The United States Navy initially worked on a sea-based variant of the US Army Jupiter intermediate-range ballistic missile, projecting four of the large, liquid-fueled missiles per submarine. Rear Admiral W. F. "Red" Raborn headed a Special Project Office to develop Jupiter for the Navy, beginning in late 1955. However, at the Project Nobska submarine warfare conference in 1956, physicist Edward Teller stated that a physically small one-megaton warhead could be produced for the relatively small, solid-fueled Polaris missile, and this prompted the Navy to leave the Jupiter program in December of that year. Soon Chief of Naval Operations Admiral Arleigh Burke concentrated all Navy strategic research on Polaris, still under Admiral Raborn's Special Project Office. All US SLBMs have been solid-fueled while all Soviet and Russian SLBMs have been liquid-fueled except for the Russian RSM-56 Bulava, which entered service in 2014.