SAN DIEGO – is preparing to repeat tests of a large, inflatable deceleration device designed to help land large payloads on Mars after incorporating lessons learned from a mostly successful initial test over the Pacific on June 28.
The test, launched by helium balloon from the U.S. Navy’s Pacific Missile Range Facility in Kauai, Hawaii, was part of’s Low Density Supersonic Decelerator (LDSD) technology demonstrator project, which is testing larger braking parachutes and inflatable drag devices at high speeds for future Mars missions. The devices will be needed to slow larger, heavier landers from the supersonic speeds of Mars atmospheric entry to subsonic approach speeds for safe, controlled landings.
Although the decelerator device inflated and deployed as planned, the parachute was ripped to shreds after being pulled from its housing into the supersonic slipstream of the test unit as it decelerated to around Mach 2. The unexpected chute failure caused the inflated device, still attached to a booster rocket, to impact the sea at far greater speed than planned. The test unit was, however, recovered successfully.
"Someday we want to get humans there and larger structures," says Mark Adler, project manager for LDSD at NASA’sin Pasadena, California. "Unfortunately we are victims of the square-cube law problem where, as the vehicles get larger in size they go up in mass as the cube of the size but they go up in area only as the square of the size." The result, he adds is that the drag area is not growing as quickly as the mass. "So you end up with vehicles with much higher ballistic coefficients, and those vehicles will hit the ground supersonically if you don’t develop something to slow them down — and that is what we are developing."
The test evaluated a 20-ft.-dia. Supersonic Inflatable Aerodynamic Decelerator designed for robotic missions (SIAD-R), which uses automobile-derived airbag generators to inflate with hot gas. It also tested a 110-ft.-dia. supersonic parachute system, which is significantly larger than the 64.7-ft. chute used for the Mars Curiosity rover. For the Curiosity mission and all previous landers, NASA relied on basic parachute system data collected in 1972 for the Mars Viking lander. "New tests were needed as we go to larger payloads to Mars," Adler says.
Speaking at the American Institute of Aeronautics and Astronautics Space 2014 conference here, Adler says a key challenge of the test was correctly simulating Mars entry conditions. "The hardest part of the test is to get them to close to the actual condition we see at Mars. So we have to get to 190,000 ft. altitude [to simulate the thin Martian atmosphere and low gravity] and get a full-size entry vehicle up there at Mach 4." Following release from the balloon at around 120,000 ft., the test unit was boosted to 190,000 ft. and Mach 4.3 by an ATK Star 48 solid rocket motor. The payload was spun to 50 rpm during the 71-sec. ascent before the SIAD was deployed.
"The first thing we wanted from the test was to show we could get the vehicle to altitude and up to speed to get to test conditions, and this mission was therefore a complete success," says Adler, who adds the SIAD deployed successfully and immediately slowed the vehicle. Following the deployment of the ballute (a device to help pull the parachute from its housing), the chute was ripped "before it could even deploy," he adds. "We would have rather seen this failure this year rather than next year because that would have had an impact on our cost and schedule. We learned a lot from this failure and because we had high-speed video we are now observing phenomena we have never seen before. We are now redesigning the parachute for the next flights."
Two units are scheduled for testing in 2015, the core structure for the first of which is already assembled at JPL.