Complex sentence: While Siebold was injured he was able to parachute to safety, but Alsbury was unable to exit the spacecraft and his remains were found still strapped to his seat in the fuselage.
Split into simpler sentences: While Siebold was injured, he was able to parachute to safety. Alsbury, however, was unable to exit the spacecraft, and his remains were found still strapped to his seat in the fuselage.