Combustion Science Experiments

Within four hours, Janice Voss and Roger Crouch entered the once-again-pristine Spacelab module to begin what they hoped would be 16 days of around-the-clock research. "It's good to be back,'' Voss radioed to Mission Control, as Crouch cartwheeled and backflipped behind her in the voluminous lab. Among the most important unfinished business from the April flight was the completion of 144 scheduled tests in the CM-1; in the event, more than 200 were actually conducted during STS-94.

Among the payloads getting a reflight was SOFBALL, whose off-the-shelf gas chromatograph performed flawlessly throughout the mission, successfully verifying the composition of a variety of premixed gases prior to combustion and determining the remaining reactant and other combustion products. Unfortunately, in light of the constraints imposed upon the chromatograph by the CM-1 hardware, it was not possible to measure the gases to the required accuracy. Improvements were made to the unit after Columbia's landing and another flight of the hardware would later be pencilled-in for a subsequent mission.

Nonetheless, SOFBALL's Principal Investigator, Paul Ronney, was overjoyed with the results, which included the weakest flames ever burned. "Ecstatic would be an understatement," he said as the mission approached its midpoint. "The results so far are just beyond our wildest dreams. We're getting more data than we know what to do with!" The science crew mixed a variety of gases, including tiny quantities of hydrogen and oxygen, which - although too small to be flammable on Earth -burned for more than eight minutes in CM-1. "We saw some flame shapes that nobody had seen before,'' Crouch said later in the mission.

During the inaugural SOFBALL run on 8 July, supervised by Linteris, the fuel mixture was so weak that it only produced what researcher Karen Weiland referred to as "flame kernels''. A later test, using a richer mixture of hydrogen, oxygen and sulphur hexafluoride, proved much more successful, burning for 500 seconds and allowing investigators to gain insights into how different concentrations of fuel and oxidiser affected the flameballs' stability and existence. "In pre-mixed gases used for combustion on Earth, we simply don't understand the mechanisms of flame extinction, what stabilises it or what keeps it going,'' said Paul Ronney.

Janice Voss works with the Combustion Module.

''These [SOFBALL] stationary spherical flameballs are the simplest to study, learn from and then apply to Earth applications.'' Such knowledge could, he added, help engineers to develop improved fire safety equipment for mineshafts, chemical plants or on board future spacecraft. Many of the flames produced during MSL-1 were so weak that they equalled only one watt of energy, compared to approximately 50 watts normally produced by a single candle on a birthday cake.

The peculiar sense of dejai vu was not lost on Columbia's crew. ''This is the first chance to refly a payload and a crew altogether in the same group so quickly,'' said Voss during a space-to-ground news conference on 3 July. ''It's been much easier to get back into the swing of things, and all the experiments are going great, and we all feel extremely comfortable and well-prepared because we've done this so recently.'' However, she added that communication between the two shifts (Red and Blue) was difficult at times. ''There's a lot of issues, like where everything is stowed, and people always find their favourite places. On a single-shift flight, where we all sleep at the same time, it's a little bit easier to negotiate, because everyone's awake and you can ask them where they put something. But on a dual-shift flight you have to be very careful to work together as a team across those few hours when you're both awake.'' Watching the conduct of the MSL-1 experiments from the ground, LSP Principal Investigator Gerard Faeth expressed surprise as he saw weightless flames almost twice the size of any produced on Earth. ''It does highlight the importance of us finding out something about what fires are like in spacecraft environments,'' he told journalists. Faeth's investigation of soot behaviour, it was hoped, could lead to a better understanding of how it forms and perhaps aid efforts to more effectively tackle forest fires and develop 'cleaner'-burning fuels.

''We have a lot of breakthroughs we're working on here,'' Don Thomas explained in an interview on 13 July, adding that ''it might be a few years before they find their way into everyday life down on Earth, but this is basic research and we're pioneering out here''. Nevertheless, Thomas' work with SOFBALL received the distinction of having a newly discovered combustion 'effect' named after him! One surprising finding from the experiment involved what happened when two small fuel droplets burn in close proximity: they initially moved away, then approached each other in a phenomenon dubbed the 'Thomas Twin Effect'.

