The most complex spacecraft ever flown were supported by an orbiter that flew every Space Shuttle mission without ever leaving the ground.
In a building at the Johnson Space Center in Houston is the Shuttle Avionics Integration Laboratory, or SAIL.
SAIL was a high-fidelity simulator of the Shuttle Orbiter, capable of simulating all electrical, logical, and aspects of an Orbiter, including all electrical and data connections to ground umbilicals and other flight hardware.
SAIL even had a tail designation reflecting its prototype and test equipment status: OV-095. You’ve likely seen that zero designation on other non-flight Orbiter hardware, which were used for such needs as fit checks, life support system, structural and vibration tests. Only one non-flight test article became a flight-worthy Orbiter after NASA deemed it was too expensive and more practical to convert OV-101 Enterprise, the first Orbiter, from its near-mockup state as an approach and landing test vehicle into a flight-worthy version. So the structural test article called STA-099 became OV-099 Challenger. SAIL’s importance gave it an honorary OV designation.
SAIL was in place from the very start of the Shuttle program, a few years before the first Orbiter was built. Before Enterprise began its drop tests, SAIL was used to verify every aspect of an Orbiter’s landing software and avionics. As Orbiters moved to flight status, SAIL was used to verify telemetry from simulated SRBs and ETs, ground test equipment of a simulated pad (including pad power), launch configuration, software and hardware that formed the general purpose computers, backup flight software and more.
SAIL tested every mission’s objectives and procedures before the actual flight, effectively making OV-095 a vehicle that has “flown” every mission.
While there were other simulators available to the crews, some Shuttle pilots preferred to use SAIL because of its greater fidelity to the actual vehicles.
As technology advanced, SAIL was the first to verify the reliability, safety and functionality of many adjustments before these were integrated into flight Orbiters. Such items included the new flight computers and their software, the “glass cockpit” refits, safety modifications after the Columbia and Challenger accidents, and the remote control Orbiter hardware and software.
When the Orbiters were scheduled to retire, several NASA officials stepped in to preserve the SAIL facility for the public. The tour shows the SAIL facility a bit more exposed to air than the operational facility, with a few elements moved for better viewing. You can see the length of the payload bay and the miles of complex power and wiring that would be present in an actual Orbiter. Likewise, rather than the white coverings of cabinets in the mid-deck of flight Orbiters, SAIL’s skeletal nature lets tourists see the complex support hardware and computers that resided behind the cabinets and below the cockpit.
Unfortunately most tours aren’t allowed to step inside the cockpit or mid-deck, although you’ll get a great view from its windows.
Visiting SAIL may be the next best thing to a close-up of an actual flown Orbiter.
Here’s a great 2014 video tour of the SAIL facility from NASA.