U.S. Conventional Prompt Global Strike: Issues for 2008 and Beyond (2008)

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5 Assessment of Conventional Prompt Global Strike Options—Synthesis This chapter synthesizes the military issues; the political, international, policy, and doctrinal issues; and the technical assessments presented in Chapters 2, 3, and 4 and presents salient advantages and disadvantages associated with each option. The chapter also discusses some issues that the committee sees in the relationship between conventional prompt global strike (CPGS) and strategic strike in general and how these issues complicate the selection of a longer-term development path at this time. Evaluation Factors The chapter begins with a review of several important aspects of the assess- ment conducted in Chapters 2 through 4—aspects that bear on conclusions to be drawn from this chapter’s synthesis of the various issues addressed. Military Assessment Factors: Requirements and Operational Concepts The Department of Defense’s (DOD’s) initial capabilities document (ICD) for CPGS identifies a number of key capabilities that it refers to as “required,” such as global reach, promptness, and high probability of achieving the desired effect on the target. These key capabilities were included along with other evaluation fac-   ffice O of the Secretary of Defense. 2006. Initial Capabilities Document, Stage for Prompt Global Strike (U), Washington, D.C., July 29 (classified). 138 

ASSESSMENT OF CONVENTIONAL PROMPT GLOBAL STRIKE OPTIONS 139 tors in the assessment in Chapter 2 of the potential military effectiveness of each proposed option. The ICD differentiates CPGS from what it calls Next Generation Long-Range Strike by virtue of CPGS’s promptness and also by limiting CPGS in volume and persistence of fire. These latter limitations make CPGS (as defined in the ICD) a “niche capability”: that is, for prompt fire limited in volume and persistence. The need for CPGS is driven in large part by the goal of a very limited strike in a time of crisis or opportunity, such as to counter terrorist activities. While Conventional Trident Modification (CTM) or CTM-2 is a relatively inexpensive potential means to meet important aspects of the CPGS need and to do so in 3 to 5 years, any longer-term, more versatile option will be a far more expensive national investment that the committee believes, for reasons discussed below, must be put into the broader context of the nation’s strategic strike policy and national security strategy. The committee believes that a comprehensive examination of future strategic strike or deep strike would likely result in an allocation of require- ments to CPGS weapons different from that defined in the ICD. This committee’s skepticism regarding requirements focuses on the use of CPGS weapons in the leading edge of major combat operations rather than on their use in very limited strike in times of crisis or opportunity, as the next paragraphs discuss. The CPGS joint analysis of alternatives (AoA) currently underway with the U.S. Air Force as lead Service appears to be in competent hands, using good analy- sis tools and responding faithfully to the CPGS ICD’s statement of key capabilities required. It also appears, however, that in adhering to the ICD, the AoA omits any consideration of the possible need for greater firepower. Here, the committee is concerned that, against a large and formidable future adversary, versatile plat- forms capable of destroying a wide range of targets may be unable to penetrate (or may suffer severe losses in penetrating) the adversary’s sophisticated air defenses unless preceded by high-volume defense-suppression attacks. The AoA also takes to heart the ICD’s statement that CPGS should be effective against any type of target: The AoA is specifically evaluating each option’s capability to destroy a very broad range of targets, including moving targets. While the committee sees this as a laudable goal, it expects that cost and technical risk will militate against requiring the same weapon to have a flight time under an hour and to hit moving targets, at least in the next decade or so. A trade-off study in a broader context encompassing all strategic strike sys- tems that might be considered candidates for development in the next decade or so might reach different conclusions from those reached by the DOD to date. Such a study might result in a concept of operation in which conventionally armed ballis- tic missiles lethal against a somewhat-limited set of targets (including air defense threats) are first used, followed by long-range, air-breathing vehicles that penetrate into denied-access areas after some defense suppression to provide versatile attack capabilities that reduce dependence on forward-deployed forces. 

