The plug door made it impossible for the astronauts to escape or be removed before their deaths. An investigation revealed the fire was probably started by a spark from a frayed wire, and fed by combustible materials that should not have been in the cabin. The crewed flight program was delayed while design changes were made to the Block II spacecraft to replace the pure oxygen pre-launch atmosphere with an air-like nitrogen/oxygen mixture, eliminate combustible materials from the cabin and the astronauts' space suits, and seal all electrical wiring and corrosive coolant lines. The Block II spacecraft weighed fully fueled, and was used in four crewed Earth and lunar orbital test flights, and seven crewed lunar landing missions. A modified version of the spacecraft was also used to ferry three crews to the Skylab space station, and the Apollo-Soyuz Test Project mission which docked with a Soviet Soyuz spacecraft. The Apollo spacecraft was retired after 1974. Retired robotic space capsules Corona Dragon FSW Foton Raduga Atmospheric Reentry Demonstrator SRE Zond/L1 L3 TKS Yantar OREX Xiaofei HSRC Active space capsules Soyuz In 1963, Korolev first proposed the three-man Soyuz spacecraft for use in Earth orbit assembly of a lunar exploration mission. He was pressured by Soviet premier Nikita Khrushchev to postpone development of Soyuz to work on Voskhod, and later allowed to develop Soyuz for space station and lunar exploration missions. He employed a small, lightweight bell-shaped reentry capsule, with an orbital crew module attached to its nose, containing the bulk of the mission living space. The service module would use two panels of electric solar cells for power generation, and contained a propulsion system engine. The 7K-OK model designed for Earth orbit used a reentry module measuring in diameter by long, with an interior volume of . The spheroidal orbital module measured in diameter by long with a docking probe, with an interior volume of . The total spacecraft mass was . Ten of these craft flew crewed after Korolev's death, from 1967 to 1971. The first (Soyuz 1) and last (Soyuz 11) resulted in the first in-space fatalities. Korolev had developed a 7K-LOK variant for use in the lunar mission, but this was never flown manned. The Russians continued to develop and fly the Soyuz to this day. Shenzhou The PRC developed its Shenzhou spacecraft in the 1990s based on the same concept (orbital, reentry and service modules) as