"Gas stations" in zero gravity: Purdue experiment establishes foundational science for cryogenic fuel depots in space

To journey and return from other planets, future spacecraft may have to do something they've never done before: refueling in space. Thanks to a Purdue University experiment, scientists are now beginning to understand how cryogenic liquids behave in zero-gravity, and how this affects the future operation of propellant depots in space.
Issam Mudawar (far left) oversees an experiment to test how microgravity affects cryogenic liquids, which is vital to the future operation of space-based propellant depots. (Purdue University photo/Jared Pike)

“Right now, almost all spacecraft are powered by cryogenic liquid fuel,” said Issam Mudawar, the Betty Ruth and Milton B. Hollander Family Professor of Mechanical Engineering. “We know how those super-cooled liquids behave on earth. But when we have propellant depots in space, and we start refueling spacecraft in orbit, how will those liquids behave in a zero-gravity environment?”

Mudawar’s Boiling and Two-Phase Flow Laboratory has a history of establishing foundational heat transfer science. In 2021, he collaborated with Ford Motor Company on a method to cool electric vehicle charging cables, to potentially charge a vehicle in 5 minutes. And a collaboration between Mudawar and NASA’s Glenn Research Center resulted in the largest phase change experiment ever launched into space, which is currently being conducted on the International Space Station.

Now, he’s once again collaborating with NASA to explore the potential of cryogenic fuel depots in space.

“Cryogenic liquids behave differently from other fluids, such as water and other common coolants,” said Mudawar.  “Not only do they have a much lower boiling point, but they have different thermophysical properties such as surface tension and viscosity. The only place to conduct sizable experiments on cryogens in zero-gravity (other than space itself) is on a parabolic flight.”

In 2020, NASA awarded Mudawar a grant to design and build a cryogenic experiment to fly on a Zero Gravity Corporation (ZERO-G) parabolic flight, in which an airplane simulates periods of 15-17 seconds of microgravity by flying up-and-down parabolas. In June 2022, Mudawar’s students conducted their custom-built experiment on the flight.

“Our test rig is the first of its kind,” said Mudawar. “We send cryogenic liquid nitrogen through a test loop, heat it to induce phase change, and capture high-speed video of its boiling flow structure. We also have a large assortment of flow, temperature, and pressure controls and measurement instruments.”

Co-principal investigator Dr. Jason Hartwig of NASA Glenn Research Center accompanied the Purdue researchers on the flights. “The test rig and operation performed flawlessly,” he said. “We were able to collect numerous highly coveted cryogenic flow boiling datasets at various flow conditions in reduced gravity.”

As they continue to analyze the data, Mudawar’s team plan to conduct another flight later in the year. “The data we are gathering will serve as the basis for computational and empirical models for future space-based propellant depots,” he said. “These systems are crucial not just for space systems in low Earth orbit, but also for fueling future missions to both the Moon and Mars.”

A joint Purdue-NASA team conduct an experiment on cryogenic liquids during a ZERO-G parabolic flight. (Left to right) Dr. Jason Hartwig of NASA Glenn Research Center, and Purdue Ph.D. students Nishad Damle, Jeongmin Lee, and Sunjae Kim (Photo courtesy of Zero Gravity Corporation)

Writer: Jared Pike, jaredpike@purdue.edu, 765-496-0374

Source: Issam Mudawar, mudawar@purdue.edu, 765-494-5705