The 20 Twenties: Alexis Harroun
Alexis Harroun figured she’d become a doctor, like her father.
She even entered a summer program doing research at a local hospital in Tacoma, Wash., while she was in high school, getting a taste of what could lie ahead as a physician.
But she also enrolled in an online program during her junior year called Washington Aerospace Scholars, which was capped by a week-long summer residency to the Museum of Flight. At the museum, students were divided into teams and competed against each other in challenges related to aerospace engineering.
“My parents were not engineers. So I think it was through that program I really learned about aerospace engineering and how much I liked it,” she says. “I liked the cool planes, rockets and all that, but I also really liked working on a team in that kind of an environment, working on technical problems with other talented individuals.
“In the middle of the program, I thought, ‘I think I’d rather do this. I like this a lot more.’”
So the path was set to become an aerospace engineer.
Harroun spent four years at the University of Washington in Seattle, the last three as part of a student team that built a hybrid sounding rocket. That project piqued her interest in propulsion. An internship at Blue Origin cemented her graduate school destination to explore that passion.
One day, all the employees were asked to wear shirts from their alma maters for a group photo. Washington was well represented, but when someone called “Purdue,” a horde of people came forward. That prompted Harroun to think, “I should look into this,” she says with a laugh.
Through a contact at Blue Origin, she was connected to Stephen Heister, a professor at Purdue’s School of Aeronautics and Astronautics. Heister and Harroun continued talking while Harroun was a senior, and then Heister was on Washington’s campus to give a presentation on his current research focus, rotating detonation engines (RDEs). As Harroun was listening to Heister’s talk, she kept thinking, “He needs to look at the rocket nozzle design,” which is now Harroun’s research area. So after the presentation, Harroun went up to Heister and told him so.
“He said, ‘Yeah, you should come and do that.’ Honestly, that’s literally the thing that has carried me to this point,” Harroun says.
Now, she’s a master’s student under Heister doing computational research into how to design a rocket nozzle for an RDE. It’s that work that prompted Heister to nominate Harroun for “Tomorrow’s Leaders: The 20 Twenties” by Aviation Week Network, in collaboration with the American Institute of Aeronautics and Astronautics (AIAA). Harroun was one of three Purdue students who were selected for the honor, and she will attend the 20 Twenties Awards Luncheon and Aviation Week’s 62nd Annual Laureates Awards on March 14 at the National Building Museum in Washington D.C.
“I think Alexis is very deserving,” Heister says. “She impresses me as being wise beyond her years. She is very contemplative, and that is quite unusual for such a young person.”
Heister says he’d been looking for someone to research RDE nozzles because “the need is great.”
And the challenges are considerable.
Harroun was drawn to the research because she wanted to work on something that was at the forefront of propulsion and rocket engine technology, and RDEs are considered the next big step, she says.
Though RDE technology was proposed in the 1960s, it has generated more interest in the last decade as computational capability and high-speed instrumentation permits researchers to capture events that are happening incredibly fast. (Detonation waves can move at speeds up to 5,000 mph.) Peak pressures behind the detonation wave greatly exceed those in today’s engines and permit greater thermal efficiency from combustion at these conditions.
“Typically for a conventional rocket engine, you have a constant pressure in the engine. It’s easier to design a nozzle when you have one feed pressure. But for an RDE, you have a traveling pressure wave going around the engine, so you have different pressures being fed into the nozzle,” she says. “Basically, that pressure ratio is the driving factor for the design. If you don’t know what the pressure ratio is, it’s an unknown. So I’ve looked into designing an aerospike nozzle for this application.”
Rocket nozzles convert the thermal energy in the engine into kinetic energy by accelerating the products in the engine. It’s the final step in producing thrust in an engine. An aerospike nozzle is a specific type of rocket nozzle that almost looks like an inverted rocket nozzle. Instead of having a bell shape, it’s a spike. Aerospike nozzles are able to capitalize on extracting better efficiency from changing pressure ratios that can help produce more thrust, making them a natural candidate for use with RDEs.
Aerospike nozzles have been used before for this application, but Harroun says “no one has really looked into how to design it correctly for a rocket engine.”
Harroun has attached one of her rocket nozzle designs to a current experimental test campaign conducted in conjunction with fellow students at Maurice J. Zucrow Laboratories. She has sensors to measure pressure on the nozzle and is collecting that pressure data to see if the design is working well.
She’s working to design a nozzle for an RDE specifically to maximize the thrust of the engine, but because the RDE feed pressure varies, there are questions how this hugely unsteady, non-uniform pressure field develops thrust.
“The whole point of looking at RDE technology is to improve the efficiency beyond our engines today,” she says. “There is something like 5 to 10 percent possible gains in thermal efficiency, which sounds small, but if you think about if you’re launching a rocket from Earth and trying to deliver payload into space, that can translate into 50 to 100 percent more payload. If you don’t have the nozzle right, then you might not get those performance benefits. It all adds up.”