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The following is the first in a series of white papers which will be issued as a result of a task to define and qualify space grade fiber optic cable assemblies. Though to qualify and use a fiber optic cable in space requires treatment of the cable assembly as a system, it is very important to understand the design and behavior of its parts. These papers will address that need, providing information and “lessons learned” that are being collected in the process of procuring, testing and specifying the final assemblies. This installment covers information on optical fiber, coatings, cable components, design guidelines and limitations, radiation and reliability.

Spacecraft radiators reject heat to their surroundings. Radiators can be deployable or mounted on the body of the spacecraft. NASA's Crew Exploration Vehicle is to use body mounted radiators. Coatings play an important role in heat rejection. The coatings provide the radiator surface with the desired optical properties of low solar absorptance and high infrared emittance. These specialized surfaces are applied to the radiator panel in a number of ways, including conventional spraying, plasma spraying, or as an appliqué. Not specifically designed for a weathering environment, little is known about the durability of conventional paints, coatings, and appliqués upon exposure to weathering and subsequent exposure to solar wind and ultraviolet radiation exposure. In addition to maintaining their desired optical properties, the coatings must also continue to adhere to the underlying radiator panel.

On Day 97, 2005, a temperature excursion of the Burst Alert Telescope (BAT) loop heat pipe (LHP) #1 compensation chamber (CC) caused this LHP shut down. It had no impact on the Gamma Ray Burst (GRB) detection because LHP #0 was nominal. After LHP #1 was started up and its primary heat controller was disabled on Day 98, both LHPs have been nominal. On Day 337, 2004, the X-Ray Telescope (XRT) thermo-electric cooler (TEC) power supply (PS) suffered a single point failure. The charge-coupled device (CCD) has been cooled by the radiator passively to -50°C or colder most of the time. The CCD temperature meets the main objective of pinpointing GRB afterglow positions. With these anomalies overcome, the Instrument Module (IM) thermal control system (TCS) is nominal during the first 2.5 years in flight.

Following the satellite-level thermal vacuum test for the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation spacecraft, project thermal engineers determined that the radiator used to cool the Integrated Lidar Transmitter subsystem during its operation was oversized. In addition, the thermal team also determined that the operational heaters were undersized, thus creating two related problems. Without the benefit of an additional thermal vacuum test, it was necessary to develop and prove by analysis a laser temperature control scheme using the available resources within the spacecraft along with proper resizing of the radiator. A resizing methodology and new laser temperature control scheme were devised that allowed, with a minimum of 20% heater power margin, the operating laser to maintain temperature at the preferable set point. This control scheme provided a solution to a critical project problem.

The Burst Alert Telescope (BAT) instrument of the Swift mission consists of a telescope assembly, a Power Converter Box (PCB), and a pair of Image Processor Electronics (IPE) boxes (a primary and a redundant). The telescope assembly Detector Array thermal control system includes eight constant conductance heat pipes (CCHPs), two loop heat pipes (LHPs), a radiator that has AZ-Tek's AZW-LA-II low solar absorptance white paint, and precision heater controllers that have adjustable set points in flight. The PCB and IPEs have Z93P white paint radiators. Swift was successfully launched into orbit on November 20, 2004. This paper presents a thermal assessment of the BAT instrument thermal control system during the first six months in flight.

In this paper, experimental work performed on a breadboard Loop Heat Pipe (LHP) is presented. The test article was built by DCI for the Geoscience Laser Altimeter System (GLAS) instrument on the ICESat spacecraft. The thermal system requirements of GLAS have shown that ammonia cannot be used as the working fluid in this LHP because GLAS radiators could cool to well below the freezing point of ammonia. As a result, propylene was proposed as an alternative LHP working fluid since it has a lower freezing point than ammonia. Both working fluids were tested in the same LHP following a similar test plan in ambient conditions. The thermal performance characteristics of ammonia and propylene LHP's were then compared. In general, the propylene LHP required slightly less startup superheat and less control heater power than the ammonia LHP. The thermal conductance values for the propylene LHP were also lower than the ammonia LHP. Later, the propylene LHP was tested in a thermal vacuum chamber.

The thermal design and modeling of NASA’s James Webb Space Telescope (JWST) Integrated Science Instrument Module (ISIM) is described. The ISIM utilizes a series of large radiators to passively cool its three near-infrared instruments to below 37 Kelvin. A single mid-infrared instrument is further cooled to below 7 Kelvin via stored solid Hydrogen (SH2). These complex cooling requirements, combined with the JWST concept of a large deployed aperture optical telescope, also passively cooled to below 50 Kelvin, makes JWST one of the most unique and thermally challenging space missions flown to date. Currently in the preliminary design stage and scheduled for launch in 2010, NASA’s JWST is expected to replace the Hubble Space Telescope as the premier space based astronomical observatory.