Summer of Discontent: 25 Years Since the Shuttle Hydrogen Leaks (Part 1)

A quarter-century ago, in the summer of 1990, the shuttle program was in chaos. Less than two years since the resumption of flight operations in the aftermath of the Challenger tragedy, the three remaining orbiters—Columbia, Discovery, and Atlantis—had successfully completed 10 missions, totaling almost eight weeks in orbit. Beginning with the STS-26 Return to Flight (RTF) mission in September 1988 and closing out with the spectacular deployment of the long-awaited Hubble Space Telescope (HST) on STS-31 in April 1990, the shuttle’s future seemed assured, as the fleet recovered from the worst disaster in its short history. Following STS-31, six more missions were planned, bringing 1990’s total to nine flights. Had this been achieved, it would have equaled 1985, with the maximum number of shuttle launches in a single calendar year. However, it was not to be, and the summer of 1990 would instead become known for a notorious series of complex hydrogen leaks which left missions grounded, astronauts frustrated, and threw the delicately planned shuttle flight manifest in disarray.

With the return of STS-31 to Earth on 29 April 1990, following the HST mission, the next scheduled launch was that of Columbia, no sooner than 16 May, to kick off the 10-day STS-35 mission, carrying the ASTRO-1 payload of ultraviolet astrophysics instruments. This flight—in its original incarnation as “Mission 61E”—would have been the next flight after Challenger and, planned for March 1986, would have formed part of the vanguard of our observations of the most recent visitation of Halley’s Comet to the inner Solar System. More than four years later, with the fabled comet long gone, ASTRO-1 was combined with another instrument, known as the Broad Band X-Ray Telescope (BBXRT), and was expected to reveal significant new insights into the nature and evolution of the cosmos at ultraviolet and X-ray wavelengths. On the heels of STS-35, Atlantis would fly the classified STS-38 mission for the Department of Defense in July, following by Columbia’s STS-40 Spacelab Life Sciences (SLS)-1 in September, the eagerly anticipated launch of the Ulysses solar explorer aboard Discovery on STS-41 in October, the deployment of the Gamma Ray Observatory (GRO) by Atlantis in November, and, lastly, the International Microgravity Laboratory (IML)-1 payload, aboard Columbia, in December. Under this scenario, Columbia would fly no fewer than four times in 1990, creating a new record for herself and tying with an equivalent number of missions accomplished by her sister Discovery in 1985.

Then came the issue with the hydrogen disconnect valves.

Deep within the belly of each shuttle were a pair of 17-inch-diameter (43-cm) propellant lines, through which liquid oxygen and hydrogen flowed from the External Tank (ET) into the cluster of three main engines. Both “sides” carried mechanical disconnect fittings, and, shortly before the separation of the ET—about eight minutes after launch, right on the edge of space—a pair of flapper valves were commanded shut by pneumatic helium pressure, thereby preventing further propellant discharge and circumventing the risk of contamination. The criticality of these disconnect valves was acute: Any inadvertent closure during powered flight would have prevented propellant flow from the ET and precipitated disaster, and, in the days, weeks, and months before STS-35, they took center-stage as one of the most difficult engineering challenges ever to face the shuttle team.

Before each mission, a Flight Readiness Review (FRR) scrutinized all pertinent documentation, before formally reaching an agreement on a target launch date. At the beginning of May 1990, Columbia was provisionally scheduled to fly on the 16th, but a problem was noted with a proportioning valve on her Freon coolant loop, requiring its replacement. A second review was convened and settled on 30 May for launch. By this time, the delays were expected to have a significant effect on the remaining flights of the year, which included the SLS-1 and IML-1 Spacelab missions by Columbia, in September and December. Then, during the lengthy process to load liquid hydrogen into the ET, a tiny leak was detected, close to the tail service mast on the Mobile Launch Platform (MLP). Further investigation revealed a much more extensive—and far more worrisome—leak, apparently coming from the disconnect hardware in Columbia’s belly.

