NASA Sets Sights on Thermal Coating
When it comes to protecting its spacecraft—and the human occupants—from both heat loss and extremely high temperatures during space travel and reentry into Earth’s atmosphere, NASA is turning to coatings to control the heat.
The agency plans to use a silver, metallic-based thermal control coating as part of the thermal protection system on its Orion spacecraft, NASA announced Thursday (Nov. 19) when releasing conceptual renderings of the coated capsule.
“Orion’s thermal protection system is essential to successful future missions,” said John Kowal, NASA’s thermal protection system lead for Orion.
“As we move toward building the system for [Exploration Mission-1], we’ve been able to take advantage of what we learned from building and flying Orion to refine our processes going forward,” he said.
The Orion spacecraft is meant to bring a new level of deep space exploration to the U.S. space program. In order to take humans farther than they’ve ever gone before—including Mars—Orion is designed to be the safest, most advanced spacecraft ever built.
A test flight in December 2014 evaluated launch and high-speed reentry systems such as avionics, attitude control, parachutes and the heat shield—which included testing performance of the thermal protection system during a high-energy return.
The heat shield, which faces into the atmosphere during reentry, assists in slowing down the craft and helps protect it from the extreme heat generated during its descent. Specialized thermal tiles cover the back shell of the craft in order to insulate against the high temperatures outside the capsule.
During the test launch, the craft returned through the atmosphere at a speed of 20,000 mph (32,000 kph), where it experienced the hottest period of its return. The heat shield faced temperatures near 4,000 degrees Fahrenheit (2,200 degrees Celsius).
According to NASA, that’s roughly 80 percent of the peak heating Orion would see during a return from lunar orbit, in which temperatures could reach 5,000 degrees Fahrenheit (2,800 degrees Celsius). This test gave engineers on the ground confidence in the heat shield design for future missions.
Next Testing Phase
During its next mission atop the Exploration Mission-1 (EM-1) rocket, scheduled for no later than November 2018, Orion will be in space for more than three weeks and return to Earth under even faster and hotter conditions than during the first test flight.
The coating will help Orion’s back shell maintain an exterior temperature from about –150 to 550 degrees Fahrenheit before entry and will protect against electrical charges during reentry, NASA said.
In this return flight, speeds are expected to reach 36,000 feet per second. While the speed difference may seem subtle, NASA said, the heating the vehicle sees increases exponentially as the speed increases.
Upcoming missions will be able to take advantage of the silver, metallic-based thermal control coating that will be applied to the crew module’s thermal protection system back shell tiles.
Similar to the coating used on the main heat shield, the insulating coating is meant to help regulate interior temperature—reducing heat loss when Orion is pointed toward space (and experiencing cold temperatures as a result), and limiting the high temperatures the crew module would experience when the craft faces the sun.
“You’re trying to hit this sweet spot because when you’re looking at the sun, you don’t want to get too hot,” Kowal said, “and then when you’re not looking at the sun and instead in darkness, you don’t want to lose all the heat that the spacecraft generates.”
The reflective coating will help Orion’s back side maintain an exterior temperature range from about –150 to 550 degrees Fahrenheit prior to entry and also will protect against electrical charges during reentry, NASA said.
The Orion spacecraft is built to support a crew of four for up to 21 days in space, a smaller crew for a longer period, or up to six astronauts for either a shorter period or with the addition of a habitat module for extended missions.
The vessel is made of aluminum and aluminum-lithium, with a friction-stir-welded pressure vessel covered in 970 tiles composing its back shell. Made of a low-density, high-purity silica fiber made rigid by ceramic bonding, the tiles are needed to protect the sides of Orion from extremely high temperatures.
The Orion spacecraft, shown here after its first test flight, is made of aluminum and aluminum-lithium, with a friction-stir-welded pressure vessel covered in 970 tiles composing its back shell.
Temperatures will climb even higher at the bottom of the Orion capsule, which will be pointed into the heat as Orion returns to Earth. Protecting the spacecraft from those temperatures requires the largest, most advanced heat shield ever built, according to NASA.
The 16.5-foot (5-meter) diameter heat shield is built around a titanium skeleton and carbon fiber skin that give the shield its shape and provide structural support for the crew module during descent and splashdown.
A fiberglass-phenolic honeycomb structure fits over the skin, and each of its 320,000 cells is filled with an epoxy novolac resin material called Avcoat. This material is designed to burn away as the material heats up, rather than transfer the heat back into the crew module. At its thickest, the heat shield is 1.6 inches (4 centimeters) thick, and about 20 percent of the Avcoat will erode as Orion travels through Earth’s atmosphere.
Lockheed Martin, the contractor working on Orion, is among those working to protect the spacecraft and its future human occupants.
Lockheed Martin recently completed a heat shield manufacturing development unit that engineers will use to test the improved manufacturing process before it is used on hardware for flight.
Additionally, engineers have found ways to reduce the mass of the heat shield’s underlying structure, NASA said, which is composed of a titanium skeleton and carbon fiber skin.
Crews were able to optimize the skeleton and skin measurements based on the pressures the different areas will experience during flight and reentry, adding more fidelity to the overall structure while allowing it to be lighter.
Teams have already begun building Orion’s heat shield for EM-1, the next unmanned test flight, as well.
“When you look at the Orion crew module for EM-1 from the outside, it will look like the spacecraft we flew on Exploration Flight Test-1,” Charlie Lundquist, Orion crew and service module manager, told the Spaceflight Insider site.
“But the spacecraft we’re building for our next mission incorporates a lot of engineering ingenuity and improved manufacturing processes,” he added. “These improved processes have also contributed to a reduction in weight of the overall spacecraft.”
Exploration Mission-2, the first crewed flight for Orion, which has been under development since about 2004, is expected to launch no later than 2023.