June 27, 2013
Protecting Spacecraft From Space Junk With Supercomputers
Lee Rannals for redOrbit.com - Your Universe Online
Researchers developed a numerical algorithm that simulates the shock of orbital debris striking materials that make up a space vehicle's outer defense.
NASA says there are more than 21,000 pieces of "space junk" roughly the size of a baseball in orbit, and about 500,000 pieces the size of a golf-ball. These pieces can strike a spacecraft at a velocity of 3 to 9 miles per second, which is about ten times faster than a speeding bullet.
"If a spacecraft is hit by orbital debris it may damage the thermal protection system," said Eric Fahrenthold, a professor of mechanical engineering at The University of Texas at Austin who studies impact dynamics both experimentally and through numerical simulations. "Even if the impact is not on the main heat shield, it may still adversely affect the spacecraft. The thermal researchers take the results of impact research and assess the effect of a certain impact crater depth and volume on the survivability of a spacecraft during reentry."
NASA uses so-called ballistic limit curves in the design and risk analysis of current and future spacecraft. Results from Fahrenthold and his colleagues' impact dynamic research detail how different characteristics of a hypervelocity collision could affect the depth of the cavity produced in ceramic tile thermal protection systems like those used in a number of spacecraft.
The researchers' simulations have been tested against real-world experiments conducted by NASA using light gas guns to launch tiny projectiles at speeds up to six miles per second. The simulations are evaluated in this speed regime to ensure they can accurately capture the dynamics of hypervelocity impacts.
Validated simulation methods can then be used to estimate impact damage at velocities outside the experimental range as well as to investigate detailed physics that may be difficult to capture using flash x-ray images of experiments.
"We validate our method in the velocity regime where experiments can be performed, then we run simulations at higher velocities, to estimate what we think will happen at higher velocities," Fahrenthold explained. "There are certain things you can do in simulation and certain things you can do in experiment. When they work together, that's a big advantage for the design engineer."
He says this method offers a fundamentally new way of simulating fabric impacts, helping to replicate the physics of projectile impacts and yarn fractures, and capture the complex interaction of the multiple layers of fabric protection system.
"Using a hybrid technique for fabric modeling works well," Fahrenthold said. "When the fabric barrier is hit at very high velocities, as in spacecraft shielding, it's a shock-type impact and the thermal properties are important as well as the mechanical ones."
The team also uses the same numerical method for the NASA simulation to model a series of experiments on layered Kevlar materials, such as those used in body armor.
"Future body armor designs may vary the weave type through a Kevlar stack," Shimek said. "Maybe one weave type is better at dealing with small fragments, while others perform better for larger fragments. Our results suggest that you can use simulation to assist the designer in developing a fragment barrier which can capitalize on those differences."
He concluded that improving their method can provide tools for engineering design, and simulation-based research to contribute in areas where experiments are very difficult to do.