Complex sentence: Article 135 was operating 300 miles from the California coast to carry out a test launch of a D-21, the aircraft was flying at Mach 3.2 + when the crew made sure that Marquardt engine on the D-21 had the required airflow.
Split into simpler sentences: Article 135 was operating 300 miles from the California coast to carry out a test launch of a D-21. The aircraft was flying at Mach 3.2 + when the crew made sure that Marquardt engine on the D-21 had the required airflow.