Complex sentence: The existence and 1950s emergence of the gravity control propulsion research has not been a subject of controversy for aerospace writers, critics, and conspiracy theory advocates, but its rationale, effectiveness, and longevity have been the objects of contested views.
Split into simpler sentences: The existence and 1950s emergence of the gravity control propulsion research had not been a subject of controversy for aerospace writers, critics, and conspiracy theory advocates. But its rationale, effectiveness, and longevity have been the objects of contested views.