The following is the Nov. 8, 2024, Congressional Research Service In Focus report, Defense Primer: LGM-35A Sentinel Intercontinental Ballistic Missile.
From the report
The LGM-35A Sentinel is an intercontinental ballistic missile (ICBM) system that is expected to replace the Minuteman III (MMIII) ICBM in the U.S. nuclear force structure. MMIII has served as the ground-based leg of the U.S. nuclear triad—land-based ICBMs, submarine-launched ballistic missiles, and nuclear-capable bombers—since 1970. (See CRS In Focus IF10519, Defense Primer: Strategic Nuclear Forces.) The Biden Administration included $3.7 billion for the Sentinel in its FY2025 Department of Defense (DOD) budget request, and $1.1 billion in the Department of Energy’s National Nuclear Security Administration (NNSA) budget request for the W87-1 nuclear warhead that is being developed for the missile. In 2024, DOD conducted a congressionally mandated review of the Sentinel program following an increase in cost projections.
What Is an ICBM?
A U.S. ICBM can reach targets around the globe in approximately 30 minutes after launch. During the first three minutes, three solid fuel rocket motors power the missile’s flight. After the powered portion of flight, the missile follows a parabolic trajectory toward its target. The missile releases its warhead during the mid-course portion of its flight, and the warhead continues to the target.
The United States began deploying nuclear-armed ICBMs in 1959 and has maintained these systems “on alert,” or able to launch promptly, since that time. The Air Force has tested MMIII missiles to a range greater than 6,000 miles, or 5,000 nautical miles. The United States bases its ICBMs solely in hardened concrete silos, known as launch facilities, located in North Dakota, Montana, Wyoming, Colorado, and Nebraska. Russia and China use both silos and road-mobile launchers for their ICBMs.
Once the President authorizes the launch of any U.S. ICBM, the missile cannot be recalled or destroyed in flight. The same is true for nuclear missiles launched from U.S. submarines. In contrast, U.S. bombers can return to their bases, without releasing their weapons, although their weapons also cannot be recalled after their release.
The Transition from Minuteman III
The U.S. Air Force first deployed Minuteman ICBMs in the 1960s. MMIII, which is currently deployed in a single-warhead configuration, entered the force in 1970. The Air Force has replaced and updated many of the component systems on the missile—a process known as life-extension—several times over the past 50 years. The Air Force has stated that some of these components may face reliability concerns as they reach the end of their intended lifespans. After conducting an Analysis of Alternatives in 2014, the Air Force decided to replace MMIII with a new missile system (originally Ground Based Strategic Deterrent) that would serve through 2075. The Air Force argued that when compared with a life-extended MMIII, the new ICBM would meet current and expected threats, maintain the industrial base, produce a modular weapon system, and reduce life cycle cost. The Air Force and Northrop Grumman, the Sentinel’s lead defense contractor, planned for the Sentinel to begin replacing MMIII in 2029.
Program Status
The Air Force plans to procure 634 Sentinel missiles, plus an additional 25 missiles to support development and testing, to enable the deployment of 400 missiles. According to the Air Force, the program also includes modernizing “450 silos and more than 600 facilities across almost 40,000 square miles.”
In January 2024, the Air Force informed Congress that the Sentinel program exceeded its initial cost projections, positing at least a 37% increase (from $118 million initial baseline cost to $162 million in 2020 dollars) in the cost per unit. This cost increase is known as a “critical” breach per the Nunn-McCurdy Act (Title 10 U.S. Code §§4371-4377), which requires the Secretary of Defense to certify that the program is essential to national security, has no cheaper alternatives, and cannot be terminated. It also mandates that DOD develop and validate new cost estimates and program milestones and submit this information to Congress.
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