(SPACE.com) HOUSTON -- As NASA has readied its retired space shuttles to set sail for their museum homes, the agency has also been quietly preparing its least-known orbiter vehicle to stay in place.

The SAIL -- or Shuttle Avionics Integration Laboratory -- is set to become the newest stop on tours of the Johnson Space Center here this fall.

The once fully-functional space shuttle simulator, which was used throughout the 30-year program to develop and test the flight software for each of the 135 missions, was designated an honorary part of the fleet with its own orbiter vehicle (OV) number.

Space shuttle Discovery, which is now on display at the Smithsonian in Virginia, was also referred to by NASA as OV-103. Enterprise, the original shuttle prototype, which is now exhibited at the Intrepid Sea, Air & Space Museum in New York City, was similarly OV-101.

Endeavour, which next month will be flown to Los Angeles for the California Science Center, was designated OV-105. And Atlantis, which is scheduled to arrive this November at the Kennedy Space Center Visitor Complex in Florida, was OV-104.

The SAIL was designated OV-095. Although it was never space-worthy, from the perspective of its flight computers, the simulated missions that it 'flew' might have just as well been in orbit. [NASA's Most Memorable Shuttle Missions]

Skeleton of a space shuttle

Filling a couple of floors inside Building 16 at the Johnson Space Center, OV-095 doesn't look like its sister ships.

Although it has a fully-accurate flight deck and is laid out to have a payload bay and aft section, the SAIL's lack of wings, tail -- and for that matter, walls -- leaves exposed the mock space shuttle's wires, switches, crawl spaces, steep stairs and ledges.

That setup worked well for the more than three decades when the SAIL was an operational laboratory, but was not ideal as a bustling tour stop. NASA needed to make the SAIL safe for visitors while keeping the historical integrity of the facility intact.

Go here to see the original:

NASA's hidden space shuttle opens in Houston