An advanced docking and berthing system (ADBS) is being developed by the NASA Johnson Space Center to support future missions in NASA’s vision for space exploration. This mechanism would serve as an interface between pressurized manned and autonomous modules to join space vehicles and structures. The ADBS will improve the functionality of mechanisms currently in use for the human exploration of space.
All docking and berthing mechanisms currently in use, including the common berthing mechanism used on the International Space Station, are composed of two nonidentical halves, which limit their functionality to mating only with structures having the opposite configuration interface. The ADBS, however, is an androgynous system, such that each system half is an identical replicate of any other, and any two vehicles or modules each having an ADBS can mate with the other without regard to gender. In addition, the ADBS system is designed to perform docking and berthing types of mating, such that both manned and unmanned autonomous vehicles and structures can be joined.
The androgynous nature of the ADBS, however, creates challenges for the sealing interface between the two pressurized modules. Since each system half is an exact replicate of its mating counterpart, the gas seals must seal against each other instead of against a conventional flat surface. The NASA Glenn Research Center is supporting this project by developing the gas seals for the ADBS system.
The environmental operating temperatures to which the ADBS will be exposed are expected to be -50 to 50 °C (-58 to 122 °F). Therefore, the elastomeric compound from which the seal is manufactured must remain compliant at temperatures below the minimum operating temperature of -50 °C (-58 °F). Since silicone rubber is the only class of elastomer that encompasses that temperature range and is commonly molded into seals, this study focused on three elastomers within the silicone class. An additional requirement is that the seal material be classified as a low-outgassing compound, per ASTM E595, having a total mass loss less than 1.0 percent and collected volatile condensable materials less than 0.1 percent.
The unconventional interaction between the gas seals makes the adhesion between two similar elastomers very important. If the adhesion forces between the seals are too great, the seals will not allow the space structures and vehicles to separate from one another without dislodging one of the two systems’ seals. The removal of one of the seals would render the attachment point useless and must be avoided, so Glenn researchers used an adhesion test to quantify these adhesion forces (see the graph).
Effect of contact period on elastomer-to-elastomer adhesion for three silicone elastomers.
Long description of figure 1.
The ADBS is also designed for a service life of 5 to 10 years. During its life-span, the system is expected to undergo many mating-demating cycles, while being continuously exposed to the space environment. Since the seals would be compressed during each mating cycle and released during each vehicle separation, the ability of the seal’s elastomer compound to retain its elastic properties after prolonged compression is important. Glenn researchers used a compression set test to quantify this ability (see the bar chart).
Compression set results for three silicone elastomers.
Long description of figure 2.
Planned future work includes exposing the silicone elastomers to atomic oxygen and ultraviolet radiation to simulate time on orbit. The samples are then to be subjected to compression set, adhesion, and leakage tests to determine the effects of space environment exposure.
Find out more about Glenn’s structural seal and thermal barriers research: http://www.grc.nasa.gov/WWW/structuralseal/
Glenn contact:
Dr. Bruce M. Steinetz, 216-433-3302, Bruce.M.Steinetz@nasa.gov
Authors:
Dr. Christopher C. Daniels, Patrick H. Dunlap, Jr., Dr. Bruce M. Steinetz, Marta Bastrzyk, and Leigh Duren
Headquarters program office:
Exploration Systems
Programs/Projects:
CEV
Last updated: October 16, 2006
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