STS-55 (Columbia / Spacelab D-2)

Background

By the early 1990s, the partnership between NASA and the German space agency (DARA, later folded into DLR) had already produced one landmark Spacelab mission. Spacelab D-1, flown aboard *Challenger* in 1985, demonstrated that a non-American partner could effectively lead scientific operations from the ground while a shuttle crew executed experiments in orbit. The success of that mission prompted planners to mount a second, more ambitious German-directed flight. Spacelab D-2 would carry a substantially expanded complement of experiments across materials science, fluid physics, life sciences, and technology development — reflecting nearly a decade of refinements to both the hardware and the operational philosophy. Germany's commitment to the program was underscored by the assignment of two payload specialists from the German astronaut corps, physicist Ulrich Walter and engineer Hans Schlegel, who would work alongside their NASA colleagues in rotating shifts around the clock.

Before STS-55 ever reached orbit, however, it attracted attention for a more unsettling reason. On 22 March 1993, during a first launch attempt, all three of *Columbia*'s main engines shut down in rapid succession within seconds of ignition — a dramatic pad abort that left the vehicle sitting on Launch Complex 39A with fully loaded propellant tanks and a startled crew strapped inside. Investigators traced the aborts to contamination in the oxidizer preburners of two engines and a related sensor reading on a third. The episode forced a lengthy stand-down for engine replacement and inspection, pushing the mission into late April.

Crew and Vehicle

STS-55 was commanded by Steven Nagel, a veteran of three previous shuttle flights, with Terence Henricks serving as pilot. Mission specialists Jerry Ross, Charles Precourt, and Bernard Harris rounded out the NASA portion of the crew, giving the flight a deep bench of experienced operators. Ross in particular brought a reputation as one of the most capable EVA astronauts in the corps, though STS-55 was not a spacewalk mission; his systems expertise made him integral to Spacelab operations. Precourt and Harris were each flying their second missions.

*Columbia* was the natural choice for Spacelab D-2. The oldest orbiter in the fleet, she had been purpose-built with the heavier structural margins needed to accommodate the long Spacelab module in her payload bay. By 1993 she had undergone extensive modification and refurbishment, and her track record with pressurized Spacelab modules was unmatched. The Spacelab long module, a pressurized cylindrical laboratory roughly the size of a school-bus interior, connected to the crew cabin through a tunnel adapter, giving crew members shirt-sleeve access to a working laboratory without suiting up.

The Mission in Orbit

*Columbia* lifted off from Kennedy Space Center on 26 April 1993. Within the first minutes of flight the vehicle performed its roll maneuver and the main engines throttled through their programmed profile, clearing the atmosphere without incident — a relief after the March abort. Approximately eight and a half minutes after launch, the orbital insertion sequence was complete and the Spacelab D-2 science program was formally under way, marking the beginning of the second German-operated Spacelab mission.

German ground controllers at the DLR facility in Oberpfaffenhofen, near Munich, assumed primary direction of the scientific payload, coordinating with NASA's Payload Operations Control Center in a joint command structure that had been rehearsed intensively. The crew operated in two overlapping shifts to keep experiments running continuously, a standard approach for Spacelab missions designed to extract maximum return from limited on-orbit time.

The D-2 payload encompassed more than 80 individual experiments. In materials science, furnaces and processing units examined solidification behavior of semiconductors and metallic alloys in the absence of convective flows driven by gravity — phenomena relevant to both fundamental physics and the manufacture of high-purity materials. Fluid-physics investigations studied surface-tension-driven flows (Marangoni convection) and the behavior of liquid bridges under microgravity conditions. Life-science experiments examined human cardiovascular and vestibular adaptation to weightlessness, the growth and orientation of plants, and the behavior of lower organisms including frogs and fertilized eggs. Several technology demonstrations evaluated sensors and materials intended for future spacecraft applications.

The crew maintained a demanding pace throughout. Walter and Schlegel, as dedicated payload specialists, spent the majority of their waking hours at the experiment racks, troubleshooting and optimizing runs in real time alongside guidance from the Oberpfaffenhofen control team. The mission accumulated what was at the time considered an exceptionally high experiment productivity rate, with most investigations completing their full planned test matrices.

After approximately ten days aloft, *Columbia* performed her deorbit burn at mission elapsed time T+239 hours, committing the vehicle to re-entry. The orbiter landed at Edwards Air Force Base in California at T+239 hours and 40 minutes, rolling out on the dry lakebed in the clear desert air. The choice of Edwards, rather than Kennedy, reflected standard contingency practice when Florida weather or scheduling considerations made the primary site less favorable.

Legacy

STS-55 reinforced the template for international science partnerships that would prove central to the International Space Station era then taking shape in treaty negotiations. The mission demonstrated that a foreign agency could exercise genuine scientific and operational leadership on a NASA vehicle — not merely contributing hardware but directing the research agenda from the ground in real time. The experiment yield from the D-2 payload contributed to peer-reviewed literature across multiple disciplines for years afterward.

The March pad abort, though never involving any danger to the crew, became a standard reference case in shuttle operational history, illustrating both the sensitivity of the main-engine health-monitoring system and the effectiveness of the redundant shutdown logic in protecting the vehicle and its crew. *Columbia* herself would go on to fly seven more missions before the tragic loss of STS-107 in February 2003, but her role as the workhorse of Spacelab science operations remained one of the defining chapters of her long career. STS-55 stands as a high-water mark of American-German cooperation in human spaceflight during the shuttle era.