Los Alamos National Laboratory in New Mexico. Production engineering began in 1965, with the first war-reserve B61-0 weapon accepted by the AEC in December 1966. However production was halted in May 1967 for design modifications to be made before resuming in January 1968. Shot Flintlock Halfbeak in June 1966 may have been a full yield test of the weapon with other tests conducted between 1963 and 1968 at the Nevada Test Site. During testing, spin motors were added to the weapon to produce a spin of 5 revolutions per second. This was to improve stability and decoupling during high speed delivery. Three alternatives to the weapon were also explored for still-classified reasons. The first alternative was halted due to its similarity to the original TX-61 design, and the Department of Defense was unfavorable to the second option which halted its exploration. The TX-61 design was ordered to "proceed along the lines" of the third alternative in May 1964, though it is unclear what that means. Total production of all versions was approximately 3,155, of which approximately 540 remain in active service, 415 in inactive service and 520 are awaiting dismantlement . 13 versions of the B61 have been designed, known as Mod 0 through Mod 12. Of these, nine have entered production. Each shares the same physics package, with different yield options. The newest variant is the Mod 11, deployed in 1997, which is a ground-penetrating bunker busting weapon. The Russian Continuity of Government facility at Kosvinsky Kamen, finished in early 1996, was designed to resist US earth-penetrating warheads and serves a similar role as the American Cheyenne Mountain Complex. The timing of the Kosvinsky completion date is regarded as one explanation for U.S. interest in a new nuclear bunker buster and the declaration of the deployment of the Mod 11 in 1997: Kosvinsky is protected by about of granite. The B61 unguided bomb should not be confused with the MGM-1 Matador cruise missile, which was originally developed under the bomber designation B-61. Deployment The B61 has been deployed by a variety of US military aircraft. US aircraft cleared for its use have included the B-1 Lancer, B-2 Spirit, B-52 Stratofortress, F/A-18 Hornet, A-6 Intruder, F-15E Strike Eagle and F-16 Fighting Falcon. As part of NATO Nuclear Weapons Sharing, German and Italian Panavia Tornado aircraft can also carry B61s. The B61 can fit inside the F-22 Raptor's weapons bays and