Complex sentence: He invented the first castable, composite solid rocket propellant, pioneered the advancement of both liquid-fuel and solid-fuel rockets, and was an early proponent of using rockets for scientific research and exploration beyond Earth.
Split into simpler sentences: He invented the first castable, composite solid rocket propellant, and pioneered the advancement of both liquid-fuel and solid-fuel rockets. He was also an early proponent of using rockets for scientific research and exploration beyond Earth.