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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 Next Generation Space Telescope (NGST) design requires a large sunshield to passively cool the telescope and detectors to temperatures in the 60° to 100° Kelvin range. The government yardstick design for the NGST observatory has baselined an inflatable sunshield. The NGST project plans to fly a one-third-scale sunshield during a Shuttle flight in late 2000. The Inflatable Sunshield in Space (ISIS) experiment will demonstrate stable deployment of a large, multilayer thin film sunshield and ridigization of inflatable struts. A new method of modeling large membrane systems will be developed, and data will be obtained in order to validate the model. The flight experiment will also demonstrate the viability of the thermal approach by verifying separation and flatness of membrane layers.

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.

The configuration and thermal analyses of NASA’s Next Generation Space Telescope (NGST) Yardstick concept utilizing a novel sunshield approach for passive cooling is described. The NGST mission concept of a large aperture optical telescope passively cooled to less than 40 K and instrument detectors passively cooled to below 30 K is unique from any other mission flown to date. Achieving such a low operational temperature requires reducing by a factor of several thousand the internal heat dissipation and environmental heating of the telescope. The techniques for achieving these requirements, i.e. orbit selection, configuration, etc., along with the supporting thermal analyses are described.