Navy Next-Generation Attack Submarine (SSN[X]) Program: Background and Issues for Congress [Updated October 19, 2021] [open pdf - 615KB]
From the Introduction: "The Navy wants to begin procuring a new class of nuclear-powered attack submarine (SSN), called the Next-Generation Attack Submarine or SSN(X), in FY2031. The SSN(X) would be the successor to the Virginia-class SSN design, which the Navy has been procuring since FY1998. The Navy's proposed FY2022 budget requests $98.0 million in research and development funding for the SSN(X) program."
CRS In Focus, IF11826
Congressional Research Service: https://crsreports.congress.gov/