A game plan for dealing with the costly Sentinel missile and future nuclear challenges

Enormous cost overruns in the Sentinel program have engendered a debate about how or if to go forward with a US intercontinental ballistic missile (ICBM) modernization program. We see five potential paths forward that might reduce costs and maintain or even improve the United States’ strategic posture. But to make the best military and financial choice, the United States government will have to consider how an updated missile force relates to evolving technology in the space and cyber realms and the implications of decisions about ICBM modernization for nuclear arms control.

Questions have been raised about the cost overruns for the Sentinel ICBM modernization program, which aims to replace the existing fleet of Minuteman III missiles beginning in the next decade. Sentinel is one part of a plan to replace all three legs of the U.S. nuclear strategic triad of land-based intercontinental ballistic missiles (ICBMs), submarine-launched ballistic missiles (SLBMs) deployed on fleet ballistic missile submarines (SSBNs), and bomber-delivered weapons. Columbia class SSBNs and upgraded Trident II D-5 missiles are intended for the next generation of sea-based strategic forces, and the B-21 Raider advanced stealth bomber is already on track to replace both remaining B-52 and B-2 bombers in conventional and nuclear roles.

Plans for modernization of the entire nuclear triad were approved in the Obama, Trump, and Biden administrations, albeit with some differences in emphasis with respect to the role of nuclear weapons in US deterrence, defense, and foreign policy. The Congressional Commission on the Strategic Posture of the United States also recommended modernization and replacement of all US strategic nuclear delivery systems.

The sticker shock associated with rising cost estimates for the Sentinel program is understandable. Estimated program acquisition costs for a “reasonably modified” Sentinel have risen to about $140.9 billion. According to the Congressional Budget Office, Department of Defense and Department of Energy, budgetary requests for fiscal year 2023 related to nuclear forces total more than $576 billion for the period 2023-2032, averaging just above $75 billion per year. The history of nuclear modernization does not suggest that complete cancellation of Sentinel is the most probable outcome. The program has the support of the Air Force, members of Congress, and various defense contractors. Given the inertia of the Sentinel program, we believe questions about its cost should focus not on eliminating it, but on the implications of strategic land-based missile modernization for US national strategy, nuclear deterrence, and arms control. Going forward, what are the options for the ICBM leg of the nuclear triad from this perspective?^[1]

Alternatives for US ICBM modernization.^[2] The first option for dealing with Sentinel’s cost overruns would involve canceling the entire Sentinel program and continuing to modernize and upgrade the existing Minuteman ICBM force. The argument for this option contends that Minuteman upgrades would be much less costly than Sentinel and still adequate to fulfill ICBM mission requirements. But the Air Force and other experts contended that Minuteman had already passed its dependable shelf life and a systemwide upgrade is unavoidable if the United States is to sustain a viable land-based strategic deterrent. Continued dependence upon Minuteman would really leave the United States with a two and one-half modernized deterrent system instead of three dependable components.

A second option would be to move to a nuclear strategic dyad instead of a triad and depend on a deterrent of submarine-based weapons and strategic bombers. One rationale for this argument involves the vulnerability of the silo-based ICBM force to a first strike, which invites attack. An associated concern about the ICBM force’s potential vulnerability: US leaders might be tempted to premature alerts or prompt launch during a crisis. Supporters of a US nuclear dyad also argue that survivable submarine and bomber forces could provide necessary coverage for prompt and delayed targets without need of the additional targeting ICBMs might provide. Against these arguments, proponents of ICBMs contend that a three-legged triad of nuclear launch systems provides synergistic survivability that cannot be accomplished by a dyad. A prospective attacker against three systems must plan conservatively against three potential retaliatory forces deployed in entirely different basing modes. A three-sided deterrent force also allows for more uncertainty with respect to future breakthroughs in technology, compared to a dyad. For example, future submersible drone swarms could make the undersea environment more transparent and nuclear missile submarines more vulnerable to discovery and attack than they have been.

In a third option, future ICBMs would be deployed on mobile platforms instead of in silos. Mobility would improve their chances of surviving a first strike and, potentially, require a smaller number of deployed missiles to cover the same targeting requirements. One could envision, for example a force of 100 to 200 mobile ICBMs with two or three warheads assigned to each, versus the current fleet of 400 Minuteman IIIs. The United States could use its extensive road or rail network (or parts of both) for mobile ICBM deployments, without having to get into the environmental and construction issues that doomed the Jimmy Carter administration’s MX/MPS system. There would be challenges with respect to management of road or rail traffic, of course, and specialized mobile launch systems would be required. But the basic principles of mobile road and rail mobility for ICBMs have been well understood for many years; both the former Soviet Union and Russia have deployed and now deploy mobile ICBMs.

