EDWARDS AIR FORCE BASE, Calif. -- When the experimental XS-1 rocket-plane dropped away from the B-29 that carried it aloft on an October morning 74 years ago today, then Capt. Charles “Chuck” Yeager fired its liquid oxygen rocket chambers to exceed the speed of sound. Even though the flight took only a few minutes in the air above Muroc Army Air Field (later Edwards Air Force Base), it represented the culmination of years of effort by a highly-driven team from the US Army (later the US Air Force), US Navy, and the National Advisory Committee for Aeronautics (NACA; the predecessor to NASA).
As far back as the 1930s, a professor at the Army Air Corps Engineering School named Ezra Kotcher had seen a need for a specialized research aircraft to test flight conditions at speeds approaching and exceeding Mach 1. Knowing that the highest-speed planes of the day could not reach that speed, nor could available wind tunnels accurately simulate it, Kotcher proposed to the Army a rocket-powered research plane in 1939.
At the time, the Army rejected the idea, but Kotcher continued to advocate for such an effort. Similarly, a number of researchers for NACA had recognized the same need. In order to investigate flight at this new frontier of speed, leading NACA experts met with representatives of the Navy and the Army Air Forces (AAF) on March 16, 1944, to lay out a plan to make a concerted effort to break the so-called “sound barrier.”
By December 1944, the interested parties had worked through a number of disagreements. The most significant of these conflicts centered on the theoretical vehicle’s propulsion system. The NACA leadership insisted that the plane should use turbojet engines, while Kotcher and other AAF engineers argued for rocket engines.
Additionally, the Army and NACA clashed over who would fly the aircraft and where they would fly it. The NACA assumed their test pilots would do so at Langley Field, Virginia, which had both the NACA’s primary laboratories and an AAF base, but the Army offered to use military test pilots at Muroc in California. This discussion exacerbated the engine question since one of the NACA’s lead scientists asserted that no NACA pilot would fly a rocket plane.
The Navy decided to withdraw to pursue a separate program with NACA that ultimately led to the creation of the D-558 Skystreak, leaving the Army and NACA to pursue the sound barrier. The Army and NACA considered proposals from Bell Aircraft Corporation and McDonnell Aircraft Company before giving the job to Bell due to its team’s track record of innovation. After examining the requirements and their options, the Bell team agreed with Kotcher’s call for a rocket-powered aircraft.
The first Experimental Sonic-1 (XS-1, later shortened to X-1) plane rolled off Bell’s production line on December 27, 1945. The XS-1’s fuselage resembled a bullet, which the designers knew from munitions researchers could travel supersonically, and had thin wings to reduce problems caused by air compressing in front of them at high speeds. Bell settled on using the XLR-11 rocket engine, which Reaction Motors had been developing for the Navy, to power the aircraft. The other major change during the design process arose from adding a high-pressure fuel system. Because this extra weight reduced fuel capacity, the XS-1 would not take off from the ground. Instead, a B-29 carried it aloft and released it for flights.
Bell test pilot Jack Woolams, an experienced test pilot who flew many of the key flights for the US’s first jet, the XP-59, flew the XS-1’s unpowered glide flights at Pinecastle Army Air Field in Florida. Other than some landing gear failures and a windscreen fogging issue, weather proved the largest problem the program encountered. These events led Woolams to recommend moving the tests to Muroc Army Airfield in southern California. With the consistently clear weather of the Mojave Desert and the miles of naturally smooth landing space on Rogers Dry Lake Bed for emergencies, Muroc proved a natural place to test aircraft. The lakebed offered miles of naturally smooth landing space if a pilot needed to make an emergency landing. Following Woolams’ death in an unrelated airplane crash, the issue of who would conduct the test flights returned to the fore.
The NACA produced a plan for the effort that emphasized an incremental advance in speed to maximize data-gathering over more than a year, but that plan was too slow for the Army. To the Army Air Forces, this program represented both a needed advancement in military technology and a potential public affairs win over the Soviet Union. In the late 1940s, any edge (in either technology or publicity) was a valuable asset. Thus, the Army began to look at other options. Bell’s chief engineer Bob Stanley suggested that Bell fly a short test program to reach Mach 1 and prove the safety of the XS-1 as a stage-setter for the NACA program. When Bell submitted its formal plan, however, it was almost as long as the NACA plan, leading the AAF to consider using Army test pilots to do the job.
This idea of using Army pilots for research and test personnel marked a key turning point. Up until this program, contractors or the NACA handled testing new planes and flying research programs. The AAF had only recently revived its test pilot corps, which had gone dormant in the late 1920s. Col. Albert Boyd, chief of the Flight Test Division and dubbed by his men “the father of modern flight test,” stated that his pilots could handle a program aimed at rapidly breaking the sound barrier while the NACA team pursued its slower, but still crucial, data-gathering mission.
