September 2006 marks the 50th anniversary of two aerospace milestones that involved both triumph and tragedy for the flight test community at Edwards Air Force Base. It was in September 1956 that the highest and fastest flights of the Bell X-2, a swept-wing, rocket-powered research aircraft were flown. Sadly, the latter of those two missions cost the life of Air Force Capt. Milburn “Mel” Apt, one of the test pilots assigned to the project.
The X-2 was flown in a joint program to investigate the problems of aerodynamic heating, stability and control effectiveness at high speeds and altitudes. Project management was shared by the U.S. Air Force and Bell Aircraft Co., but the National Advisory Committee for Aeronautics’ High Speed Flight Station, now the Dryden Flight Research Center, participated in supporting research, wind-tunnel and rocket-powered model tests and data analysis.
Bell built two X-2 aircraft, designed to be carried to launch altitude beneath a modified B-50 bomber and released for unpowered and powered research flights. In 1952, while Bell technicians fitted the first vehicle with its rocket engine, the second X-2 began glide tests at Edwards Air Force Base, Calif. After being returned to Bell in 1953 for engine installation, the second X-2 was lost in a catastrophic accident during a captive-carry flight.
During a series of high-speed flights in 1955 and 1956, Air Force Lt. Col. Frank K. “Pete” Everest piloted the remaining X-2 in increasing speed increments in an attempt to reach three times the speed of sound, or Mach 3. He eventually achieved a speed of Mach 2.87 in the X-2, just short of the Mach 3 goal. Dubbed the “Fastest Man Alive,” Everest was destined to hold the title but not the speed record in the X-2.
The Air Force next initiated a series of altitude expansion flights with Capt. Iven C. Kincheloe as project pilot. On Sept. 7, 1956, he began his third high-altitude mission with release from the B-50 at an altitude of 29,000 feet.
Kincheloe ignited the XLR-25 rocket engine and pulled the X-2 into a climb, eventually reaching a speed of 1,700 mph. The engine shut down at 90,000 feet as its fuel was depleted and the X-2 coasted to an altitude of 126,200 feet, experiencing a state of near weightlessness as the X-2 passed the top of its semi-ballistic arc.
The airplane, well above 90 percent of the Earth’s atmosphere, rolled into a left bank as its aerodynamic controls lost effectiveness due to the thin air. Once the X-2 descended to 40,000 feet, Kincheloe returned the vehicle to level flight and glided to a landing at Edwards. This first manned fight above 100,000 feet earned him the sobriquet “First of the Space Men,” even though the altitude reached was well below the levels later defined by military and civil authorities as being in space.
Following the Air Force test program, NACA director Dr. Hugh L. Dryden requested that the X-2 be transferred to the NACA for aerodynamic and structural-heating studies, but the Air Force delayed turning it over to the civilian agency in the hope of attaining Mach 3. The service requested and received a two-month extension to qualify another Air Force test pilot in the airplane.
Capt. Milburn “Mel” Apt was assigned to the X-2 program in February 1956 and flew several chase missions in support of Kincheloe’s altitude flights. Finally, in September 1956, he was offered the opportunity to fly the X-2 himself. After several ground briefings in the simulator, Apt – with no previous rocket plane experience – was scheduled to make his first flight on Sept. 27. Just before the flight Kincheloe, who would perform chase duties, expressed his confidence in Apt’s abilities.
“You’ve got it hacked, Dad,” he told Apt.
After being lofted to altitude and released, Apt raced the X-2 away from the B-50 under full power, quickly outdistancing the F-100 chase planes. At high altitude, he nosed over, accelerating rapidly. The X-2 reached Mach 3.2 (2,094 mph) at 65,500 feet and Apt became the first man to fly more than three times the speed of sound.
He never got to celebrate his victory.
Still above Mach 3, Apt began an abrupt turn back toward Edwards. The maneuver proved fatal as the X-2 began a series of diverging rolls and tumbled out of control. Apt was unable to successfully complete the escape sequence and perished in the subsequent crash. He had flown a perfect flight profile and broken Everest’s record, but was unable to claim the title of “Fastest Man Alive.”
Despite difficulties throughout the X-2 program, the NACA was able to salvage useful data regarding the challenges of high-speed and high-altitude flight, aerodynamic heating and aircraft control. Even Apt’s fatal accident provided valuable lessons about aerodynamic design problems for supersonic airplanes, including the inertial coupling problem that resulted in Apt’s loss of control and cost him his life.