Thirty years ago, this week, one of the most significant Space Shuttle science missions ever undertaken hung—for the merest of minutes—in the balance, suspended on a knife-edge of success and failure, some 67 miles (108 km) above Earth. Heading towards low-Earth orbit at more than 9,300 mph (15,000 km/h) on the afternoon of 29 July 1985, Challenger was in the process of delivering her eighth human crew on the Spacelab-2 mission to explore the Sun and the cosmos in unprecedented detail, using a battery of telescopes and instruments in her payload bay. Three weeks earlier, on 12 July, the crew of Mission 51F had also suffered a hairy shutdown of their three main engines on the pad, seconds before liftoff. If the crew believed to have weathered their run of bad luck, they could not have been more mistaken. Today, with the shuttle now a figure of history, Mission 51F stands alone as arguably the most significant near-miss in the program’s 30-year operational lifespan.
Aboard Challenger that morning was one of the oldest crews ever launched into orbit, with an average age of 47, and just two previous space missions between them. In command was veteran astronaut Gordon Fullerton, joined on the flight deck for ascent by pilot Roy Bridges, flight engineer Story Musgrave—recently interviewed by AmericaSpace’s Emily Carney—and the then-oldest man in space, Karl Henize. Downstairs, on the shuttle’s darkened middeck, were fellow astronauts Tony England, Loren Acton and John-David Bartoe. For Musgrave, flying his second mission, his duty during ascent was to assist Fullerton and Bridges with monitoring Challenger’s systems and reading back procedures to the pilots in the event of an off-nominal situation.
And 29 July 1985 certainly proved to be one such situation.
At an altitude of 67 miles (108 km), and almost six minutes after leaving Pad 39A at the Kennedy Space Center (KSC), the shuttle had long since shed her twin Solid Rocket Boosters (SRBs) and was racing towards low-Earth orbit under the impulse of her three liquid-fueled main engines, fed by the giant External Tank (ET). Suddenly, temperature readings for the No. 1 engine’s high-pressure turbopump indicated “above” its maximum redline, prompting Challenger’s General Purpose Computers (GPCs) to shut it down. Mission Control made the call “Limits to Inhibit”, advising the crew that they were seeing a potentially show-stopping malfunction and the imminent necessity of an abort. At this stage of ascent, the vehicle was too high and traveling too fast to accomplish a Return to Launch Site (RTLS) abort. One of two options remained open to the 51F crew: either a Transoceanic Abort Landing (TAL) in Europe or a tricky maneuver, called an Abort to Orbit (ATO), whereby the shuttle would pulse her twin Orbital Maneuvering System (OMS) engines to augment the two remaining main engines and limp into a low, but stable orbit.
Sitting behind and between Fullerton and Bridges, the first instinct of Musgrave was to flick to the page on his knee-mounted checklist which dealt with a TAL abort to a place called Zaragoza Air Base, a joint-use military and civilian installation with a NATO-equipped bombing range, in the autonomous region of Aragon in north-eastern Spain. This particular site had been assigned to 51F for a TAL scenario because the mission’s orbital inclination of 49.5 degrees placed it close to the nominal ascent ground track and enabled the most efficient use of available main engine reserves and cross-range capability.
Next to Musgrave was Karl Henize, who looked on with a measure of nervousness. He was keenly aware that TAL encompassed the six-minute period following the closure of the RTLS “window”, through SRB separation and main engine cutoff, and he knew that it would only be selected in the event of a major malfunction, such as a serious cabin pressure leakage or cooling system failure. Had Mission Control issued the instruction to “Abort TAL” that day, Fullerton would have rotated the abort switch on his instrument panel to the TAL/AOA position and depressed the abort push button next to the selector switch. Challenger’s computers would then have automatically steered the orbiter towards the plane of the European landing site.
Henize could see Musgrave’s checklist open at the page headed “SPAIN”.
“Where we going, Story?”
“Spain, Karl.” Then he retracted it. “We’re close, but not yet.”
