The pressure of at operating depth of the torpedo was great, and newly developed aluminum alloy was used and successfully passed the tests. By the end of 1966, special torpedo test range was also authorized to be built, resulting from lessons learned from the development of Yu-1 torpedo and Yu-2 torpedo. In order to guarantee the development, the Central Military Committee of the Communist Party of China issued “Special Official Letter“ in 1967 to prevent the any disruptions from the political turmoil that plagued China at the time. The original plan of high speed approach with low speed search and engagement had severely limited the speed of the torpedo, because the original acoustic homing seeker was very limited in its effectiveness and prone to noise radiated by the engine. In 1967, this problem was finally solved when a new multi-beamed acoustic homing seeker was successfully developed jointly by the Acoustic Research Institute of the Chinese Academy of Science and 705th Institute, under the leadership of Mr. Hou Chaohuan (侯朝焕). By 1969, the 750-metre-deep deepwater torpedo testing range was established with the help of over 80 enterprises and named as 750 Testing Range (750试验场) in Kunming, after the greatest simulated depth of the range, and a total of 4 sample torpedoes were produced. In the same year, the fire control and launching system for Yu-3 torpedo were also successfully developed and installed on board the nuclear submarine.