Karel Jagr, First man to fly faster than sound
CSDS leader Josip Broz personally asked Major Karel "Karčo" Jagr, who was Danubia's top fighter ace in the Second Great War, having scored 23 confirmed kills, to be the chief test pilot of the Raketoplan program. Jagr instantly agreed and he, along with the three rocket planes, a small maintenance crew, two backup pilots and a team of scientists headed by Col. Koco Arsovski and Prof. Nemanja Djordjevic, headed to Mali.
Major Jagr had selected as his technical adviser and crew chief his wartime squadronmate Captain (dipl. ing.) Petru Gabresku. Gabresku and Jagr were entrusted with developing a test plan and to conduct the research at the pace they see fit. They developed a plan which entailed three unpowered and five powered flights, gradually building up speed step by step until the sixth powered flight, in which the supersonic attempt would be made.
The Raketoplan was designed to be dropped in mid-flight like a bomb. For this purpose, a Erakles heavy transport helicopter (which was the world's first aeroplane-helicopter hybrid, entering service in 1959) was modified with extended-length landing struts (to allow space for the rocket plane) was built. This would carry the rocket plane, slung under its belly with shackles, to an altitude of 30,000 feet, where, at a speed of 210 versts per hour, the shackles would be released and the Raketoplan would fall away and begin gliding.
Thus were the unpowered flights conducted; they served to familiarize Jagr thoroughly with the Raketoplan, as all landings would have to be unpowered due to the volatility of the fuel used. The powered flights also began the same way.
The first five powered flights were all flown at subsonic speeds, each setting new air speed records; the fifth flight reached Mach 0.97. There were some difficulties along the way which had to be resolved before testing could continue, but in the event the sixth powered flight took place as scheduled on 23 May 1961, and with this flight, Major Jagr became the first - and only - man to fly faster than sound.