The US Navy plans to build one of the two Virginia-class boats procured in FY2019, and all Virginia class boats procured in FY2020 and subsequent years, with an additional mid-body section, called the Virginia Payload Module (VPM). The VPM, reportedly about 70 feet in length (earlier design concepts for the VPM were reportedly about 94 feet in length), contains four large diameter, vertical launch tubes that would be used to store and fire additional Tomahawk cruise missiles or other payloads, such as large-diameter unmanned underwater vehicles (UUVs).
The four additional launch tubes in the VPM could carry a total of 28 additional Tomahawk cruise missiles (7 per tube), which would increase the total number of torpedo-sized weapons (such as Tomahawks) carried by the Virginia class design from about 37 to about 65—an increase of about 76%. The Navy wants to start building Virginia-class boats with the VPM in FY2019. Building Virginia-class boats with the VPM would compensate for a sharp loss in submarine force weapon-carrying capacity that will occur with the retirement in FY2026-FY2028 of the Navy’s four Ohio-class cruise missile/special operations forces support submarines (SSGNs).
Each SSGN is equipped with 24 large-diameter vertical launch tubes, of which 22 can be used to carry up to 7 Tomahawks each, for a maximum of 154 vertically launched Tomahawks per boat, or 616 vertically launched Tomahawks for the four boats. Twenty-two Virginia-class boats built with VPMs could carry 616 Tomahawks in their VPMs.
SOURCES – congressional research service