'Safing In Work': Remembering Discovery's Almost-Liftoff, On This Day a Quarter-Century Ago

After a month-long wait for launch, following a harrowing pad abort on 12 August 1993, STS-51 thunders into space on 12 September. Photo Credit: NASA

“T-minus 30 seconds…”

The words of launch commentator George Diller at the Kennedy Space Center (KSC) in Florida at 9:12 a.m. EDT on 12 August 1993—a quarter-century ago, this morning—were calm and measured, as all eyes focused on shuttle Discovery in the final moments of her countdown to fly STS-51. The planned nine-day mission was destined to deploy an advanced NASA communications satellite and release and retrieve an ultraviolet telescope on a German-built Shuttle Pallet Satellite (SPAS), as well as perform a six-hour spacewalk. Yet STS-51 had struggled to get off the ground.

Video Credit: CNN

On 17 July, a flaw in a pyrotechnic initiator controller, needed to trigger the release of the shuttle’s twin Solid Rocket Boosters (SRBs) from the launch pad, had forced a scrub. Troubleshooting identified a thermal instability issue in a solid-state switch card in the Ground Support Equipment (GSE) and after replacement and testing the launch of STS-51 was rescheduled for the following week.

“Standing by to activate the sound suppression water system in five seconds…”

The next attempt to get Discovery and her five-man crew—Commander Frank Culbertson, Pilot Bill Readdy and Mission Specialists Jim Newman, Dan Bursch and Carl Walz—into space on 24 July also came to nothing, following a turbine “underspeed” condition in a Hydraulic Power Unit (HPU) in the right-hand SRB. “The specification requires the turbine speed to be between 66,200 and 77,800 rpm,” noted NASA’s STS-51 Mission Report, “and the speed at that point was 65,000 rpm.” The countdown was automatically halted at T-19 seconds and the launch was scrubbed. Due to the impending July-August Perseid meteor shower, associated with Comet Swift-Tuttle and known to produce a rate of up to 450 meteors per hour at its peak, NASA opted not to make another launch attempt until mid-August. Predictions showed that the Perseid debris field would pass closest to Earth on the 11th, creating a one-in-a-thousand chance of an impact with the shuttle, and liftoff was scheduled for 9:10 a.m. EDT on 12 August. However, CNN’s John Zarrella reported only about a dozen meteors on the evening of the 11th, far fewer than expected.

Commander Frank Culbertson leads his crew out to the launch pad for STS-51. The five men suffered a last-second Redundant Set Launch Sequencer (RSLS) pad abort on 12 August 1993, the fourth such event in the shuttle program’s 30-year history. Photo Credit: NASA, via Joachim Becker/SpaceFacts.de

Early on the 12th, Culbertson and his men were ready to go. Aboard Discovery, they had closed and locked their visors and were forced to sit through a 2.5-minute additional hold in the countdown at T-5 minutes, caused by a lost synchronization “lock” between the Mission Control Center (MCC) at the Johnson Space Center (JSC) in Houston, Texas, and the Merritt Island Launch Area (MILA). From the pilot’s seat, Readdy reached over and switched on the three Auxiliary Power Units (APUs), which hummed perfectly to life, bringing muscle and control to the shuttle’s hydraulics. In the succeeding minutes, the on-board computers commanded a final check of the three Space Shuttle Main Engines (SSMEs) and the elevons on the wings. At T-31 seconds, control of the countdown was handed off to Discovery.

“T-minus ten, nine, eight, seven…”

By now, the astronauts had been training for almost 18 months, since their assignment in March 1992. With Culbertson and Readdy—the only veterans among the crew—seated at the front of the flight deck, STS-51 was unique for a post-Challenger shuttle mission in that none of its Mission Specialists had flown before. Also on the flight deck were Bursch in the flight engineer’s seat, behind and between the Commander and Pilot, and Newman, abutting his right shoulder. Downstairs on the darkened middeck was Walz.