140 U.S. Conventional Prompt Global Strike Political Assessment Factors: Ambiguity and Overflight Chapter 3 discusses the issue of “nuclear ambiguity,” that is, the possible misidentification of a launch of a conventionally armed delivery vehicle as an attack with a nuclear-armed weapon, an issue that has been one of the key issues associated with CPGS weapons. For the reasons explained in Chapter 3, it is the committee’s view that there is no “bright line” between conventional and nuclear capabilities based on the physical characteristics of a CPGS option, or on a mis- sile’s prior association with an exclusively nuclear role, because any long-range vehicle capable of carrying an effective conventional payload could alternatively carry a nuclear weapon. Accordingly, all CPGS options could have ambiguity issues; those issues cannot be avoided simply by insisting that the system be without a nuclear “legacy.” However, it is also the view of the committee that there are many plausible cases in which there would be no risk of the misinterpretation of a CPGS attack (and those cases can be strengthened by various cooperative and unilateral measures), so that the possibility of circumstances in which even a remote probability of misinterpretation would make it imprudent to use a CPGS weapon should not be regarded as a reason to forgo the capability altogether. Similarly, Chapter 3 assesses the political risk of overflying third-country ter- ritory as not being decisive in option selection. Nor, for the reasons stated there, does the committee believe that arms control or other political considerations are decisive reasons not to go forward with CTM (or an alternative CPGS system). Technical Assessment: Options in Broader Operational Concepts As noted earlier, in a broader study exploring the full range of strategic strike developments feasible in the next decade or so, requirements might reasonably be apportioned so that a ballistic missile with limited warhead capacity and versatility would handle situations truly requiring flight times of 1 hour or initial penetration into highly defended areas, while long-range air-breathing vehicles with more general utility would handle situations in which a wide range of targets must be struck. This would introduce new options beyond those now being considered for prompt global strike, such as subsonic options that have low technical risk and are capable of more versatile use, making the significant investment more worthwhile. As discussed in Chapter 4, a large, low-altitude, subsonic cruise missile could be designed with low technical risk for launch from a nuclear-powered guided missile submarine (SSGN) (or a nuclear-powered ballistic missile submarine [SSBN]). It is noteworthy that, for a given number of missiles per launch tube, the subsonic cruise missile can carry a given warhead weight a longer distance than can either an intermediate-range ballistic missile (IRBM) or a hypersonic cruise missile. Its flight time to the desired range, however, will generally be much longer than 1 hour, possibly as long as 8 hours, and it may have difficulty surviving future 

ASSESSMENT OF CONVENTIONAL PROMPT GLOBAL STRIKE OPTIONS 141 sophisticated air defenses, although its low-altitude terrain-following capability may give it some advantage over the boost-glide missiles and hypersonic cruise missiles in the latter parts of their trajectories. It should be possible to design the subsonic cruise missile to have considerable warhead versatility. If the United States were required today to put conventional ordnance on a previously unrecognized target in a corner of the globe distant from current disposition of forces as quickly as possible, the platform most likely to be used for the operation would be a manned bomber (e.g., a B-2) with a great amount of tanker support. Given that part of its long flight time could be in parallel with targeting, mission planning, and decision making, it seems to the committee that this versatile platform type will continue to be considered for the future. For air- breathing weapons delivery systems that must penetrate sophisticated air defenses in the future, the DOD should put greater emphasis on unmanned vehicles, as previous National Research Council (NRC) committees have recommended. A possible combination option is a manned or unmanned bomber that launches either a high- or low-speed cruise missile. Some basing options for CPGS systems have been considered and rejected. Launching large, long-range weapons from combatant ships would require an expensive and lengthy development (compared with that needed for other basing options), whether it involved a new launcher system designed for multiple legacy ship classes or a new ship class. Manned or unmanned combat aircraft capable of being launched from aircraft carriers would have inadequate range for some CPGS missions. Assessment Synthesis This section summarizes the primary pros and cons of options for a CPGS delivery system, synthesizing the detailed discussion in previous chapters. Conventional Trident Modification The Conventional Trident Modification (and possibly its variant CTM-2) is the only potentially viable near-term option available 3 to 5 years from program start. Assuming that tests confirm that CTM would meet planned performance goals, it provides some important capabilities in the relatively near term, is very inexpensive by DOD standards, has low technical risk, and builds on a well-proven and comprehensive system—the Fleet Ballistic Missile force and its infrastruc- ture. Remaining technical uncertainties require that the range of military effective- ness of the CTM be established by testing the delivery accuracy and the effects on target of the munitions prior to deployment. The kinetic energy projectile (KEP)   ee, S for example, National Research Council, 2005, Autonomous Vehicles in Support of Naval Operations, The National Academies Press, Washington, D.C. 

