modified Boeing 747s. Either could fly an orbiter from alternative landing sites back to the Kennedy Space Center. These aircraft were retired to the Joe Davies Heritage Airpark at the Armstrong Flight Research Center and Space Center Houston. A 36-wheeled transport trailer, the Orbiter Transfer System, originally built for the U.S. Air Force's launch facility at Vandenberg Air Force Base in California (since then converted for Delta IV rockets) would transport the orbiter from the landing facility to the launch pad, which allowed both "stacking" and launch without utilizing a separate VAB-style building and crawler-transporter roadway. Prior to the closing of the Vandenberg facility, orbiters were transported from the OPF to the VAB on their undercarriages, only to be raised when the orbiter was being lifted for attachment to the SRB/ET stack. The trailer allowed the transportation of the orbiter from the OPF to either the SCA "Mate-Demate" stand or the VAB without placing any additional stress on the undercarriage. The Crew Transport Vehicle (CTV), a modified airport jet bridge, was used to assist astronauts to egress from the orbiter after landing. Upon entering the CTV, astronauts could take off their launch and reentry suits then proceed to chairs and beds for medical checks before being transported back to the crew quarters in the Operations and Checkout Building. Originally built for Project Apollo. The Astrovan was used to transport astronauts from the crew quarters in the Operations and Checkout Building to the launch pad on launch day. It was also used to transport astronauts back again from the Crew Transport Vehicle at the Shuttle Landing Facility. The three locomotives serving the NASA Railroad, used to transport segments of the Space Shuttle Solid Rocket Boosters, were determined to be no longer needed for day-to-day operation at the Kennedy Space Center. In April 2015, locomotive No. 1 was sent to Natchitoches Parish Port and No. 3 sent to the Madison Railroad. Locomotive No. 2 was sent to the Gold Coast Railroad Museum in 2014. See also List of human spaceflights List of Space Shuttle missions List of spaceflight-related accidents and incidents List of Space Shuttle crews Simulation Physics Other spaceflight programs Similar vehicles Dream Chaser – Commercial space glider for flying either cargo or a crew to the ISS ReferencesFootnotesCitations''' Further reading Shuttle Reference manual Orbiter Vehicles Shuttle Program Funding 1992 – 2002 NASA Space Shuttle News Reference – 1981 (PDF document)