Assertion: liquid oxygen's use | /r/HasProperty | experimented by late 1930s
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text: By the late 1930s, use of rocket propulsion for manned flight began to be seriously experimented with, as Germany's Heinkel He 176 made the first manned rocket-powered flight using a liquid-fueled rocket engine, designed by German aeronautics engineer Hellmuth Walter on June 20, 1939.