devices are promising candidates for space-based systems. However, lasing gain per pass through the optical resonator cavity increases as the cube of the length of the resonator, and gains thus far demonstrated with multi-meter resonators have been only a few percent. Resonator mirror coatings have also demonstrated short lifetimes when exposed to ultraviolet radiation. Power requirements for such systems could range from 200
to 729 MWe according to one estimate(2), while another study suggested a range from 143 to 316 MWe.(3) Depending on the brightness of the laser device, from several dozen up to a hundred FEL battle stations, each weighing several hundred tons, would be required for an operational constellation. A number of configurations have been suggested for such weapon platforms (Figures II-45 through II-50). One of the key constraints on FEL platform design is the required distance between resonators, with higher power devices requiring greater separations. One analysis suggested the use of:(4)
"... four separate but coherently-phased FEL devices with the output beams combined onto a common single-output aperture... to minimize the distance (170 m) between the forward and aft resonators positioned on either side of the respective accelerator/wiggler assemblies. If the total output power were to be generated by a single FEL device, that 170 m distance would grow to an estimated 600 m which would exceed the bounds of reason for a total required platform length."
Neutral particle beam weapons could destroy boosters, decoys and reentry vehicles through structural damage. Weapons of lower power could be used to negate nuclear warheads by damaging nuclear and electronic components (which would require an energy deposition of about 10 Joules per gram), and by detonation of high explosives in the device (which might require an energy deposition of about 200 Joules per gram). Approximately 40 particle beam battle stations, each weighing several hundred tons kilograms would be deployed in low Earth orbit. These would generate a 200 MeV neutral Hydrogen beam for boost and post-boost phase interceptions. Power requirements for such systems would be
approximately 200 MWe(5) and could range from 170 to 375 MWe.(6) As with FELs, depending on the brightness of the laser device, from several dozen up to a hundred FEL battle stations, each weighing several hundred tons, would be required for an operational constellation. A number of configurations have been suggested for such weapon platforms (Figures II-51 through II-54).
2 - Space-Based Kinetic Energy Weapons
The Space-Based Electro-magnetic Launcher uses an electromagnetic accelerator, analogous in concept to a particle beam accelerator, to propel projectiles to very high velocities ranging from 8 to over 25 kilometers per second. These projectiles would be comparable in design to the heat-seeking hit-to-kill warheads used by rocket interceptors. This system offers the prospect of very high rates of fire, and is in a sense an `anti-missile gatling gun.'
Present electromagnetic guns have demonstrated the ability to accelerate projectiles weighing hundreds of grams to velocities of in excess of 4 km/sec. Maximum demonstrated velocities with much smaller projectiles are about 8 km/sec. However, these systems are capable of firing at rates on the order of one shot per hour.
Capable space-based weapons systems would require improvements over this performance of between one and two orders of magnitude. Velocities of between 8 and 25 kilometers per second are called for, as are firing rates of about one shot per second. Projectile weights of several kilograms are needed, and a tolerance to accelerations of 100,000 Gravities are required. Pointing accuracies required would be on the order of tens of microradians, in contrast to the thousand-fold greater accuracy required for most directed energy weapons.
The total number of space-based platforms required and the total on-orbit mass is highly dependent on maximum kill vehicle velocities achieved. Preliminary estimates of the likely range of kill vehicle velocities suggest that several dozen to several hundred platforms could be required, each weighing several hundred to a few thousand tons. A number of configurations have been suggested for such weapon platforms (Figures II-55 and II-56). Power requirements for such systems could range from 1,600 to 2,963 MWe according to one estimate(7), while another study suggested a range from 400 to 1,500 MWe.(8)
3 - Alternative Space-Based Weapon Concepts
Although these concepts have been leading candidate for the space-based boost-phase engagement mission, a range of other approaches have also been considered.
The hallmark of the SDI since 1983 has been an initial layer of space-based interceptors that home in on the hot exhaust plumes of hostile missiles during the first few minutes of their flight. This boost-phase layer is intended to destroy missiles before they can deploy multiple warheads and decoy warheads that would stress the performance of subsequent layers of the defense.
