In the aftermath of the sad decision making that led to the re-entry loss of OV-102 Columbia, the Space Shuttle program was compelled to consider new contingency options in the event of a similar failure of an Orbiter’s thermal protection system (TPS) or other flight-critical system once in orbit.

Prior to the loss of Columbia, options existed for the crew of a crippled Orbiter visiting the ISS to take refuge at the station and await a not-so-hasty arrival of a second Orbiter to take them home. The crew would dock to the ISS, where both crews would salvage as much of the Orbiter’s resources for the station and the stranded crew, conserving ISS resources but also to drain the potentially volatile materials aboard the spacecraft.

Before that trip, the crew would make what automatic preparations they could to undock the stricken Shuttle. From there, Mission Control would send what commands they could to aim the empty Shuttle at earth for an uncontrolled re-entry and destruction. There were two pressurized mating adapters (PMAs) on the ISS for docking but two Orbiters could not be docked to the ISS simultaneously due to space constraints.

Columbia herself was too heavy to help with ISS construction, so she usually flew lab missions, lifted low-orbit satellites, and even completed the second Hubble Space Telescope repair.

Although the ISS could keep a crew hale and hearty until rescue, NASA officials weren’t keen to lose another $1.7 billion Orbiter if it could help it. So engineers dusted off some notes and created a brilliant idea that, thankfully, was never used.

Remote Control Orbiter

The idea of automating manned spacecraft isn’t new, of course. Gemini missions 1 and 2, as well as Apollo missions 4, 5 and 6 tested their respective spacecraft systems completely by ground control.

Gemini 1 was intentionally destroyed after mission objectives were complete (even going so far as drilling holes in its heat shield), and Apollo 5 flew Lunar Module 1, a true spacecraft without any TPS and so couldn’t return. Gemini 2, Apollo 4 and the bat-crap crazy failure-plagued Apollo 6 brought their man-capable spacecraft home safely.

The most recent automated tests of man-qualified spacecraft were EFT-1, the Orion spacecraft’s Apollo 4-like high-orbit test of its heat shield system, and SpaceX’s Crew Dragon pad abort test.

While the pre-2003 Space Shuttle Orbiters had several flight systems that supported a nearly computer-controlled de-orbit and landing, certain features required human intervention, such as powering up the auxiliary power units (which give hydraulic power to the spacecraft’s aerodynamic surfaces), activating the landing gear and the drag chute. The best that Mission Control could do to for a crew-less Orbiter at the time was aim it to ditch in an ocean.

So the RCO, or remote control Orbiter option was formulated.

STS-3xx RCO cableA special interface was designed, a complex and very long cable with multiple connectors called, naturally, the RCO cable. It was first tested in Houston in NASA’s Shuttle Avionics Integration Laboratory (SAIL), a fully-functional Orbiter mockup for simulations with all electronics and computers found in the actual vehicle.

The flight-article RCO cable was carried up on STS-121 in July 2006, the second Return-to-Flight mission after STS-107. The cable kit was left aboard the ISS for the duration of the Shuttle program.

What Would Happen

The very first thing that an approaching Orbiter did in close proximity to the ISS, starting with STS-114, was a rendezvous pitch maneuver. The RPM was a slow somersault of the Orbiter for the ISS crews to record the condition of the Orbiter’s TPS by camera. After analysis of the footage by the NASA ground teams, if a significant TPS breach is suspected, NASA would declare a “safe haven” contingency. It’s not clear if the general mission objectives would be performed, but the crew would prep the Orbiter for a remote control mode. It’s possible that supply or construction EVAs may have proceeded since the ISS mission was the central purpose. In addition, anything that would reduce the mass of the automated Orbiter during re-entry and landing as well as putting expensive materials to good use would’ve been preferred.

It’s probable that any TPS repairs could’ve been accomplished in conjunction with the automation installation to increase the chances of the Orbiter’s survival.

