The development and flying of the Bell X-1 was no picnic, but in the newly formed USAF’s hands it raced through test objectives: breaking records, setting standards and un-shrouding the mysteries of supersonic flight. By the end of the forties, it, and aircraft such as the X-1, D-558, and YP-86, had vastly increased man’s knowledge of high speed, high altitude flight. Engineers had a greater understanding of the mechanical and aeronautical challenges, but the USAF had no intention of stopping at Mach 1.
To that end, Bell was once again contracted for an experimental aircraft, the X-2. The principal goals for the X-2 were to investigate the structural effects of operating at higher mach numbers, with the goal of reaching Mach 3.
Engineers already knew they would encounter greater stresses and aerodynamic stability problems, but worst of all was a dramatic increase in the surface temperature the aircraft.
This increase in temperature was referred to as the “thermal thicket.” Even though the air temperature at high altitude is in the region of -60C, the friction effects of air on the surface of the aircraft were predicted to raise the skin temperatures many hundreds of degrees. Aluminum, a typical metal used for aircraft skin, softens around 120C. So entering the thermal thicket was going to require new materials along with more powerful engines and more effective control surfaces.
The challenges were significant, and the program overran by years, but gradually the challenges were met. K-Monel, a stainless steel/copper/nickel alloy was developed for the skin, and Reaction Motors developed a throttleable two chamber rocket, the XLR25.
Two X-2 “Starbuster” aircraft were built in Wheatfield, New York. Like the X-1, they were designed for air launch, this time from a B-50. Jean “Skip” Ziegler performed the first flight (unpowered) on 27th June 1952, seven years after the program started. Sadly, in 1953, Skip Ziegler and the aircraft were lost when the it exploded on a captive-carry flight.
Testing of the remaining aircraft was taken up by Lt Col Frank “Pete” Everest. In 1956 he reached Mach 2.87 (1,900mph), but found the controls only marginally effective. The engineers knew they were approaching the stage where the aircraft was going to become unstable, and with feeble control surfaces the prospect wasn’t great.
The X-2 was intended to be handed over to NACA for civilian scientific study, but the USAF requested an extension to allow them time to qualify two new pilots and push the envelope further.
Everest handed testing of the aircraft over to Captains Iven Kincheloe and Milburn “Mel” Apt. Kincheloe and Apt were both excellent pilots, but neither had any experience of rocket powered aircraft.
In the X-2, Kincheloe, a Korean war ace, became the first pilot to exceed 100,000ft on 7 September 1956, when he reached at 126,200ft.
Twenty days later, Mel Apt seated himself in the X-2 for his first flight, a flight with an ambitious goal, Mach 3.
After release from the B-50, Apt lit both rocket chambers and quickly left his F-100 chase plane behind.
He flew a flight path designed to maximize the aircraft speed as the air thinned out. The rocket motor burned slightly longer than expected and Apt had flown the optimum path, at 65,500ft he reached Mach 3.2 or 2,094mph.
126,200ft and Mach 3.2. It had taken far longer to get there than the program originally planned, but at last USAF had their achievements, the engineers had their data and the country had gained another first. There were celebrations all round.
But the euphoria was short lived – less than a minute. After reaching Mach 3.2, Apt started a turn. It’s not known why he began a turn, perhaps he thought he would stray too far from the lakebed. Whatever the reason, the result was disastrous. The aircraft began to experience inertial coupling and began violently tumbling. Yeager had experienced the same effect in the X-1, and only just recovered in time, but he had extensive experience in the handling qualities of his aircraft, Mel Apt was on his first flight. That ambitious goal must have quickly looked foolish and ill-concieved.
The X-2 was equipped with an ejection capsule, not an ejection seat. Apt must have determined he was not going to recover the aircraft and ejected the capsule. The idea of the capsule was that when the pilot reached a safe altitude and speed, he would bale out of the capsule and return to earth on his own parachute. However, Apt must have been incapacitated by this point, because he never baled out of the capsule and it plummeted straight to the desert floor.
The tragedy of Mel Apt’s death stopped the program. No further X-2 aircraft were built. The USAF/NACA plans to explore the Mach 2 – 3 region with a manned aircraft were shelved until the X-15 came along in 1959.
The X-2 had numerous technical and operational problems. More than one rough landing required extensive rebuilds. One aircraft was lost in captive carry testing. You might consioder it an omen, but either way, it was a a wake-up call to the aviation community. High speed flight would never again give up its secrets gladly.
What do you think? had the sound barrier be conquered too easily, and the pilots and engineers been lulled into a false sense of security? Or was the USAF/NACA team pushing too hard before the proper research had been done?
(Images courtesy of NASA Dryden Flight Research Center)