The B-52H shared its crew and structural changes with the B-52G, but carried the most significant upgrade in the whole family: the switch to Pratt & Whitney TF33-P-3 turbofan engines, which despite initial reliability problems (corrected by 1964 under the "Hot Fan" program) offered considerably better performance and fuel economy than the J57 turbojets. Electronic countermeasures and avionics were updated, a new fire-control system was fitted, and the rear defensive armament changed from machine guns to a 20 mm M61 Vulcan cannon, removed between 1991 and 1994. The final 18 aircraft were built with provision for the ADR-8 countermeasures rocket, later retrofitted to the rest of the G and H fleet. It first flew on 10 July 1960 and entered service on 9 May 1961; it is the only variant currently in use. 102 were built; the last B-52H, serial 61-40, left the factory on 26 October 1962. It can carry up to 32,000 kg of mixed ordnance — bombs, mines and missiles — including the AGM-86B nuclear cruise missile in its nuclear role and, conventionally, AGM-142, AGM-154, AGM-158, JDAM guided munitions and, in maritime roles, Harpoon anti-ship missiles and Quickstrike naval mines; it carries no civil payload.