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Composite Crew Module (CCM) Permeability Characterization

In January 2007, the NASA Administrator chartered the NASA Engineering and Safety Center
(NESC) to form an Agency team to design and build a composite crew module in
18 months in order to gain hands-on experience in anticipation that future exploration systems
may be made of composite materials. One of the conclusions from this Composite Crew Module
(CCM) Primary Structure assessment [ref. 1] was that there was a lack of understanding
regarding the ability for composite pressure shells to contain consumable gases, which posed a
technical risk relative to the use of a metallic design. After the completion of the CCM test
program, the test article was used in a new program to assess the overall leakage/permeability
and identify specific features associated with high leak rates. The “International Space Station
(ISS) to Commercial Orbital Transportation Services Interface Requirements Document”
specifies that the maximum leakage rate cannot exceed 0.01 kilograms (kg) per day of air at
14.7 pounds per square inch differential (psid) (760 millimeters of mercury (mmHg)) while the
crew module is mated to the ISS [ref. 2].
The CCM was built as two halves spliced together around the circumference using a double lap
shear joint cured under vacuum bag pressure with a heater. The majority of the construction was
honeycomb sandwich with unvented aluminum honeycomb. The unvented core was selected to
provide double redundancy in pressure containment with the two skins of the sandwich. Six
large openings were included for windows and hatches. The minimum gage sandwich skin and
the minimum gage laminate consisted of 4 plies and 10 plies of fabric, respectively. Metallic
fittings bolted through the pressure shells were used for attachment points for the Service
Module/Alternate Launch Abort System (SM/ALAS) and parachutes. An external bracket was
attached to the sandwich structure using 20 two—piece inserts that penetrated both face sheets. In
addition to these penetrations, two repairs were made using through bolts in the lobed bottom of
the lower pressure shell.