The following is the Congressional Research Service, Defense Primer: LGM-35A Sentinel Intercontinental Ballistic Missile on Jan. 10, 2023.
From the report
The LGM-35A Sentinel is expected to replace the Minuteman III (MMIII) Intercontinental Ballistic Missile (ICBM) in the U.S. nuclear force structure. MMIII has served as the ground-based leg of the U.S. nuclear triad—land-based ballistic missiles, submarine-launched ballistic missiles, and nuclear-capable bombers—since 1970. The Air Force expects the Sentinel (originally Ground Based Strategic Deterrent—GBSD) to begin replacing MMIII in 2029. Although some have debated whether to continue the program, the Biden Administration included $3.6 billion for the GBSD program in its FY2023 budget request and endorsed the program in its Nuclear Posture Review, which says Sentinel will replace the MMIII missiles “one-for-one to maintain 400 ICBMs on alert.” It also says the Sentinel will “field the W87-0/Mk21 and W87-1/Mk21A warheads and aeroshells.”
The Air Force plans to acquire 642 missiles to support testing and the deployment of a force of 400 missiles. The Air Force expects the program to reach its initial operational capacity, with nine missiles on alert, by 2029; it expects to complete the deployment in 2036. The Sentinel program encompasses both the missile itself and its associated infrastructure, including launch and flight-related capabilities. The FY2023 NDAA authorized funding the program at request and would prohibit any reduction in alert levels or reduction of the quantity of deployed ICBMs below 400 total.
What Is an ICBM?
The United States began deploying nuclear-armed intercontinental ballistic missiles in 1959, and has maintained these systems “on alert,” 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. Although some countries use road or rail mobile launchers for their ICBMs, the United States bases its ICBMs in hardened concrete silos, known as launch facilities, located in North Dakota, Montana, Wyoming, Colorado, and Nebraska. An 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. It releases its warhead during the mid-course portion of its flight, and the warhead continues to the target.
Once the President authorizes the launch of any U.S. nuclear-armed missile, it cannot be recalled or destroyed in flight. The same is true for nuclear missiles launched from U.S. submarines. In contrast, U.S. bombers could return to their bases after launch, without releasing their weapons, although the weapons could not be recalled after their release from the bomber.
Status of Minuteman III
The U.S. Air Force first deployed Minuteman ICBMs in the 1960s. MMIII, the first of the class to carry multiple warheads, entered the force in the early 1970s. 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 most recent life-extension program occurred in the late 2000s and included, among other things, a replacement booster and a new missile guidance computer. The Air Force has noted that both of these components may face reliability concerns as they reach the end of their intended lifespans over the next decade (see Error! Reference source not found.). After conducting a comprehensive Analysis of Alternatives (AOA) in 2014, the Air Force determined that it would replace MMIII with a new missile system. The Air Force states that when compared with a life-extended MMIII, the replacement system (the LGM-35A Sentinel) would meet current and expected threats, maintain the industrial base, insert more reliable technology, produce a modular weapon system concept, and reduce life cycle cost. The Department of Defense commissioned an independent study on future ICBM options from the Carnegie Endowment for International Peace. It was published in 2022 and recommended further study of MMIII life-extension, specifically regarding technical and cost feasibility.
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