Potential Implications of the SM-3 Block IIA’s Counter-ICBM Capabilities


An SM-3 Block IIA missile during production. Note the significantly larger diameter relative to Block IA/B SM-3s. Raytheon image.

Two years ago, missile defense expert George Lewis of mostlymissiledefense penned an article on the prospect of employing Standard Missile 3 (SM-3) Block IIA interceptors against intercontinental ballistic missiles (ICBMs). His piece struck me as remarkable because the Block IIA has not been advertised as a homeland defense system. Raytheon’s website describes the SM-3 as “a defensive weapon the U.S. Navy uses to destroy short- to intermediate-range ballistic missiles,” which is echoed by official MDA materials. Moreover, the SM-3 Block IIA is slated for production in relatively large numbers, so its employment in a strategic capacity would significantly alter the landscape of nuclear deterrence (as will be discussed).

At the time, I viewed George Lewis’ claims with a certain degree of skepticism due to their radical implications. Though he provided copious and irrefutable evidence that military leaders had at least considered using the Block IIA against ICBMs, most published materials from both industry and the DoD declined to mention the interceptor’s strategic potential. As such, there were few concrete indications that the SM-3 Block IIA would actually be tested against ICBMs or that performance would be satisfactory. And, of course, strategic SM-3 Block IIA employment would be somewhat redundant; the continental United States (CONUS) already has an entrenched strategic missile defense system in the form of Ground-based Midcourse Defense (GMD).


Slides such as this one (dated 2009) and various statements made by defense officials indicate that the DoD has been aware of the SM-3 Block IIA’s potential strategic utility. From mostlymissiledefense.

The situation has since been clarified by language in the FY18 budget requiring the MDA to test the Block IIA against “longer-range threats;” given that tests against IRBM-class targets have already been conducted, this clearly refers to an ICBM. As a result of this development (and others), George Lewis posted an updated article with additional evidence in support of his claims. I have decided to further explore his conclusions regarding deterrence and speculate on a few other matters he does not consider, namely: how would strategic SM-3 Block IIA deployment affect the US Navy and how would it coexist with GMD?

The SM-3 Block IIA is the newest variant of the SM-3, an exo-atmospheric ballistic missile interceptor designed for launch from the Mark 41 Vertical Launch System (VLS). Describing the Block IIA as a mere “variant” is somewhat misleading; it features a new 21-inch diameter propulsion section that significantly improves kinetic performance and it incorporates substantial upgrades to the seeker and kill vehicle. Crucially, the Block IIA’s seeker appears to have explicit counter-ICBM capabilities, as it is slated for use on Raytheon’s Redesigned Kill Vehicle (RKV). As a result of the propulsion improvements, the Block IIA’s kill vehicle can reach speeds of approximately 4.5 km/s, with maximum engagement range more than doubled versus Block IA/B. According to figures drawn up by George Lewis and others, three offshore Block IIA launch positions could cover all of CONUS, including the northern approach.

Let us consider a hypothetical scenario in which the SM-3 Block IIA is tested against an ICBM-representative target and passes with flying colors. The effects on the nuclear status quo would be quite alarming, especially for China. Unlike Russia, China’s arsenal is far smaller than America’s — the Bulletin of Atomic Scientists estimates that China deploys around 280 warheads. More than half of these are mated to short and medium-range missiles not capable of reaching the United States. Moreover, China’s sea-based deterrent leg, which possesses about 50 SLBMs, does not offer a robust second strike capability; its missiles do not currently have the range to reach CONUS without patrols well outside of Chinese waters (striking the East Coast would require sailing in the mid-Pacific) and its nuclear submarines are relatively noisy. At the moment, China’s nuclear capability against CONUS depends on around 50 active land-based ICBMs, of which around 30 are road-mobile solid-fuel DF-31As. These missiles are relatively survivable. The remainder are liquid, silo-based DF-5 missiles that have a lengthy fuelling sequence. About ten of these are DF-5As with one warhead and the other ten are DF-5Bs with three warheads per missile, per the Bulletin of Atomic Scientists.

According to Donald O’Rourke of the CRS, 48 SM-3 Block IIAs will be delivered by FY23, of which a few will have been used in tests. Let us assume that, in an extreme crisis scenario, around 30-35 IIAs could be surged to homeland firing positions aboard Aegis BMD cruisers and destroyers. There would be a maximum of 44 GBIs active as well. This would give the United States around 70-80 interceptors capable of counter-ICBM duties. However, one must remember that single-interceptor kill probabilities are not stellar; current missile defense doctrine calls for expending multiple interceptors per target to achieve satisfactory performance. Thus, at least two interceptors would be expended per incoming warhead; nominally, around 35-40 targets could be countered.

