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Updated December 2, 2022
Defense Primer: Ground Based Strategic Deterrent (GBSD)
The Ground Based Strategic Deterrent (GBSD)—recently
Status of Minuteman III
designated the LGM-35A Sentinel missile—is expected to
The U.S. Air Force first deployed Minuteman ICBMs in the
replace the Minuteman III (MMIII) Intercontinental
1960s. Minuteman III, the first of the class to carry multiple
Ballistic Missile (ICBM) in the U.S. nuclear force structure.
warheads, entered the force in the early 1970s. The Air
MMIII has served as the ground-based leg of the U.S.
Force has replaced and updated many of the component
nuclear triad—land-based ballistic missiles, submarine-
systems on the missile—a process known as life-
launched ballistic missiles, and nuclear-capable bombers—
extension—several times over the past 50 years. The most
since 1970. (For details, see CRS Report RL33640, U.S.
recent life-extension program occurred in the late 2000s and
Strategic Nuclear Forces: Background, Developments, and
included, among other things, a replacement booster and a
Issues, by Amy F. Woolf.) The Air Force expects the
new missile guidance computer. The Air Force has noted
Sentinel (GBSD) to begin replacing MMIII in 2029.
that both of these components may face reliability concerns
Although some in Congress and outside government have
as they reach the end of their intended lifespans over the
debated whether to continue the program, the Biden
next decade (see Figure 1). After conducting a
Administration has included $3.6 billion for the GBSD
comprehensive Analysis of Alternatives (AOA) in 2014, the
program in its FY2023 budget request and endorsed the
Air Force determined that it would replace MMIII with a
program in its Nuclear Posture Review, which says Sentinel
new missile system. When compared with a life-extended
will replace the MMIII missiles “one-for-one to maintain
Minuteman III, the replacement system (the Ground Based
400 ICBMs on alert.” It also says the Sentinel will “field
Strategic Deterrent, GBSD) would meet current and
the W87-O/Mk21 and W87-1/Mk21A warheads and
expected threats, maintain the industrial base, insert more
aeroshells.”
reliable technology, produce a modular weapon system
concept, and reduce life cycle cost.
The Air Force plans to acquire 642 missiles to support
Figure 1. Projected Decrease in Operational
testing and the deployment of a force of 400 missiles. The
Minuteman III Missiles
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 Senate
version of the FY2023 NDAA 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. These missiles have been tested to
Source: Mark Gunzinger, Carl Rehberg, and Gil ian Evans, Sustaining
a range greater than 5,500 km, or roughly 3,400 miles.
the U.S. Nuclear Deterrent: The LRSO and GBSD, Center for Strategic
Although some countries use road or rail mobile launchers
and Budgetary Assessments.
for their ICBMs, U.S. ICBMs are based in hardened
concrete silos, known as launch facilities, located in North
Capabilities of GBSD
Dakota, Montana, Wyoming, Colorado, and Nebraska. An
ICBM can reach targets around the globe in approximately
Modularity: What Is It and Why Is It Important in
30 minutes after launch. During the first three minutes, the
Lowering Lifecycle Costs?
missile’s flight is powered by three solid fuel rocket motors.
In contrast with MMIII missiles, the GBSD (Sentinel) will
After the powered portion of flight, the missile follows a
use a modular design and open architecture, allowing for
parabolic trajectory toward its target. It releases its warhead
the replacement of aging and outdated components.
during the mid-course portion of its flight, and the warhead
According to the Air Force, this modular approach would
continues to the target.
reduce the lifecycle cost and provide flexibility for
improvements throughout the life of the weapon system.
Once the President authorizes the launch of any U.S.
Open systems architectures allow the Air Force to control
nuclear-armed missile, it cannot be recalled or destroyed in
the intellectual property of the system, including the
flight. The same is true for nuclear missiles launched from
system’s source code. This allows multiple vendors, in
U.S. submarines. In contrast, U.S. bombers could return to
addition to the contract winner Northrop Grumman, to
their bases after launch, without releasing their weapons,
compete for and complete future upgrades and
although the weapons could not be recalled after their
improvements to the system. These types of upgrades might
release from the bomber.
become important as technology evolves and could allow
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Defense Primer: Ground Based Strategic Deterrent (GBSD)
for improvements in the safety and reliability of the missile
MMIII. Most notably, this will increase the missile’s throw
system. They could include better guidance systems or new
weight, which is a measure of the weight of the payload that
types of countermeasures that might allow the missile to
the missile can deliver to a particular range. The Air Force
penetrate an adversary’s ballistic missile defensive systems.