Originally, plans for this layer of the system called for Space-Based Interceptor (SBI) rockets, each weighing about 100 kilograms, with between five and ten interceptor rockets carried on a satellite that would also carry target tracking sensors. The 1987 plan called for approximately 3,000 interceptors to be carried on approximately 300 Carrier Vehicle satellites, while the 1988 plan called for about 1,500 interceptors deployed on about 150 Carrier Vehicle satellites.(9)
A major change in these plans came in early 1989 with adoption of the "Brilliant Pebbles" (BP) concept (the name implying improved capabilities compared with the SBI "smart rocks").(10) Each Brilliant Pebble would orbit separately, making a less attractive target for Soviet attack. This dispersal, as well as advanced construction techniques, would also permit each Brilliant Pebble to weigh about 40 to 50 kilograms, less than half that of the traditional SBI. Each Brilliant Pebble would have its own missile tracking sensors, eliminating the need for the BSTS satellite sensor. And computers on-board each Brilliant Pebble would direct each Pebble to its target, reducing reliance on expensive communications systems for ground control.(11) The initial plan for Brilliant Pebbles called for 4,614 to be procured at a cost of between $1.1 million and $1.4 million each.(12)
Space-based chemical lasers have also been considered for the boost-phase mission. Unlike the previously discussed directed energy devices, which require large amounts of electrical power for laser beam generation, chemical laser beam generators produce laser light though the combustion of reactants, such as deuterium and fluorine. A deployed space based laser derived from the Triad technology would weigh on the order of 100,000 kilograms, and have a mirror 15 meters in diameter. The laser would have a power output of about 25 Megawatts, and carry sufficient fuel for about 100 seconds of operations. The brightness of the system would be on the order of 1.0 X 10^23 watts/steradian.
4 - Ground-Based Directed Energy Weapon Concepts
Although space-based directed energy weapons have received extensive study, by the late 1980s increasing attention was being given to ground-based systems. High performance short wavelength systems include a concept where a beam of about 10 Megawatts would be generated on the ground and propagated to one or more mirrors in space, and then focused on the target. For a space-based system, a total of between 10 and 40 mirrors, each with a diameter of about 10 meters, would be required in orbits of about 1000 kilometers altitude. Alternatively, a very large number of mirrors could operate in unison, much like a phased array radar, to increase the energy deposited on the target. This would permit ground-basing of the mirrors to enhance survivability. The mirrors would be launched into space on warning of attack. Target destruction would either be by thermal or impulse kill.
A ground-based laser weapon system would ultimately consist of a number of ground sites where high energy laser devices and the appropriate acquisition, tracking, pointing, and advanced beam control subsystems to provide the compensation necessary to transmit the laser beam through the atmosphere to space through "bounces" from space relay and mission mirrors to the target. Pointing and tracking accuracies of about 20 nanoradians would be required.
Such system architectures achieve the access to boost-phase of space based systems by basing mission mirrors in space, but reduce the cost, maintenance and survivability problems associated with basing beam generators in space.
5 - Space-Based Sensors
The Strategic Defense Initiative includes work on ground-based, air-based and space-based sensors for the surveillance, acquisition, tracking and kill assessment of ballistic missiles in all phases of their flight: boost phase; post-boost phase; mid-course; and terminal. These sensors would be required to perform their duties with high confidence in the face of an attack consisting of thousands of missiles, tens of thousands of warheads, hundreds of thousands of decoys, and billions of bits of chaff and aerosols.
Surveillance requires the continuous coverage of likely missile launch locations and regions in space through which missiles and their warheads would pass in order to reach their targets.
Acquisition requires that targets be identified as such in the presence of natural background noise and hostile interference, and that threatening objects such as warheads be discriminated from non-threatening decoys.
Tracking these targets requires that their precise location and trajectory be determined and frequently updated so that interceptor forces can be properly assigned. Target tracking also assists in the further discrimination of targets from decoys.
Kill assessment is required to determine whether a target has in fact been negated, or whether further action is called for. The different responses of warheads and decoys to some types of directed energy weapons such as X-Ray lasers allows the kill assessment function for further contribute to the decoy discrimination effort.
