Updated March 19, 2024
Defense Primer: LGM-35A Sentinel Intercontinental
Ballistic Missile
The LGM-35A Sentinel is an intercontinental ballistic
The Transition from Minuteman III
missile (ICBM) system that is expected to replace the
The U.S. Air Force first deployed Minuteman ICBMs in the
Minuteman III (MMIII) ICBM in the U.S. nuclear force
1960s. MMIII, the first of the class to carry multiple
structure. MMIII has served as the ground-based leg of the
warheads, entered the force in the early 1970s. The Air
U.S. nuclear triad—land-based ICBMs, submarine-
Force has replaced and updated many of the component
launched ballistic missiles, and nuclear-capable bombers—
systems on the missile—a process known as life-
part of the U.S. nuclear deterrent since 1970.
extension—several times over the past 50 years. The most
recent life-extension program included, among other things,
The Sentinel program encompasses the missile and its
a replacement booster and a new missile guidance
associated infrastructure, including silos, communication
computer. The Air Force has noted that both of these
equipment, and facilities for launch control, maintenance,
components may face reliability concerns as they reach the
and weapons storage. The Department of Energy’s National
end of their intended lifespans over the next decade. After
Nuclear Security Administration (NNSA) is also
conducting a comprehensive Analysis of Alternatives in
developing a W87-1 warhead for the missile.
2014, the Air Force determined that it would replace MMIII
with a new missile system. The Air Force argued that when
The Biden Administration endorsed the program in its 2022
compared with a life-extended MMIII, the new missile
Nuclear Posture Review (NPR), which says Sentinel will
replace the MMIII missiles “one
would meet current and expected threats, maintain the
-for-one to maintain 400
industrial base, insert more reliable technology, produce a
ICBMs on alert.” Since the FY2017 NDAA, Congress has
modular weapon system concept, and reduce life cycle cost.
required that no less than 400 on-alert U.S. ICBMs be
deployed. The Administration included $3.7 billion for the
Air Force officials have stated that the Sentinel would have
program in its FY2025 Department of Defense (DOD)
numerous advantages to MMIII. These include the
budget request. The NNSA FY2025 budget request
following:
included $1.1 billion for the W87-1 program.
What Is an ICBM?
• The use of a modular design and open architecture that
would reduce cost and provide flexibility for
The United States began deploying nuclear-armed ICBMs
improvements throughout the life of the weapon system.
in 1959, and has maintained these systems “on alert,” or
Open systems architectures would allow the Air Force
able to launch promptly, since that time. The Air Force has
to control the intellectual property of the system,
tested MMIII missiles to a range greater than 6,000 miles,
including the system’s source code, enabling the Air
or 5,000 nautical miles. Although some countries use road
Force to complete future upgrades and improvements.
or rail mobile launchers for their ICBMs, the United States
The modular design, in turn, would also allow
bases its ICBMs in hardened concrete silos, known as
maintenance in ways that mitigate security risks and
launch facilities, located in North Dakota, Montana,
potentially enabled a reduction in the number of
Wyoming, Colorado, and Nebraska.
required security forces.
An ICBM can reach targets around the globe in
• The increased throw weight of the Sentinel compared to
approximately 30 minutes after launch. During the first
the MMIII, due to the use of much lighter composite
three minutes, three solid fuel rocket motors power the
missile’s flight
material, would allow the new missile to carry different
. After the powered portion of flight, the
payloads. As adversaries develop ballistic missile
missile follows a parabolic trajectory toward its target. It
defenses in the future, the increased throw weight,
releases its warhead during the mid-course portion of its
which is a measure of the weight of the payload that the
flight, and the warhead continues to the target.
missile can deliver to a particular range, could
Once the President authorizes the launch of any U.S.
potentially allow the Air Force to develop
nuclear-armed missile, it cannot be recalled or destroyed in
countermeasures that would help the missile overcome
flight. The same is true for nuclear missiles launched from
the defenses.
U.S. submarines. In contrast, U.S. bombers can return to
their bases after launch, without releasing their weapons,
The Air Force planned for the Sentinel (originally Ground
although the weapons cannot be recalled after their release
Based Strategic Deterrent, or GBSD) to begin replacing
from the bomber.
MMIII in 2029 and to complete the deployment in 2036.
https://crsreports.congress.gov

