NASA Developing New Heat Shield for Orion
NASA's new spaceship of the future must endure searing temperatures capable of melting iron, steel or chromium as the spacecraft streaks into the Earth's atmosphere on the way back from the moon.
Faster than the fastest bullet, the spaceship – called the Orion – will enter Earth's atmosphere at 6.8 miles (11 kilometers) per second, generating surface temperatures equivalent to more than 4,800 degrees Fahrenheit (2,649 degrees Celsius.)
¹To illustrate how hot that is, rapidly rubbing your hands together generates warmth. In contrast, a speeding space vehicle creates not just warmth, but tremendous heat, minimally due to friction between the air and surface of the spaceship as it moves at mind-boggling speed. (Most of the generated heat is due to rapid air compression without time for the air to cool off.)
Image left: Heat shield testing.
The shuttle enters the atmosphere at lesser speeds, 4.7 miles (7.5 kilometers) per second, generating a lower maximum temperature of 2,900 degrees Fahrenheit (1,593 degrees Celsius) -- still hot enough to melt nickel and iron. The crew vehicle will see temperatures of as much as 3,400 degrees Fahrenheit (1,871 degrees Celsius) when re-entering from low-Earth orbit. The length of spacecraft re-entries is also a variable. Because re-entry times vary, the duration of these periods also affect how a heat shield reacts to high temperatures.
Scientists and engineers at NASA Ames Research Center, located in California's Silicon Valley, are leading NASA's Orion advanced heat shield development effort. It is officially called the Thermal Protection System Advanced Development Project (TPS ADP).
The advanced heat shield development team includes engineers not only from NASA Ames, but NASA Johnson Space Center, Houston; NASA Kennedy Space Center, Fla.; NASA Langley Research Center, Va.; NASA Jet Propulsion Laboratory, Pasadena, Calif.; and NASA Glenn Research Center, Cleveland, Ohio. The advanced heat shield work is part of a NASA-wide, cooperative endeavor, called the Constellation Program, jointly conducted to develop a new set of spaceships and launch vehicles to carry tomorrow's astronauts into space. The final flight version of the heat shield and ancillary support systems will be designed and manufactured by the Orion prime contractor when that contract is awarded.
"We don't know what the final (advanced heat shield) material will be until the testing and analysis is complete," said George Sarver, manager of Ames' Orion/ Ares Support Project. According to Sarver, NASA must complete the advanced heat shield development work by 2009 in order to be ready for Orion's first flight that possibly could be in 2012, but no later than 2014.
"Because of the short amount of time that we have to develop the Orion, we're only testing and analyzing materials that we know can tolerate the heat rates present during a lunar return mission," said James Reuther, who is based at NASA Ames and who is the agency's project manager for TPS (heat shield) advanced development work. Heat rate is the amount of heat energy transmitted to a material in a given time. Only a few materials can tolerate the higher heat rates needed for lunar return, according to Reuther.
NASA is working to create a Frisbee-shaped heat shield 16.5 feet (5 meters) in diameter that can be manufactured in one piece. It will be attached to the base of the Orion's cone-shaped crew capsule. The shield must protect the capsule during both low-Earth-orbit returns and very fast moon-mission re-entries into Earth's atmosphere, when it carries astronauts back home.
Comparing the amount of thermal protection that the space shuttle needs with what the new Orion spaceship will require, Reuther noted that engineers designed the shuttle only to come back from low Earth orbit, which is about 150 to 250 miles up. "However, it's not really the altitude that matters, but, instead, the velocity at which the vehicle enters the Earth's atmosphere," he noted.
"The difference between 7.5 kilometers per second (shuttle re-entry speed from low-Earth orbit) and 11 kilometers per second (Orion capsule re-entry speed from the moon) translates into a factor of five in increase of heat rate (for the Orion)," Reuther explained.
Image right: Model instrumentation setup in the NASA Ames Interaction Heating Facility (IHF) arc jet.
"The material's temperature has nothing to do with the performance," Reuther observed. "The surface temperature at a given heat rate is completely material-dependant," he added.
"Most of these TPS (advanced heat shield) materials have only been tested in small, hockey-puck-sized samples -- about four inches in diameter," Reuther said. "Unfortunately, because the heat shield we need for the Orion will be five meters in diameter, more than arc jet testing will be required to validate the design," he said.
Two large NASA 'arc jets,' equivalent to room-size blowtorches cooled by thousands of water lines, are helping scientists and engineers at NASA Ames and NASA Johnson test advanced heat shield materials that may be used in the Orion's heat shield.
"The temperatures are high enough (during a Orion capsule's return from the moon in the final part of the flight through Earth's atmosphere) that you have to go to what's called an ablating material," said John Balboni, a NASA Ames engineer who works with the Ames arc jet facility. "An ablating material is designed to slowly . . . burn away in a controlled fashion, and in a way in which this ablation actually carries away some of the heat from the surface and protects (it) from the superheated gases on top," he explained.
Engineers say that it is difficult to predict a material's temperature during re-entry because the temperature depends on how the particular material ablates or burns. "The real key to understanding the performance capability of a candidate material is to subject it to the expected heat rate Orion will see on (a) return trip from the moon," Reuther observed.
