Productive Albeit Short Flight

Certainly, in spite of the difficulties and the tireless efforts of Halsell, Still, Gernhardt and Mission Control to stabilise and save the ailing fuel cell, the research work in the Spacelab module was proceeding in a superb manner, with a significant 'first' in combustion science late on 6 April. ''Six burns were successful and, for the first time, we're burning free droplets,'' said DCE Principal Investigator Forman Williams of the University of California at San Diego. ''We can't get this kind of information from ground-based experiments. We have burned at two different atmospheres of oxygen concentration and calculated the burning times of free fuel droplets at each.''

Meanwhile, Crouch busied himself with the SOFBALL investigation, designed to explore the conditions under which a 'stable' flameball can exist and if heat loss is responsible, in some way, for its stability while burning. ''The two completed runs were successful beyond my wildest dreams,'' Principal Investigator Paul Ronney of the University of Southern California at Los Angeles said on 7 April.

During the first SOFBALL run, a mixture of hydrogen, oxygen and carbon dioxide burned in the facility for the entire 500-second limit; this was particularly significant because ''these are the weakest flames ever burned - lowest temperature, weakest, most diluted mixtures'', according to Ronney. ''These mixtures will not burn in Earth's gravity. We have known that burning weaker mixtures increases efficiency, but not much is known about the burning limits of these mixtures.'' As well as providing a clearer understanding of the combustion process, it was anticipated that SOFBALL results would help to improve theoretical models.

''Combustion models give different results for these types of flames,'' said Ronney. ''This is an acid test to show which, if any, current combustion modules should be used.'' His research could ultimately lead to developments in spacecraft fire-safety systems and had already, by the time of MSL-1, identified anomalous

'flameballs' - essentially stable, stationary, spherically symmetric flames in combustible gas mixtures - from which the experiment received its name. By examining the behaviour of such flameballs in microgravity, including their stability and propagation, more efficient combustion engines, emitting fewer atmospheric pollutants, could be produced.

During typical SOFBALL runs, a 26-litre chamber was filled with a weakly combustible gas (hydrogen and oxygen, highly diluted with an inert gas) and ignited. The resulting flames and their unusual behaviour were then imaged and recorded using a sophisticated battery of video cameras, radiometers, thermocouples and pressure transducers. Unfortunately, the shortened STS-83 mission meant that only two of some 17 planned SOFBALL runs were completed.

Elsewhere in the Spacelab module, other experiments were underway in the area of materials processing, as part of efforts to develop techniques for stronger and more resilient metals, alloys, ceramics and glasses in space. Two facilities used to support these experiments - the Japanese-built Large Isothermal Furnace (LIF) and Germany's Electromagnetic Containerless Processing Facility (TEMPUS) - had already flown on board IML-2 in July 1994. The LIF was activated by Thomas on 5 April and was intended for studies of the diffusion of liquid metals, which cannot be adequately duplicated on Earth because of gravity-induced fluid movements.

One of the first investigations to use the high-temperature furnace was a study of the diffusion of impurities in molten salts. Provided by Tsutomu Yamamura of Tohoku University in Japan, it sought to reveal ideal conditions for the electrolysis of molten salts. Later, Thomas began the Liquid Phase Sintering experiment, which tested theories of how liquefied materials form a mixture without reaching the melting point of the new alloy combination. It was expected that results from this investigation would provide researchers with a clearer understanding of liquid phase sintering in microgravity and compare these findings with theoretical predictions.

Other LIF experiments diffused molten semiconductors - causing two or more types of atoms or molecules to intermingle, in much the same way as a sugar cube would dissolve in a cup of coffee - as part of efforts to explore how uniformly their constituents mixed during the sample-cooling process. Diffusion studies have many terrestrial applications: from very small movements in plasma to massive depletions of the ozone layer. The MSL-1 experiments focused on the usefulness of diffusion processes in support of manufacturing technologies.

Mike Gernhardt shoots photography from Columbia's overhead windows.

The carbonisation of steel and fabrication of transistors have already pointed to the economic and technological importance of the diffusion process; in the semiconductor industry, for example, 'dopants' - small additions of impurities -of antimony, gallium or silicon are routinely added to semiconducting materials such as silicon or germanium to greatly improve the performance of electronic components fabricated from them. Since solid crystals of most semiconductors are grown from their melts, knowledge of diffusion processes in semiconducting melts is important for this industry.

A key problem, however, was that the liquid state of the process was imperfectly understood in terrestrial experiments; however, in the microgravity environment, buoyancy-driven convection is drastically reduced and high-precision diffusion measurements are possible. The MSL-1 experiments, provided by David Matthiesen, explored the diffusion characteristics of gallium, silicon and antimony dopants in samples of germanium. Other investigations focused on the diffusion of liquid lead-tin telluride, which holds great potential as a base material for building infrared lasers and detectors. After Columbia's landing, the sample columns were cut into segments to determine how well their components mixed while cooling.