“We got to about T-4 hours, when they start the fast-fill of the hydrogen,” recalled STS-35 Mission Specialist Jeff Hoffman in his NASA oral history. “We had had the party for the launch guests. Everybody comes down; that’s a tradition. Then there’s another nice tradition, where we would go out and stand on one side of the trench, out by the launch pad, and the buses would come and the families and friends would be on the other side, so we could wave at them.” With the discovery of the leak, the launch attempt was immediately called off and the ET was emptied and inerted.

Since the STS-35 crew would be working in two 12-hour teams in orbit, they had already begun “sleep-shifting,” and the “Blue Team” of Mission Specialists Hoffman and Mike Lounge, together with Payload Specialist Sam Durrance, was awake when the news of the launch delay came through. Hoffman recalled waking up his “Red Team” counterparts—Pilot Guy Gardner, Mission Specialist Bob Parker, and Payload Specialist Ron Parise—and STS-35 Commander Vance Brand, with the unwelcome news.

A few days later, on 6 June, a “tanking test” was conducted to identify the exact location of the leak. It soon became clear that Pad 39A was not an appropriate place for exploratory work to be undertaken, and on the 12th the STS-35 stack was rolled back to the cavernous Vehicle Assembly Building (VAB). Columbia was removed from the stack and returned to the Orbita Processing Facility (OPF) for repairs on the 15th. Launch was provisionally rescheduled for mid-August. The shuttle “side” of the disconnect hardware was replaced with a set borrowed from the new orbiter, Endeavour, which was then undergoing final construction as Challenger’s replacement, and apparatus for the ET “side” arrived shortly thereafter from its assembly facility at Michoud in New Orleans, La. Certainly, Hoffman remembered that it was a tough time on his crewmates, but not so much for himself. He had already been assigned to another mission, STS-46 with an Italian-built tethered satellite, and there was plenty of work for him during the enforced down time. “I carried on working with the tethered satellite,” he said, “in addition to the maintenance training that we had to do for ASTRO.” For the others, it was harder, since they were all fully trained, at the peak of mental and physical readiness, and yet forced to sit through endless simulator sessions with no revised launch date.

As this work was being undertaken, Atlantis progressed smoothly—or so it seemed—toward her own launch on STS-38 in July. As a precautionary measure, shortly after the STS-38 stack arrived at Pad 39A on 18 June, NASA decided to perform a tanking test on Atlantis, to verify that she was not similarly affected. Two modes of propellant loading were followed; the first, termed the “slow-fill,” served to chill down the pipework and the structure of the tank, so that when the cryogenic hydrogen was pumped in at a higher rate, known as the “fast-fill,” it would not boil and generate excessive amounts of gas. On the 29th, liquid hydrogen was duly pumped into her ET … and to engineers’ dismay the same problem appeared: After the fueling process moved from “slow-fill” to “fast-fill,” concentrations of gas were found in the vicinity of her disconnect hardware. It was small and was described by NASA as “both temperature and flow-rate-dependent,” but the agency was convinced that the two leaks in Columbia and Atlantis represented nothing more than coincidence.

To identify the source, more instrumentation was fitted around the disconnect and a second tanking test was performed on 13 July. Sealants were added in a bid to stop the leak, but a third test on the 25th revealed that the problem persisted. Two weeks later, in the pre-dawn darkness of 9 August, the STS-38 stack rolled back to the VAB, providing a unique photo opportunity in the process, as the STS-35 stack passed it on its way back out to Pad 39A. Despite the similarities between the problems, NASA managers were convinced that the hydrogen leaks were unrelated. “Incorrect torqueing of bolts around the flange interface between the tank and the orbiter caused the Atlantis mishap,” Flight International told its readers on 8 August. “The Columbia leak was caused by a faulty seal in the drive mechanism used to close the flapper valve in the disconnect.”

With an apparent end to the hydrogen leak problems in sight, STS-35 was rescheduled for launch on 1 September. However, as will be explored in tomorrow’s article, misfortune was far from finished with Columbia and her hapless crew.

The second part of this article will appear tomorrow.

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