'Engines in Post-Shutdown Standby': 20 Years Since STS-68 (Part 1)

One day in the summer of 1993, astronaut Tom Jones was made an offer that he could not refuse. More than 18 months earlier, Jones had been assigned as a mission specialist on STS-59, the first shuttle-based Space Radar Laboratory (SRL-1) flight, tasked with imaging virtually the entire Home Planet with powerful synthetic aperture radar. Repeatedly postponed, the mission eventually settled on a targeted launch date in April 1994, with a follow-up flight, STS-68 and SRL-2, manifested for four months later, in August. Summoned to the office of Chief Astronaut Robert “Hoot” Gibson, Jones was offered the chance to fly aboard both missions, acting as payload commander for SRL-2. The dual assignments followed typical NASA practice of “carrying over” an experienced astronaut from one payload to the next on important science missions, but Jones’ double duty also meant that he came tantalizingly close to securing a new record for the shortest time between two space missions. His crewmate, Dan Bursch, succeeded in securing another record, for entirely different and wholly unwanted reasons.

Originally, the SRL-1 and SRL-2 missions were supposed to fly about a year apart, as illustrated by NASA’s February 1991 and January 1992 shuttle manifests, which anticipated an approximately 15-month gap between them. However, when the decision was taken in mid-1992 to advance Shuttle Endeavour’s critical Hubble Space Telescope (HST) servicing mission (STS-61) ahead of SRL-1, the two radar science flights drew much closer—within four months of each other—on the planning charts.

As a result, when Gibson named Jones as SRL-2 payload commander in August 1993, he was still eight months from flying SRL-1, his first shuttle mission. “Hoot laughed at my startled reaction,” Jones wrote in his memoir, Sky Walking, “but he wasn’t kidding. What else could I say but yes?” Two months later, in October 1993, NASA announced the names of the remainder of the STS-68 crew: Mike Baker in command, joined by pilot Terry Wilcutt and mission specialists Steve Smith, Dan Bursch, and Peter “Jeff” Wisoff. The five men pestered Jones mercilessly during his SRL-1 training and even whilst he was in orbit.

“Don’t forget you start sims with us next week,” read one note, authored by Baker. It was signed off with simplicity: “Your STS-68 Associates.”

Endeavour was tasked with flying both SRL-1 and SRL-2, and as soon as she returned from STS-59 on 20 April 1994, technicians immediately set to work preparing her for a return to space as early as 18 August. If she launched STS-68 on time, it promised to give Jones the new record for the shortest interval between two flights by any astronaut or cosmonaut in history. He would launch on his second shuttle flight just 120 days after touching down from his first, an achievement which would slightly pip the incumbent record-holder, U.S. astronaut Steve Nagel, who had achieved 128 days between the landing of his first and second missions in June and October 1985.

Little did any of them realise that SRL-2 would only get off the ground after a particularly hair-raising on-the-pad engine abort, less than two seconds ahead of liftoff. In the pre-dawn gloom of 18 August, Endeavour sat on Pad 39A at the Kennedy Space Center (KSC), bathed in million-candlepower xenon floodlights, as the final seconds of the countdown to her next voyage into orbit evaporated. Like SRL-1, the SRL-2 payload was chiefly sponsored by the Jet Propulsion Laboratory (JPL) of Pasadena, Calif., and featured the Shuttle Imaging Radar (SIR)-C and the X-band Synthetic Aperture Radar (X-SAR). On STS-68, Endeavour would repeat the SRL-1 mission to monitor changes between late spring and late summer. “JPL had always planned to fly the new instrument at least two and preferably three times,” explained Tom Jones, “testing the radars’ ability to monitor seasonal variations in rain forests, croplands, wetlands, ocean currents, sea ice, soil moisture, glaciers and snow cover.” Similarly, the Measurement of Air Pollution by Satellite (MAPS) experiment sought to track ongoing changes in carbon monoxide production. “JPL would have preferred six months between missions,” wrote Jones, “to capture a full seasonal swing, but competing demands on the shuttle schedule moved SRL-2 up to late summer.”

Liftoff of STS-68 was scheduled for 6:54 a.m. EDT. After strapping in and checking their equipment, Jones and Jeff Wisoff—seated on Endeavour’s darkened middeck—killed a little time by playing rock, scissors, paper. As the countdown resumed ticking from the T-9 minute hold, the astronauts were heartened to learn that Range Operations had given them a green light to go. Weather was good, as were the systems and payloads aboard the orbiter herself. Launch Director Bob Sieck wished the crew good luck, to which Mike Baker responded with a heartfelt thanks for getting them ready to fly this important “Mission to Planet Earth.” A few minutes later, Terry Wilcutt reached over and activated the ship’s Auxiliary Power Units (APUs). “Here comes the vibration of the vehicle,” Jones wrote, “flight control surfaces moving, engines are cycling now with the hydraulics.”