Yet another option would be to deploy ICBMs in so-called deep underground basing. In this approach, ICBMs would no longer be the front end of a prompt retaliatory force. Instead, they would be buried in mountainsides in the US West and rolled out for retaliation only after the first wave of an enemy attack had been completed. They would be a type of doomsday force that is presumably untargetable and therefore guaranteed to respond. In this approach, much of the US prompt counterforce mission would now be assigned to the sea-based deterrent, although the submarine missile force and bombers would also form part of the survivable retaliatory force. The challenge for this option would be to provide for some enduring command and control in the aftermath of an attack that destroyed significant parts of the nuclear command, control and communications system.

A fifth option for the ICBM force would be “conventionalization” of strategic land-based missile launchers. This would change the mission of that force into one of conventional global precision strike (CGPS). Instead of nuclear warheads, the ICBM force would be equipped with conventional weapons of various yields and improved accuracies across international distances. Russia would doubtless claim that any missile of this type overflying its territory would be assumed to be nuclear armed, but the United States could allow international inspections to verify the non-nuclear status of these launchers. In some situations, conventionally armed ICBMs would be more threatening to potential adversaries than nuclear-armed missiles. A state that doubted American willingness to attack with a nuclear first strike against one or more of its military targets might conclude that an American president would more easily be persuaded to strike with precision conventional weapons, avoiding the collateral damage attached to nuclear use.

Domain challenges to strategic stability: space and cyber. Options for a future ICBM force will have to be considered within the larger context of evolving technology related to deterrence. The domains of space and cyber now form part of the context for military planners.^[3] A putative future nuclear attacker must first establish domain awareness and operational control over the requirements for space and cyber warfare. Space assets provide the defender with early warning; command, control, and communication; geolocation; and other capabilities necessary to functioning as a cohesive military organization. Cyberwar provides a means of disabling and-or confusing the opponent’s decision-making process and paralyzing infrastructure necessary for timely military response to threats. The first wave of any nuclear first strike will doubtless consist of threatening behavior in space and the activation of hostile malware previously implanted within enemy military and civilian computing networks. Complete physical destruction of enemy space and cyber assets is not necessary; only a sufficient measure of confusion would be needed on the tail end of a crisis that is heading over the cliff into war.

Hypersonic weapons cast another shadow of concern over deterrence and crisis stability.^[4] Hypersonic glide vehicles are already being deployed on some Russian ICBMs, and other major powers can be expected to deploy similar weapons or hypersonic cruise missiles in their future arsenals. Compared to slower-arriving attackers, hypersonics allow less time for political leaders and their military advisors to receive early warning, to establish attack characterization, and to decide upon an effective response before their nuclear forces or other targets are struck. Awareness of this possible vulnerability may encourage hasty decision making or partial delegation of decisions to pre-programmed AI systems. The concern about capabilities of hypersonics may also lead, in the case of the US ICBM force, to a decision in favor of launch on warning instead of riding out the attack and retaliating. The problem of hypersonics is also related to uncertainty in the space and cyber domains. Destruction of, or interference with, a state’s space assets and cyber defenses, combined with expectations of hypersonic attacks against forces and command systems, invites preemptive attack justified as a “defensive” move based on worst case assumptions.

Finally, there is the issue of strategic nuclear arms control and its potential demise under the pressures of US-Russian political disagreement, of China’s apparent ambition to become a nuclear superpower, of growing political and military alignments between Beijing and Moscow, and of the wobbly status of the last major Russian–American strategic nuclear arms control agreement (New START), originally signed in 2010 and now extended only until February 2026.^[5] Senior Biden administration officials have warned that, given developments in Russian and Chinese nuclear strategy and Russian President Vladimir Putin’s recurrent threats of nuclear first use in the war against Ukraine, the United States may be forced to expand its nuclear arsenal instead of simply modernizing it. Absent a prompt peace settlement of the war in Ukraine, New START may be permanently stalled or ditched in principle. Such an outcome would be contrary to US, Russian, and possibly Chinese interests (officially recognized or otherwise) in avoiding nuclear war, in forestalling expensive and unnecessary arms races, and in containing global nuclear proliferation. Between now and the official expiry date for New START, the United States and Russia should conduct unofficial talks between their arms control and military experts, avoid provocative demarches with respect to the possibility of nuclear first use in Europe, and act together to discourage the emergence of new nuclear weapons states (e.g., Iran).

