Space Shuttle It weighed 3.4 metric tons (7,500 lb) less than the last run of lightweight tanks, allowing the Shuttle to deliver heavy elements to ISS's high inclination orbit. As the Shuttle was always operated with a crew, each of these improvements was first flown on operational mission flights. The solid rocket boosters underwent improvements as well. Design engineers added a third O-ring seal to the joints between the segments after the 1986 Space Shuttle "Challenger" disaster. Several other SRB improvements were planned to improve performance and safety, but never came to be. These culminated in the considerably simpler, lower cost, probably
Space Shuttle of "Atlantis"s final flight on July 21, 2011. The U.S. has since relied primarily on the Russian Soyuz spacecraft to transport astronauts to the International Space Station. The Space Shuttle was a partially reusable human spaceflight vehicle capable of reaching low Earth orbit, commissioned and operated by the U.S. National Aeronautics and Space Administration (NASA) from 1981 to 2011. It resulted from shuttle design studies conducted by NASA and the U.S. Air Force in the 1960s and was first proposed for development as part of an ambitious second-generation Space Transportation System (STS) of space vehicles to follow the Apollo program