Julius Hatry (30 December 1906 – 7 November 2000) was a German aircraft designer and builder. He is remembered for his contributions to sailplane development in the early twentieth century and for building the world's first purpose-built rocket plane, the Opel RAK.1.
Hatry was born in Mannheim as son of Katharina and Julius Hatry senior, a successful Mannheim real estate entrepreneur. Julius Hatry junior developed an early interest in aviation, joining the Mannheim flying club in 1922. With the club, he became a frequent visitor to the annual gliding competitions held at the Wasserkuppe during the 1920s. Between 1927 and 1928, he assisted in the construction of the Kakadu (then the largest sailplane ever built) with the Munich Akaflieg (student flying) group. References from Alexander Lippisch and Oskar Ursinus helped him gain his first design contract, for a motor glider which never actually flew due to problems with its powerplant. His association with Lippisch also allowed him to participate in the 1928 rocketry experiments carried out for Fritz von Opel within Opel RAK, the world's first rocket program. This led to von Opel commissioning him to build him a rocket plane for a public demonstration the following year.
Hatry and the world's first rocket program Opel RAK
As early as 1922, Julius Hatry, nicknamed "Uss", became a member of the Mannheim Aviation Club and through this came into contact with the first Rhön competitions. In 1927 he did the Abitur and passed the glider pilot C exam. His certificate with the number 409 was the first in the German state of Baden. He began studying engineering at the Technical University of Munich and worked as a flight instructor in Rossitten (East Prussia), where he also designed large aircraft models for the first time. In 1927/28, under August Kupper, Hatry was significantly involved in the construction of the Mü 3 "Kakadu", the largest glider at the time. In 1928 Hatry himself took part in a Rhön competition for the first time with the two-seater "Mannheim". He had met Alexander Lippisch and Oskar Ursinus in the aviation scene and was therefore commissioned to design a cell for a motor glider. However, the engine did not pass the type approval test. Then he developed the "Water Rat", a water glider with a boat-like fuselage and raised tail unit, with which he was awarded 1200 Reichsmark in the technical competition in the “Preisfliegen Rossitten 1928/1929”. In the meantime, Lippisch had carried out the first rocket model tests in 1928 on befalf of Fritz von Opel's Opel RAK program, using Friedrich Sander's rockets. Hatry evaluated the flight curves mathematically for months and was able to prove physical relationships between the thrust axis, center of gravity and the flight curves: The unsatisfactory results of the early experiments prompted Hatry to first analyze the problem theoretically and examine the flight behavior of a rocket plane more closely. While the space flight pioneers like Oberth and Goddard created their basic calculations for missile flights outside the earth's gravitational field, Hatry set up the basic equations of aircraft with rocket propulsion within the gravity range for the first time. This was Hatry's pioneering achievement in aeronautical science. His flight-mechanical calculations provided the requirements for the documents for the rocket aircraft model he built. Experiments with this model confirmed Hatry's calculations. This refuted Lippisch's wrong deduction about the causes of the unstable flight in his early Opel RAK experiments with Stamer as test pilot of the "Lippisch Ente". Lippisch found this so uncomfortable that he asked Hatry not to publish his calculations. Out of respect, Hatry followed Lippisch's request that this fundamental theoretical work would not be published in a specialist journal. As a scientific publication, it remained known only to a narrow circle of specialists and it was not until 1983 that Hatry published his equations, when they were already the subject of historical research.
After this preliminary work, Lippisch suggested the construction of new dedicated manned rocket aircraft. Hatry began construction in June 1929. Although initially working on his own, he later also entered the Opel RAK team, the industrialist Fritz von Opel provided the financial means for the construction of the RAK.1 within the OPEL RAK program framework.