Complex sentence: The M-60 project was extensive, with variants featuring stealth or UAV capabilities, with some variants designed to carry a crew of 10-15 for 8 hours at an altitude of 20 kilometers.
Split into simpler sentences: The M-60 project was extensive, with variants featuring stealth or UAV capabilities. The aircraft was designed to carry a crew of 10-15 for 8 hours at an altitude of 20 kilometers.