While the rest of the world was still marveling at the Curiosity landing on Mars two years ago, NASA’s Jet Propulsion Laboratory (JPL) was already working on ways to safely land even larger vehicles there.
The problem: The Martian atmosphere is much thinner than Earth's, so falling objects above a certain mass need to be slowed down to a speed where a parachute can handle their descent. Retrorockets require too much fuel and add too much weight. So JPL engineers had to rethink their longstanding design of descent technology and ways to test it.
“For 40 years, we’ve been living off the decelerator technologies that Viking gave us [in the mid-'70s],” says Ian Clark, JPL’s Low Density Supersonic Decelerator principal investigator. “In those 40 years, we’ve successfully landed several Mars missions with the parachute as the primary decelerator, but it’s come time that we can no longer use them, because we want to land things that are bigger, heavier, from higher altitudes, and with more accuracy than we have in the past.”
And so, after nearly four years of planning, JPL scientists have finally managed a new kind of decelerator system--a saucer-like vehicle containing a heat shield and specially developed technologies, called the Low Density Supersonic Decelerator (LDSD), unveiled to reporters last month--that, if successful, could herald the next phase of Martian landings. On June 3, the team will engage a test at the U.S. Navy’s Pacific Missile Range Facility in Kauai, Hawaii, at altitudes where the thinness of the Earth’s atmosphere approximates that of Mars, to see how the LDSD holds up at supersonic speeds and 1,300˚F temperatures.
During the test, a giant balloon will carry the LDSD to 120,000 feet and release it, at which point rockets propel it to 180,000 feet and Mach 3.5 (2664 mph). At that speed, the saucer's decelerator will inflate, slowing the vehicle, when a parachute deploys to carry it to the ocean.
“We needed ways of creating more drag by creating more surface area,” says Clark. “Parachutes have limits in terms of the environments, speeds, and conditions in which you can use them. So we developed new technologies that go beyond the capabilities of parachutes, that get you to the point where you can start to use one.”
The LDSD accomplishes this through three specially developed technologies--called Supersonic Inflatable Aerodynamic Accelerators (SIADs). The first is an inflatable Kevlar package that expands like an inner tube around the LDSD, increasing the saucer diameter from roughly 15 to 20 feet, thus creating more resistance. The second is a parachute deployment device.
The third is a new type of supersonic parachute. At 100 feet in diameter, it’s 29 feet larger and produces 2.5 times the drag of any parachute flown on Mars in the past, but too large to test in existing wind tunnels, so the team devised an innovative test. A helicopter dropped the chute from half a mile up, and, once it opened, a rocket sled pulled the chute by a rope around a pulley to speeds of 300 mph to replicate the aerodynamic stresses.
“It’s a test that nobody had thought of or had done anything like in the past,” says Clark. “But if you’re going to develop these technologies and convince yourself they’re going to work, you’re going to have to find new ways of doing those tests.”
The same held true for the design, which needed to be lightweight, economic, and withstand extreme conditions. “We launch from a high altitude balloon, and there’s a certain amount that the vehicle could weigh,” says Clark. “The lightweight chute is made from the nylons you’d see in a camping tent, and Kevlar, which you’d find in a bulletproof vest. But we’ve developed these technologies to be bigger and used in more extreme environments than they have in the past, which means new test methods. That’s some of the challenges we’ve had.”
The vehicle housing the SIADs was made of a carbon fiber composite protected by a heat shield of cork, fiberglass coated with fireproofing Vermiculite, and filled with insulation. It also contains a Star 48 solid rocket motor, Harrier jet engine spin motors, along with Go Pro cameras and flight imagery recorder to supply images of the vehicle in operation.
“We were able to get away with some very simple, off-the-shelf choices to try and save money,” says Brant Cook, JPL’s Supersonic Flight Dynamics Test Vehicle mechanical lead engineer.
“These are the technologies to get us to the point that we could then get the payload on the ground,” adds Clark. “For a Mars mission, the aeroshell would have a back end containing the payload. After it slowed down, it would drop, just like [Curiosity], the heat shield would come away, and we would deliver the payload to the ground.”
The team will return next summer for another two tests, followed by a couple of months of analysis. The LDSD project will conclude by the end of 2015 and be ready for flight projects, like Mars 2020 or the Mars Sample Return missions.