Review of Directed Energy Technology for Countering Rockets, Artillery, and Mortars (RAM): Abbreviated Version (2008)

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Summary This study report was prepared by the National Research Council’s Com- mittee on Directed Energy Technology for Countering Indirect Weapons. The report provides results of the committee’s assessments and committee recom- mendations concerning the U.S. Army’s efforts to develop and demonstrate a high-energy, solid-state laser weapon system that could be used to defend an area a few kilometers in diameter against incoming rockets, artillery, and mortars (RAM). Specifically, as requested by the Army’s Space and Missile Defense Com- mand/Army Forces Strategic Command, the committee considered the quality and complementarities of the Command’s laser program and related technical efforts in counter-RAM applications. In performing this task, the committee addressed several issues, including the effectiveness of solid-state laser weapon system concepts, the technological matu- rity of various optical subsystems of the laser itself, and risk to overhead airborne and space assets. The committee also considered complementarities of various pieces of the technology effort, related systems engineering and integration, and the adequacy of related supporting technologies (such as power supplies and ther- mal management). It also evaluated the adequacy of the phenomenological base. Finally, the committee considered benefits that could accrue from maturation of related technical efforts outside the Army and the sufficiency of Army budgets and schedules to ensure adequate technological maturity and to evaluate a weapons prototype. The full statement of task is given in the report’s preface.  

 REVIEW OF DIRECTED ENErGY TECHNOLOGY FOR COUNTERING RAM OVERARCHING FINDINGS AND A RECOMMENDATION The Army’s development program is aimed at demonstrating a mobile 100 kilowatt (kW) solid-state laser weapon system concept that has the potential of performing usefully against RAM attacks. It is clear that the various pieces required to demonstrate a mobile 100 kW solid-state laser weapon system have relatively low technological maturity and relatively high risk and involve chal- lenging engineering and integration issues. For this reason a transportable, rather than mobile, system was also considered. For a technology-paced program of this type, it is likely that substantially more money than the Army currently has programmed will be required to realize the demonstration. Indeed, the committee estimates that over the period of the program $100 million more than the amount currently planned will be needed. The rudimentary effectiveness assessments made during this study reveal the clear benefits of higher laser power than is provided by the 100 kW demonstrator to counter more stressing raids and hedge the need to destroy future hardened RAM projectiles. Accordingly, the committee endorses the Army’s longer-term goal to eventually develop and field a multi-hundred kW solid-state laser (e.g., a 400 kW laser weapon system). In addition to assessing the Army’s current technology-paced program to demonstrate a 100 kW system, the committee examined a three-element sequen- tial program of the committee’s own design that could proceed as follows: 1.  arly on, ruggedize and integrate into a transportable or mobile test-bed E a previously developed, good-beam-quality 25 kW solid-state laser to demonstrate the ability to use laser technology of this type under realistic field conditions rather than in the laboratory. This test-bed would primarily reduce the development, engineering, and integration risks in spiraling to the 100 kW and 400 kW demonstrations and very likely pay for itself. 2.  roceed with a 100 kW demonstrator, only at reduced risk and cost com- P pared to the current Army program because of lessons learned and data gathered with the 25 kW test-bed; the 100 kW demonstrator would also likely give the Army some useful military capability. 3.  ully fund the continuing longer-term 400 kW effort to follow the 100 kW F demonstration; the 400 kW laser, which could be tested by 2018 under this sequential program, would offer much greater military effectiveness. The committee’s coarse estimate of the cost of the above sequential program is approximately $470 million. This kind of program would provide early and frequent opportunities for testing and evaluation as well as clear decision points Although the ultimate goal of the Army is a multikilowatt system, that does not mean that a 100 kW demonstrator will have no credible weapons capability or that it is not useful militarily. The 100 kW lasers could do some useful things, and 400 kW lasers could do even more. 

SUMMARY  for off-ramps if needed. Its major drawback is the higher peak funding required during the 3-year period when it must proceed in parallel (about 20 percent more in FY 2011 through FY 2013 compared to the Army’s current development plans for the 100 kW system and a multi-hundred kW system). This program allows the Army to choose between higher middle-program development costs or increased program risks. Recommendation: The Army should consider changing its high-energy laser tech- nology development and demonstration program to reflect the three-phase (25 kW, 100 kW, and 400 kW) spiral approach of the proposed sequential ­program. OTHER KEY FINDINGS AND RECOMMENDATIONS Effectiveness estimates were briefed to the committee during this study. The committee’s own assessments, although necessarily limited because of the time frame of this study, revealed several aspects of effectiveness that need thorough analysis to better illuminate the military utility of future high-energy lasers as the Army’s high-energy laser technology development and demonstration program proceeds. Recommendation: The Army should perform a detailed, quantitative study of the effectiveness of high-energy, solid-state laser weapon systems against future threats. That study should address a comprehensive range of parameters and is- sues, including various power levels (e.g., 100 kW and 400 kW), the effects of obscurants, weather, atmosphere (including turbulence with and without adap- tive optics, scattering, and absorption), resistance to countermeasures that would increase the hardness of incoming RAM, and deployment tactics, concepts of operation, and associated training. With respect to the maturity of various laser approaches, the committee de- veloped Table S-1, which summarizes its assessments. Although the committee identified ceramic slabs as the most promising near- term technical approach for solid-state lasers, other approaches hold promise over the longer term. Since laser efficiency is the single most important determinant of overall weapon size, a very significant improvement in efficiency over that demonstrated to date is required for a single-vehicle, mobile, high-power laser system to be feasible. Recommendation: The government should continue to pursue several competi- tive approaches for solid-state lasers for the next few years. The Army should The committee was briefed on single-vehicle (mobile) concepts, but none involved shoot-on-the- move capability. A transportable system involves one or more large trucks and relatively long set-up times. 