The surveillance and acquisition processes are intended to be conducted independently by each sensor. This will increase the resistance of the system to direct attack. Tracking and kill assessment functions will be conducted on a consultative basis; that is, the observations from different sensors will be combined and compared. This consultation will increase the confidence in the assessment, enhance decoy discrimination, and provide continuous target coverage throughout the engagement regime.
SDI sensors are divided into three classes: passive, active and interactive. The primary difference among these classes is their potential for discrimination during mid-course.
Passive sensors (such as infrared detectors) are used for surveillance, and tracking, as well as discrimination, by detecting thermal signature differences between targets and decoy. For example, during mid-course, light balloon decoys will cool off much more rapidly that heavy warheads, and this temperature difference can be detected. However, offense countermeasures, such as placing small heaters in the empty balloons or surrounding the threat complex with aerosols that will mask the position and signature of reentry vehicles, have a significant potential to defeat this form of discrimination. Power requirements for such sensors range from a few kilowatts up to 100 kilowatts, depending on the size and complexity of the infrared detector systems.
Active sensors (such as radar or ladar) are used for tracking, and discrimination of real targets from decoys by emitting a signal that is modified when it reaches the target or decoy, with targets producing different changes in the signal than are produced by decoys. For example, some types of decoys will produce a much weaker reflection of a radar beam than is produced by a warhead. Power requirements for such large space-based sensor systems (Figure II-57) can range from 1 MWe for laser radars to 5 MWe for space based microwave radars.(13)
Interactive sensors (such as a neutral particle beam generator of an X-Ray Laser) are used for discrimination by generating emissions that interact with
a target or decoy, with targets changing in a fashion that is observably different from the changes in the decoy. For example, a neutral particle beam will penetrate through several centimeters of aluminum before it interacts with aluminum atoms to produce secondary radiation. The NPB will thus produce observable secondary radiation when it strikes a warhead which is much thicker than this depth, but it will pass through the very thin skin of a balloon decoy without interacting and producing observable secondary radiation.
When the SDI program was initiated in 1983, there was considerable optimism that sensitive thermal sensors could detect minute differences in the heat emitted from real warheads and decoys, enabling the system to attack the real warheads and ignore the decoys. Although subsequent work on using laser radars to detect slight differences in the vibration patterns of warheads and decoys showed some promise, over the years the mid-course discrimination problem seemed to grow increasingly intractable.
These concerns effectively stalled the development the Space Surveillance and Tracking System (SSTS), the primary mid-course passive IR discrimination sensor. By the end of the decade, relatively little progress had been made toward deploying this network of up to twenty satellites, orbiting at an altitude of approximately 5,000 kilometers, which would use cryogenically refrigerated long-wave infrared sensors for tracking and discrimination.
The decision in 1989 to add the GSTS and GBR midcourse sensors marked a further move away from mid-course discrimination toward simply tracking targets during midcourse. The Ground-based Surveillance and Tracking System (GSTS), would use long-wavelength sensors similar to those on SSTS, that would be lofted into space on ballistic trajectories upon warning of an attack.(14) The Ground Based Radar (GBR) was been divided into three related projects.(15) A GBR-TMD (Theater Missile Defense)was added to support theater defense operations. The GBR-T (Terminal) will support strategic interceptors, and the GBR-M (Midcourse) would track reentry vehicles during the later part of their trajectory, before they reenter the atmosphere.(16)
The "Brilliant Eyes" concept marks a further step in the SDI's evolution away from discrimination of real warheads from decoy warheads during the mid-course phase of their flight, as they coast through space prior to reentering the Earth's atmosphere. The Brilliant Eyes constellation of satellites could replace some or all of the three mid-course sensors (SSTS, GSTS and GBR).(17) These 50 to 80 spacecraft would orbit at altitudes about twice that of the Brilliant Pebbles, or somewhat less than 1,000 kilometers.(18) Each spacecraft would be equipped with a combination of long-wavelength infrared, visible light and laser radar sensors, for tracking targets in mid-course.(19) According to SDIO Director Henry Cooper, these satellites "would provide cuing and/or other information that would be helpful in accomplishing the discrimination task."(20)
6 - Orbital Transfer Vehicles
Orbital transfer vehicles are required to provide space logistics support for the deployment and maintenance of space-based weapon and sensor systems. These could include deployment of platforms with masses ranging from 100 to 300 tons to orbits with altitudes ranging from 750 to 3000 km.(21) Preferred propulsion concepts include magnetoplasmadynamic and ion engines (Figure II-58). Power requirements range from 1 to 17 MWe according to one estimate,(22) while another assessment place requirements in the range of 1 to 5 MWe.(23)
1. Pop-up X-ray lasers are perhaps the only class of weapon that can potentially conduct boost-phase engagements of ICBMs using ground-based interceptor, but even this system faces major physical and operational constraints. Directed and kinetic energy weapons can potentially engage SLBMs during their boost phase, but this requires continuous airborne patrol of suspected submarine deployment areas, which has largely obviated interest in this approach.