Given these assumptions, China’s nuclear deterrent could become uncomfortably marginal if it is not expanded by the time SM-3 Block IIAs begin to enter service en masse. Assuming that the US could eliminate most of the silo-based DF-5s (not inconceivable, as Trident D5 reentry vehicles (RVs) have a circular error probable (CEP) of around 90 m), keep the SSBNs at bay, and destroy half of the DF-31s, China would be left with fewer than ~35 warheads capable of reaching CONUS. In this scenario, the survival of Chinese warheads would depend on the efficacy of penetration aids and the performance of US missile defenses.

This, of course, is a worst-case scenario for the PRC; it is exceedingly unlikely that the US could neutralize China’s deterrent in practice. Testing has revealed significant shortcomings in both the SM-3 Block IIA and GMD even in highly simplistic intercept scenarios; a real-life saturation attack with decoys and other debris would be daunting. Moreover, mustering Block IIAs around the homeland would require significant foresight or persistent strategic patrols, since the missiles are normally carried aboard a variety of ships deployed around the world. And China’s nuclear deterrent may be survivable enough that eliminating a significant portion is simply not feasible; only those with access to high-level intelligence (especially silo survivability information) could supply anything beyond a rough estimate here.

However, the mere prospect that such an event could theoretically take place is significant in and of itself. The current balance of nuclear power between Russia, China, and the United States is characterized by mutually assured destruction, not mutually probable destruction. If China perceives a worst-case-scenario threat to its second strike capability, it will have little choice but to increase the efficacy of its nuclear forces. Indeed, China has already taken steps to this effect by investing in hypersonic weapons, fielding MIRV missiles, and deploying more decoys. A successful demonstration of the SM-3 Block IIA’s counter-ICBM capabilities would result in even more pressure to modernize the PRC’s nuclear arsenal.

And this problem becomes far more acute when one considers longer timeframes. The above hypothetical was set in FY23, a time when less than 50 deliveries of SM-3 Block IIAs will have taken place. George Lewis projects that, were the MDA to continue funding the SM-3 program at current levels and begin purchasing only Block IIA missiles, there would be almost 500 Block IIAs in service by 2040 (assuming no missiles are retired, which is a fair assumption given periodic upgrades and service life extension programs (SLEPs)). As such, the SM-3 Block IIA inventory could become quite large without any overt elevation of the program’s priority. Were the DoD to increase SM-3 procurement rates, it is not inconceivable that 800 or more SM-3 Block IIAs could be fielded by 2040. The United States’ Arleigh Burke class destroyers have more than enough vertical launch cells to facilitate such a scenario. This would, without a doubt, necessitate massive quantitative and qualitative improvements in China’s strategic nuclear force, especially assuming advances in discrimination, fielding of the Multi-Object Kill Vehicle (MOKV), and further upgrades to the SM-3 Block IIA in the intervening time. Even Russia’s 1,500-deployed-warhead strong arsenal could see its assured second-strike capabilities threatened in such a scenario.

And quantity is not the only concern associated with the SM-3 Block IIA. GMD operates from two large bases and stores its interceptors in large, easily identifiable silos. Any alterations made to the system, including an increase in the number of interceptors, would be relatively difficult to hide. Thus, adversaries have a good sense of how GMD impacts the nuclear balance. SM-3s, on the other hand, can be launched from the Mk 41 VLS system of any Aegis BMD destroyer or cruiser. Since all Mk 41 VLS cells look identical regardless of what they contain, adversaries have no guaranteed way of knowing how many SM-3 Block IIAs are deployed or where they are deployed. The DoD has thus far self-reported the number of SM-3s in its inventory, but there is no certainty this will continue (or that available figures are completely accurate). Moreover, SM-3 Block IIAs are relatively small, can be produced quickly, and could (given sufficient volume) be roughly equivalent in price to the weapons they defeat. As a result, they present a breakout threat; if the United States suddenly redirected tens of billions per year towards SM-3 Block IIA production, the number of interceptors deployed could rapidly reach levels concerning to China and Russia. All of these factors make the SM-3 an opaque and potentially escalatory weapon.

