The KC-10 Extender is a multi-mission aircraft operated by the U.S. Air Force Air Mobility Command. In the 1970s, the Air Force began to look for a higher-capacity air-to-air refueling tanker. Although they had a couple of variants already in service, their primary 'large' tanker, the KC-135, was based on an airframe that was no longer in production (the Boeing 707). They decided on one which (like the KC-135) had commonality with an existing civilian design - the McDonnell Douglas DC-10. The DC-10, a tri-engine airplane (two on the wings and one on the vertical stabilizer) had a higher payload than the KC-135. Although the airplane required modifications to allow it to both perform its refueling mission and to operate in a military environment, the plane was nevertheless a relatively cheap option compared to designing a purpose-built aircraft from scratch. The Air Force ended up with 59 of them, at a cost in 1998 dollars of around $89 million each.
The main capability the KC-10 offered that the KC-135 did not was that the KC-10 could use its larger internal volume to transport both cargo and fuel, either for delivery in the air or on the ground. It has six total fuel tanks (three in the wing structure like the DC-10, and three in the lower deck cargo area) which can be used for fuel for itself and its client aircraft, and 27 standard pallet positions on the main deck for cargo. Optionally, 75 passengers can be accommodated along with 17 pallets. This is most useful in one particular circumstance - that of deploying combat aircraft overseas. Since most combat airplanes don't have the range, even the ferry range, to fly from the CONUS to some of the more distant spots they have been put to use, they need tanker support in order to deploy. The KC-10 can fly 3,800 nautical miles while carrying a full cargo load. This means that it can fly with the fighters at Mach 0.8, refueling them as necessary, as well as carry support personnel and equipment for them internally.
The KC-10 has a refueling boom and a drogue system along its centerline, which enables it to refuel nearly all U.S. and allied aircraft with aerial refueling capability.
Contractor: Boeing, who bought McDonnell Douglas, the original builder.
Engines: Three General Electric CF6-50C2 turbofans developing a 52,500 lbs. (23,625 kg) of thrust each
Length: 181 feet, 7 inches (54.4 meters)
Height: 58 feet, 1 inch (17.4 meters)
Wingspan: 165 feet, 4.5 inches (50 meters)
Operating Speed: 619 mph (Mach 0.825)
Maximum Takeoff Weight: 590,000 lbs (265,500 kg)
Maximum cargo payload: 170,000 pounds (76,560 kg)
Range: WIth cargo: 3,800 nautical miles; Without cargo, 10,000 nautical miles
Crew: Four base including boom operator
Deployed: March 1981