Laser

A Layered Defense System

SDI Applications
Source: Unknown

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 systemcomplexity, 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 l

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