AAF headquarters formally signed off on the idea in June of 1947. Boyd had already begun the difficult job of planning the program and choosing the team to carry it out. He selected Capt. Jackie L. Ridley, an Oklahoman with a Masters in Aeronautical engineering, as the engineer-in-charge for the project, relying on his ability to effectively communicate theoretical and abstract concepts in common terms to provide expert guidance to the pilots. 1st L. Robert Hoover would serve as the secondary pilot. Maj. Robert L. Cardenas and 1st Lt. Edward L. Swindell would fly the B-29 which would carry the XS-1 up for its flights.
For the role of primary pilot of the XS-1, Boyd selected the 24-year-old Yeager, a World War II ace and one of the best instinctive pilots to graduate from the young Test Pilot School at Wright Field. Though not a college graduate, Yeager’s skills in adapting to new and unusual aircraft made him a sound choice for this experimental program. Boyd’s choices in all of these roles proved wise and fruitful as the team moved to Muroc to begin its assault on the sound barrier.
After spending time at Bell’s production facility and receiving familiarization lessons on the aircraft from Bell’s Dick Frost, the small team gathered at the isolated Rogers Dry Lakebed in late July 1947. On Aug. 6, 1947, the B-29 carried both Yeager and the XS-1 aloft for his first unpowered flight, which went without incident. They conducted further glide flights on the 7th and 8th of August. The only incident was a mock dogfight Yeager conducted with Hoover flying one of two FP-80 Shooting Star chase planes during the flight on the 8th. Due to parts shortages, a three-week delay followed before the powered flights began on August 29th.
Though Yeager departed radically from the plan laid out for that first powered flight (much to the consternation of Boyd and the NACA team on site), he had gained a significant grasp on the handling characteristics of the rocket-powered plane. His second powered flight saw the XS-1 reach Mach 0.89 on Sept. 4.
On his fourth flight, on Sept. 10, the plane reached Mach 0.91. On his fifth flight, reaching Mach 0.92, Yeager encountered a tendency for the plane’s nose to rise and to experience significant buffeting at that speed. After a few modifications to the rear stabilizer, Yeager performed a number of powered flights which offered increasing data on the plane’s performance in the Mach 0.8-0.95 region. The eighth powered flight on October 10 included a loss of elevator controls and the first appearance of a serious icing issue on the windscreen. Guided by Frost, flying one of the chase planes, Yeager made an instruments-only landing on the lakebed.
Over the following weekend, the Air Force (having stood up as a separate service the month before) and NACA teams worked to examine the data and determine a means of controlling the plane at these high speeds. Thanks to inclusion of an adjustable rear stabilizer, Ridley believed that Yeager could retain control by moving the stabilizer as the speed increased. Yeager, believing whole-heartedly in Ridley’s skills, accepted the solution and prepared for what would become the famous flight on Oct. 14, 1947.
That morning, as the team performed final checks, crew chief Jack Russel addressed the screen-icing problem by spreading a thin layer of Drene shampoo over the inside of the windscreen. With all preparations complete, Cardenas flew the B-29 to 20,000 feet and, at 10:26 AM, pulled the lever to release the XS-1. Yeager conducted a test-firing of all four rocket chambers in sequence and began to climb with two of them still burning. During that climb, he tested the moving stabilizer and found Ridley’s advice spot-on.
Upon reaching 42,000 feet, Yeager fired off the third of four chambers and accelerated rapidly. Within moments, his Mach-meter went off the scale and he correctly assumed he had crossed the sound barrier. Analysis confirmed that he had reached Mach 1.06 at roughly 43,000 feet. He concluded the flight with a clean glide in to land on Rogers Lakebed. As Yeager noted, he had experienced no flash of buffeting or chaos as he broke Mach 1.0 and he needed the mach-meter to tell him he had just made history.
While the NACA continued to use the other two XS-1 craft for their detailed, data-gathering flight plans in the months ahead, the Air Force team had broken the sound barrier in nine powered flights and under four months. In doing so, the team had also proven that Air Force Test pilots could handle such aggressive and experimental flying. They had also demonstrated, especially on that dangerous eighth flight, just how perfect Muroc was for flight testing experimental aircraft.
By 1951, the new Air Force Flight Test Center and the Air Force Test Pilot School relocated to the renamed Edwards Air Force Base. Though that small team could not know it when they broke Mach 1 in 1947, Air Force experimental planes would break Machs 2, 3, 4, 5, and 6 above Edwards in less than 15 years, a proud legacy of flight test and experimentation started by the team that built and tested the Bell XS-1.