Complex sentence: Leading up to the early 1960s, there was a rapid growth in government demand in the United States for helium fuelled by the military, especially for aerospace applications such as liquid fuel rockets for defense and space exploration.
Split into simpler sentences: Leading up to the early 1960s, there was a rapid growth in government demand in the United States for helium. It was fuelled by the military, especially for aerospace applications such as liquid fuel rockets for defense and space exploration.