In addition to threatening deterrent, strategic SM-3 Block IIA deployment would pose a dilemma for the Navy. Will surface combatants be diverted to homeland defense if the SM-3 Block IIA is employed against ICBMs? The Navy has already expressed displeasure at having to perform BMD missions in the West Pacific; homeland BMD patrols would be even further from potential conflicts and from the demands of combatant commanders. Plus, Aegis destroyers and cruisers possess many capabilities (a >30 knot top speed, a hangar, etc.) that would be squandered by using them as homeland defense batteries. And having three Aegis BMD ships constantly on patrol at the three locations required for CONUS coverage would require at least six (more likely nine) ships permanently assigned to the mission. Aegis BMD vessels have a limited number of VLS cells as well; deploying more than ~300 SM-3 Block IIAs around the homeland would require even more ships. Thus, homeland patrols would represent a substantial reduction in the number of surface combatants available for traditional duties.

On the other hand, the Navy could assign strategic patrols to a bespoke missile defense vessel like the LPD-based behemoth Huntington Ingalls showed off at Sea-Air-Space 2017. Such a design would be ideal for the task thanks to its large radar and hundreds of VLS cells, but the costs related to designing, building, and sustaining a whole new class may not be justifiable. Aegis Ashore could be used as well, but that solution comes with survivability drawbacks and would lessen the SM-3’s value as an opaque weapon (for better and worse). Moreover, Aegis Ashore would tie up interceptors in a land-based installation, whereas sea-based deployment (especially aboard Aegis BMD ships) would allow SM-3s to be rapidly concentrated wherever they are needed.

Another important question: how would strategically-employed SM-3 Block IIAs coexist with GMD? The Block IIA’s range and interception altitude are inferior due to the missile’s smaller size; does this mean it would be employed as a lower layer in the event that GMD fails to intercept? This seems an appealing option, but there will be far more SM-3 Block IIAs than GBIs deployed unless production of the latter is significantly increased. Obviously, the opposite should be the case if the SM-3 is to be used as a backup interceptor.

Then there is the issue of funding cannibalization; would it make sense to buy SM-3 Block IIAs for homeland defense when various GBI improvements are in the pipeline? If the MOKV is successfully deployed and GBI production rates increase, the SM-3 Block IIA could lose much of its cost edge. And, at projected buy rates, it will take decades for the SM-3 Block IIA inventory to reach >400 interceptors. It is entirely conceivable that missile defense researchers will find a more effective means of handling the bulk of the missile threat (high-energy lasers, boost-phase interception, hypersonic projectiles, space-based kill vehicles, etc.) before then.

In making decisions about the SM-3 Block IIA, policymakers will have to grapple with a core question of American missile defense: who and what should the program defend against? North Korea and Iran? Or should it have some capabilities against near-peer competitors such as China and Russia? The former is the DoD’s official stance, but one could easily read certain MDA objectives as a hint at the latter.

Of course, this all presumes that the SM-3 Block IIA is actually useful in a strategic capacity, which is yet to be determined. The SM-3 has theoretical capabilities against ICBMs which the MDA plans to evaluate, but this notional capability may not translate well into an actual one. Even in an age of extremely sophisticated simulations, there are still no substitutes for real-life testing; one need look no further than the repeated failures of the SM-3 and GMD as evidence.

Discrimination is another ever-persistent variable; the strategic impact of the SM-3 Block IIA will be blunted severely if the MDA cannot find a way to reliably distinguish warheads from clutter. Many missile defense naysayers focus on this issue, citing decoys and other penetration aids as a reason why missile defense can never be feasible against a saturation attack. However, I am not comfortable assuming (as these commentators do) that countermeasures are an insurmountable challenge; large and unforeseen advances in technology can occur over the course of decades, and solutions based on space-deployed sensors are currently in the pipeline.

In any case, the first Block IIA vs. ICBM test is not mandated until the end of 2020, so it will be a little while before those not intimately involved with the program get further information. China and Russia could be left guessing until then (and even long after, since a single test is unlikely to be fully conclusive). Thus, concerns surrounding the SM-3 Block IIA could factor into near-term weapons development decisions in Russia and China; in fact, I would argue this has already occurred. And, now that the US has withdrawn from the Intermediate-range Nuclear Forces (INF) treaty, one can expect missile defense broadly — and the SM-3 Block IIA in particular — to factor into any attempts at renegotiation. Even the linchpin US-Russia nuclear treaty, New START, could see its renewal in 2021 impacted by developments in missile defense, including the evolving capabilities of the SM-3 Block IIA.

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