Leduc O.20 Millénaire
When the first Leduc ramjet flew, it decisively smashed all previous speed records for level flight - but it was clear that higher speeds yet were still possible. Between 1941 and 1944, Leduc and his airframe-designing partner, Breguet-Nord, made several incremental improvements on the O.10; but their interest increasingly lay towards a whole new airframe, one capable of surpassing the sonic barrier. It was known by mid-1943 that British designers at Miles Aircraft were working on similar projects, and the STA was highly interested in retaining and expanding the level-flight speed records they had set earlier with the O.10. Breguet-Nord and Leduc made extensive use of ONERA's supersonic wind-tunnel at Modane-Avrieux, opened in June 1942, to develop a new airframe, code-named O.20 Millénaire, with the goal of unleashing it in late 1943. Unfortunately, this timeline for construction and test-flight of the O.20 proved highly optimistic.
The Millénaire was to be a tour-de-force of technological marvels. Powered by one of Leduc's largest and most capable ramjets to date, the aircraft had narrow, blade-like wings swept to twenty-five degrees, and experimentation in the wind-tunnels resulted in Breguet-Nord's development of a fuselage designed using a mathematical equation to minimize drag at higher speeds. In order to limit weight and increase strength, many vital portions of the aircraft would be manufactured from lightweight aluminium alloys, some designed specifically for aviation purposes. In order to enhance pilot safety, the pilot received the first ejection seat installed aboard a French aircraft, which used hydraulics to assist the pilot clear of a doomed plane. The necessity for this was made clear as a result of several earlier close calls with the Leduc O.10 and other testbed aircraft.
Although the STA intended to construct and fly an airframe in the latter end of 1943, this simply proved impossible: the technical requirements for the construction of the aircraft were very difficult for Breguet-Nord to meet. A single full-sized mockup, made of regular aluminium, was handed over in great secrecy to ONERA where it was tested at the 700kph wind-tunnel in August 1944. The tests revealed grave issues with the design of the tailplane, which became useless at higher speeds. This same problem had previously challenged the Breguet-Nord designers with the O.10, but the problem grew more acute with higher speeds. In February of 1945, Breguet-Nord started testing a variable-incidence tailplane which alleviated the issue in testing. Construction of the premier aircraft, which had stalled since February, recommenced; however, the delays continued to mount, primarily due to the great complexity and novelty of virtually every single piece of the aircraft, ranging from the engines to the metalwork in the fuselage. By September 1944, a chagrinned Breguet-Nord reported to the STA their embarrassing figures: they had overspent their research and development budget four times over and had yet to produce a complete airframe.
More alarmingly, several engineers involved in a related program began to raise concerns about whether or not the aircraft could in fact break the sound barrier as designed. While Breguet-Nord struggled with development of the Millénaire, another design team from the company built a radio-controlled drone to serve as a small-scale ramjet engine testbed. As the supersonic Modane-Avrieux wind-tunnel was too small to test a full-scale O.20, one of the drones was modified to resemble an O.20, fitted with booster rockets to assist the small ramjet, and tested at the CEV. In August 1944, the two-meter long drone launched successfully from an MB.161 mothership, but then broke up spectacularly just before it reached the speed of sound. This result dismayed many of the Millénaire's project developers, and STA paused the project through September and October while the drone's failure was evaluated and the airframe design was modified.
Despite being horrifically over-budget and behind schedule, the project nevertheless had already yielded valuable results even without a functional aircraft. By August 1944, Breguet-Nord's designers had accumulated four thousand hours of test data at the STA's supersonic wind-tunnel, building a massive body of knowledge about high-speed aerodynamics that proved to be highly useful in the later 1940s and 1950s. The STA also acquired two of the first ten computers in France solely in order to support supersonic research and development. The program also pushed new methods for designing and manufacturing airframes, and developed several new composites for high-strength and low-weight uses.
Throughout 1945, Breguet-Nord worked hard to redesign the Millénaire, and finally produced the first airframe in June 1945; a second airframe was delivered in January 1946. Aircraft #1 was again tested at the 700kph windtunnel at Modane-Avrieux between June and September. In September, the windtunnel was remodeled for supersonic testing, and by March of 1946 the aircraft was cleared to begin actual flights at CEV, with the objective of a full supersonic flight by May 1946. The Millénaire's first test flight occurred May 24th of 1946, when Aircraft #2 successfully launched from its Bloch MB.161 mothership. Between the two test aircraft, over four dozen test flights occurred with the engine at lower power settings, building up to a planned October supersonic flight. On July 31st, during one of these preliminary test flights, test pilot Capitaine Louis-Antoine Messiaen believed he had achieved mach one during a gentle dive, a claim supported by two of the ground-based test personnel, who later reported a 'sonic boom'. However, the CEV's ground-based radio-teledector used to measure absolute airspeed was not in use during Messiaen's flight, and did not taking any speed readings. At the time, Messiaen did not claim that he had passed the sound barrier, since he had exceded his instructions for the carefully planned and closely-monitored test flights.
In early August, however, the STA learned that the rival British program (with the Miles M.52) was preparing for a supersonic test flight, and the Millénaire team was directed to attempt to seize the record first. On August 19th, Capitaine Jean Seigner took the controls of Aircraft #1 for the first official attempt to achieve supersonic speeds. The aircraft successfully launched from the MB.161 mothership. However, several critical engine alarms went off as the aircraft approached the sound barrier and encountered transonic buffetting. As a result, Capitaine Seigner chose to terminate the test flight. Close inspection of the aircraft failed to find any serious issues, and the CEV eventually determined the safety alarms had been set off by shockwaves. According to the aircraft's instruments, the aircraft exceeded the speed of sound, but once again the ground-based radar could not verify Seigner's absolute airspeed due to the short duration of the high-speed run. The STA would only certify a genuine supersonic flight on the basis of ground-based instrumentation, since Breguet-Nord's designers and the STA technical staff were aware that their onboard airspeed indicators were inaccurate at transonic speeds. Seigner and a number of other personnel believed that he had briefly passed the sound barrier during his flight, but were unable to press the claim further.
While the STA inspected the aircraft to determine the cause of the false engine alarms, the British Miles M.52 successfully broke the sound barrier, to the dismay of Leduc and the rest of the Millénaire flight team. However, on September 3rd, Capitaine Messiaen took the Millénaire #2 for a second high-speed flight. Transonic buffetting again set off the aircraft's engine warning alarms, but Messiaen chose to ignore them and continued flying. On this occasion, ground-based radar successfully tracked the Millénaire for four minutes at a top verified speed of 1,369 kilometers per hour (or mach 1.12). This decisively exceeded the speeds achieved two days prior by the Miles M.52. As Messiaen cut the throttle to slow back down, however, he encountered further shockwaves that caused a malfunction in the landing gear actuators. The aircraft's landing gear locked up and refused to deploy, forcing Messiaen to dump his fuel and belly out the aircraft; he walked away with only a sprained ankle.
In future flights, both Messiaen and Seigner each broke the sound barrier on multiple occasions.
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