The Benjamin Franklin class of US ballistic missile submarines were in service from the 1960s to the 2000s. The class was an evolutionary development from the earlier James Madison class of fleet ballistic missile submarine. Having quieter machinery and other improvements, it is considered a separate class. A subset of this class is the re-engineered 640 class starting with USS George C. Marshall. The primary difference was that they were built under the new SUBSAFE rules after the loss of USS Thresher, whereas earlier boats of the class had to be retrofitted to meet SUBSAFE requirements. The Benjamin Franklin class, together with the George Washington, Ethan Allen, Lafayette, and James Madison classes, comprised the "41 for Freedom" submarines that were the Navy's primary contribution to the nuclear deterrent force through the late 1980s. This class and the James Madison class are combined with the Lafayettes in some references.