Assertion: liquid oxygen's use | /r/HasProperty | experimented
Facets:
value: seriously
type: MANNER
Source sentences:
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.