Usmp4 A Great Mission

The two spacewalks and the Spartan fiasco had so far distracted many from the highly prized research that was going on elsewhere on a battery of experiments that formed the fourth United States Microgravity Payload (USMP-4), which was flying its final mission. The payload was activated, under Chawla's supervision, late on 19 November. Despite having to make adjustments to their science-gathering schedules to accommodate the initial one-day postponement of Spartan, Assistant Mission Manager Jimmie Johnson told journalists on 20 November that from preliminary results, ''it looks like we're going to have a great mission''.

Three of the USMP-4 experiments were already familiar from earlier flights: the

AADSF and MEPHISTO solidification furnaces and the IDGE dendritic-growth facility. Another, the Confined Helium Experiment (CHeX), was making its first sojourn into space to test theories of the influences of boundaries on matter by measuring the heat capacity of helium while confined to two dimensions. A clearer understanding of the effects of 'miniaturisation' on material properties was expected to lead to smaller and more efficient and cheaper electronic devices, including highspeed computers.

Almost daily, CHeX's Principal Investigator John Lipa pointed out, the semiconductor industry reduces the size of devices such as computer chips in a bid to improve transmission speeds and power consumption and reduce costs. However, electrical performance is also affected by size, thickness, surface irregularities and changes in a given material's physical properties. As these changes take effect, they often result in defects. Lipa's team from Stanford University sought to discover what caused the changes, using a sample of liquid helium, which conducts heat a thousand times more efficiently than any other material.

''We are studying the novel properties of matter when it becomes very thin,'' said co-investigator Talso Chui of NASA's Jet Propulsion Laboratory. ''Hopefully, we'll learn some secrets of nature.''

CHeX consisted of a refrigerated dewar, coated with a magnetic shield and holding 392 crystal silicon disks, each just two-thousandths-of-an-inch thick, which forced the liquid helium into very thin layers. Through these layers, Lipa's team took precise temperature measurements of within a tenth of a billionth of a Kelvin. ''The temperature resolution'', said USMP-4 Mission Scientist Peter Curreri, ''is equivalent to the ability to measure the distance between Los Angeles and New York to the thickness of a fingernail! The information from this experiment can be applied to future generations of microprocessors, where we'll approach the finite size limit.''

It was a pity that the media focused so intently on the Spartan fiasco, because the USMP-4 investigators were quietly gathering their own treasure trove of scientific data. The AADSF processed two different alloys - lead-tin telluride and mercury-cadmium telluride which could form the basis of advanced infrared detectors or lasers. ''We completed processing one sample in both the low-temperature and high-temperature modes of operation,'' said Archie Fripp of NASA's Langley Research Center, one of AADSF's principal investigators. ''The low-temperature sample looked at thermal convection, or flows, and the high-temperature sample studied the changes caused by manipulating the growth rate of the crystal.''

Unfortunately, operations with the furnace ended earlier than planned on 30 November, when the science team noted unexpected readings from several temperature sensors used to control the solidification process. This prevented investigators from completing the second growth of three lead-tin telluride samples. ''Prior to this mission, the furnace was modified to allow the exchange of samples,'' said Johnson. ''Although we were unable to complete the third run, the two we've completed will potentially yield more science from this mission than the previous two combined.''

In particular, a single, unique electrical crystal with exceptional uniformity of composition was grown. ''We could only get this uniformity in the microgravity of space,'' said Marshall investigator Don Gillies. Added lead researcher Sandor Lehoczky: ''One of our objectives on this flight was to get benchmarks: near-perfect materials that can be used to compare with and judge materials made on Earth.'' After Columbia's landing, the crystals were polished and etched in the hope that they could be used to detect previously undetectable infrared energy levels. ''We're working on the materials for the future,'' said Marshall's Dale Watring.

Meanwhile, the French-built MEPHISTO furnace processed three identical bismuth-tin alloy samples, while sensors monitored temperature fluctuations and the position of the solid-to-liquid border, 'marking' them with electric pulses. Its data was correlated with measurements from the SAMS device to determine the impact of Shuttle motions on the solid-to-liquid interface. Once again, the furnace performed like a champ. ''We are very happy with the MEPHISTO hardware,'' said Gerard Cambon of the French space agency, CNES. ''The French team has participated in this experiment for the last 18 months and we have worked in a cooperative effort to enhance the science gains.''

On its two previous flights, the IDGE device had used ultra-pure succinonitrile (SCN), an organic crystal that mimicked the solidification of so-called 'body-centred-cubic' materials, such as ferrous metals. For USMP-4, however, it employed a different test material: pivalic acid (PVA), a 'face-centred-cubic' material that solidified like many non-ferrous metals. Its transparent nature and low melting point made it an ideal sample for fundamental 'benchmark' observations. According to IDGE Principal Investigator Marty Glicksman on 20 November, the information gathered ''is providing extremely valuable insight[s] into the solidification behaviour of [PVA]. It will doubtless lead to new scientific findings about dendrite-growth dynamics.''

Whether melting and resolidifying bismuth-tin, lead-tin telluride or mercury-cadmium telluride, monitoring the heat characteristics of liquid helium or growing PVA dendrites, the presence of the two acceleration monitors - SAMS and the Orbital Acceleration Research Experiment (OARE) - helped to identify the impact of vibrations and tremors, some of which could be caused by crew movements on their middeck ergometer. ''If we can identify what's causing a disturbance to an experiment,'' said Melissa Rogers of NASA's Lewis Research Center, ''a science team can then decide whether to ask for that interfering fan, pump or whatever to be turned off.''

Still other USMP-4 investigations were conducted in Columbia's cabin, using the versatile Middeck Glovebox facility, which Chawla used to investigate ways of creating uniform mixtures of liquids to form specific metal alloys, for possible use as a basis of future semiconductors. Liquids that do not mix well - called 'immiscibles' - were deliberately chosen as part of this experiment to pinpoint the causes of their separation. Another study, provided by Doru Stefanescu of the University of Alabama at Tuscaloosa, examined the solidification of liquid-metal alloys. Lastly, a combustion experiment to investigate laminar gas flows was carried out.

Such flows are a key phenomenon in combustion processes, such as those used to power aircraft engines, natural-gas power plants and modern ship. The MGBX experiment was expected to provide clues of why jet engines occasionally 'flame-out'

when fuel injected into a moving airflow moves out of the combustion chamber and extinguishes. ''We hypothesised that flames would remain stable at a higher forced airflow in the microgravity environment, and that's exactly what we saw,'' said Principal Investigator Dennis Stocker of NASA's Lewis Research Center.

''This research will have a direct bearing on engine safety and furnace efficiency,'' added researcher Lea-Der Chen of the University of Iowa. ''This data will also allow us to critically test our computer simulations by comparing them to the data we received from the experiment.''

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