RCO cable to Landing Gear
RCO cable attachments to the exposed landing gear cabling after the cockpit panel is removed. (NASA)

Automating the Orbiter, called In-Flight Maintenance (IFM) began with the ISS crew breaking out the RCO cable. The flight crew would grab a screwdriver to open several panels on the flight cockpit consoles, making connections with one end of the RCO cable to the wiring inside the cockpit that command the landing gear, the drag chute, the fuel cell reactants,  the air data probes and the auxiliary power units (APUs).

The other end of the RCO cable connectors are linked up to the Ground Control Interface Logic avionics as well as the the Load Control Assembly units (located on the mid-deck) which were critical for the APUs to do their work in moving the aero surfaces and deploying the landing gear.

After some special software related to these IFM operations is loaded into the Orbiter flight computers and verified, the command and pilot make a few last switch flips to enable the Orbiter for departure before leaving the spacecraft.

From here, Mission Control would have complete autonomous control of all flight critical Orbiter functionality. After undocking and moving the Orbiter away, they’ll send commands to close the payload bay doors, fire the orbital maneuvering engines at the correct time, and target the Orbiter’s automated re-entry flight software to a landing at Vandenberg or Edwards AFB as primary and secondary sites.

Vandenberg was preferred because, should control be lost or the Orbiter otherwise loses too much energy to make it to the runway, the spacecraft could be ditched in the Pacific. A wayward landing towards Edwards could’ve endangered populations along the flight route. Vandenberg required some hardware on-site to support automated landings (Edwards already had this equipment as a backup to the Kennedy Space Center runway).

All the usual functions of a Shuttle landing would occur with this automated landing–except crew departure from the vehicle.

Rescue Missions With and Without the ISS

The remarkable sight of an automatic Shuttle landing would be overshadowed by the events of the rescue mission.

Rescue missions had a special mission number. STS-3## (the last two numbers would be unique) were always prepared to fly within 40 days of a safe-haven contingency for all post-Columbia missions to the ISS.

Initially, post-Columbia restrictions prohibited any further missions that did not take the vehicle to the station. But with the popularity and benefits to science of the Hubble Space Telescope, NASA approved one non-ISS mission, the third and last Hubble refurbishment, STS-125 with Atlantis in 2008.

STS-125_and_STS-400_on_launch_pads
STS-125 (foreground) and STS-400 in September, 2008. (NASA)

Shuttle Endeavour, provisionally named as rescue mission STS-400, was on the second pad in a rare photo of both pads of Launch Complex 39 occupied with Shuttles. This was necessary because Atlantis would have limited power and resources than a powered-down and docked Orbiter at the ISS, so Endeavour would have to leave within a few days maximum. Atlantis, on flight day 2, would use its remote manipulator arm and a special extension, the Orbiter Boom Sensor System, to inspect all critical areas of its TPS.

Some concerns arose when ground crew spotted some damage on the forward edge of the Orbiter’s right wing, collaborated with data from sensors embedded along the wing that required something in the first two minutes of ascent. Thankfully, after close scrutiny, the damage was determined not critical, and STS-400 would eventually not be needed.

STS-124 272152main_027_sts_400_hold_position
A NASA depiction of how the Orbiters would manage a stable but tricky crew transfer.

If a rescue mission were required, Discovery and Atlantis would’ve oriented above each other, payload bays facing each other for the rescue Orbiter’s arm to grapple Atlantis’s arm to stabilize the vehicles. Tethers and the RMS on both vehicles would be used to retrieve some crew members at a time after another change in vehicle orientation. A complex dance of space suit transfers, empty and occupied, between the Orbiters would’ve been conducted to get the crew moved to Discovery.

The only flight-qualified RCO cable was aboard the ISS. With insufficient time or resources to make any repairs to a Shuttle with fatally compromised TPS, Discovery would’ve made a quick check of its own TPS and depart. Atlantis would’ve been commanded to de-orbit by the ground controllers and break up over an area of the central Pacific.

With only two Orbiters left in the fleet in that scenario, the Space Shuttle program might’ve ended right then and there.

You can read the NASA slide presentation on RCO here as a PDF file.