142 U.S. Conventional Prompt Global Strike warhead (tungsten rods) and the relatively limited payloads that it would carry limit the lethality of CTM to relatively small, relatively soft, and (usually) accu- rately geolocated targets that are not going to move for at least the “end-to-end” time line for acquisition, decision, and attack. (One possibly significant limitation arising from the flight geometry of the current CTM concept could be eliminated if the Navy’s current proposal were modified, possibly without significant program delay, by adding a bent-nose reentry vehicle to enable attacks on the back side of hills or buildings.) While at some point in the future, “peer competitors” may develop missile defenses that would be effective against current U.S. long-range ballistic missiles (at least in small numbers), a Trident-based CPGS system would have an extremely high probability of defense penetration for many years into the future. The committee is confident that the Navy has proposed adequate technical and procedural safeguards against the accidental launch of a nuclear weapon in mixed loads (with the proviso that an expert red team critically review them) and that mixed loads would have no detrimental effect on the nation’s nuclear deter- rent capability for the next decade or more. Because it is launched from a limited number of SSBN tubes, the CTM is not suitable for high-firepower operations, especially if, as proposed, CTM occupies only two tubes per SSBN. The fall of CTM’s early rocket stages may cause overflight issues in certain cases, but the fall is predictable and can be diverted to low-density areas, and the potential dam- age can be assessed for the decision maker. The CTM is indistinguishable from the nuclear-armed Trident externally or in-flight, but the committee believes that the risks of misinterpretation of a CTM launch as a nuclear attack are very low (even given the stakes) for a wide range of plausible cases and can be lowered still more by cooperative measures and by experience with development, testing, wargaming, exercising—and potentially using—the system. In the committee’s view, CTM offers an excellent growth path, as discussed in the next subsection, and its development would provide technology and flight experience valuable for many of the longer-term options should the country choose to pursue them. Conventional Trident Modification-2 Whether a growth path from CTM or a first-step development itself, the CTM- 2 concept proposed in Chapters 2 and 4 would be a significant advance beyond CTM because it would have greater payload capacity. It seems possible that CTM- 2 could be available in 5 to 6 years from program start, although a full engineering analysis of the option would be needed to confirm the timescale. The two-stage propulsion of CTM-2 could distinguish it from the three-stage Trident, but with high confidence today probably only when tracked by the systems of the United States itself. Some members of the committee see a possible evolutionary path for CTM-2 that in its next generation could incorporate a reentry vehicle capable of delivering (yet to be developed) air-breathing vehicles into theater for the provi- sion of capability against moving targets and for post-attack assessment and reat- 

ASSESSMENT OF CONVENTIONAL PROMPT GLOBAL STRIKE OPTIONS 143 tack. This evolutionary path appears to those members to be a possible approach to balance technical risk with the rapid fielding of improved CPGS capabilities. Land-Based Ballistic Missiles Land-based intercontinental ballistic missiles (ICBMs) have the range, fast time of flight, and survivability for CPGS, as well as relatively low technical risk. One could probably adapt the warhead technology under development for CTM to meet the challenge associated with the land-based ICBM’s greater reentry speeds, but this has not been demonstrated. Assuming that this adaptation proved work- able, a conventionally armed modification of Minuteman could be operational in perhaps 6 years, given sufficient priority and organizational interest (which does not now exist). Several hundred Minuteman boosters are available in storage. Building more of these boosters for high-volume operations would add substan- tially to system costs. In the longer term, land-based ICBMs may be able to carry very large conventional warheads capable of penetrating hard and buried (but not deeply buried) targets. Basing conventionally armed ICBMs in Hawaii would reduce overflight issues but would raise “not-in-my-backyard” issues and could be done only with the consent of signatories to (or the termination of) the Strategic Arms Reduction Treaty (START). The ICBM’s high-speed terminal approach limits its lethality to a subset of potential target classes, usually accurately geolo- cated. A conventional Minuteman modification would be indistinguishable from Minuteman externally or in flight, but ambiguity could be resolved cooperatively had there been prior inspection of the vehicle in the silo. The U.S. Air Force decided not to pursue the land-based ICBM option, and the committee supports its decision, as discussed in Appendix I. The Submarine-Launched Global Strike Missile and Similar Submarine-Launched, Intermediate-Range Ballistic Missiles The Submarine-Launched Global Strike Missile (SLGSM) or similar sub- marine-launched IRBM (SLIRBM) has a number of attractive features as a potential mid-term CPGS weapon. Its principal advantage over several other mid-to-long-term options is its relatively low technical risk and relatively early initial operational capability (IOC), perhaps 6 years from program start if the CTM is developed, 8 years if not. The Navy’s Strategic Systems Programs (SSP) presented to the committee a point design (specific configuration) for the SLGSM, and Chapter 4 presented the results of a conceptual design study trading range, warhead size, and firepower (number of tubes and number of missiles per tube) applicable to SSGN- or SSBN-launched IRBMs. If based only on SSGNs, the SLGSM point design has a range that can cover most of the areas of interest on the globe. To make it more nearly global, SSP proposes basing it on SSBNs as well. This would restrict the SSBN’s operating areas, which could in the future raise 

