The Big Picture

Sacco's earlier comment, in relation to his zeolite investigations, of USML-2 pointing scientists in the right direction for space station research, was certainly not

The 'big picture' 245

the last time Columbia's astronauts would remark on this 'bigger picture', of which their mission was only a small constituent part. ''This is a kind of pathfinder for the kind of investigations we'll have on the space station,'' Kathy Thornton told journalists during a space-to-ground news conference on 2 November. ''There are very complicated experiments onboard, but they're working beautifully.''

Thornton had been training for this mission for 19 months and took a leading role in activating the USML-2 experiments within hours of Columbia reaching space. As with most Spacelab missions having a microgravity-research emphasis, the module was outfitted with a number of devices to measure the effect of Shuttle accelerations on very sensitive experiments. As Thornton powered-up USML-2 on the afternoon of 20 October, Sacco switched on the mission's two accelerometers, which, said Project Scientist Alex Pline, ''allow us to make a judgement as to whether to wait for external movements to settle down before beginning an experiment run''.

One of the accelerometers' sensor heads had been deliberately positioned next to a major facility known as the Surface Tension Driven Convection Experiment (STDCE), which had also been on USML-1. This enabled scientists to view in great detail the behaviour of fluid flows in microgravity when there were temperature differences along their interfaces. Such 'thermocapillary' flows occur in many industrial processes on Earth, and when they manifest themselves during melting or resolidification they can create defects in crystals, metals, alloys and ceramics. Gravity-driven convection overshadows the flows in ground-based tests, making them difficult to measure. High above the Earth, in a microgravity environment, on the other hand, it became possible to explore them in greater detail. Fluid physicists expected this knowledge to provide clearer insights into bubble and drop migration, which in turn could aid the development of better fuel-management and life-support systems. As with its previous mission, STDCE used lightweight silicone oil as the test fluid and a laser diode and several cameras measured its transition from a 'steady', two-dimensional flow into an 'oscillatory', three-dimensional one; similar, ground-based tests had shown periodic variations in the fluid's motion speed and temperature.

By comparing conditions for the onset of this oscillation in microgravity and on Earth, it was hoped to identify its cause. During the USML-1 experiment runs, investigators concentrated on steady-state fluid flows, with the results confirming many theoretical predictions from scientists at Case Western Reserve University in Cleveland, Ohio, although no oscillations were actually observed. For the second mission, however, the apparatus had been modified to carry three different-sized experiment containers and several viscosities of oil to create more favourable conditions for oscillation. The facility's imaging system was also upgraded to improve observations of any oscillations.

The new optics, developed by H. Philip Stahl and students from Rose Hulman Institute of Technology in Terre Haute, Indiana, provided precise images of oil surface shapes and flow patterns. ''Once we understand when and how oscillations occur,'' said STDCE Co-Investigator Yasuhiro Kamotani of Case Western, ''we should eventually be able to design processes to control them.'' On 21 October, a day into the mission, Leslie reported the onset of oscillations during the very first

STDCE run, as ground controllers watched three different views simultaneously downlinked by Spacelab's new television system.

Two days later, the astronauts drew down the volume of silicone oil to create a concave surface. As a laser gradually heated the surface, investigators identified the transitional point where oscillations began to occur in each run. ''We've never seen this kind of transition before,'' said Pline of NASA's Lewis Research Center, ''because we have no way to create a large curved liquid surface on the ground.'' After watching four STDCE runs, Principal Investigator Simon Ostrach believed that the onset of oscillations was affected by heat sources, temperature distribution on the fluid's surface and the dimensions of its container.

In subsequent runs, Coleman lowered and increased oil levels to create deeply concave, then convex, surfaces. The 'size' of the laser beam on the oil surface was also adjusted to determine its effect on the direction and nature of fluid flows, as well as lifting its temperature to introduce oscillations. This led to several never-before-seen phenomena. During one test on 28 October, investigators watched erratic flows with no apparent organisation or pattern as Thornton increased oil temperatures beyond the point at which flows began to oscillate. Later, Sacco stepped aside and let ground controllers run the facility remotely via telescience.

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