Friction Measurements on a Low Earth Satellite

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Abstract
The coefficient of sliding friction for a number of materials was measured during the flight of Ranger 1 spacecraft. Flat disks of materials of interest were rotated at a speed of 8–14 inches per minute while in contact with 1/8-inch diameter hemispherical riders. Because of the low orbit achieved by Ranger 1, the experiment was exposed to vacuum in the range of 3 × 10−6 to 8 × 10−9 mm Hg. For unlubricated metals sliding on metals, the friction coefficient averaged about 0.5; for some combinations of metals, it occasionally exceeded 1.0. Lower values were observed with lubricants of grease or gold-plate and for ceramics sliding against metals. The coefficient of friction was very low, averaging 0.04, for metallic pairs lubricated with molybdenum disulfide and for polytetrafluoroethylene sliding against metals and ceramics. Relatively low friction coefficients were found for metallic materials sliding against unlubricated metallic and ceramic materials when at least one member of the pair was of high hardness. The coefficients observed for unlubricated metal pairs were not inconsistent with the hypothesis that high friction tends to correlate with high mutual solid solubility. In general the coefficients in space and in a laboratory vacuum of 5 × 10−6 mm Hg were not systematically different. For unlubricated metallic materials, friction in vacuum was higher than in air at shorter running times.

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