 TABLE S-1  Status and Committee Assessment of Various Laser Approaches Technology/Advocate Key Issues Comments TRL Risk Estimate NGST coherent beam combining Coherent beam combining Improve efficiency TRL 4 Medium amplifier chains, slab complexity Complexity leads to system issues Textron thin Zag, slab Thermal lensing Thermal lensing will require TRL 3 Medium to high Low efficiency adaptive optics Thermal management Complexity leads to major Ceramic materials system issues technology Unstable resonator not-yet- proven approach DOE/LLNL heat capacity, slab Heat capacity limited System concept requires TRL 3 High BQ not stable multiple slabs with Difficult thermal complex loading system management issues and thermal management system 

DARPA/HELLADS, slab Thermal management with Lasing media are thin TRL 3 High index-matched coolant slabs with index-matched BQ coolant flow between Optical efficiency slabs To maintain index-match, laminar flow necessary Specific power difficult to maintain Thin disk Poor BQ at high power Promises high efficiency COTS, High Thermal shock in gain media Problems with coupling TRL 9 Difficult to scale to high power and beam combining Other, levels expected TRL 3 Optical fiber, single mode Single fiber limited Coherent and incoherent TRL 2 High Nonlinearities limit power beam combining scaling Optical fiber, multimode BQ Difficult to propagate long COTS, High distances due to poor TRL 9 beam quality NOTE: TRL, technology readiness level (see Appendix D for more information); BQ, beam quality; HELLADS, High Energy Liquid Laser Area Defense System; NGST, Northrop Grumman Space Technology.  

 REVIEW OF DIRECTED ENErGY TECHNOLOGY FOR COUNTERING RAM concentrate on a transportable system until efficiency is improved sufficiently to allow for a mobile system. The committee’s consideration of necessary supporting technologies revealed that thermal management will be a substantial part of the total mass and volume of a high-energy laser weapon system; thus, complete system designs that include all aspects of the supporting technologies (e.g., total system weight, volume, power) are necessary to ensure truly mobile systems. Hybrid electric power systems being developed by the Army have ample energy but currently lack sufficient power capability for solid-state laser weapon systems; also, the hybrid-electric timelines do not match those of the laser development and demonstration program. Rug- gedization will be a key issue for 100 kW transportable and mobile systems and will require intensive engineering. Recommendation: A transportable system should be implemented first, and complete system designs should be ensured for follow-on mobile systems. Army capabilities in power, energy, and thermal management should be utilized and interfaced with the laser program. To reduce risk for the 100 kW system and a higher power follow-on, ruggedization should be demonstrated early on at a lower power level. Systems engineering and integration for a solid-state laser weapon system must be comprehensive, encompassing all aspects of all pieces of the system and their numerous interfaces. The current acquisition approach by the Army has made the government the de facto system engineer and integrator through the first phases of the High Energy Laser Technology Demonstrator effort, yet there is no evidence that the Army has established an organization or hired people to accomplish this critical function. Recommendation: The Army should establish a systems engineering and integra- tion team to develop the top-down performance allocations, error budget tracking, engagement timeline management, and integration plans for the high-energy laser system. Valuable lessons can be learned from the Tactical High Energy Laser (THEL) program. The approach to testing and diagnostics is also important. Recommendation: THEL lessons learned should be widely distributed and taken into account in the solid-state laser programs. The Army should establish a team to ensure that necessary systems engineering and integration functions are ac- complished, and the approach for testing and diagnostics should be defined. Adequacy of the phenomenological database is critical for a laser weapon system. A key characteristic is lethality (in other words, hardness, or the amount of laser energy per unit of surface area that is needed to destroy or disable an incoming projectile). The ability to destroy RAM targets by a laser depends on 

SUMMARY  many factors, such as engagement geometry and the type of munition as well as the laser wavelength and dwell time on the incoming round. Recommendation: Current efforts to characterize lethality at the solid-state laser wavelength should be pursued aggressively, and robust modeling and simulation under a range of threats in a variety of conditions, including potential counter- measures, should be undertaken. The committee found substantial benefits for the Army’s solid-state laser weapon system program from other programs outside the Army (e.g., ceramic materials in Japan, progress on diode arrays by DARPA and Lawrence Livermore, low-absorption-loss coatings in the United States and Europe, power electronics by the Navy and DARPA, and advanced energy storage by U.S. and Japanese companies). Recommendation: The Army should continue to support research and develop- ment in advanced ceramics materials for lasers. The Army should also continue participation in U.S.-based and international research on various other elements of high-energy lasers, including related equipment for mobile laser systems (e.g., energy storage). The use of lasers necessarily raises concerns about the safety of airborne platforms that may be in the vicinity, of any manned or unmanned spacecraft that may be in the laser’s field of view, and of persons either on the ground or in the air who might suffer ocular damage from exposure to direct or scattered laser light. In all of the above situations, the probability of illumination is small, but the consequences could be significant. Risk assessments must take into account both probability and consequences, and deconfliction between laser firing and local air- borne platforms must be included in the weapon system’s battle management. Recommendation: The Army should study eye safety for military operators and for civilians (collateral damage) and integrate the results into its development of concepts of operation. Predictive avoidance for space platforms should also be incorporated into the laser weapon system’s battle management. The Army should start with the predictive avoidance approach of the Airborne Laser Program and should work with the operational communities and U.S. satellite agencies to establish rules of engagement for the laser weapon.