144 U.S. Conventional Prompt Global Strike issues with respect to the nation’s nuclear deterrence capability. The SLGSM’s warhead capacity and versatility are less than those of the conceptual CTM-2 and other longer-term options, but it has an advantage over the CTM in these regards. Especially if it adopts terminal countermeasures as defenses improve, the SLGSM should be highly survivable for decades after its introduction. Proposed at two per canister, SLGSM is suitable for high-volume operations, and the committee believes that a three per canister IRBM may be feasible to increase firepower. The SLGSM and other IRBM designs could be differentiated in flight from Tridents, but as with CTM-2, with high confidence today probably only by the systems of the United States itself. In the committee’s opinion, nearly ­constant-heading, largely ballistic trajectories (such as flown by CTM, CTM-2, and land-based ICBMs) have the advantage that they permit a tracker to predict with confidence a weapon’s destination, reducing uncertainty about the nature of an attack. The SLGSM’s 700-mile glide range increases that uncertainty. A glide range that long also implies about 5 minutes of atmospheric heating, which begins to stress a reentry vehicle’s thermal protection system (TPS), as discussed in the next subsection. An SSGN carrying SLGSM could face compromises with its Special Operations Forces mission. Land-Based Boost-Glide Missiles The Air Force Space Command has programmed about $480 million over the next 6 years to develop and demonstrate in three flight-tests a hypersonic boost-glide vehicle that it has named the Conventional Strike Missile (CSM). The weapon would be based at Vandenberg and Cape Canaveral Air Force Bases (raising START issues). Another design concept defined in the CPGS AoA is a shorter-range but forward-based (in Guam, Diego Garcia, and Puerto Rico) boost-glide vehicle named Advanced Hypersonic Weapon (AHW). The principal advantages of proposed boost-glide missiles are their (as-yet-unproven) range, warhead capacity, versatility, and maneuverability. These missiles are envisioned to have the capability to deliver KEP or penetrator warheads or to dispense virtu- ally all of the Air Force’s air-launched weapons. If these capabilities were proved possible (and the near-real-time targeting necessary in many cases were in fact present), it would make the boost-glide missiles lethal against a very wide range of targets and would often significantly reduce the requirement on target geoloca- tion accuracy. Its ability to alter its trajectory significantly would reduce overflight issues, enable significant cross-range diverting capability for in-flight retargeting, and permit tailoring the end-game approach angle for improved weapons system effectiveness. A boost-glide missile would be easy to differentiate from a ballistic missile late in flight, but its maneuverability raises uncertainty about its destina- tion until then. The principal disadvantages of the boost-glide missile are its high technical risk, late availability, and high cost. Hypersonic boost-glide vehicle proposals push the performance envelope in the areas of thermal protection and management 

