These studies led to initial requirements for the CX-Heavy Logistics System ( CX-HLS ) in March 1964 for an aircraft with a load capacity of 180,000 pounds ( 81,600 kg ) and a speed of Mach 0.75 ( 500 mph or 805 km\/h ) , and an unrefueled range of 5,000 nautical miles ( 9,260 km ) with a payload of 115,000 pounds ( 52,200 kg ) .
It also had to be able to fly for 5,000 nautical miles ( 9,260 km ) while carrying 115,000 pounds ( 52,200 kg ) of cargo .