Mk21A Reentry Vehicle

History

The Mk21A is a developmental reentry vehicle being developed by Lockheed Martin for the LGM-35A Sentinel intercontinental ballistic missile. It is based on the Mk21 reentry vehicle used on the Minuteman III and is distinct from the W78 or W88 warheads employed on earlier systems. The Air Force Nuclear Weapons Center awarded Lockheed Martin a $996 million contract for the Mk21A in October 2023, followed by a $453 million contract modification in November 2025 as part of a program rebaseline intended to decouple the schedule from external programs.

The first announced flight test of the Mk21A took place on June 17, 2024, when an unarmed unit was launched aboard a Northrop Grumman Minotaur I rocket from Vandenberg Space Force Base, California; that mission successfully flight-tested the vehicle's heat shield. On October 9, 2026, the Air Force conducted a further in-flight environmental test, again using a Minotaur I launch vehicle from Vandenberg Space Force Base under oversight of the U.S. Space Force's Rocket Systems Launch Program. The test carried no warhead and generated data on heat, vibration, and aerodynamic loads experienced during reentry. The Air Force has not publicly released detailed performance measurements from the mission.

Under the Air Force's phased modernization strategy, Sentinel is expected to be fielded initially with the existing Mk21 reentry vehicle before subsequently transitioning to the next-generation Mk21A.

Design

The Mk21A is a cone-shaped chamber that holds the nuclear warhead and sits atop the three-stage solid-fuel missile. It separates from the boosters during reentry from space and impacts or detonates above the target. The Pentagon acquisition report identifies the vehicle's components as a shape-stable nose tip, high impulse transducer, arming and fuzing subsystem, aeroshell forward and body sections with rear cover, radio frequency subsystem with antennas, RV spin subsystem, in-flight disconnect cable, and other electrical cables.

Specifications

When operational, the Mk21A is designed to carry the W87-1 nuclear warhead with a yield of up to 475 kilotons. For comparison, the current W87-0 warhead on the Minuteman III has a yield of 300 kilotons.

Program Status

The Mk21A is in its engineering and manufacturing development phase. A critical design review for the Mk21A system is planned by March 2027. The Air Force says it is on track to flight test a completely assembled Sentinel with an unarmed warhead in 2027. The Air Force Nuclear Weapons Center at Kirtland Air Force Base, New Mexico, leads the acquisition effort. The Air Force's fiscal 2027 budget request includes approximately $540 million for Mk21A development, an increase of $184 million from the 2026 allocation as Sentinel integration and test activities ramp up.