April 11, 2013
Aerospace engineering PhD candidate Christopher Combs hopes to change the future of space access as he works to understand how space vehicles can safely re-enter the Earth’s atmosphere at hypersonic speeds. Collaborating with his faculty advisor, Professor Noel Clemens, the duo is gathering data to gain a better understanding of what occurs to the heat shield of a space vehicle during atmospheric entry. Their work could ultimately lead to improved and more cost efficient designs of new spacecraft.
Combs is a recent recipient of the prestigious NASA Space Technology Research Fellowship, a program that allows graduate student researchers who show significant potential to contribute to NASA’s goal of creating innovative new space technologies for our nation’s science, exploration and economic future.
As part of the fellowship, Combs conducts research at NASA Centers and U.S. Research and Development laboratories in addition to his work here at The University of Texas at Austin. The training grant covers his tuition, stipend and travel expenses.
Combs came to UT after receiving a bachelor’s degree in mechanical engineering from the University of Evansville. While his mechanical engineering degree gave him a broad perspective, his heart has always been in aerospace engineering and he has long dreamed of working at NASA as a researcher.
Researchers still have much to learn about the physics involved in the re-entry process and currently have a difficult time accurately predicting the effects of re-entry. This is in large part because ablation – the removal of material from a surface by friction, heating, and other aerodynamic forces – is extremely complex and has proved to be challenging to model computationally. Combs and Clemens are studying how the products of ablation are transported in the boundary layer over the heat shield.
During an actual re-entry event, the Orion capsule will be traveling at several miles per second and experiencing temperatures of thousands of degrees Fahrenheit. These conditions are extremely difficult to recreate in a university laboratory. Therefore, instead of employing a conventional ablative heat shield material designed to withstand these extreme conditions, the duo opted to use a low-temperature ablator: naphthalene. Naphthalene – a solid aromatic hydrocarbon – is the main component of ‘moth balls,’ which is used to protect clothing from insects.
The substance is ideal for this application for two reasons. First, it fluoresces when excited with ultra-violet laser light, allowing the naphthalene to be imaged. Second, naphthalene readily sublimes—transitions directly from the solid phase to the gas phase—at temperatures experienced in the Mach 5 wind tunnel facility at UT’s Pickle Research Campus.
Using a technique called planar laser-induced fluorescence (PLIF), Combs is working to develop an improved understanding of the ablation process. PLIF allows the naphthalene vapor emanating from the heat shield to be visualized by fluorescing the vapor with a sheet of laser light. The naphthalene fluorescence is then imaged with a scientific-grade camera. These images can be used to verify computations that aim to predict the effects of the re-entry process and improve the design of ablative heat shields.
“Developing a better understanding of the ablation process will allow space vehicles to travel with a more efficient design and at a lower cost,” Combs said. “Heat shields are often overdesigned as researchers do not know how large they need to be for a safe re-entry. This heavy design makes it more expensive to travel to space. By developing lighter, more compact heat shields, scientists will be able to send more vehicles into space for the same cost.”
Clemens and Combs were recently recognized for their research when they won the 2013 AIAA Aerodynamic Measurement Technology Best Paper Award.
Combs plans to graduate in May 2015 and hopes to pursue a career at NASA as a research engineer.