2. Martin Marietta Astronautics Group, Space Power Architecture Study (SPAS), Final Report, Report Number TR-87-16, June 1988, page 3.
3. TRW, Inc., Space & Defense, Space Power Architecture Study, Task 3 Report, Volume 3, Executive Summary, Report No. SN48083-FR-05, 4 September 1987, page 7.
4. Martin Marietta, op cit, page 104.
5. Martin Marietta, op cit, page 3.
6. TRW, op cit, page 7.
7. Martin Marietta Astronautics Group, Space Power Architecture Study (SPAS), Final Report, Report Number TR-87-16, June 1988, page 3.
8. TRW, Inc., Space & Defense, Space Power Architecture Study, Task 3 Report, Volume 3, Executive Summary, Report No. SN48083-FR-05, 4 September 1987, page 7.
9. Stroble, Warren, "Ex-Head of SDI Touts Brilliant Pebbles Plan," The Washington Times, 14 March 1989, page A4.
10. Bennet, Ralph, "Brilliant Pebbles," Reader's Digest, September 1989, page 128-132 provides a useful though uncritical background.
11. This description of Brilliant Pebbles is based on Wood, Lowell, "Brilliant Pebbles Missile Defense Concept Advocated by Livermore Scientist," Aviation Week & Space Technology, 13 June 1988, page 151-155, and Wood, Lowell, "Concerning Advanced Architectures for Strategic Defense," Lawrence Livermore National Laboratory Preprint UCRL-98424, 13 March 1988.
12. "SDIO Plans to Buy 4600 Brilliant Pebble Interceptors," Defense Daily, 13 February 1990, page 231.
13. TRW, Inc., Space & Defense, Space Power Architecture Study, Task 3 Report, Volume 3, Executive Summary, Report No. SN48083-FR-05, 4 September 1987, page 70.
14. Gilmartin, Trish, "McDonnell Douglas to Build Surveillance and Tracking System for SDI Effort," Defense News, 12 September 1988, page 21.
15. "Strategic Defense Command Plans Ground Based Radar Effort," Aerospace Daily, 18 January 1991, page 102.
16. Foley, Theresa, "Raytheon Proposes Rail-Mobile Radar for Midcourse SDI Sensing," Aviation Week & Space Technology, 11 January 1988, page 22-4.
17. Grossman, Elaine, "Small and Light 'Brilliant Eyes' Could Replace Three SDI Surveillance Systems," Inside the Army, 28 May 1990, page 15.
18. Strobel, Warren, "Limited SDI Program Might Cost $9 Billion," The Washington Times, 1 February 1991, page A3.
19. "SDI Constellation Grows in Brilliance," Military Space, 14 January 1991, page 3-4.
20. Bates, Kelly, "SDIO's Cooper Says U.S. Could Deploy Strategic Defense System for $40 billion," Inside the Pentagon, 20 December 1990, page 10-11.
21. Martin Marietta Astronautics Group, Space Power Architecture Study (SPAS), Final Report, Report Number TR-87-16, June 1988, page 3.
22. Martin Marietta, ibid, page 3.
23. TRW, Inc., Space & Defense, Space Power Architecture Study, Task 3 Report, Volume 3, Executive Summary, Report No. SN48083-FR-05, 4 September 1987, page 7.