Washington, D.C. – NASA has explored much of the solar system, but only a handful of missions have entered the atmospheres of planets other than our own and successfully touched down on their surfaces. Most of our neighboring planets have never seen the wheels of a rover, and never before has a robot we’ve sent to another planet returned to Earth.
NASA needs technology to protect spacecraft as they enter the atmospheres of worlds near and far, from Venus to Uranus. And when we finally bring samples back from places like Mars, NASA needs to ensure those precious pieces of alien worlds are safe as they come back to Earth.
The Heatshield for Extreme Entry Environment Technology, also known as HEEET, is a system to protect a probe against the extreme heat generated when passing through a planet’s atmosphere. HEEET’s development is now complete, and was led by NASA’s Ames Research Center in California’s Silicon Valley, the agency’s hub for thermal protection systems technology.
Heat shields themselves have existed for almost as long as NASA has sent spacecraft into orbit. Safe re-entry into Earth’s atmosphere is made possible by the blunt body design, created by “Harvey” Julian Allen of Ames. It was used for the Apollo program, and every human spacecraft since.
The farther our journeys take us away from Earth, the faster we need to travel, and the hotter things will get when punching through an atmosphere. Heat shields will need to be more robust and versatile for missions to the edge of our solar system, allowing entry into atmospheres even more intense than our own.
New Materials for Future Missions
Going farther, faster and hotter means innovating how NASA constructs the materials used for heat shields. For HEEET, this results in the use of dual-layer, three-dimensional, woven materials capable of reducing entry loads and lowering the mass of heat shields by up to 40%.
The outer layer, exposed to a harsh environment during atmospheric entry, consists of a fine, dense weave using carbon yarns. The inner layer is a low-density, thermally insulating weave consisting of a special yarn that blends together carbon and phenolic materials. Phenolic materials are also used for flame resistance in aircraft to keep passengers and pilots safe.
Heat shield designers can adjust the thickness of the inner layer to keep temperatures low enough to deal with the extreme heat of entering an atmosphere. That lower temperature allows the heat shield to be bonded onto the structure of the spacecraft itself and insulates the cargo inside – whether that’s an astronaut or a sample from another world.
The outer and inner layers are woven together in three dimensions, mechanically interlocking them so they cannot come apart. To create this material, manufacturers employ a basic weaving processes that isn’t too different than what is used to weave cloth or a rug.
For HEEET, computer-controlled looms precisely place the yarns making this kind of complicated three-dimensional weave possible. The materials are woven into flat panels that are formed to fit the shape of the capsule forebody. Then the panels are infused with a low-density version of phenolic material that holds the yarns together and fills the space between them in the weave, resulting in a sturdy final structure.
There are some limits to this method. The size of each finished piece of HEEET material is limited by the size of the loom used to weave the material, thus the HEEET heat shield is made out of a series of tiles. At the points where each tile connects, there’s a gap that needs to be filled in order to bond the tiles together.
The gap filler was the most difficult part of HEEET’s development. The material not only needed to tie the tiles together, but it also must survive the structural loads of launch and entry. It needed to be prepared to face extreme temperatures as well, from the heat of atmospheric entry to the cold of deep space, which averages at 455 degrees Fahrenheit below zero. But through inventive designs, the HEEET team surmounted the challenge – with the help of rigorous testing facilities.
Extreme Testing for Extreme Environments
These materials and systems need to be tested here on Earth before they are used to shepherd million- or billion-dollar projects to other worlds. A facility known as the Arc Jet Complex at Ames is one of the few in the world that can reproduce the extreme conditions of atmospheric entry in a controlled way.
Using an arc jet test chamber, HEEET has been shown to withstand 3500 watts of energy per square centimeter, and five times the atmospheric pressure at sea level. For comparison, that’s over 17 times more energy and over 12 times more pressure than was required of the heat shield used to safely land Curiosity on Mars.
Probes to Saturn and Uranus, landers on Venus, retrieving samples from Mars – these are the kinds of missions that will become reality in future decades. From protecting science instruments sent to other worlds to unlocking mysteries about our solar system, heat shields are vitally important. Because of years of development and testing, HEEET can handle the heat.
- Helen Hwang (NASA image feature, June 2020)
- There and Back Again: How NASA’s Journey to the Moon Paved a Path Home (NASA story, June 2019)
- Ancient Art of Weaving Ready to Head to Mars and Beyond (NASA story, March 2017)