Solar Probe Shield Testers Can Take the Heat
August 13, 2009
When Solar Probe Plus soars through the Sun’s atmosphere —facing temperatures 20 times higher than the hottest summer days on Earth—a thin carbon-foam shield will be all that separates the spacecraft from fiery obliteration. So it’s no surprise that testing and perfecting the probe’s Thermal Protection System tops the list of mission design priorities.
“No one has done anything quite like what we’re trying to do with Solar Probe: fly within 4 million miles of the Sun and study the solar wind where it originates,” says Andy Dantzler, Solar Probe Plus project manager at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. “Which is why it’s so important to test the shield designs and materials now, and why it’s so exciting to see the progress we’ve made.”
Engineers at the Johns Hopkins University Applied Physics Lab combined tests of materials for the Radiation Belt Storm Probes and Solar Probe Plus. The trials included acoustic tests to simulate the noise and vibrations of launch.
The APL engineering team has tested various combinations of the carbon foam that will make up the 9-foot-diameter, 4½-inch-thick shield and the reflective coatings that will repel the Sun’s heat and energy. Solar Probe Plus will study the solar wind where the processes that produce these streams of charged particles actually occur, just 3.7 million miles above the Sun’s “surface,” some seven times closer than any spacecraft has come before. At closest approach the probe will zip past the Sun at 125 miles per second; its shield must withstand up to 2,600 degrees Fahrenheit, allowing the payload of science instruments to operate at or near room temperature.
The team has pulled design ideas from heat-resistant technologies some have used on reentry vehicles—like the tiles that protect the space shuttle when it glides through Earth’s atmosphere—and on the APL-built MESSENGER spacecraft, which has flown past Mercury twice and in 2011 will become the first spacecraft to orbit the broiling, innermost planet. But with Solar Probe Plus designed to weigh about 1,000 pounds—less than half of MESSENGER’s launch weight—its protective materials will have to be tough and lightweight.
“Mission reentry vehicles get hot, but they have to deal with Earth’s atmosphere and tend to be heavier,” says APL’s Betsy Congdon, materials testing lead for the Solar Probe thermal protection system. “Our system has to be much lighter, so the mechanical tests help us understand how some of these materials will act under new uses.”
The carbon foam-coating combinations are designed to keep the shield together under a range of conditions, most of which would melt traditional spacecraft part-connectors like metal bolts, fasteners or adhesives. Congdon and her APL colleagues bake samples of the Solar Probe shield materials in thermal-vacuum chambers that simulate the conditions in space, shake them on vibration tables and blast them with acoustic waves to replicate the rumble of a rocket ride.
“We’re looking closely at how they break, bend and compress, and we make changes based on what we see,” she says.
The design team is working toward a 2015 launch, mainly to save NASA funds and take advantage of overlapping timelines with the European Space Agency’s Solar Orbiter mission. So with launch less than six years away, Congdon says the heat-shield team is staying cool under schedule pressure.
“When you say out loud that you’re flying to the Sun, it sounds crazy,” Congdon says. “But we can do it. We have the technology and it’s all about putting it together.”
Solar Probe Plus shield materials face a searing, simulated “Sun.”
Solar Probe Plus Gets Technical
On July 31, the Solar Probe Plus project team submitted — on schedule – five “final” reports to NASA on a variety of technical topics. The reports cover progress made on the Thermal Protection System, the near-Sun dust environment, the Solar Array Cooling System, all spacecraft mechanisms, and the Transition Structure Assembly that connects the thermal shield to the spacecraft. More than 600 pages combined, the reports offer detailed information on trade studies, modeling and hardware testing.
“Although termed ‘final’ reports in the pre-phase A statement of work, the team’s work on these and other technical topics isn’t finished,” says Dantzler, the project manager. “We’ll continue to reduce technical, cost and schedule risks as the project proceeds through the formulation and development phases.”
