when does its crew comprised of?
The Space Shuttle crew varied by mission. The test flights only had two members each, the commander and pilot. The on-orbit operations were conducted primarily by the mission specialists.
Space Shuttle
The Space Shuttle crew varied by mission. The test flights only had two members each, the commander and pilot, who were both qualified pilots that could fly and land the orbiter. The on-orbit operations, such as experiments, payload deployment, and EVAs, were conducted primarily by the mission specialists who were specifically trained for their intended missions and systems. 
where does the us launch space shuttles from Kennedy Space Center (KSC) in Florida. what are these shuttles actually? It was a partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program.
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Space Shuttle [SEP] Crew
The Space Shuttle crew varied by mission. The test flights only had two members each, the commander and pilot, who were both qualified pilots that could fly and land the orbiter. The on-orbit operations, such as experiments, payload deployment, and EVAs, were conducted primarily by the mission specialists who were specifically trained for their intended missions and systems. Early in the Space Shuttle program, NASA flew with payload specialists, who were typically systems specialists who worked for the company paying for the payload's deployment or operations. The final payload specialist, Gregory B. Jarvis, flew on STS-51L, and future non-pilots were designated as mission specialists.