Complex sentence: Spaceflight became an engineering possibility with the work of Robert H. Goddard's publication in 1919 of his paper' A Method of Reaching Extreme Altitudes'; where his application of the de Laval nozzle to liquid fuel rockets improved efficiency enough for interplanetary travel to become possible.
Split into simpler sentences: Spaceflight became an engineering possibility with the work of Robert H. Goddard's publication in 1919 of his paper "A Method of Reaching Extreme Altitudes". His application of the de Laval nozzle to liquid fuel rockets improved efficiency enough for interplanetary travel to become possible.