Task: Question Generation
Input (English): Salyut 1's orbit was increased to prevent premature reentry, but further piloted flights were delayed while the Soyuz was redesigned to fix the new safety problem. The station re-entered the Earth's atmosphere on October 11, after 175 days in orbit. The USSR attempted to launch a second Salyut-class station designated Durable Orbital Station-2 (DOS-2) on July 29, 1972, but a rocket failure caused it to fail to achieve orbit. After the DOS-2 failure, the USSR attempted to launch four more Salyut-class stations through 1975, with another failure due to an explosion of the final rocket stage, which punctured the station with shrapnel so that it wouldn't hold pressure. While all of the Salyuts were presented to the public as non-military scientific laboratories, some of them were actually covers for the military Almaz reconnaissance stations.
Output (English): Why did The Durable Orbital Station-2 not reach orbit?