Meanwhile, the TEMPUS facility was used to study the 'undercooling' and rapid solidification of metals and alloys. Such undercooling typically takes place when a solid is melted into a liquid, then cooled below its normal freezing point without solidifying. The phase change is delayed, and the material is in a metastable state. ''When the metal or alloy is solidified, it occurs very rapidly, forming new types of materials we cannot manufacture or study in any other way,'' said the facility's Project Scientist Jan Rogers. A clearer understanding of the process was expected to yield new insights into better casting, welding and soldering techniques and the improved products that will inevitably follow.

Late on 4 April, Crouch activated TEMPUS and configured it for operations. After the decision to bring Columbia home early, William Hoffmeister, the facility's assistant investigator, told journalists that several experiments were pushed up in the timeline to acquire as much scientific data as possible in the short time available. The crew was able to activate, observe and complete an experiment run by melting a zirconium metal sample and levitating it, as part of efforts to study the relationship between internal flows in liquids and the amount of undercooling that a sample can tolerate before it solidifies.

''To understand this experiment,'' said Robert Bayuzick of Vanderbilt University in Nashville, Tennessee, ''imagine if you cooled a glass of distilled water. The temperature could go below freezing without the water actually becoming ice. That is undercooling. However, if the glass were tapped or disturbed, then the water would freeze very quickly. This process may have many benefits to industry. New, enhanced properties in never-before-seen materials could become possible.''

As with its first flight on IML-2, TEMPUS provided the means for physically manipulating samples, controlling rotations and oscillations and even 'squeezing' them through the application of an electromagnetic field. The experiments involving zirconium, in particular, were expected to help determine the behaviour of this strong, ductile refractory metal, which has found applications in nuclear reactors and chemical processing equipment.

With the impending landing, a mere quarter of the way through the planned 16-day mission, the TEMPUS team had good prior knowledge of how to reprioritise their research schedules to make the best of unexpected situations. On the facility's maiden flight in July 1994, a misaligned coil forced the investigators to shorten and replan several of their key experiments. "It's a smart group of people,'' Mike Robinson said of the team before Columbia lifted off on 4 April. "I have a lot of faith in them.''

Other areas of research on MSL-1 included plant growth, which could lead to the development of life-saving medicines and other important compounds. "A fundamental objective of this research'', said investigator Gerard Heyenga of NASA's Ames Research Center, "is to evaluate whether microgravity may be used to alter specific metabolic pathways in plants, and ultimately apply this technology for Earth-based benefits.'' He hypothesised that extended exposure of plants to the microgravity environment could reduce their expenditure of energy on structural components, thereby increasing flow through other metabolic pathways, many of which yield materials of important medicinal value.

Of even greater significance, potentially, was developing a clearer understanding of how such pathways are controlled at genetic levels. "Such knowledge would allow us to manipulate or genetically engineer plants with desired metabolic traits,'' Heyenga added. "For example, this information could be applied to the lumber industry in the production of trees with a low lignin content, greatly reducing the cost of paper production both economically and environmentally.'' Conversely, it could also be applied to improving timber quality in fast-growing softwoods, thus reducing the need to harvest slow-growing hardwoods.

"If this hypothesis is correct and achievable,'' he said, "it obviously represents the basis for a multi-billion-dollar industry and certainly highlights the value of space-related research and such facilities as the space station.'' To conduct the experiments, Columbia carried an advanced piece of hardware developed by BioServe - one of NASA's Commercial Space Centers, situated at the University of Colorado at Boulder - known as the Plant Generic Bioprocessing Apparatus (PGBA). Previously flown on the STS-77 mission in May 1996, it "produced a particularly high quality of plant material, which provided a good basis for further research'', said Heyenga.

As well as offering a highly controlled environment with lighting, temperature and gas-exchange functions, the unit was fitted with a 'nutrient pack' to supply water and other nutrients to nine different plant species on MSL-1. ''This technology will open an entirely new area of space plant physiology, allowing the study of issues not previously possible,'' said Heyenga. ''It is likely to lead to some very exciting results.'' For Columbia's mission, the plant species included a member of the black pepper family, selected through a cooperative project with a Brazilian research group.

