Complex sentence: The jet pods were fitted with intake doors that were kept closed when the J-34s were not running; this prevented windmilling, allowing for economical piston-engine-only long-endurance search and patrol operations.
Split into simpler sentences: The jet pods were fitted with intake doors that remained closed when the J-34s were not running. This prevented windmilling, allowing for economical piston-engine-only long-endurance search and patrol operations.