Sending spacecraft into low Earth orbit could transform next-generation telecommunications, space exploration and national security. But that altitude - which begins roughly 60 miles above the planet's surface - is a fickle place due to atmospheric drag, and spacecraft would require a new kind of propulsion system to remain in orbit there.
"At the boundary of space, there's still enough residual atmosphere that a spacecraft traveling at hypersonic speeds is going to be slowed down by the atmosphere, and it needs a propulsion system to keep it aloft, otherwise those collisions with residual air will deorbit your spacecraft very quickly," said Elaine Petro, assistant professor of mechanical and aerospace engineering in Cornell Engineering. "And there's no good propulsion solutions right now to do that."
But maybe not for long. Petro and her Cornell collaborators have been using 3D printing to custom build high-efficiency, low-cost electric rockets that, combined with novel propellants, will keep small satellites in low Earth orbit.
The project was launched through the Department of Defense's Defense Advanced Research Projects Agency (DARPA), which sought a handful of teams to compete to devise radical new solutions to the problem - and to do so in a radically short time frame: three years.
Petro, center, conducts a meeting with her research team, including doctoral candidate Stefan Bell, in Upson Hall.
"To go from an idea that nobody had ever tested before in the lab to something that's prototyped on orbit in the course of three years is a difficult challenge, but an exciting challenge," Petro said.
It's one that Petro was well-suited to tackle. Her early interest in math and physics and understanding humanity's place in the universe led to several years at NASA's Goddard Space Flight Center. At NASA, she worked on the MAVEN Mars Orbiter, James Webb Space Telescope and Hubble Space Telescope missions. In 2020, she came to Cornell and established the Advanced Space Transit and Architectures (ASTRA) Lab, which focuses on plasma science and sustainable space exploration, including the design of small satellites.
"We found that small satellites in particular can be really economically efficient. You can accomplish a lot with one or a few small satellites that are much cheaper to build, that otherwise would take a lot longer to accomplish with one large satellite," Petro said. "But the thing that's really hard to do for the small satellite is the propulsion solution."
The team's novel approach is twofold. Working with Sadaf Sobhani, assistant professor of mechanical and aerospace engineering, the group is using additive manufacturing to build features into the system that weren't previously possible. That allows for much cheaper, rapid prototyping, and improved functionality.
The other part of the project involves synthesizing a new propellant that is more robust, provides higher thrust and will be more efficient at keeping small satellites in low Earth orbit.
The traditional propellant, ionic liquids, have numerous benefits: They are storable and don't evaporate in orbit, and they can be ejected with reasonable ease to produce thrust. But the amount of thrust they provide is not enough to keep the spacecraft aloft for long. In collaboration with Emmanuel Giannelis, the Walter R. Read Professor in the Department of Materials Science and Engineering, Petro's team has developed a radical new approach in which they accelerate discrete nanoparticles, instead of molecules, to get the optimal thrust and miles-per-gallon performance in the system, reaching unprecedently high exhaust velocities of 30,000 miles an hour.
"We're able to much more specifically tune the properties of the propellant," she said. "We can control the size of the nanoparticle versus controlling the chemistry of the propellant itself. That's a harder problem to solve."
Developing such an approach requires better understanding of how these novel chemicals behave in an electric rocket and affect its efficiency. To fill those gaps in fundamental knowledge, Petro received a Young Investigator Program Award from the U.S. Air Force in 2023.
"I think it's important to acknowledge the impact that fundamental research has, because that can get lost when we're trying to really frantically come up with new technological solutions," Petro said. "If you don't answer the fundamental questions, you're never going to understand the core problem that's holding your technology back."
That effort, which moves at a far more traditional pace than the DARPA project, received a stop-work order in April. While the funding is now in the process of being restored, its interruption reinforced the importance of federal support for scientific innovation.
"The questions that we're able to answer through government funding are the ones that will allow us to be competitive with technologies over the next few decades of exploration," Petro said. "If we're not investing in these solutions, then I fear that we will fall behind in a new space race. It's very hard to improve a technology without having the fundamental insight that comes through our universities and our basic research."
Space may seem like a distant and often irrelevant place, but the benefits of these technological breakthroughs are decidedly earthbound. The closer small satellites can get to Earth, the more high-quality data they can transmit, at lower cost and with greater efficiency, whether it involves imaging the melting of glaciers, enabling GPS systems or aiding in national defense. And all of that relies on new forms of propulsion, which also can extend the lifespan of the satellites.
"The thing that's sort of radically different between satellite technology and technology on Earth is its lack of reusability. So our satellites are not designed to be refueled. That's a paradigm that we see changing," Petro said. "You need to make the spacecraft itself reusable, and we'll get a lot more out of our investment in technology. It'll make internet cheaper. It'll make GPS more effective. It'll make satellite imagery more accessible and effective to everyone. So it will help all of us, but you might not see the satellite itself."