ASSESSMENT OF CONVENTIONAL PROMPT GLOBAL STRIKE OPTIONS 145 and the dispensing mechanisms of air-launched weapons. While a ballistic reentry body typically spends about 1 minute in the oxidative hypersonic environment, the CSM under development by the U.S. Air Force would have to fly in it for more than 10 minutes, stressing current technology. In the CSM’s next evolutionary step, in order to expand area coverage, it is proposed to fly for nearly 1 hour in that environment, which will require the development of new thermal protection and management technologies. The CPGS AoA is modeling the hypersonic boost- glide vehicles as slowing to Mach 5 to dispense their weapons. This speed seems at the same time aggressively high for dispensing weapons and questionably low for surviving strong area air defenses. A boost-glide missile appears to be an expensive weapon for high-firepower operations. In the committee’s view, the CSM effort planned and funded by the U.S. Air Force is optimistic for a program intended to result in a highly reliable, highly effective presidential-release weapon. The committee’s judgment of a prudently scheduled, well-funded program with adequate testing has an IOC of about 2017 for a version of CSM that has a 10-minute-plus glide phase and delivers a penetrator warhead, and (with high technical risk) an IOC of about 2022 for a second version that has nearly a 1-hour glide phase and can dispense a wide variety of air-launched weapons at high speed. Hypersonic Cruise Missiles Hypersonic cruise missiles share some of the same promise (e.g., warhead capacity and versatility) and some of the same high technical risks (thermal pro- tection and management and high-speed dispensing of weapons) as those associ- ated with boost-glide missiles. The committee again wonders, as with boost-glide missiles in a weapons-dispensing mode, about the hypersonic cruise missile’s survivability against strong regional and local air defenses. The CPGS AoA is modeling a very large air-launched hypersonic cruise missile. A similar missile could also be launched from SSGNs, and Chapter 4 includes a conceptual design study trading range, warhead size, and firepower, complementing the previously mentioned study for IRBMs. Compared to IRBMs, cruise missiles (high- or low-speed) have the advantage that their cross-section can be designed so that multiple missiles fit snugly in an SSGN (or SSBN) tube—for example, three mis- siles pie-shaped at an angle of 120 degrees to fill the 360-degree circular volume. IRBMs require round rocket-motor chambers, and so multiple IRBMs cannot fill the volume of a tube. This snug fitting enables a hypersonic cruise missile to have about the range of an IRBM when designed for launch from SSGN tubes. As an example, for three missiles per tube and a warhead weight of 1,500 lb, a two-stage IRBM can fly 2,400 miles, including glide range extension, as can a hypersonic cruise missile, including its glide path. Compared with an IRBM or a boost-glide missile, a hypersonic cruise missile can be more easily differentiated from a nuclear-armed ballistic missile in all phases of its trajectory. Of course, it cannot be differentiated from a nuclear-armed cruise missile, and the United 

146 U.S. Conventional Prompt Global Strike States has a history of such weapons (albeit subsonic), just as it has a history of nuclear-armed ballistic missiles. A disadvantage for the hypersonic cruise missile compared with an IRBM or boost-glide missile is that the hypersonic cruise mis- sile will not fly high enough to escape national sovereignty issues. Conclusions In the near term, CTM (or, with perhaps 2 years’ more delay, CTM-2) is the only option. It has the potential to meet important aspects of the immediate need, is inexpensive, and has low technical risk, although its military effectiveness must be evaluated by flight-tests of targeting accuracy and munitions lethality. The com- mittee is confident that issues such as ambiguity, overflight, impact on nuclear deterrence, and accommodation with arms control are manageable and do not constitute a reason to forgo the capability. Moreover, testing and evaluation of the CTM will provide a foundation of information useful for many future options. In the longer term, submarine-launched ballistic missile options continue to appear attractive, in single or multiple missiles per launch tube, and to offer evolutionary paths that balance technical risk with rapid fielding of improved capabilities. Boost-glide missiles and hypersonic cruise missiles hold promise of versatile capabilities but have high technical risk and questionable survivability against defenses. Land-based ballistic missiles offer little that sea-launched mis- siles do not, although land-basing avoids concerns about impacting the sea-based nuclear deterrence mission. In the committee’s opinion, the selection of a long-term CPGS deliv- ery system—able to engage more-demanding targets and/or to do so in large volumes—must be put into the broader context of the nation’s policy on strategic strike, which requires a new look based on careful analysis and thoughtful debate. In turn, the policy on strategic strike must be consistent with the national secu- rity strategy, which a new administration must review beginning in 2009. Any option beyond the CTM or CTM-2 will be an expensive national investment and needs to be well aligned with these yet-to-be-defined national policies. A more comprehensive examination of strategic strike or deep strike and an allocation of requirements to potential future weapons systems would allow a better perspec- tive on CPGS. What CPGS should encompass and which CPGS delivery system option is best are but two of many serious, interrelated questions concerning stra- tegic strike. How much firepower is needed against deep, well-defended targets and how should it be delivered? Can we be confident that air-breathing missiles and boost-glide missiles late in flight will be able to survive future sophisticated air defenses without prior defense suppression? Are the operational advantages of long-range conventionally armed ballistic missiles worth their political risks? What is the future of the SSBN and SSGN? Should they be the basis for a more formidable conventional strategic force in numbers of platforms and in their individual capability?