Measurements of Parachute Dynamics in the World's Largest Wind Tunnel by Stereo Photogrammetry
2018 Aerodynamic Measurement Technology and Ground Testing Conference
Between 2012 and 2017, parachutes for four NASA Projects were tested in the 80-by 120-Ft test section of the National Full-Scale Aerodynamic Complex (NFAC) at NASA Ames Research Center. These projects were: (1) Low-Density Supersonic Decelerator (LDSD); (2) Capsule Parachute Assembly System (CPAS, for Orion); (3) Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight, a Mars mission); and (4) Mars 2020. In all tests stereo photogrammetry was used to measure
... ed to measure time-dependent positions of features on the canopies. For the LDSD and CPAS tests, where the purpose was to study the trade-off between stability and drag of different parachute designs, the pendulum motion of the canopies about the riser attachment point was measured by calibrated cameras in the diffuser. The CPAS test also included static measurements where the inflated parachutes were pulled to the side by a system of tethers. The Insight tests were structural qualification tests where each canopy was packed in a bag and launched from a mortar. Cameras in the diffuser measured the trajectory of the bag and the stripping of the bag from the canopy. The Mars 2020 test was a workmanship verification test where the canopies were either launched from a mortar or deployed from a sleeve stretched along the tunnel axis. The deployments were recorded from many directions by thirteen high-speed cameras distributed in the diffuser and test section. Photogrammetry was not planned; however, after a tunnel-related accident ended the test prematurely, photogrammetric measurements were bootstrapped from the images to support the accident investigations. This paper describes how the photogrammetry measurements were made in each test and presents typical results. ARACHUTES for four NASA programs were recently tested in the 80-by 120-Ft test section of the National Full-Scale Aerodynamic Complex (NFAC) at NASA Ames Research Center. In each test, the time-varying positions of the parachutes were measured by stereo photogrammetry. The first test was for the Low-Density Supersonic Decelerator (LDSD) Project . This project was developing Mars-entry decelerator systems for large payloads that includes both an inflatable aerodynamic decelerator and a supersonic parachute. A goal of the LDSD project was to improve the state-of-the-art of supersonic parachute design. To date, all Mars landers-from Viking I and II in 1976 to Mars Science Laboratory in 2012-have used the same parachute design-Disk-Gap-Band (DGB) -to decelerate from supersonic to subsonic speeds. The purpose of the NFAC tests was to compare the performance (drag and stability) of other parachute designs-in particular Ringsail and Ringsail-like parachutes-to the performance of DGB parachutes and down-select a design for future testing at full scale in supersonic flight. Thirteen 38%-scale parachute configurations were tested. The riser for each parachute was attached to the top of a strut near the upstream end of the test section, and the time history of the position of the parachute vent, which was near the downstream end of the test section, was measured by two cameras mounted in the diffuser. The tests were conducted during two tunnel entries, the first in late 2012 and the second in early 2013. The second test supported development of the Capsule Parachute Assembly System (CPAS) for the Orion spacecraft  . CPAS is designed to decelerate the Orion vehicle during the final stage of re-entry through the Earth's atmosphere. The system includes eleven parachutes that deploy sequentially beginning with three parachutes that pull off Orion's forward bay cover and ending with inflation of three large main chutes. The test in NFAC was conducted in preparation for drop tests to identify modifications to the main parachutes that would improve stability without decreasing drag. Of particular concern was undesirable pendulum motion observed during earlier drop tests of a mock capsule under two parachutes. Three 35%-scale parachutes were tested. Modifications to the canopies included changes in the distribution of porosity, reefing, and suspension line lengths. The test configuration for the CPAS was very similar to that used for LDSD. The principal difference was that most CPAS runs included "static" test points where a system of tethers pulled the inflated canopy laterally through a sequence of angles of attack. After the last static point, the tethers were released and "free-flight" data were acquired. The CPAS tests occurred in early 2015. The third test was for the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission to Mars, launched on May 5, 2018 and scheduled to land November 26, 2018 . This mission will place a geophysical lander on Mars to study the planet's deep interior. Upon entry in the Martian atmosphere the vehicle will be decelerated from supersonic to subsonic speeds by a DGB parachute. Full-scale parachutes for this mission were tested during two entries in NFAC, the first in early 2015, immediately after the CPAS test, and the second in late 2015. During both entries, the parachutes were packed in bags and fired from a mortar mounted atop the same strut as was used for the LDSD and CPAS tests. After the suspension lines were fully extended, they pulled the canopy out of the bag ("bag strip") and the canopy inflated. The first entry was conducted as part of a structural qualification program, and the deployments were documented by high-speed cameras. Photogrammetry was not required for this test; however, because the cameras used for CPAS were still in place, images from these cameras were also acquired and used to compute the trajectories of the bags. The high-speed imagery revealed that the canopies twisted as they were being pulled out of the bag, and there was concern that this twisting could interfere with proper inflation of the canopies. Therefore, the second entry was conducted to observe how a different method of packing the parachutes might affect this twisting and inflation of the canopies. The CPAS cameras were replaced by high-speed cameras that were calibrated for photogrammetry. The fourth test was a workmanship verification test of full-scale parachutes for the Mars 2020 mission  . Mars 2020 is part of NASA's Mars Exploration Program and is very similar to the successful Mars Science Laboratory (MSL). It will use the same landing system, including a similar parachute, as MSL. The parachutes tested for Mars 2020 were significantly larger than the parachutes tested for LDSD, CPAS, or Insight, and they flew much further downstream-in the diffuser rather than near the downstream end of the test section. The parachutes were deployed either by firing from a mortar or by sleeve deployment whereby the canopy was wrapped in a narrow sleeve and stretched along the tunnel centerline. The deployments were recorded from many directions by 13 high-speed cameras fielded by teams from both Ames and JPL. Photogrammetry measurements were not planned; however, after a tunnelrelated accident ended the test prematurely, an effort was made to extract photogrammetric information from the images to support the accident investigations. P 3 The purpose of this paper is to describe how the photogrammetry measurements were made in each test and to present typical results. All of the tests had much in common. They are described in chronological order, with differences and how those differences were accommodated pointed out.