The four engine KC-135 carries 90 tons of fuel and can transfer up to 68 tons per sortie. Typically, aerial tankers have to service B-52s, which carry over 140 tons of jet fuel, and fighters like the F-15 (over five tons). The KC-135 has also made itself useful carrying cargo and passengers, as well as fuel. But the elderly KC-135s are difficult to keep running, and many of them still have to serve another twenty years until completely replaced by the new KC-46A. Operating aircraft this old is unexplored territory because this is the first time in history that so many large, and fast, aircraft have gone on flying for so long. Commercial freighter aircraft fly more frequently than their military counterparts, putting more strain on them and forcing their operators to develop new maintenance techniques the air force can use. The KC-135s are the oldest transports the air force is still using, and keeping them working is proving to an expensive and challenging effort.
The basic problem is that, despite constant maintenance and careful monitoring, unexpected failures still occur with elderly aircraft. Nothing that is cause for alarm but it is more expensive to keep them flying and problems are more difficult to predict. Older aircraft are grounded if any unexpected failure seems imminent. While that just about eliminates these aircraft having fatal failures while in the air, it also makes older aircraft less available for service. For a long time it was considered more cost effective to keep the old birds flying than to buy new ones. Eventually advanced age made replacement a necessity, not an option.
Thus after over a decade of effort, the U.S. Air Force managed to finally select a replacement two years ago. The air force procurement process has been cursed with corruption, incompetence, meddling politicians, and litigious suppliers, all combining to prevent the acquisition of a new tanker. One was finally selected and the initial order was for 18 aircraft (at about $150 million each), to be delivered within six years. That initial order also comes with about a billion dollars for development work. The air force might order over a hundred KC-46As, but the exact number depends on what kind of future aircraft the air force will be using. If there are a lot of unmanned aircraft (UAVs), fewer tankers will be needed (because UAVs are smaller and need less fuel). There is, however, a lot of resistance in the air force and Congress to any further squabbling over who should build the replacement for the KC-135.
The KC-46A is based on the Boeing 767-200 airliner, which sells for about $120 million. The 767 has been in service since 1982, and over 800 have been manufactured so far. Boeing developed the KC-767, at a cost of nearly a billion dollars, on its own. Boeing also developed the original KC-135 tanker in the 1950s, and has since built over 2,000 of these.
The KC-46A was selected partly because it is about the same size as the KC-135 (wingspan is 50.3 meters/156 feet, 6.8 percent larger than the KC-135). Thus the new tanker can use the same basing and repair facilities as the 135. The KC-46A can carry up to 94 tons of fuel. It can also carry up to 114 passengers or 18 cargo pallets or 58 patients (24 on litters).