'Shields to Maximum, Mr. Scott'
- Sunday, Jul 14, 2013
According to NASA, there are more than 21,000 pieces of “space junk” roughly the size of baseballs (larger than 10 centimeters) in orbit, and about 500,000 pieces that are golf ball-sized (between 1 and 10 centimeters).
Sure, space is big, but when a piece of space junk strikes a spacecraft, the collision occurs at a velocity of 5 to 15 kilometers per second — roughly 10 times faster than a speeding bullet.
“If a spacecraft is hit by orbital debris it may damage the thermal protection system,” said Eric Fahrenthold, 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. 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 re-entry.”
Only some of the collisions that may occur in low earth orbit can be reproduced in the laboratory. To determine the potential impact of fast-moving orbital debris on spacecraft, and to assist NASA in the design of shielding that can withstand hypervelocity impacts, Cockrell School of Engineering’s Fahrenthold and his team developed a numerical algorithm that simulates the shock physics of orbital debris particles striking the layers of Kevlar, metal and fiberglass that make up a space vehicle’s outer defenses.
Running hundreds of simulations on the Ranger, Lonestar and Stampede supercomputers at the Texas Advanced Computing Center, Fahrenthold and his students have assisted NASA in the development of ballistic limit curves that predict whether a shield will be perforated when hit by a projectile of a given size and speed. NASA uses ballistic limit curves in the design and risk analysis of current and future spacecraft.
Results from some of the Fahrenthold group’s impact dynamics research were presented at the April 2013 American Institute for Aeronautics and Astronautics’ (AIAA) meeting, and have recently been published in the journals Smart Materials and Structures and International Journal for Numerical Methods in Engineering. In the paper presented at the AIAA conference, they showed in detail how different characteristics of a hypervelocity collision, such as the speed, impact angle and size of the debris, could affect the depth of the cavity produced in ceramic tile thermal protection systems.
The development of these models is not just a shot in the dark. Fahrenthold’s simulations have been tested exhaustively against real-world experiments conducted by NASA, which uses light gas guns to launch centimeter-size projectiles at speeds up to 10 kilometers per second.
“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,” Fahrenthold said. “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.”
Back on land, Fahrenthold and graduate student Moss Shimek extended this hybrid method to study the impact of projectiles on body armor materials in research supported by the Office of Naval Research. Some of the same materials used on spacecraft for orbital debris protection, such as Kevlar, are also used in body armor.
According to Fahrenthold, this method offers a fundamentally new way of simulating fabric impacts, which have been modeled with conventional finite element methods for more than 20 years. The model parameters used in the simulation, such as the material’s strength, flexibility and thermal properties, are provided by experimentalists. The supercomputer simulations then replicate the physics of projectile impact and yarn fracture, and capture the complex interaction of the multiple layers of a fabric protection system — some fragments getting caught in the mesh of yarns, others breaking through the layers and perforating the barrier.
Moss Shimek’s dissertation research added a new wrinkle to the fabric model by representing the various weaves used in the manufacture of Kevlar and ultra-high molecular weight of another leading protective material, polyethylene (another leading protective material) barriers, including harness-satin, basket and twill weaves. Each weave type has advantages and disadvantages when used in body armor designed to protect military and police personnel. Layering the different weaves, many believe, can provide improved protection.
Fahrenthold and Shimek (currently a post-doctoral research associate at Los Alamos National Laboratory) explored the performance of various weave types using both experiments and simulations. In the November 2012 issue of the AIAA Journal, Shimek and Fahrenthold showed that in some cases, the weave type of the fabric material can greatly influence fabric barrier performance.
“Currently, body armor normally uses the plain weave, but research has shown that different weaves that are more flexible might be better, for example in extremity protection,” Shimek said.
What can researchers learn about the layer-to-layer impact response of a fabric barrier through simulation? Can body armor be improved by varying the weave type of the many layers in a typical fabric barrier? Can simulation assist the design engineer in developing orbital debris shields that better protect spacecraft?
The range of engineering design questions is endless, and simulation can play an important role in more efficient development of improved impact-protection systems.
"We are trying to make fundamental improvements in numerical algorithms, and validate those algorithms against experiment,” Fahrenthold said. “This can provide improved tools for engineering design and allow simulation-based research to contribute in areas where experiments are very difficult to do or very expensive."
Written by Aaron Dubrow. A version of this story originally appeared on the Texas Advanced Computing Center’s website.