A vital part of NASA's Mars 2020 mission has passed an important milestone, with the unmanned probe's heat shield passing its final static load test under the thermal conditions it will encounter as it enters the Martian atmosphere. Designed and built by Lockheed Martin, the 15 ft (4.5 m) ablative shield is one of the largest ever built.
When the Mars 2020 spacecraft reaches the Red Planet on February 18, 2021, it will face the most dangerous phase of the mission – hitting the atmosphere at interplanetary speeds. Sealed inside its protective aeroshell, the probe will be traveling at 12,000 mph (19,300 kph) as it plunges into the thin Martian atmosphere, piling up the air in front of it in a shockwave that will slow it down to supersonic speeds.
As it does so, the trapped air will heat until it becomes an incandescent plasma that is so hot that it would burn through the spacecraft as if it were tissue paper if it weren't for the large, saucer-shaped heat shield that covers the leading surface of the aeroshell. The 10th such aeroshell shield made by Lockheed for a NASA mission, its phenolic plastic structure protects the vehicle by carrying off the heat as the shield burns away layer by layer.
According to Lockheed, the size of the shield makes the Mars 2020 landing one of the most challenging ever attempted by a Mars mission. Even with decades of experience, the company has to conduct very careful tests to make sure that the shield can not only withstand the heat, but also the thermo-mechanical stresses set up by encountering air so hard that it's like hitting a wall made out of tar bricks.
The latests and final static tests of the shield were completed on April 25. These used vacuum pumps to simulate the stresses of 140,000 lb (63,500 kg) on the shield's structure or the equivalent of 120 percent of the expected flight load. One notable part of the test was that instead of conventional strain gauges and extensometers, the effects of this stress was measured using a new form of photogrammetry called Digital Image Correlation.
Digital Image Correlation involved the shield structure being sealed in a special, decal-like vinyl wrap made up of dark random speckles over a white background. As digital cameras monitored, the stresses applied to the structure caused the speckles to shift, creating a 3D map of how the load produced displacements and surface strains. This allows the engineers to assess the stress factors across the entire structure rather than in a few key locations.
"While we have used this full-field photogrammetry technique on test articles in the past, this is the first successful implementation on official flight hardware," says Sotiris Kellas, NASA Langley aerospace engineer and lead for the technical demonstration. "This technology will allow us to safeguard hardware during testing but more importantly provide data for test analysis correlation and improvement of our design and analysis tools."
Lockheed says that with the completion of the load tests, the company will now apply the Phenolic Impregnated Carbon Ablator (PICA) thermal protection system tiles to the structure before mating the shield with the rest of the aeroshell.
Source: Lockheed Martin