A game plan for the future of strategic stability. With regard to strategic nuclear arms control, we outline this possible US post-New START Interim Holding Force for the time period 2026-2030:

• 300 Minuteman ICBMs LE (Life Extension), of which 200 would be silo based, and 100 mobile, for a total of 300 land-based missile warheads.
• 12 Columbia class nuclear ballistic missile submarines (SSBN) with Trident II D-5 LE ballistic missiles. At any given time, 10 SSBNs would be operational, providing a total of 1080 deployed warheads.
• 60-100 B-21 Raider bombers, carrying mixed loads of air-launched cruise missiles and gravity bombs, yielding a total of 60-100 warheads using New START counting rules.

This notional force falls within current or prospective New START limits if the United States and Russia decide to reboot and extend that treaty beyond 2026. It should provide for assured retaliation against a Russian force that is similarly constrained or against a Chinese force that levels off at numbers of operationally deployed warheads on strategic launchers between 1,000 and 1,500. Any longer-term follow-on force to the interim force suggested above would have to take into account the possibility of a future combined Russian-Chinese attack, but that possibility is not necessarily the most logical basis for realistic US planning. China will be very cautious about getting into a nuclear war as Russia’s tail gunner: Chinese perceptions of their own national interests will determine Beijing’s nuclear deterrence and defense priorities.

US, Russian, and Chinese arms control efforts should also reach into the space and cyber domains. As indicated above, these domains are on the front end of possible vulnerabilities that could tempt nuclear first strike or first use. The near-term issue in space is a regime that restrains attacks on space-based assets for warning, for nuclear command, control, and communications, for navigation, and for other capabilities vital to deterrence and to the avoidance of escalation by miscalculation.^[6] In the cyber realm, the chaos that could result from an unconstrained digital misanthropy, especially during a nuclear crisis, could create “analysis paralysis” followed by distorted perceptions of immediate danger and insufficient awareness of available alternatives.

The Sentinel ICBM system is one option among many for modernizing the ICBM force, but it has inertia—based on support from members of Congress, from the US Air Force, and from defense contractors—that will be difficult to reverse. Regardless of the preferred launch system, at least some of the ICBM force should be assigned to mobile basing to reduce incentives for prompt launch based on mistaken or ambiguous warning. The war in Ukraine should not obviate US-Russian efforts to revive the New START and post-New START arms control discussions, and a rising Chinese nuclear power must eventually be brought into strategic arms limitation talks—if not immediately about force reductions, then at least about concepts of military strategy, deterrence, and defense. The space and cyber realms are no longer sidebars or afterthoughts for deterrence and arms control, but candidates to be prime movers for rethinking nuclear stability in decades ahead.

Notes

[1] Additional expert commentary on Sentinel appears in: Hans M. Kristensen, Matt Korda, Eliana Johns, and Mackenzie Knight, “United States Nuclear Weapons, 2024,” Bulletin of the Atomic Scientists, May 7, 2024, https://thebulletin.org/premium/2024-05/united-states-nuclear-weapons-2024/

[2] For an overview of this issue, see: Lauren Caston, Robert S. Leonard, Christopher A. Mouton, Chad J.R. Ohlandt, S. Craig Moore, Raymond E. Conley, and Glenn Buchan, The Future of the U.S. Intercontinental Ballistic Missile Force (Santa Monica, California: RAND Corporation, 2014), https://www.rand.org/pubs/monographs/MG1210.html

[3] See, for example: Erik Gartzke and Jon R. Lindsay, “Thermonuclear cyberwar, Journal of Cybersecurity (2017), pp. 1-12, <doi:10.1093/cybsec/tyw017>; and David E. Sanger, The Perfect Weapon: War, Sabotage, and Fear in the Cyber Age (New York: Crown Publishing, 2018); U.S. Department of Defense, Space Policy Review and Strategy on Protection of Satellites (Washington, D.C. Department of Defense, September 2023), https://media.defense.gov/2023/Sep/14/2003301146/-1/-1/0/COMPREHENSIVE-REPORT-FOR-RELEASE.PDF

[4] This issue is discussed by US Air Force Col. Stephen Reny in “Nuclear-Armed Hypersonic Weapons and Nuclear Deterrence,” Strategic Studies Quarterly, no. 4 (Winter 2020), pp. 47-76.

[5] A useful assessment appears in Daryl G. Kimball, “Does the United States Need More Nuclear Weapons?” Arms Control Today, July – August 2024, https://www.armscontrol.org/act/2024-07/features/does-united-states-need-more-nuclear-weapons

[6] For further discussion, see: Lawrence J. Korb, “The focus of US military efforts in outer space should be…arms control,” Bulletin of the Atomic Scientists, no. 4 (2019), pp. 148-150, DOI: 10.1080/00963402.2019.1628471