https://crsreports.congress.gov
Updated December 2, 2022
Defense Primer: Ground Based Strategic Deterrent (GBSD)
The Ground Based Strategic Deterrent (GBSD)—recently
designated the LGM-35A Sentinel missile—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. (For details, see CRS Report RL33640, U.S.
Strategic Nuclear Forces: Background, Developments, and
Issues, by Amy F. Woolf.) The Air Force expects the
Sentinel (GBSD) to begin replacing MMIII in 2029.
Although some in Congress and outside government have
debated whether to continue the program, the Biden
Administration has 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-O/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 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
a range greater than 5,500 km, or roughly 3,400 miles.
Although some countries use road or rail mobile launchers
for their ICBMs, U.S. ICBMs are based 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, the
missile’s flight is powered by three solid fuel rocket motors.
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. Minuteman III, 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 Figure 1). After conducting a
comprehensive Analysis of Alternatives (AOA) in 2014, the
Air Force determined that it would replace MMIII with a
new missile system. When compared with a life-extended
Minuteman III, the replacement system (the Ground Based
Strategic Deterrent, GBSD) 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.
Figure 1. Projected Decrease in Operational
Minuteman III Missiles
Source: Mark Gunzinger, Carl Rehberg, and Gillian Evans, Sustaining
the U.S. Nuclear Deterrent: The LRSO and GBSD, Center for Strategic
and Budgetary Assessments.
Capabilities of GBSD
Modularity: What Is It and Why Is It Important in
Lowering Lifecycle Costs?
In contrast with MMIII missiles, the GBSD (Sentinel) will
use a modular design and open architecture, allowing for
the replacement of aging and outdated components.
According to the Air Force, this modular approach would
reduce the lifecycle cost and provide flexibility for
improvements throughout the life of the weapon system.
Open systems architectures allow the Air Force to control
the intellectual property of the system, including the
system’s source code. This allows multiple vendors, in
addition to the contract winner Northrop Grumman, to
compete for and complete future upgrades and
improvements to the system. These types of upgrades might
become important as technology evolves and could allow
