In a little under seven weeks, the wonderfully complicated Sky Crane will deliver the Mars Science Laboratory (MSL) rover “Curiosity” to the surface of Mars.
But before the descent module lowers the rover on a tether and uses retrorockets to place it gently in Gale Crater, a parachute will slow the whole payload to subsonic speeds. In finalizing the design of MSL’s parachute, NASA looked to mission requirements and tests carried out nearly 50 years ago.
MSL’s parachute is the main source of atmospheric drag. It’s a 64.7 foot (19.7 meter) disk-gap-band style chute deployed by a mortar. The main disk is a dome-shaped canopy with a hole in the top to relieve the air pressure. A gap below the main canopy also lets air vent out to prevent the canopy from rupturing. Under the gap is a fabric band designed to increase its lateral stability by controlling the direction of incoming air.
It’s not easy testing this important piece of hardware on Earth since Mars’ atmosphere is one percent as thick and its gravity is only a third as strong.
Simulating these conditions on Earth is possible but expensive, so much so that any high altitude hypersonic tests were deemed prohibitively expensive early on in the MSL development process. So JPL engineers broke the parachute’s job into stages that could be tested individually: mortar deployment, canopy inflation, inflation strength, supersonic performance, and subsonic performance. Luckily, NASA had data on high-altitude hypersonic parachute tests from the late 1960s for parachutes exactly the size of MSL’s.
When NASA began working out the details for the 1976 Viking missions to Mars, the agency was enjoying the inflated budgets that came with the push to land a man on the moon before the end of the 1960s. In this environment, NASA established three programs dedicated to testing parachutes: the Planetary Entry Parachute Program (PEPP), the Supersonic Planetary Entry Detector Program (SPED), and the Supersonic High Altitude Parachute Experiments (SHAPE).
The PEPP program ran sixteen high altitude supersonic deployment tests, eight of which used the DGB style and one tested a 64.7-foot chute at hypersonic speeds in the upper atmosphere.
The method was simple. The parachute had 72 gores or sections (MSL by comparison uses 80 gores to reduce fabric stress and allow the use of lightweight fabric) with 72 suspension lines connecting the parachute to the bridle. The bridle was in turn connected to the payload. In this test’s case, the payload was a 15-foot-diameter analogue spacecraft.
On July 28, 1967, the whole configuration — payload and parachute packed in an aeroshell — was launched in a 26,000,000 foot cubed balloon from Walker Air Force Base in Roswell, New Mexico. It took three hours for the payload to reach its test altitude close to 130,000 feet over the White Sands Missile Range. At this altitude, the test was an adequate simulation of the environment a parachute would encounter in Mars’ upper atmosphere.
Once at launch height, the payload separated from the balloon. Rocket motors ignited 3.8 seconds later and propelled the stand-in spacecraft above Mach 1.5. The bridle unfurled, pulling the parachute out of its casing. The chute inflated, briefly collapsed, then filled with air again. Within three seconds it was fully inflated and stable.
The only major problem was a tear in two sections of the canopy — a loss of less than 0.5 percent of nominal surface area — after part of the casing ripped through the fabric. But overall the test was successful and proved the feasibility of deploying a 64.7 DGB parachute at supersonic speed in a thin atmosphere. It would inflate and produce enough drag to slow its payload’s rate of descent to a target planet’s surface.
DGB parachutes have been a staple of NASA’s Mars missions for decades. The Viking landers, Mars Pathfinder rover, both MER rovers, and the Mars Phoenix lander all used this type of parachute to reach the surface safely. The Sky Crane might be the most sophisticated, precise, and intricate landing system ever sent to the red planet, but its parachute has nearly 50 years of success behind it.
Image: The parachute for Curiosity passing flight-qualification testing in March and April 2009 inside the world’s largest wind tunnel, at NASA Ames Research Center, Moffett Field, Calif. Credit: NASA/JPL-Caltech