By Gregory H. Canavan
April 12, 1999
Defenses viewed as “theater” elsewhere are strategic defense within Asia. While THAAD is still in development, and has had more than its share of problems, Patriot has now achieved the hit to kill capability demonstrated earlier in Homing Overlay Experiment (HOE) and the US-Israeli Arrow. Modern radars should largely negate theater decoys, and improved internetting of sensors, battle management, and command and control have multiplied combined effectiveness. Satellite, Patriot, Aegis, and THAAD data are now internetted in real time on operational platforms, and should suffice for projected unitary warheads.
Issues
However, aggressors could challenge such theater defenses with advanced technologies. The earliest could be chemical and biological weapons, which must be intercepted at higher altitudes to avoid effective dispersal multiple munitions. Commercial dispensers are generally designed to release below THAAD and PAC 3 intercept altitudes, but could be modified at modest penalties. If the agents are in individual containers, hit to kill is less effective; achieving adequate lethality against them is an unsolved problem. Nuclear weapons require very low leakage defenses. Theoretically, they could be produced by compounding the kill probabilities of current concepts, but it is likely that they will require more direct counters.
In the Gulf War, missiles breaking up during reentry accidentally generated stressing maneuvers. In the future, aggressors will intentionally program their weapons to generate such maneuvers. Interceptors might be able to overcome them with better accuracy, range, and guidance, but as yet the penalties are not known, other than through computer simulations.
Multiple munitions can also be released immediately after the missile’s boost phase, which occurs far out of THAAD and Patriot range. That could saturate current theater defenses or render them cost-ineffective, much as multiple reentry vehicles (MRVs) overcome early NMD systems. The technologies for release are readily available or adaptable. At theater speeds, reentry measures amount to little more than a coat of plastic on each munition. Such dispersal is a benefit for explosive, chemical, and biological weapons, as their effectiveness increases with the total area covered, and hence with the number of munitions.
Boost phase intercept could address these issues, but requires lasers in aircraft or in space, interceptors in space, or fast interceptors on the ground, each of which has technical and cost issues. The largest lasers on aircraft currently have powers—-MW, which have to propagate through—-500km of turbulence (including standoff for self protection) to hold launch areas at risk. Scientific issues in propagation, engineering issues in lasers, and economic issues in packaging and effectiveness,[1] and concerns about hardening missiles against lasers[2] must be overcome through a -$1B/yr development program that is a decade from completion.
For space lasers, propagation concerns are reduced, but the vulnerability of the laser becomes more of an issue, as its capabilities, orbit, and hardness are known. The fraction of the constellation overhead must suffice for the engagement, which is a penalty for a defense for a limited geographic area, as most of the lasers are in the “wrong” place at any given time. Because deploying technology in space is more difficult than on airplanes or ground platforms, space lasers are less technically mature than airborne lasers and will become available later.
Space-based kinetic energy interceptors (SBI) were developed for cold war threats. They are generally unnecessary for theater threats, and avoided because of ABM Treaty compliance issues. They face additional technical challenges in theaters. On average, they start several hundred kilometers from the missile and must reach it before it burns out to negate early release, so their acceleration must be much higher than that of strategic SBIs. For <500 km missiles, the acceleration required would be several 10s of g’s. That would require expensive SBIs, which would be inferior to air- or space-based lasers. For 500-1,000 km ranges, SBIs would be preferred, if they were not given credit.
Reaching theater missiles in boost with ground based interceptors (GBI) generally also requires large velocities and accelerations. However, in Asia, many engagements would involve launches over large bodies of water, which could allow the launchers to be placed closer to the launch area.[3] On a 1,000 km trajectory, a 3g missile has an acceleration time of ~90 s, during which it cannot release its weapons. If the GBI was placed between the launch area and the target and fired promptly at launch, a current 5g, 4 km/s interceptor could reach it by burnout from a range of —325 km. This range degrades if time is required for detection and release: 15 s would cut the range to ~ 260 km, and 30 s to ~220 km, which is probably not enough for survivability. However, to the same degree of accuracy, 10g, 7km/s fast GBIs would have the ranges of ~600, 450, and 360 km. Such delays could be supported by current satellite systems and improved by planned ones. The interceptors could be command rather than self-guided during much of their flight; not necessarily by the launch platform. They would look for a bright plume rather than a dim body, so they could use developed plume detection and hard-body transfer, which would reduce the mass of the kill vehicle and the interceptor motor proportionally. Because all of the interceptors would be placed around the aggressor, there would be no geometric penalty.
Overall, airborne lasers appear to be preferred technically if a single theater of modest size is at issue, the aggressor has limited means to attack the aircraft, and his missiles cannot be hardened. Space lasers are penalized for a single theater, and sensitive to launch area, hardening, and survivability. SBIs are insensitive to target hardening but more sensitive to launch area, rate, range, and burnout times. Fast GBIs could be preferred when water permitted survivable access. The advantages of the concepts are largely complementary. It is too early to say which would be the best choice technically.
Summary and conclusion.
Current defenses should be adequate for projected unitary threats, but could be offset by multiple, chemical, biological, nuclear, maneuvering, and early deployed weapons, which are at roughly the same level of technology. Additional development is required. Boost phase detection and interception have promise, but require development. Airborne lasers are more developed than space lasers, but have more serious propagation issues. Space based lasers interceptors are not optimal for theater launches. Fast ground based interceptors could be for engagements where the launchers could be based survivably and detection and command provided promptly. At current development rates, it is unlikely that any will be an effective deterrent on the time scale of the evolving threats.
References
[1] Congressional Research Service (US Congress, Washington DC. Spring 1999), excerpted Defense Week 8 & 15 March 1999.
[2] G. Canavan, N. Bloembergen, and C. Patel, “Debate on APS Directed-energy Weapons Study,” Physics Today, November 1987, pp. 48-53.
[3] R. Garwin, “Effectiveness of Proposed National Missile Defense Against ICBMs from North Korea,” FAS: Garwin Archive (http://web.archive.org/web/20020110101001/http://www.fas.org/rlg/990317-nmd.htm).
Gregory H. Canavan is a science advisor and senior fellow at the Los Alamos National Laboratory. He has served at Los Alamos since 1981, previously as director of the Office of Inertial Fusion and special assistant to the chief of staff at the Department of Energy. Dr. Canavan received his Ph.D. in applied science from the University of California.