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Defense Primer: LGM-35A Sentinel Intercontinental Ballistic Missile
Program Status
Force to meet targeting requirements with a smaller number
The Air Force plans to procure 634 missiles, plus an
of deployed missiles.
additional 25 missiles to support development and testing,
Considerations for Congress
to enable the deployment of a force of 400 missiles by
Some members of Congress have been concerned about the
FY2036 through at least FY2075. As of early 2024, the
missile’s forward and aft
growing cost and delayed schedule of the program. In the
sections, as well as its rocket
past, some members of Congress have questioned the need
motor, have been undergoing testing, according to Northrop
Grumman, the Sentinel’s
to fund and deploy new ICBMs; others have also suggested
lead defense contractor.
that the Air Force consider, again, whether it could life-
According to the Air Force, the program also involves the
modernization of “
extend the MMIII. They, along with other commentators,
450 silos and more than 600 facilities
have argued that a delay or cancellation of Sentinel could
across almost 40,000 square miles over 6 states, 3
ease financial and other pressures caused by the
operational wings and a test location.” To provide program
simultaneous recapitalization of U.S. land-based, sea-based,
coordination, the Air Force recently stood up an ICBM
and air-delivered nuclear weapons. In 2021, DOD
Modernization Directorate in the Air Force Global Strike
commissioned an independent study on future ICBM
Command, as directed by the FY2023 NDAA.
options from the Carnegie Endowment for International
Peace. It recommended further study of MMIII life-
In January 2024, the Air Force reportedly informed
extension, specifically regarding technical and cost
Congress that the Sentinel program exceeded its initial cost
feasibility. However, the 2022 NPR said that any alternative
projections, noting a 37% increase (from $118 million
to Sentinel replacing the MMIII would “would increase risk
initial baseline cost to $162 million in 2020 dollars) in the
and cost.” Air Force officials also stated in 2023 that they
cost per missile. This cost increase is known as a “critical”
do not see a “viable” long-term life-extension for MMIII,
breach per the Nunn-McCurdy Act, which requires that the
though they are “committed” to doing “everything [they]
Secretary of Defense certify that the program is essential to
can to keep it in the field.”
national security, has no cheaper alternatives, and cannot be
terminated. It also mandates that DOD develop and validate
In the past, some analysts have suggested that the United
new cost estimates and program milestones and submit this
States reduce or eliminate its ICBMs because their
information to Congress.
vulnerability to attack could make them destabilizing in a
Air Force officials have also suggested that the program
crisis and could incentivize adversaries to attack the United
could be delayed by as much as two years and stated that
States first. Alternatively, other commentators have asserted
cost and schedule challenges have arisen primarily from
that every NPR since the end of the Cold War has endorsed
updates to the missile’s supporting infrastructure, including
the nuclear triad. The 2022 NPR stated that the three triad
silos, launch control facilities, and below-ground
legs are “complementary,” with each one “offering unique
communications cabling, as well as from supply chain
attributes.”
disruptions. A June 2023 GAO report also warned of

potential risks to cost and schedule involving immature
The 2023 report of the Congressional Commission on the
technologies, software development, and cybersecurity.
Strategic Posture of the United States, created by the
Warheads
FY2022 NDAA, proposed strategies to mitigate the risk of
modernization delays of the U.S. nuclear triad. These
The Sentinel will initially deploy with the W87-0 warheads
included a proposal to upload additional warheads to part of
currently mounted on the MMIII. NNSA is in the process of
the MMIII force if Sentinel deployment is delayed while
developing the W87-1 warhead, which, according to
some of the deployed MMIII age out, and a proposal to
NNSA, “is slated to deploy between FY2031 and FY2032.”
This warhead is the “first newly manufactured nuclear
deploy the Sentinel in a MIRVed configuration. It noted,
warhead in three decades,”
however, that these activities would need to be done in
according to its design agency
compliance with the New START Treaty, which sets
Lawrence Livermore National Laboratory. NNSA is also in
warhead and launcher limits on U.S. and Russian strategic
the process of establishing production capacity and
nuclear forces. To mitigate the dangers of growing Russian
recapitalizing infrastructure to meet the requirement of the
and Chinese nuclear threats, the commission also
2019 NDAA to produce 80 plutonium pits by 2030, and
recommended the possible fielding of a portion of the
anticipates the first certified pit for the W87-1 warhead in
“future ICBM force in a road-mobile configuration.”
2024. Lockheed Martin is developing the Mk21A reentry
vehicle that will house the W87-1 atop the Sentinel.
The FY2024 NDAA included a number of provisions and
reporting requirements related to the Sentinel program. It
The Air Force planned to deploy the Sentinel with one
directed the Air Force, in coordination with U.S. Strategic
warhead per missile. However, with the greater throw
Command and others, to “develop a plan to decrease the
weight available on the missile, the Air Force could,
amount of time required to upload additional warheads to
potentially, instead deploy two or three warheads in a
the [ICBM] force in the event Presidential direction is given
multiple-independently targetable reentry vehicle (MIRV)
to exercise such a plan.”
configuration. A MIRV enables the delivery of multiple

warheads deployed on one missile to different targets.
This In Focus was originally authored by Amy F. Woolf,
Currently, the United States disperses single-warhead
Specialist in Nuclear Weapons Policy. Benji Johnson,
MMIIIs across the upper Midwest, which, according to
former CRS U.S. Air Force Fellow, contributed to the
DOD, reduces the value of each individual missile and
complicates an adversary’s ability to attack the e
research and writing of this In Focus.
ntire force.
Some argue that MIRVed ICBMs could enable the Air
Anya L. Fink, Analyst in US Defense Policy
https://crsreports.congress.gov

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Defense Primer: LGM-35A Sentinel Intercontinental Ballistic Missile

IF11681
Alexandra G. Neenan, Analyst in U.S. Defense
Infrastructure Policy


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https://crsreports.congress.gov | IF11681 · VERSION 9 · UPDATED

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