The purpose of this project is to develop a new class of reaction/momentum wheels for spacecraft applications. This innovation involves the use of retainerless (i. e., no cage) bearings and unidirectional composite wheels. Most anomalies with conventional wheels are associated with the bearing retainer. These include: retainer squeal (high frequency vibrations), instability, pocket wear, retainer fractures and retainer lubricant impregnation problems. Metal rim masses are used to produce the desired momentum characteristics for conventional wheels. New wheel designs operating at higher speeds will exceed the strength characteristics of the metal systems. Composite materials can be substituted to insure proper strength. Incorporation of the innovations should improve reliability, increase longevity, reduce cost, reduce weight, size and complexity.
Moving mechanical assemblies on spacecraft require reliable lubrication for long term durability. In particular, ball bearings that support scanning instruments, momentum wheels, filter wheels, solar array drives and gyroscopes are lubricated with liquids and greases. These lubricated mechanisms have been identified as life-limiting components on multi-year missions. Traditionally, the components have been cleaned by Freon 113 prior to lubrication. The Montreal protocol mandated the termination of Freon 113 use, thus requiring the development of a new series of non-ODC cleaning techniques. There is a concern that some of these new techniques may reduce bearing lifetimes compared to those qualified using standard Freon cleaning. The objective of this program is to determine which techniques produce adequate cleanliness with no adverse effects on mechanism lifetimes.
During the next 11 years, over 100 spacecraft launches are being planned. All of these spacecraft, including the space station, will utilize hundreds of mechanisms, including momentum wheels, control moment gyros, solar array drives, scanners, gimbals, slip rings, etc., all of which require lubricants. Historically, every project selects a number of lubricants based on heritage, physical and chemical properties. Unfortunately, many of the selections are not ideal for the various applications because most are complex mixtures of many components. This leads to compromises in performance. For example, applications with wide temperature ranges, may be too viscous at low temperatures, but too volatile at high temperature. In addition, most common lubricants used today, PFPEs (perfluoropolyethers) are inherently unstable under some conditions, resulting in premature failures. In contrast, tri (CxHySi3) and penta (CxHySi5) silahydrocarbons represent a unique class of unimolecular lubricants that can be tailored for all applications. In addition, these materials readily solubilize conventional additives, unlike the PFPEs. Successful development of the silahydrocarbon class of lubricants would greatly simplify lubricant selection, thus reducing costs and schedules while providing a superior lubricant tailored for each application.
Perfluoropolyethers (PFPE) are widely employed as lubricants for space applications because of their excellent thermal and chemical stability and are particularly effective in the elastohydrodynamic range. In addition, this class is a candidate for advanced gas turbine engine applications. However, when used as a boundary lubricant, PFPE performance is less satisfactory. The most significant problem encountered in the boundary lubrication regime is the unavailability of soluble additives. In high temperature applications, certain alloys catalyze the decomposition of these materials. Successful development of soluble antiwear additives and degradation inhibitors would allow this unique class of materials to be used extensively in space and in advanced aircraft.
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. . . Last updated April 11, 2001 -fsh