Oast A Technology Testbed

Operations with the $30-million OAST-2 payload commenced immediately after USMP-2 deactivation and followed an earlier mission flown on Space Shuttle Discovery in the summer of 1984. Since that flight, the NASA sponsor had been renamed the 'Office of Advanced Concepts and Technology', but nevertheless OAST-2 retained its original designation. It comprised six experiments mounted on a cross-bay Hitchhiker bridge structure and was specifically detailed to conduct advanced-technology experiments - including evaluations of new-specification solar cells and energy-storage systems - and investigate plasma and atomic oxygen interactions with the Shuttle.

All but one of those experiments - the Thermal Energy Storage (TES) - were controlled directly from an avionics unit attached to the Hitchhiker bridge. TES, which was designed to provide new data about the behaviour of thermal energy storage salts in microgravity, was operated by one of the crew members via a laptop computer on Columbia's aft flight deck. As NASA moved into high gear in its plans to develop and build the International Space Station, it was realised that a low-mass, low-cost and high-efficiency electrical power generation and storage system would become increasingly necessary.

The TES experiment consisted of two GAS canisters affixed to the Hitchhiker bridge, one of which contained a 'salt' of lithium fluoride and the other of lithium fluoride and calcium difluoride eutectic. After activation by Mission Specialist Pierre Thuot early on 5 March, TES collected and stored solar energy which was converted into electricity while in Earth's shadow. As the salts in the GAS canisters absorbed thermal energy, they slowly melted and expanded by up to 30%. Then, when cooled, they solidified and shrank, creating 'voids' in the salts which affected their heat-absorption rates.

It was expected that research conducted by TES would assist with the development of future solar dynamic power receivers being considered for the space station. Typically, the experiment underwent a five-hour 'heat-up' period, 10 hours for completing four separate thermal 'cycles' and a lengthy period of cooling. After Columbia's return to Earth, CAT scans were taken of the canisters to provide data on the sizes and distribution of the voids in the salts. Overall, the experiment was highly successful and yielded the first long-duration, high-temperature melting-and-freezing tests with lithium-based heat-storage salts.

Another OAST-2 investigation with its eyes firmly on the future International Space Station was the Solar Array Module Plasma Interaction Experiment (SAMPIE) which exposed several samples of solar array material to the harsh environment of low-Earth orbit to evaluate their performance and degradation over time. Although there is no 'atmosphere', in the normal sense, in the region of space from which the station would operate, there is still the ionosphere high above Earth comprising a plasma of widely spaced ionised atoms of oxygen and nitrogen. These are known to have potentially damaging effects on high-voltage spacecraft surfaces.

Conducting surfaces, whose electrical potential is highly negative with regard to this plasma, are prone to 'arcing' that severely damages solar cells and causes major electromagnetic interference. Intermittent power loss can also be a problem. Before STS-62, engineers had used ground-based plasma chambers to simulate temperatures and conditions in low-Earth orbit, but were unable to replicate them accurately because of differences in pressure, plasma flow, electron temperature and ion species. During the mission, SAMPIE exposed four different types of solar cells to the space environment, with Columbia's payload bay alternately facing the direction of travel (the 'ram side') and away (the 'wake side').

Generating and storing heat energy, and then routeing it to different parts of a spacecraft, were also expected to prove useful as space station construction got underway. The Cryogenic Two Phase (CRYOTP) experiment evaluated the performance of a nitrogen 'heat pipe', the techniques for which had already been tested on several previous Shuttle missions, in cooling electronic components and sensors. Heat pipes are essentially fluid-filled closed tubes which absorb heat at one 'end' and evaporate the liquid into a vapour. This condenses and moves towards the other end of the pipe, from which the heat is released. The condensate then returns to the heated end of the pipe by capillary forces formed in a porous 'wick' and the cycle is repeated. On STS-62, a device called the Brilliant Eyes Thermal Storage Unit (BETSU) took the procedure a step further by absorbing thermal energy from a heater using a 'phase-change' material known as methyl-pentane in a pair of cryo-cooler devices. After activation, the devices cooled the material down to 120 Kelvin and the heater was switched on to melt it. Several melting and freezing cycles were run throughout the mission to better quantify ground-based models.

Shielding astronauts from the harsh radiation environment of low-Earth orbit would also be essential for station operations and another OAST-2 experiment, known as Emulsion Chamber Technology (ECT), sought to collect data on its impact on a series of photographic plates. Highly sensitive X-ray films were interleaved with sheets of lead and the 'tracks' left in them by radiation particles were examined after the mission. It was hoped that these particle patterns would yield more information about how to safeguard spacecraft from such radiation. Additional objectives included testing the film's sensitivity against deterioration caused by heat, mechanical vibrations and unwanted 'background' radiation.

Not only radiation, but also particles of atomic oxygen and nitrogen in the near-Earth environment, had long been known to potentially have an adverse influence on the Shuttle itself as they ram into it at high velocity. Typically, the impacts of these particles create strange auras of light around the leading or front-facing edges of orbiting spacecraft and can interfere with measurements taken by sensitive optical or other scientific instruments. Two experiments on the OAST-2 payload - the Experimental Investigation of Spacecraft Glow (EISG) and the Spacecraft Kinetic Infrared Test (SKIRT) - were designed to explore these impacts in greater detail.

A set of pressurised nitrogen gas canisters were mounted beneath the sample plate on EISG and used to produce ionising atoms that generated artificial 'spacecraft glow' around Columbia's OMS pods and vertical stabiliser fin. Far-ultraviolet and visible imaging spectrometers then gathered data on a series of samples coated with paints of differing thermal qualities. Infrared sensors also monitored the samples' behaviour. To conduct the experiment, Casper and Allen manoeuvred the Shuttle into seven different elliptical orbits and four circular orbits, one of which took the crew on 16 March to their lowest-yet altitude of just 170 km.

The lowering of the Shuttle's orbit in this way increased the yield of glow-related data. Glow measurements were usually taken during 'nighttime' portions of Columbia's orbit for periods up to about 30 minutes at a time, yielding three-and-a-half hours' worth of data altogether. Additional measurements were taken during scheduled thruster firings and also from Earth's own atmospheric 'airglow'. Meanwhile, the adjoining SKIRT experiment complemented EISG with an infrared-imaging capability. During the course of the mission, the astronauts were

The strange 'Shuttle glow' phenomena envelopes Columbia's OMS pods and tail fin in this view from STS-62.

required to perform six manoeuvres: four involved nose-down, 360-degree 'rolls' of Columbia and two used the Moon for calibration purposes.

Overall, the experiments were highly successful, although a problem surfaced with EISG's far-ultraviolet spectrometer, which had gathered some good spectral data on 11 March but its capabilities later degraded. Ground-based scientists suspected that it was either operating at a lower gain or was partially blocked. Mission Specialist Marsha Ivins used Columbia's RMS camera at one stage to look at the device as part of troubleshooting efforts. Nevertheless, in general, operations with the two glow experiments ran relatively smoothly and Thuot remarked that the phenomenon was much more pronounced from the lower orbit than when the crew was at a higher altitude.

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