Boeing KC-135 Stratotanker airplane model. Because the KC-135A's original engines are of 1950s technology, they don't meet modern standards of increased fuel efficiency, reduced pollution and reduced noise levels. By installing new, CFM56 engines, performance is enhanced and fuel off-load capability is dramatically improved. In fact, the modification is so successful that two-re-engined KC-135Rs can do the work of three KC-135As. This improvement is a result of the KC-135R's lower fuel consumption and increased performance which allow the tanker to take off with more fuel and carry it farther. Since the airplane can carry more fuel and burn less of it during a mission, it's possible to transfer a much greater amount to receiver aircraft.