Endeavour was primed and ready to go. Or so it seemed.

“Go for Autosequence Start. Endeavour’s on-board computers now have primary control of all the vehicle’s critical functions … ”

As the countdown ticked to the all-important T-31 seconds, control of the final stages was handed off from the Launch Control Center (LCC) to Endeavour’s on-board General Purpose Computers (GPCs). It was they, and they alone, which would monitor hundreds of separate sensors and execute decisions as to whether the mission would fly. The disembodied voice of the launch announcer echoed from loudspeakers, crisply acknowledging the seconds as they worked their way backwards toward the ignition of the three main engines, the ignition of the twin Solid Rocket Boosters (SRBs) and a date with a high-inclination (though relatively low-altitude) orbit to radar-map the Home Planet for the next 10 days or so.

“Twelve, 11 , 10 , nine, eight, seven … ”

At 10 seconds, the darkness was punctuated by the bright cascade of sparks from the swirling hydrogen burn igniters, as they fired off to disperse unburnt gas in the vicinity of the main engine nozzles.

“We have a Go for main engine start … ”

With a familiar rumble and a sheet of translucent orange flame, Endeavour’s three main engines thundered to life. From the roof of the LCC, Tom Jones’ wife, Liz, together with their two children, the other crew families and a handful of astronaut escorts braced themselves for the upcoming crescendo of sound and vibration. “In the growing light of dawn,” Jones recalled, “she saw the gout of orange exhaust flare beneath the orbiter and saw the steam billow from the flame trench as the engines spooled up to full power.”

“We have three main engines running … ”

All seemed normal. Then, with shocking abruptness, something went wrong.

“Three, two, one … and … we have main engine cutoff. GLS safing is in progress.”

As the Ground Launch Sequencer automatically kicked in to safe the vehicle, the three blazing engine bells suddenly fell dark and silent. For the third time in less than 18 months (and only the fifth occasion in the shuttle’s operational history), a Redundant Set Launch Sequencer (RSLS) abort had been called, after engine start, necessitating a highly hazardous on-the-pad shutdown. However, whereas previous aborts had occurred at around T-3 seconds, STS-68 had gotten down to a mere 1.9 seconds ahead of SRB ignition. Almost immediately, cooling water was sprayed onto the hot engines and the attention of everyone in the LCC was riveted upon the Main Propulsion System (MPS) fire detectors; if any of them had tripped, the abort carried the prospects of turning into a bad day, for an invisible hydrogen fire at the base of the main engines could easily spread across the launch pad and trigger an explosion. And that would necessitate a “Mode One Egress”: a hairy evacuation of the six astronauts from the orbiter.

“We have a cutoff of the main engines. The countdown clock has stopped.”

In the seconds which followed, an urgent flurry of acronym-laden communications passed between engineers, controllers and managers in the LCC, and with the astronauts themselves aboard Endeavour: “We have main engine cutoff … RSLS safing is in progress … All three main engines are in post-shutdown standby … GLS is Go for orbiter APU shutdown … ”

The gathered spectators at the Cape watched in alarm as the famous countdown clock starkly read T-00:00:00, yet no shuttle ascended into the heavens and only a large smudge of gray cloud rose ominously above Pad 39A.

Then came the call which brought a measure of calm to the proceedings: “No MPS fire detectors tripped.” There was no evidence of fire on the pad, meaning a “Mode One Egress” of the vehicle would probably be unnecessary. The “white room” was moved back into position alongside Endeavour’s crew access hatch, to facilitate the departure of Baker and his men. Pilot Terry Wilcutt shut down the three APUs. Through Endeavour’s tiny side hatch window, Tom Jones could clearly see the Pad 39A gantry visibly swaying backwards and forward; the vehicle was still rocking from the “twang” effect induced by the ignition of her main engines.

Had the engine shutdown been triggered a couple of seconds later—after SRB ignition—it would have placed the crew in an unenviable situation of having to perform the shuttle program’s first Return to Launch Site (RTLS) abort, riding the boosters for two minutes until their expiry, then separating from them and the External Tank (ET) to perform an emergency landing, back at the Shuttle Landing Facility (SLF).

“The RTLS is a daunting prospect for the crew,” wrote Tom Jones. “We would have to fly the orbiter and attached ET through half an outside loop, then ride backward through our exhaust plume at Mach 5. Ditching the empty tank, we would then try to make it back to the Kennedy runway. No shuttle crew had ever flown such an emergency approach. None wanted to be the first to try.”

For poor Dan Bursch, who had sat through a similar abort in August 1993, prior to his previous mission, STS-51, it gave him the unenviable record of being the only astronaut to have endured two RSLS aborts in his career. It was hardly an achievement that he would have wished for himself.

The second part of this article will appear tomorrow.

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