First delivered to the USAF in June 1957, the KC-135 served for more than sixty years as the USAF's principal refueling aircraft. Originally developed to extend the range of Strategic Air Command's bomber fleet, the Stratotanker went on to support a broad range of aircraft. In the 1980s, roughly half of the 732 KC-135s were upgraded, enabling them to offload fifty percent more fuel, improve fuel efficiency, and reduce operational costs. The aircraft transfers fuel through a telescoping boom, guided by an enlisted crewmember using small control surfaces called ruddervators to steer the nozzle into a receptacle on the receiving aircraft, a task requiring precise and steady flying by both planes. The NMUSAF's KC-135 received the Mackay Trophy for a mission flown on 31 May 1967, when its crew responded to the rescue of a downed U.S. Navy pilot in Southeast Asia by refueling Navy tankers as they simultaneously refueled two Navy F-8 fighters, then fueling two more Navy F-4s on the return to base.