KC-135R Stratotanker / 77th Air Refueling SquadronBoeing KC 135R Mahogany wood 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