Complex sentence: During World War II, Fulton invented the first ground-based aerial flight trainer, but when the military wasn't interested, he modified it into a training aid for aerial gunners, the first fixed aerial gunnery trainer, called the "gunairstructor".
Split into simpler sentences: During World War II, Fulton invented the first ground-based aerial flight trainer. But when the military wasn't interested, he modified it into a training aid for aerial gunners, the first fixed aerial gunnery trainer, called the "gunairstructor".