Specially modified for the SR-71A mission, the Qs were the unsung hero of the blackbird programme. Most Blackbird missions required at least one and sometimes as many as six aerial refuelings. Of the 803 KC-135s produced, 56 were converted to Q variants (and these became later on KC-135T). The major different between a Q and a standard KC-135, is that the payload fuel in the body of the aircraft is isolated from the KC-135s own fuel load in the wings. This is necessary because while almost all aircraft in the USAF inventory use JP-4 jet fuel, the Blackbirds used JP-7.