Meanwhile, Faeth's LSP experiment also progressed well, using ethylene gas as part of research into more environmentally friendly, fuel-burning engines. Already, the April mission had given him an important 'first look' at how soot particles formed in microgravity and the shortened flight enabled his team to enhance LSP for STS-94. ''We've learned that as we increase pressure, the amount of soot in the flame and the amount of soot it emits increases,'' he said. ''It's probably one of the reasons that high-pressure combustion processes - such as what goes on in a bus engine -tend to emit a lot of soot.''

One LSP run on 3 July involved a propane-fuelled study of soot, producing a ''beautiful and steady flame'', according to Linteris. ''We will use the first few experiment runs to set parameters for the remaining runs,'' said Faeth. ''Determining the settings for future runs will improve the efficiency of the experiment operations, which are designed to determine under what conditions soot is produced by flames and what the composition of soot is.'' The benefit of flying for 16 days, said Voss, was that ''you get faster. By the end of the flight, we were getting very efficient running these experiments.''

As well as using different fuels - propane, then ethylene - the experiment also burned them under different atmospheric pressures, ''because soot is very sensitive to pressure'', explained Faeth. ''The higher the pressure, the more soot produced. Different fuel types also make a big difference. Natural gas tends to make little soot, propane produces more soot and ethylene, used in diesel engines, produces even more.'' By the time LSP operations concluded, the experiment had conducted 17 tests ''and yielded good data'', according to Faeth.

Other, related research focused once again on Vedha Nayagam's Droplet Combustion Experiment (DCE), which involved several 'phases' of observations of the burning characteristics of heptane fuel droplets under a range of atmospheric pressures. ''In each phase,'' Nayagam said, ''we are keeping the pressure the same and slowly reducing the oxygen to see if the fuels can still burn and, if so, how they burn. On Earth, we encounter low combustion scenarios - for instance, in gas turbines - so it's important to know what happens when pressure is reduced.''

The three 'phases' were 'normal' atmospheric pressure - the same as that found at sea-level on Earth - as well as half, then one-quarter, of Earth's atmospheric pressure. Characterising the results from the experiment, Principal Investigator Forman Williams told journalists that ''the crew had a tougher time igniting the droplets at this lowered pressure. We expected that. But when the fuel droplets did ignite, they burned stronger and more vigorously than we expected.''

''In one atmospheric pressure, the flame burned out, leaving a residue of fuel,'' added Nayagam. ''At the lower pressure, the flame is larger so the same results were expected, but the flame collapsed back on the droplet, completely consuming it. This tells us something about the extinction mechanisms.'' Typically, fuel droplets were formed by squirting heptane through a pair of injectors on opposing sides of a test platform within the chamber. When the drop formed, the injectors were retracted and the fuel was sparked by two hot-wire igniters; the entire procedure was carefully recorded by video cameras and high-resolution photography.

On the whole, the DCE apparatus performed admirably, with the minor exception of three brief malfunctions of the computer responsible for overseeing all of the experiments on board the Spacelab module; it was quickly rebooted and normal operations resumed. This prompted Assistant Mission Scientist Patton Downey to praise ''great teamwork by the crew and science teams on the ground [which] has worked around some anomalies, enabling us to collect very valuable data''. Another problem with one of the experiment's imaging cameras was soon resolved.

By the time the DCE hardware was finally shut down by Voss and its control computer removed on the evening of 14 July, heptane droplets ranging in diameter from 2 to 4 mm were successfully burned. ''The large droplet extinguished early in its burn time, due to loss of significant combustion energy by radiation,'' said Fred Dryer. ''The small droplet burned to near-completion because less energy is lost by radiation as the droplet's initial diameter is decreased. We would have received opposite results if we'd conducted this same experiment on Earth.''

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