Ames' arc jet, known as the Interaction Heating Facility (IHF), is where Orion advanced heat shield material tests are being conducted. IHF is the largest arc jet facility of its kind in the United States; a similar size facility is located in Italy. The Ames arc jet, in combination with an arc jet at NASA Johnson Space Center, can simulate the range of expected heat rates that the Orion capsule will encounter while re-entering Earth's atmosphere.
Both NASA arc jets have a long history that includes testing thermal protection systems for all NASA space vehicles. The Ames arc jet uses up to 60 megawatts of electrical power, and can focus a large amount of heat energy onto a sample of heat shield material. Despite, the arc jet's size and ability to use a huge amount of electrical power, the Ames facility can test only 'coupons,' or small panels of material that might be used in the new Orion advanced heat shield.
"When you walk into the Arc Jet Laboratory (at NASA Ames), you'll see about a room-sized version of a blowtorch," Balboni said. "It sits flat on a bench. It's surrounded by thousands of water hoses - water being used to cool the heating device because the temperatures are very, very high. The temperatures are two to three times as high as the surface of the sun," he added.
According to Balboni, the apparatus shoots hot gas into a large vacuum chamber, "one that, after a test, you can open a door and walk into. But during a test, it's closed up. It's evacuated by a gigantic vacuum pumping system. So, (during a test) hot, superheated gas (flows) into a vacuum (chamber), and that's where the test then occurs because the vacuum is used to simulate very high altitudes at which the heating occurs," Balboni continued.
Image left: Teflon calibration in the NASA Ames Interaction Heating Facility.
Thermal arc jet testing will subject material samples "to simulated entry (into Earth's atmosphere) over a period from seconds all the way up to several minutes or even as long as for half an hour," according to Balboni. "These materials are then heated in (a way that closely approximates) the way that they'll be heated when they're flying through the atmosphere. And in that way, the engineers have the right data to design that heat shield so that you have confidence that it's going to protect the spacecraft from those severe temperatures, and it will survive the entire heating environment."
"This summer, we will be issuing contracts to TPS (heat shield) material providers for the production of manufacturing demonstration units (MDUs) of the heat shield at full scale," Reuther said. Companies will take about one year to make the demonstration units, according to engineers.
"We will have multiple contractors producing the manufacturing demonstration units as well as the material samples needed for further, more exhaustive arc jet testing," Reuther said. "While the MDUs are being built, we will be performing thermal, structural and environmental testing over a wide range of conditions."
Structural testing will involve bending and tension tests, as well as vibration and acoustic testing. The vibration and acoustic, or sound tests, simulate the extreme conditions experienced during launch. Environmental testing will include thermal-vacuum tests that simulate the Orion orbiting Earth and cycling from the cold of night to the heat of day.
Current plans call for full-size manufactured demonstration heat shield units to be shipped to NASA Kennedy Space Center for evaluation. Engineers will conduct various non-destructive tests to assess the quality of construction of the heat shield units.
"We're doing this advanced development under the direction of the Crew Exploration Project at NASA Johnson Space Center," noted Sarver. "NASA's expertise in the field of thermal protection, across all of NASA's centers, is world class," Sarver concluded.
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¹Technical write-up provided by John Balboni of NASA Ames Research Center, Moffett Field, Calif., to explain how rapid air compression causes extreme heat during spacecraft entry into an atmosphere:
Technically speaking, the fraction of heating to a spacecraft that is derived from friction is generally less than about 1 percent. The velocity of the spacecraft is the source of the heat that is applied to the heat shield during atmospheric entry. At high speed the gas undergoes adiabatic compression in the bow shock. The bow shock is a compression wave of gas the builds up in front of the vehicle due to its motion. Higher speeds produce stronger bow shocks, meaning the compression is much greater at higher speeds, producing higher temperature gas and thus higher heating to the spacecraft.
The compression of the gas in the bow shock heats the gas, same as in the bicycle pump in your garage. The compression can be a factor of 500 or more for shuttle, even higher for Orion because of its higher speeds. Of course, at the altitudes where a spacecraft experiences peak heating, the atmospheric pressure is low, about 0.00002 atmospheres at 250,000 ft. altitude. Therefore, the pressures on the surface of the spacecraft are still low, about 0.1 to 1 atmosphere for most NASA spacecraft entering Earth's atmosphere. When rapid compression of a gas occurs without allowing it enough time to cool (called adiabatic compression), the temperature of the gas will increase. Factors of 20 (shuttle) to 30 (Orion) higher temperature can occur behind a normal shock, producing gas temperatures up to 10,000 degrees K, or greater, inside the bow shock. Since the compressed gas is hotter than the body of the spacecraft, the gas transfers heat to the surface of the heat shield as it flows over it. Certainly it is true that viscous forces are required to transfer the heat energy across the boundary layer, (the boundary layer is the thin layer of slower-moving gas at the surface of the spacecraft). But the viscous forces that give rise to friction and frictional heating in the boundary layer produce no more than about 1 percent of the heating to the vehicle.
NASA Ames Research Center
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