Discovery rolls out to the launch pad, ahead of STS-51. Photo Credit: NASA, via Joachim Becker/SpaceFacts.de

Before he became an astronaut, Newman worked as a Simulation Supervisor, nicknamed a “Sim Sup”, part of the training team which earned a measure of good-natured notoriety for keeping shuttle crew’s skills sharp. According to fellow astronaut Tom Jones, in his memoir Skywalking, Newman had once sat in on a training session with Jones and legendary astronaut John Young and had mischievously pushed the main engine shutdown switch with his “swizzle stick”. This triggered an abort and required Young to execute an abort landing in a simulated Ben Guerir, Morocco. Jones wondered what Newman was playing at. “Why would you want to kill an engine on the most senior astronaut, the first man to command the shuttle?” he wrote with incredulity. “Finally, Newman lost it; he burst out laughing as he confessed that he was the real culprit. Newman, the former instructor, had just been keeping his hand in, feeding Young one last malfunction. What surprised me was that John loved the practical joke.”

“Go for main engine start…”

Now, on 12 August 1993, Newman was experiencing the final seconds of a countdown for real. Inside the cabin, the astronauts felt the immense vibration as turbopumps awoke, liquid oxygen and hydrogen flooded into the engines’ combustion chambers and they roared to life…and then, suddenly and shockingly, were arrested by the blaring sound of the master alarm. All three had been automatically shut down. Something had gone badly awry.

“…Three…We have a cutoff…”

STS-51 crew patch, designed by the five astronauts. Image Credit: NASA, via Joachim Becker/SpaceFacts.de

At Newman’s left shoulder at the rear of Discovery’s flight deck, Bursch’s eyes were as wide and saucers as he realised that his first mission had just suffered a potentially disastrous pad abort. Nor was it the only time in Bursch’s career that this happened; it occurred again on the first attempt to launch his next flight, STS-68, in August 1994.

“We have a main engine cutoff. Safing in work.”

If Bursch and Newman and, up front in the pilots’ seats, Culbertson and Readdy, could see the red lights on the instrument panel and catch a glimpse through Discovery’s windows as she rocked to and fro in the first few seconds after engine shutdown, one man who had to rely upon his other senses was Walz. Seated downstairs in the darkened middeck, he heard the roar of the engines and the abrupt silence when they cut off.

A month after the last-second launch abort, Discovery and the STS-51 crew flew a highly successful mission, which included two satellite deployments, a payload retrieval and a spacewalk. Photo Credit: NASA, via Joachim Becker/SpaceFacts.de

The communications loop from the Launch Control Center (LCC) provided a flurry of messages, verifying that the three main engines were in post-shutdown standby and requesting Readdy to shut down the APUs. No fire detectors on Pad 39B were tripped during the incident, which would later be traced to a faulty fuel-flow sensor in the No. 2 main engine. The engine had posted a ‘major component failure’, caused by the sensor glitch, about 0.6 seconds after ignition. “This condition,” noted NASA’s official STS-51 Mission Report, compiled and published after the flight, “caused a miscompare which violated the Launch Commit Criteria…As a result of the failure, the engines were shut down and safing activities were initiated.”

“Orbiter access arm now back in position…”

Shortly thereafter, the five disappointed astronauts disembarked from Discovery, aware from previous RSLS events that their launch had been called off for several weeks at best. The main engines were replaced and an attempt was provisionally scheduled for 10 September, but this was itself slipped by two days, as a result of the failure, on 21 August, of NASA’s Mars Observer, shortly before its arrival at the Red Planet. During the early investigation into the loss of Mars Observer, it was revealed that the spacecraft’s Transfer Orbit Stage (TOS)—a near-identical booster to that of STS-51’s primary payload, the $363 million Advanced Communications Technology Satellite (ACTS)—had exhibited a transistor failure. During the additional two days’ delay, engineers and managers verified that there was no commonality between the Mars Observer TOS fault and the ACTS booster. Launch of STS-51 occurred smoothly a month after the abort, on 12 September 1993, and the mission proved a spectacular success…and a stark reminder that shuttle missions could never be routine.



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