A heat shield that you can scorch with a blow torch until it lights red on one side and still comfortably touches the other will protect a NASA probe flying within 4 million miles from the sun’s surface.
The shield is the culmination of years of work by engineers to solve what they call the thermal issue of the soon launched Parker Solar Probe.
“Thermal Problems” is a concise way of referring to the extraordinary complications of a record dive dive directly in our star’s outer atmosphere or corona.
While the probe circuits the sun and records data with built-in instruments, its thermal protection system protects the spacecraft from heat that is more intense than any spacecraft has ever experienced. Combined with a water-powered cooling system, TPS will maintain most of the probe’s instruments at approximately 85 degrees Fahrenheit equivalent a nice summer day ̵
1; while TPS itself holds a temperature of 2,500 degrees Fahrenheit.
Without TPS, there is no probe.
“This was the technique that made it possible for us to do this mission – so it could fly,” said Elisabeth Abel, TPS thermal leader at Johns Hopkins University Applied Physics Laboratory. “It will be incredibly exciting to see something you spend a lot of energy and hard work on to see it actually fly. It will be a big day.”
Parker Solar Probe is expected to start from Kennedy Space Center in Cape Canaveral, Florida, in August. The launch window opens on Saturday, August 11, and runs through August 23rd.
Engineers have spent more than a decade creating the heat shield to deflect the worst of the sun’s energy.
During its seven-year mission, the probe will explore some of the sun’s greatest mysteries: Why is the sun wind a wind closer to the sun but a supersonic torrent farther away? Why is the corona millions of degrees warmer than the surface? What are the mechanisms behind the amazing fast-moving solar particles that can interfere with spacecraft, interfere with communications on Earth and threaten astronauts?
Engineers have spent more than a decade creating the heat shield to deflect the worst of the sun’s energy. The front and back are made of carbon carbon, a lightweight material with superior mechanical properties that are especially suitable for high temperatures.
In less than a tenth of an inch, the two carbon-carbon plates are thin enough to bend. Between them are about 4.5 inches of carbon foam, which is usually used in medicine for bone exchange. This sandwich design reinforces all rough corrugated cardboard while allowing 8-foot heat shield to weigh only 160 pounds.
Carbon itself leads heat, but coal dust is 97 percent air. There is just not much material for heat to travel through. The heat shield will be 2500 degrees Fahrenheit on the side of the sun, but only 600 degrees Fahrenheit on the back.
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Even the cold foam’s impressive heat-dispersing properties were not enough to keep the spacecraft at its desired temperature. Since there is no air in space to provide cooling, the only way for materials is to expel heat to spread light and emit heat in the form of photons. For that, another protective layer was necessary: a white coating that would reflect heat and light.
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After extensive tests, the layer was rendered on a coating of light white alumina. This coating can react at high temperatures, but with the heat shield’s carbon and become gray, so the engineers add a layer of tungsten, thinner than a hairline, between the heat shield and the coating to stop the two from interacting. They added nanoscale dopants to make the coating whiter and prevent the expansion of alumina when subjected to heat.
Then engineers must decide how to best create and apply the coating.
“It all struggled to find a ceramic coating that reflects both light and emits heat,” said Dennis Nagle, principal researcher at the Center for Systems Science and Technology . 19659002] Normally, when working with enamel, says Nagle, a hard, non-porous coating is preferred, one that will crack when hit with a hammer. But under the temperatures that Parker Solar Probe will face, a smooth coating would be crushed like a window window with a rock.
Instead, the target was a uniformly porous coating to withstand extreme environments. When cracks start in a porous coating, they will end when they hit a pore. Several coarse, granular layers make up the coating – enough that a set of ceramic grains would reflect light as another layer lacks.
“I always tell people it works, because it’s a bad occupation,” Nagle joke. “If you want to make a good coating, it will fail.”
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After launching the probe, it will fly repeatedly past Venus, which gradually comes closer and closer to the sun, far closer than Mercury, the innermost planet that is closest still more than 28 million miles out. In the next approach, spacecraft will travel around the sun at about 430,000 mph.
Source: Tracy Vogel of Johns Hopkins Universitynews