Testing an Impenetrable Heat Shield
When NASA’s Orion re-enters Earth’s atmosphere, it will endure temperatures of 5,000 degrees. How do we ensure the spacecraft —and those inside—survive the ride?
If all goes according to plan, NASA’s Orion space capsule will transport its first crew of astronauts to the moon by 2024. When it comes screeching back through Earth’s atmosphere, it will generate temperatures hot enough to melt rock. To protect the capsule and its inhabitants, Orion is equipped with an advanced heat shield designed to endure the extreme conditions of the rapid descent. In fact, it’s so effective, even sound has a hard time penetrating it.
That’s a problem, at least here on Earth. The heat shield is a critical component and needs to be thoroughly tested before flight, as the slightest flaw can be catastrophic. NASA tried many methods to probe the heat shield material, with limited success. Traditional nondestructive methods, such as x-ray and ultrasonic inspection, were simply not up to the task.
NASA then asked experts in nondestructive evaluation (NDE) at The Aerospace Corporation to test Orion’s shielding. According to Dr. Shant Kenderian, Director, Space Materials Laboratory, Aerospace has a reputation for tackling the most frustrating testing problems. “The NDE staff routinely takes on inspection challenges that other experts have deemed undoable,” he said. “The easy ones don’t come to us.”
NASA supplied samples of the heat shield material with known defects present. Working with these samples, Kenderian’s team arrived at a unique technical solution: they combined a special low-frequency wave generator with signal processing techniques that allowed them to tailor the frequency, duration, size, and shape of ultrasonic bursts. By processing multiple echoes from a single scan, they were able to detect relative changes in the bonding region, regardless of the material uniformity. Using additional information from the phase of the reflected waves, they were able to outline each flaw with high precision.
Incorporating all these techniques into a single prototype system enabled the team to create highly accurate maps showing the location, size, and shape of poorly bonded regions, simply by performing a two-dimensional ultrasound scan of the heat shield’s exterior.
Developing the technique further, Aerospace built and demonstrated a portable unit for use in the field. The heat shield material cannot be removed easily from the vehicle without causing damage, so NASA engineers use the hand-held probe to scan the structure and get images back in real time.
The new tool will enable NASA to verify and repair, as necessary, the heat-shield construction and make well-informed risk assessments for the future. It may also save astronauts lives.