Also making its second Shuttle flight was the Middeck Glovebox (MGBX), which supported a variety of fluid physics, materials research and combustion experiments. One study, provided by Peter Voorhees of Northwestern University in Evanston, Illinois, investigated coarsening in metallic mixtures at high temperatures. In

Don Thomas works at the Glovebox inside the Spacelab module.

ground-based research, during coarsening, small particles shrink by losing atoms to larger particles, resulting in a lack of uniform particle distribution and thus weakening the resultant materials and shortening their lifespans. Findings from such research could lead to improved manufacturing processes and stronger, longer-lasting materials.

Other Glovebox investigations included the Fibre-Supported Droplet Combustion experiment: a study of fundamental phenomena associated with liquid-fuel-droplet combustion in air - such as how atmospheric pollutants are formed - and had previously flown on USML-2 in late 1995. On that mission, it had demonstrated that data for fuel droplets as large as 5 mm in diameter are useful for testing droplet-burning theories and verified predictions that methanol droplets would be extinguished at diameters that increase with increasing droplet diameter. However, USML-2 data overpredicted the extinction diameter, leading to the addition of radiometers for MSL-1 to monitor radiation emissions.

The behavioural characteristics of bubbles and droplets in response to ultrasonic radiation pressure - perhaps leading to new ways of preventing complications caused during materials processing - were carefully examined in other Glovebox studies, as scientists assessed their ability to control bubbles' locations, manipulate 'double bubbles' and maximise their shapes. Another experiment explored the performance, including the mechanisms leading to unstable operations and failure, of specialised heat-transfer devices in microgravity.

Such technologies are especially important as a means of providing passive thermal control of various spacecraft components, whose electronics are packed closely together and generate heat which can often limit their performance. Mounted in Columbia's payload bay was the Cryogenic Flexible Diode (CRYOFD), attached to a Hitchhiker mounting plate, which tested a pair of experimental heat pipes. One, provided by NASA's Goddard Space Flight Center in conjunction with the US Air Force's Phillips Laboratory at Kirtland Air Force Base in New Mexico, evaluated a unique flexible 'wick' to permit easier integration into future spacecraft and even help point instruments.

REFLIGHT

With such a multitude of investigations packed on board Columbia, it was a pity that the mission should have been curtailed and, even before Halsell's crew returned home, informal plans were already afoot to refly MSL-1 later in the year. Ironically, delays in beginning construction of the International Space Station - the very project for which this mission was providing a technological testbed - made the reflight possible! Originally, Space Shuttle Endeavour was slated to take the first station components into orbit on STS-88 in December 1997, but delays of a crucial Russian service module postponed it until the summer of 1998, at least.

By pushing Endeavour's mission back until the following year, NASA had more leeway to turn Columbia around to fly MSL-1 again in the summer of 1997 and complete another, previously scheduled 16-day flight (STS-87) before the year's end. As Endeavour vacated the December launch slot, ample time was provided for Columbia to fly on two more occasions in 1997, hopefully causing as little disruption to the Shuttle manifest as possible. However, despite significant advances in the space agency's ability to process Space Shuttles more quickly and with fewer problems than ever before, meeting an early July launch date would be challenging.

So it was that, as he climbed down the airport-style steps from Columbia on 8 April, after a picture-perfect 6:33 pm touchdown on KSC's Runway 15, Halsell shook hands with Director Roy Bridges and received the news that ''we're going to try to give you an oil change and send you back''. All seven astronauts, but particularly the members of STS-83's science crew, welcomed the possibility. ''In some ways, this could make for a more meaningful flight in the long run,'' admitted Crouch, ''but, certainly, this one was a bummer.''

NASA managers discussed the plan over the next couple of weeks, juggling the launch schedule for the remainder of the year in a bid to slot Columbia in again somewhere around 1 July. Atlantis' two trips to the Mir space station, scheduled for mid-May and mid-September, remained on target, while Discovery's 11-day mid-July mission was postponed until the second week of August. Assuming Columbia was ready for the reflight - a mission originally named 'STS-83R', but later STS-94 -on 1 July or shortly thereafter, she should be more than capable of being ready in time for a late November STS-87 launch.

''We're ready to go fly,'' said Halsell as the initial plans were being wrung out. ''If it were up to me, I'd like to give the guys a week or two off to let them decompress from this flight and then we'll come back and start ramping up again for the next flight. I think we should make it clear that this is still under study by everybody; it's not a done deal yet. But certainly, this crew is ready to support with relatively minor additional training.''

The schedule was incredibly tight, but according to Shuttle manager Tommy Holloway, who made the official announcement on 25 April, ''we now are in a position to do everything possible to complete the mission with minimal impact on downstream flights. Also, it provides us with a unique opportunity to demonstrate our ability to respond to challenges such as this one.'' From a technical standpoint, the three-month turnaround was feasible, particularly since Columbia would carry the same payload and would not need to go through the lengthy removal of her 'old' payload and installation and checkout of a 'new' one.

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