What if every new car you bought came with a full tank of gas and no way to refill it? Once the gas ran out, you would have to abandon the car and buy another one. Sound preposterous? Environmentally unsound and economically wasteful? Maybe so, but it is exactly the philosophy behind spacecraft design.
“Every spacecraft is designed for one mission or job, which is limited by the amount of propellant you can put onboard,” says Elaine M. Petro, Mechanical and Aerospace Engineering. “Typically, the vehicle has to be in a certain orbit to accomplish its mission, but no orbit is inherently stable. To stay there, the spacecraft has to use its onboard propulsion system. When the propellant runs out, there’s no way to refuel.”
Consider that the cheapest spacecraft costs over $1 million from conception to orbit yet only lasts two years, and the overall unsustainability of this approach becomes obvious, says Petro. “Everything else on the vehicle is still working,” she points out. “It’s just that it’s run out of propellant.”
In Search of New Propulsion Solutions
Petro wants to overcome challenges in space propulsion that limit our ability to explore space—things such as the one-off spacecraft dilemma. “People are starting to see a big opportunity to change the paradigm for the way we build spacecraft,” she says. “Instead of being limited by what can fit in the package, we have to create a whole ecosystem in orbit. We absolutely need the ability to refuel in orbit, for instance, so creating propulsion systems that are compatible with that is a priority.”
In 2020, Petro joined the Cornell faculty and founded ASTRAlab (Advanced Space Transit and Architectures lab)—intending to push her research beyond that of the usual space propulsion lab. “I want to think about how propulsion and the technologies we study fit into space architecture overall,” she says. “You can’t design any one system or look forward to what you need from future systems without understanding the rest of the spacecraft or, more broadly, the contexts in which you’re designing the vehicle.”
Petro focuses on high-efficiency electric propulsion systems—the Teslas of the spacecraft world. This type of spacecraft hitches a ride as the payload of a launch vehicle that uses hydrocarbons as fuel. Once in space, the spacecraft is released into orbit and relies on its electric propulsion system to move around. Unlike an electric vehicle on Earth, however, the spacecraft still needs a propellant because there is nothing else in space to push off of to gain momentum. “The trick is to design the system to expel air at ten to one hundred times faster than the best chemical rocket can manage,” Petro explains. “So you need a lot less propellant to get the same response from your spacecraft.”
Propellants pose a critical challenge in modern spacecraft design. Water is the third most plentiful molecule in the universe—present on many planets and moons, in the rings of Saturn and in the tails of comets. Using water as fuel seems to make a lot of sense, but is it feasible? Petro tackled that question in her PhD research at the University of Maryland where she compared the performance of water to other more traditional propellants.
“We switched out the propellant from pressurized gas to water and modeled the performance of the propulsion system to establish the vehicle parameters necessary to achieve comparable performance,” Petro says. “And where the use of water wasn’t favorable, we were able to specify the design changes needed to make it better.”
Now, at Cornell, Petro is continuing to explore water-as-fuel, working with departmental colleague Mason A. Peck and his lab. While Petro’s original research looked at the performance possibilities of actual water, Peck’s lab is focusing on designing a propulsion system that breaks apart the hydrogen and oxygen in water molecules, then recombines and ignites them with a spark that causes combustion, generating a hot exhaust plume.
“We’re looking at ways to combine the two systems,” Petro says. “The water-based electric propulsion system gives continual thrust so you might want to use it for drag control—trying to counteract a long, steady force. But a system that breaks apart water [like the one Peck is designing] gives short bursts of thrust. So that might be best for a quick maneuver, like changing the vehicle’s orientation or moving out of the way of something quickly.”
“We are getting to the point where it is possible to build an entire propulsion module on a silicon chip.”
As a first step, Petro and Peck have joined with Andrew van Paridon, visiting research scientist with ASTRAlab, to survey all the potential technologies that use water as a propellant. Their effort is meant to benefit the entire aerospace community. “We’re doing an exhaustive publication in the summer of 2021 looking at the performance of these different systems,” says Petro. “People trying to design a hybrid system might want to use our survey to pick the best system for their applications from each category.”
Micropropulsion for Small Spacecraft
Petro also designs micropropulsion systems for small spacecraft, which can be anywhere from the size of a loaf of bread down to a penny. These vehicles have a range of potential uses: Fleets of camera-equipped, cell phone–sized vehicles could explore planets, moons, and asteroids in the solar system, for instance. Tiny spacecraft the size of a wafer could be sent to nearby stars.
“The smaller the spacecraft, the less propellant it needs,” Petro says. “And the less mass you have, the more acceleration you can get. You would need an incredibly small spacecraft to be able to go fast enough to get to a nearby star on a human-lifetime timescale.”
The technology for small spacecraft propulsion systems looks fundamentally different from a traditional rocket or spacecraft system, however. “We are getting to the point where it is possible to build an entire propulsion module on a silicon chip,” Petro says. “We store the propellant as a liquid and use capillary forces to move it around. The liquid is electrically conductive so when we want to fire the system, we use electrical forces to turn the liquid into a high-velocity plume.”
Petro is currently modeling and creating simulations of the plume. “There are a lot of unanswered questions about optimizing the performance and interactions of these new propellants with the rest of the spacecraft,” she explains. “I want to get a really good model of the application of various parameters and the results we’ll get, which will then help us achieve a finely tuned design.”
To Make the Unfeasible Feasible
Petro earned her undergraduate degree in aerospace engineering and worked for a few years at the National Aeronautics and Space Administration’s Goddard Space Flight Center before deciding to pursue spacecraft propulsion in her graduate studies.
“I wanted to try to advance technology so we could do missions that weren’t feasible yet,” she says. “And a key to creating spacecraft that can offer fundamentally new opportunities or capabilities is the propulsion system. It’s the heart of the vehicle, and by working on it, I thought I could really make a difference in what we are able to do and to learn about the solar system.”