Historically, parachutes have been load-tested by various methods including release from an aircraft, deploying in a wind tunnel, dragging through water, and shooting out of an air cannon. Each type of testing has its own advantages and drawbacks. Due to the loading mechanics particular to parachutes deploying in a very thin atmosphere, none of the testing methods was appropriate for testing the next generation of Mars’ full-scale parachutes.
The new approach to parachute testing involved a helicopter lifting the parachute into the sky with a rope trailing down to a pulley on the ground. The rope would go through the pulley and attach to a rocket sled. The parachute would be released and inflate as it dropped to the ground. Once the parachute was fully inflated, the rockets would be fired, and the sled would tow the parachute down towards the ground. By tuning the sled weight and rocket thrust profile, the desired loads can be delivered to the parachute in the fully open shape, as it would take above Mars, aiding in verifying its design.
During the course of two years, the Parachute Design Verification (PDV) test series was refined. As this was a novel test approach, a myriad of test configuration determinations needed to be made, both from the airdrop perspective as well as the parachute loading perspective. Extreme rigor was taken in personnel safety during the design process. The first focus was on the development of the specialized airdrop equipment necessary for the test, including the release platform, load platform, a launch support structure, and a method to attach and release the packed parachute from a helicopter. An airdrop test was performed to verify the function of the newly developed equipment, as well as determine the motion characteristics of a 4000-ft (≈1220-m), 3000-lbm (≈1361-kg) rope suspended by a helicopter.
After this successful test, significant development and fabrication began on the parachute loading apparatus. It was determined that the structure that supports the pulley would have to withstand 200,000 lb (≈889 kN) of force. To support the pulley, a large tripod was designed with 16 16-in. (≈41 41-cm) square beams spanning 50 ft (≈15 m). A large funnel to guide the rope to the pulley would be placed on top of the tripod, bringing the total height to 19 ft (≈6 m). Two million pounds (≈907 metric tons) of concrete was poured into the ground in order to anchor the tripod and react the loads.
The rocket sled was designed to allow the rocket motors to be tilted up or down to fine-tune the amount of pull force on the parachute. Concrete ballast blocks were placed in the sled to bring the total weight up to 136,000 lb (≈605 kN). This weight was needed to keep the acceleration and deceleration of the sled low while keeping a controlled tension on the tow rope. Behind the rocket motors and ballast portion of the sled was a 120-foot-long (≈37 m) tow bar sled. This was necessary to keep the hot rocket motor plume from melting the tow rope.
More than a dozen layers of safety were designed into the firing system so that there was no possibility of inadvertently pulling down on the helicopter. Thirteen inhibits, three man-in-the-loop safety switches, 11 live camera feeds, a new firing sequencer, and a handoff from the helicopter to the winch to the rocket sled helps ensure aircrew safety.
Full-functional testing of this architecture has successfully been demonstrated in the development of parachutes for the Low-Density Supersonic Decelerator project. This facility is now fully operational and available for other large parachutes that have unique parachute design-verification requirements.
This work was done by John C. Gallon and Michael B. Meacham of Caltech for NASA’s Jet Propulsion Laboratory. For more information, visit www.techbriefs.com/tv/parachute. NPO-49692