Eventually, the call came from Mission Control: “Abort ATO; Abort ATO.” Challenger had achieved sufficient velocity and altitude to undertake the next available option: the Abort to Orbit. In fact, she had missed the closure of the TAL “window” by just 33 seconds! At 4:06:06 p.m. EDT, some six minutes and six seconds into the ascent and hurtling towards space, Gordon Fullerton fired the OMS engines for 106 seconds, consuming a large quantity of much-needed propellant, but permitting the shuttle to continue into a lower-than-planned orbit. Two minutes later, at 4:08:13 p.m., the No. 3 main engine data indicated excessively high temperatures. If the “Limits to Inhibit” had not already been applied, the computer would have it shut down. The “inhibit” command effectively instructed the computers to ignore the over-temperature signals and prevented them from shutting down the No. 3 engine. The two remaining engines fired for an additional 49 seconds, shutting down nine minutes and 20 seconds after launch. “We never did get the call for the transoceanic emergency landing,” said Musgrave, “and we ended up making it to orbit and finishing the mission.”
The hair-raising incident on 29 July 1985 represented the only in-flight shutdown ever experienced by the shuttle, and it came as a surprise because all main engine parameters had been normal during the countdown, ignition sequence, and the first few minutes of the flight. At approximately two minutes into Challenger’s ascent, at about the same time as the SRBs were jettisoned, data from Channel A—one of two measurements of the No. 1 engine’s high pressure fuel turbopump discharge temperature—displayed characteristics indicative of the beginning of failure. Its measurement began to drift and, at three minutes and 41 seconds after launch, the Channel B sensor failed. However, its sibling continued to drift, approaching and then exceeding its own redline limit some five minutes and 43 seconds into the flight, which triggered the shutdown.
The high pressure fuel turbopump discharge temperature data from Channel B of the No. 3 engine, meanwhile, began to climb and passed its own redline just over eight minutes after liftoff. Measurements from its Channel A remained within prescribed limits and, according to NASA’s post-mission report, all other operating parameters relating to the No. 2 and 3 engines were deemed normal. Post-mission analysis suggested that the problem was not with the No. 1 engine itself, but faulty sensors that incorrectly indicated an overheating situation. According to Bill Taylor, then-head of the main engine project at the Marshall Space Flight Center (MSFC) in Huntsville, Ala., these sensors were extremely thin wires, whose electrical resistance changed as they heated up. Otherwise, the performance of the SRBs in propelling Challenger into orbit was described as “nominal”. However, gearbox nitrogen pressures in one of the shuttle’s Auxiliary Power Units (APUs) had exceeded their maximum allowable levels and, during a post-launch sweep of the Cape Canaveral beaches, a fragment of spray-on foam insulation, apparently from the ET, was discovered.
As they settled into orbit, the seven-man crew barely had chance to reflect on what had been not just been an eventful day, but a crisis-filled month. Originally scheduled to head for space at 3:30 p.m. EDT on 12 July, they had been thwarted by the second on-the-pad Redundant Set Launch Sequencer (RSLS) main engine shutdown, only seconds before liftoff. “At T-7 seconds,” recalled Bridges, “the main engines start with a rumble from far below. As the person in charge of all engines, I watch the chamber pressure indicators come to life and surge towards 100 percent. I think ‘Wait, what’s this?’ The left engine indicator seems to be lagging behind. Before I can say a word, it falls to zero, followed by the other engines. With less than three seconds before our planned liftoff, we have an abort. The groans from the rest of the crew are now audible. I take a quick look around to see if there’s anything else to be done and notice Gordon Fullerton turning to look at me. The thought crosses my mind: ‘Gordo probably is thinking I’ve done something to screw it up.’ I show him both hands, palms up, and say ‘Gordo, I didn’t touch a thing. It was an automatic shutdown.’”
The 12 July shutdown, executed because the No. 2 engine’s chamber coolant valve was slow in closing from 100 percent open to the 70 percent required for startup, necessitated a 17-day wait for a second launch attempt. When one of two command channels failed to execute the closure, fortunately, the backup took over without incident. However, flight rules dictated that both channels had to be fully functional for the countdown and liftoff to proceed. It was a shutdown which might have saved the lives of the crew and which gave them the opportunity to try again, 17 days later. And despite the challenges faced during that second—and ultimately successful—launch attempt, Mission 51F would recover from near-disaster to stage one of the most successful shuttle science flights in history.
The second part of this article will appear tomorrow.