asserts that the greater throw weight will allow the new
missile to carry different payloads and give it more
Consequently, modularity may provide benefits in the
flexibility for future missions. Specifically, as adversaries
maintenance of a weapon system because it would allow
develop ballistic missile defensive systems in the future, the
the Air Force to modify and possibly improve the initial
increased throw weight could potentially allow the Air
design of the missile by upgrading and replacing smaller
Force to develop countermeasures that would help the
systems, or modules, without redesigning the entire weapon
missile overcome the defenses.
system. This could potentially be a more cost-effective way
to support the missile’s intended 50-year life cycle than the
The Air Force plans to deploy the GBSD (Sentinel) with
life extension programs that replaced aging parts in the
one warhead per missile. However, with the greater throw
MMII. Also, the Air Force would not have to go back and
weight available on the missile, the Air Force could,
pay the original vendor to open software to add the new
potentially, deploy it with two or three warheads in
piece into the system architecture in the future.
response to changes in the international security
Improved Security
environment. Moreover, some argue that if the Air Force
deployed multiple warheads on each missile, it might be
The Air Force has noted that, with MMIII, most of the
able to meet targeting requirements with a smaller number
maintenance conducted on the warhead or the Missile
of deployed missiles. Currently, the United States disperses
Guidance Computer currently requires that the launcher
single-warhead missiles across a large area of the upper
closure door (the access door directly above the missile) be
Midwest, which both reduces the value of each individual
open. This introduces a security vulnerability by increasing
missile and complicates an adversary’s ability to attack the
the possibility of unauthorized observation or access. To
entire force. A smaller number of multiple warhead missiles
counter this, during MMIII maintenance operations, the Air
could change this calculus but also might provide a less
Force assigns additional Security Forces to the crew to help
costly alternative for the ICBM force.
protect the warhead. With the modular design of GBSD
(Sentinel), much of the maintenance can be conducted with
Considerations for Congress
the launcher closure door closed. The Air Force states that
Some Members of Congress have questioned the need to
this would mitigate the security risks during maintenance
fund and deploy new ICBMs; some have suggested that the
compared to the current MMIII.
Air Force consider, again, whether it could life extend the
Potential Manpower Savings
Minuteman III instead. They, along with analysts outside
government, have argued that a delay in GBSD (Sentinel)
The three current MMIII bases in the Air Force (Minot
could ease financial pressures caused by the simultaneous
AFB, Malmstrom AFB, and FE Warren AFB) require
recapitalization of U.S. land-based, sea-based, and air-
greater numbers of security forces personnel compared with
delivered nuclear weapons. The Congressional Budget
other units in the Air Force. The GSBD’s modularity that
Office (CBO) has estimated that the three major
enables most maintenance to be done with the launcher
programs—the GBSD, the Columbia Class submarines, and
closure door closed might also allow for a reduction in the
the B-1 bomber—could cost $234 billion over the next
number of Security Forces personnel required at the bases.
decade. Some analysts outside government and some
In addition to fewer required Security Forces, the Air Force
former defense officials have also suggested that the United
expects the maintenance needs of a new weapon system to
States reduce or eliminate its ICBMs because their
be greatly reduced. Finally, although the final layout of how
vulnerability to attack could make them destabilizing in a
the system will be set up has not been publicized, there are
crisis. On the other hand, those who support the new missile
indications to suggest that fewer Launch Control Centers
program have noted that every Nuclear Posture Review
(LCCs) will be required. Current requirements have 15
conducted since the end of the Cold War has endorsed the
LCCs at each of the three missile bases for a total of 45
nuclear triad, with President Obama’s 2010 NPR stating
LCCs. Each LCCs is manned continuously by two missile
that “retaining all three Triad legs will best maintain
combat crew members. If fewer LCCs are needed in GBSD,
strategic stability at reasonable cost, while hedging against
it could lead to the need for fewer missile operators. It is
potential technical problems or vulnerabilities.” The 2022
premature to estimate the potential total manpower savings,
NPR says any alternative to Sentinel replacing the MMIII
but it may be reasonable to assume there will be some.
would “would increase risk and cost.”
Improved Throw Weight
This In Focus was originally authored by Amy F. Woolf,
The MMIII engines use heavy steel casings to house the
Specialist in Nuclear Weapons Policy. Benji Johnson,
missile propellant. These casings add to the weight of the
former CRS U.S. Air Force Fellow, contributed to the
missile and affect its flight range and payload capabilities.
Modern rocket boosters, like the Navy’s D5 Submarine
research and writing of this In Focus.
Launched Ballistic Missile, use composite material to save
Mary Beth D. Nikitin, Specialist in Nonproliferation
weight and increase potential payload. The boosters
designed for the new missile will use a composite material,
IF11681
making GBSD (Sentinel) significantly lighter than the
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Defense Primer: Ground Based Strategic Deterrent (GBSD)
Disclaimer
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