For decades, a human mission to Mars has been dreamed, discussed, and even worked toward—but it hasn’t happened yet. And there are many reasons including the significant technical challenges that stand in the way.
For one, it takes a lot of fuel to get there and viable techniques haven’t yet been developed to successfully harness enough energy to launch a rocket on a 33.9 million-mile road trip—and then bring it back.
Also, a trip like that would take a long time. More time spent in space means more potentially harmful effects on the astronauts’ health. Living in low gravity and being exposed to space radiation for long periods of time changes the human body, as NASA is now finding out, thanks, in part, to a recent year-long space mission by UT alumnus Scott Kelly.
But, UT engineering students led by UT-ORNL Governors Chair for Nuclear Materials Steve Zinkle are working on overcoming these challenges by peering into “exotic” materials that can withstand extreme environments—as in those created by nuclear-powered thermal propulsion.
That’s because one promising approach to get a rocket to Mars and back in a shorter time is by going nuclear.
A nuclear energy reactor can be the answer.
Here’s how it would work: cryogenic hydrogen would flow through a heat source causing it to rapidly expand and release a huge thrust that can propel a rocket. The science behind this propulsion approach is well established but a key challenge is identifying materials that can withstand exposure to corrosive hydrogen at the extreme temperatures required for the reaction. The necessary temperatures are roughly 2,500 Kelvin—above the melting temperature for many elements in the periodic table and hot enough to melt steel by nearly a thousand degrees.
This is where exotic materials come in. Shortly after arriving to UT in 2015, Zinkle’s doctoral student Kelsa Benensky began work on her prestigious NASA Science Technology Research Fellowship. Her work centers around investigating the compatibility of silicon carbide and ultra-high temperature ceramics to high-temperature liquid hydrogen. She conducted her testing at a specialized facility at NASA’s Marshall Space Flight Center in Huntsville, Alabama, where she exposed the materials to temperatures as high as 2,750 Kelvin. Then she brought back the samples to UT-ORNL’s Joint Institute for Advanced Materials to be characterized using the high-powered electron microscopes and other advanced characterization tools to reveal how well they performed.
“Using precision weight change, glancing x-ray diffraction, Raman spectroscopy, and scanning electron microscopy, Kelsa obtains the materials’ microstructural and chemical fingerprints to see if there is a change in the surface composition, for example, if it eroded or corroded. Transmission electron microscopy can then be used if anything needs further scoping,” Zinkle said.
Benesky’s results have been promising. No one had tested the compatibility of these materials with hydrogen at temperatures above 1900 Kelvin before she started her PhD research. Several of the materials she is investigating are doing well at resisting hydrogen corrosion up to at least 2500 Kelvin.
“I’m very grateful to have an advisor who encourages me to step up to the plate on my own projects and allows me the freedom to follow my own research path,” shared Benensky, who added she came to UT because of Zinkle’s experience working on materials development programs for nuclear space applications.
Another doctoral student, Taylor Duffin, is also doing research in Huntsville analyzing the hydrogen compatibility of ceramic-refractory metal, or “cermet,” materials to high temperatures. The plan is for these composite materials to be used as fuel in a nuclear rocket. Zinkle is helping Duffin organize experiments, conduct follow-up characterization, and report his findings.
Without Zinkle and the electron microscopes, Duffin says his research wouldn’t be possible.
“Dr. Zinkle has been essential to my project,” Duffin shared. “And, so have the electron microscopes. Without them I would have to rely on only bulk data like mass loss or size changes without understanding the processes taking place.”
There are many reasons for a mission to Mars: curiosity, a future need for resources, and, potentially, even survival. The work Zinkle and his students are doing is getting us closer to one day making such space travel a mission accomplished.
“Before we started these projects, it was a blank sheet of paper,” Zinkle shared. “We wondered, ‘are some of these high temperature materials a viable option for this extreme operating environment?’ Thanks to the work by Kelsa and Taylor, we have solid experimental results that say ‘yes, these are viable options.’ And that puts us one more step up the ladder to making this a reality.”