Mars Science Laboratory Parachute Inversion Phenomenon and Flight Risk Assessment

During full-scale wind tunnel testing of the parachute for the Mars Science Laboratory two of the test parachutes experienced an inversion. These were unexpected occurrences and required an analysis of the phenomenon as observed in test and an assessment of what risks it may imply for flight on Mars. The test conditions and the inversion morphology are discussed as they relate to the wind tunnel tests and the impacts on the test effort. The timeline for the formation of the inversions is described and compared to the timeline expected in flight. Differences between the physics and behavior in test and flight are discussed as they relate to the threat of inversion. Mitigating actions are described as well as arguments used in determining the final configuration for use in flight. Nomenclature Do = parachute reference diameter DGB = disk-gap-band parachute M = Mach number MD = mortar deployment SD = sleeve deployment I. Introduction HE Mars Science Laboratory (MSL) mission will use a 21.5 m reference diameter (Do) Viking scaled disk-gapband (DGB) parachute as a critical component of its entry, descent, and landing system. Structural testing of the MSL parachute was performed in the 80’x120’ test section of the National Full-Scale Aerodynamics Complex (NFAC) operated by the Arnold Engineering Development Center (AEDC) at NASA’s Ames Research Center, Moffett Field, California. These developmental tests were conducted in five different tunnel entries over a ten month period from October of 2007 through July of 2008, while the final flight lot qualification tests were conducted in April of 2009. During the second tunnel entry in December, 2007, the parachute that was used in the second of three planned mortar deployment (MD) tests inverted below the band of the canopy. This was the first time that a DGB parachute had ever inverted in any nominally executed test which includes all wind tunnel, low altitude, high altitude, subsonic, supersonic, and Mars flight condition deployments. Following the MD2 inversion a Risk Assessment Workshop (RAW) was convened in January, 2008, at Pioneer Aerospace in South Windsor, Connecticut, to discuss possible mitigations to the inversion phenomenon both for NFAC testing as well as for flight. The historical flight and test experience was reviewed and a number of options were considered before a down selection exercise reduced the design space to two modifications which were implemented for the third NFAC entry in March, 2008. The two modifications were shown to have no ill effects on the mortar deployment of two 21.5 m and two 23 m parachutes and all four of the openings were observed to be within expectations. Observations made during the second and third tunnel entries showed that the weakest point in the design was at the suspension line to band joint and this junction could fail under certain conditions during test. In order to build some added strength margin into the parachute the suspension line material was changed from Kevlar-29 to Technora-T221 which necessitated a fourth tunnel entry to verify the new material margins. In addition, the testing