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 retirement and then be replaced by a new vehicle. When the concept of the U.S. space station evolved into that of the International Space Station, which suffered from long delays and design changes before it could be completed, the service life of the Space Shuttle was extended several times until 2011 when it was finally retired. In 2010 the Shuttle was formally scheduled for retirement with "Atlantis" being taken out of service first after STS-132 in May of that year, but the program was once again extended when the two final planned missions were delayed until 2011. Later, one additional mission