Feb 19, 1999

A low-gravity "Gift for the future"

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Scientists discuss results from the U.S. Microgravity Payload 4

Feb. 19, 1999: A peek at a "gift for the future" was presented recently as scientists discussed the results of the fourth U.S. Microgravity Payload (USMP-4) which flew Nov. 19-Dec. 4, 1997.

"Overall it was a very successful mission," said Dr. Ed Ethridge, the assistant mission scientist at NASA's Marshall Space Flight Center. Etheridge who chaired the L+1 - launch-plus-one-year - results conference to be held at NASA/Marshall. This brought the USMP-4 science team together for the last time.

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Above: In-flight video image of the payload bay during USMP-4 shows the larger experiment gear mounted on a support structure.

Anticipating the benefits from USMP-4's research, mission manager Sherwood Anderson said, as the mission ended in 1997, "My gift to my children is the research we're doing today."

USMP-4 had two principal elements, a cluster of four large experiment devices mounted on an exposed platform in the payload bay, and a separate series of experiments conducted by the crew in a small glovebox mounted in the shuttle middeck (see box, below).

While the bulk of the results will be discussed at the review, a couple of previews are available from inside the Shuttle and out in the payload bay.

"We got some very interesting results," said Dr. Doru Stefanescu of the University of Alabama in Tuscaloosa. "We were able to prove our hypothesis." Stefanescu is principal investigator for the Particle Engulfment and Pushing (PEP) experiment, which the astronauts conducted in the glovebox.

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Particle pushing occurs for example, when ice forms and pushes small objects out of the way. Thus, we notice the pavement cracking because ice pushes sand out (frost heaving). Particle engulfment, however, occurs in metals where the solidifying alloy incorporates ceramic particles (such as oxides or carbides) to make a new composite material. The fundamental physics involved is important to a range of industrial processes for making composites and other materials.

In PEP, Stefanescu and his team used transparent liquids that mimic metals in their behavior, and glass or plastic beads to mimic ceramics. As with all the USMP-4 experiments, conducting PEP in space eliminated certain effects due to gravity so scientists can see more clearly how materials behave and form.

"We saw a lower critical velocity in space as compared to Earth," he said, referring to the rate at which the freezing front in the liquid will overtake rather than push solid particles.

Photograph from a ground test of PEP shows how particles can be engulfed or pushed by a solidification front.

One model, using succinonitrile and polystyrene beads, matched nicely with preflight theories. The other, using biphenyl and glass beads, did not fit as well and requires more analysis to understand what was happening.

"Our main goal is science," Stefanescu said, "to understand the interaction between a particle and the solid-liquid interface. Part Two is, once we understand an effect, to control it in a process developed on earth."

Meanwhile, the team that ran the most picturesque science - the Isothermal Dendrite Growth Experiment (IDGE) - found vindication for their design, and a big surprise in their data.

"From our perspective, it was a rip-roaring success," said Dr. Martin Glicksman, the principal investigator at Rensselaer Polytechnic Institute in Troy, N.Y.

Web Links
A Christmas present for the 21st century. Dec. 4, 1997. Wrapup story on USMP-4.

USMP-4 daily science updates. A listing of all the science stories from USMP-4. Background information and tutorials about microgravity
"We cleared up an issue that was bothering us before the flight," Glicksman said. "Another group said that pivalic acid was not a good analog for face-centered cubic crystals like aluminum, silver, and gold." Pivalic acid is a transparent liquid. When cooled, it grows crystals shaped like tree limbs - dendrites (below left) - that mimic the growth of crystals in molten metals. Again, studying such models helps scientists understand the behavior of other materials.

If the critics were right about pivalic acid not being a good model, Glicksman said, then the team would have seen the crystals growing at different rates because of an effect called kinetic hindrance.

"We were able to supercool [cool the liquid below its normal freezing point] the sample to 1.25 deg. C and saw the crystals growing at 700 to 800 microns (more than 1/30th inch) a second," he continued. "That fell right on the transport line we had predicted. We knew right away that this kinetic hindrance effect was not true."

Left: Scanning electron microscope image of dendrites shows their tree-like shape and hints at how they interlock when all of a melt crystallizes.

The pivalic acid also had a surprise in store. Detailed examination of 35mm photographs of the crystal tips showed they are not parabolas (an open curve), but something closer to a hyperbola (another open curve that follows a different formula).

"This is still puzzling us as to how to analyze these shapes because they don't fit," Glicksman said. "We were surprised by this a bit."

More surprises may lie in IDGE's other treasure trove, some 16 videotapes of 117 experiments that were downlinked from the shuttle. Even though the tapes were available right away, the team has not played them back yet.

Right: This animated GIF shows dendrites growing during an early IDGE experiment.

"When you play and then replay, you run the risk of stretching the tape and introducing errors into the speed measurements," Glicksman explained. The IDGE team recently acquired a special tape digitizing system with a 100 gigabyte hard drive that will let them dub a complete tape onto a disk and then onto CD-ROMs for distribution and repeated playback.

"That's going to be a real gold mine," Glicksman said. "We expect to find out a lot of neat stuff dealing with the dynamic of dendrites."

Better than most presents, science never stops giving. You're always unwrapping the gift.

Payload bay

Isothermal Dendrite Growth Experiment (IDGE) grew dendritic (treelike) crystals under the gaze of video and film cameras to provide valuable data on how crystals form within metals, and to demonstrate new remote-control operations - telescience - that will be important to International Space Station.
Advanced Automated Directional Solidification Furnace (AADSF) melted and resolidified infrared detector crystals in a study of the best condition for growing sensitive electronics in space.
Confined Helium Experiment (CHeX) used sheets of liquid helium between silicon disks to simulate 2-dimensional conditions that are expected to control the design of ultrasmall electronics in a few years.
Materials for the Study of Interesting Phenomena of Solidification on Earth and In Orbit Experiment (MEPHISTO) melted and resolidified metal samples as electrical currents were passed through to measure growth rates.
Space Acceleration Measurement System (SAMS) and the Orbital Acceleration Research Experiment (OARE) monitored vibrations and motions throughout the mission.

Middeck Glovebox

Enclosed Laminar Flame (ELF) experiment studied flame growth and stability. Results will help in understanding and controlling flames in jet engines.
Particle Engulfment and Pushing (PEP) experiment looks for the critical velocity where an object is no longer pushed by a freeze front but becomes engulfed. This helps determine whether a material is incorporated in a new mixture or pushed out.
Wetting Characteristics of Immiscibles (WCI) may answer questions about why experiments that should have produced even dispersions of one material in another (like oil droplets flash frozen in water) instead produced a single mass of one material coated by another.

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Author: Dave Dooling
Curator: Linda Porter
NASA Official: Gregory S. Wilson