MIT engineers are developing a new spacecraft propulsion system that combines the advantages of traditional chemical rockets with the efficiency and precision of electric thrusters.
This technology gives small satellites far more flexibility in space. Instead of relying on separate fuel systems for different types of maneuvers, future spacecraft will be able to use a single propellant to perform both rapid movements and slow, highly controlled adjustments.
At the heart of this approach is a specialized fuel that works in both chemical and electric propulsion systems. Previously, these technologies typically required separate propellants and hardware, increasing weight and complexity.
“If you can put chemical and electrical propulsion into one small package, you get the best of both worlds,” says Amelia Bruno, a former postdoctoral fellow in MIT’s Department of Aeronautics and Astronautics (AeroAstro). “This allows small satellites to do more science, more observations, and more interesting missions, all on smaller, cheaper platforms.”
Bruno is the lead author of the new study published in the journal. Journal of Propulsion and Power. The research demonstrated that a “green monopropellant” originally developed by the U.S. Air Force for chemical propulsion can also successfully power small electric thrusters known as electrospray thrusters.
Combination of chemical space propulsion and electric space propulsion
Electrospray thrusters are small rocket engines about the size of a dime. An electric field is used to charge particles in a liquid propellant and eject those particles into space to create thrust.
These thrusters are very fuel efficient and suitable for progressively precise maneuvers. For example, a spaceship can slowly make long journeys between planets while consuming little fuel.
Chemical thrusters serve a different purpose. These provide powerful bursts of thrust, allowing the spacecraft to quickly accelerate, decelerate, ascend, descend, or change position.
MIT researchers believe that identifying a propellant that can power both systems could greatly expand the capabilities of small satellites.
The team is currently working with NASA on a green propulsion dual-mode mission for a briefcase-sized CubeSat with one chemical thruster and four electrospray thrusters. They all draw fuel from a single tank. This mission will be the first to test this type of dual-mode propulsion system on a small spacecraft.
If successful, the technology could allow small satellites to venture far beyond Earth’s orbit.
“We can use electrospray thrusters to send CubeSats to Mars or the asteroid belt, where they can travel slowly,” says study co-author Paulo Lozano, the Miguel Aleman Velasco Professor of Aeronautics and Astronautics at the Massachusetts Institute of Technology. “Then you can use the chemical thrusters to move quickly and observe interesting features. It gives you more flexibility and allows you to do more.”
Why ionic liquid propellants are important
Lozano’s lab develops, manufactures, and tests electrospray propulsion systems for satellites of various sizes, from lunch boxes to small carry-on suitcases.
Compared to large spacecraft, these small satellites are much cheaper to launch. However, their small size requires an equally compact propulsion system.
Electrospray thrusters fit that requirement well. The device created in Lozano’s lab is about the size of a thumbnail. Each thruster is installed above a reservoir containing ionic liquid propellant. When connected to a battery, the ions in the liquid are charged. These charged particles are ejected through small openings in the thruster and generate thrust.
Over the past decade, Lozano’s group has tested numerous designs using different ionic liquid fuels under different operating conditions.
“Ionic liquids are very stable and can even remain liquid in space, but not many materials can do this,” Bruno said. “And it’s basically an ocean of ions. So we can base our technology around that and pull those ions out into electrospray.”
MIT researchers are also collaborating with the U.S. Air Force, which has developed a new ionic liquid fuel known as Advanced SpaceCraft Energetic Non-Toxic propellant (ASCENT). This propellant was originally designed for chemical propulsion systems.
ASCENT was created as a safer alternative to hydrazine, a highly toxic fuel traditionally used in many spacecraft propulsion systems.
“ASCENT happens to be an ionic liquid mixture,” says Bruno. “And we said, this is what we normally use. In theory this should work. Let’s figure out a way.”
Testing ASCENT on an electrospray thruster
To evaluate the fuel, Bruno, Lozano, and former MIT graduate student Matthew Corrado conducted a series of experiments using an ASCENT-powered electrospray thruster.
Each thruster was attached to a small cube-shaped reservoir about the size of a Lego block. The researchers filled each reservoir with 1 gram of ASCENT, a liquid with a viscosity similar to baby oil.
The thruster was mounted on the opposite side of the CubeSat, which was placed on a custom magnetic levitation test platform known as MagLev. The setup is located in a large vacuum chamber that can reproduce conditions similar to those found in space.
During the test, the researchers remotely changed the voltage supplied to the thrusters. The resulting electrospray generated enough force to spin the CubeSat like a floating top.
By measuring the thrust produced and operating the thruster continuously for up to 100 hours, the team was able to assess the performance and efficiency of the fuel.
The results showed that ASCENT successfully powered the electrospray thruster. The fuel exhibited performance comparable to conventional ionic liquid propellants typically used in electric propulsion systems.
“Compared to our regular electrospray propellants, ASCENT can offer similar performance in terms of thrust,” says Bruno. “Now that we know that our thruster works in conjunction with ASCENT, we can start thinking about all the ways we can further improve the thruster.”
NASA mission tests shared fuel tank in space
Now that ASCENT has proven capable of supporting both chemical and electric propulsion, researchers envision future spacecraft carrying a single fuel tank that powers both systems.
The concept will soon face its first real-world test through NASA’s Green Propulsion Dual-Mode Mission, scheduled for launch in November.
“This is the first time that satellites share a propellant tank,” Lozano said.
Beyond deep space exploration, this technology could also improve missions closer to Earth. Lozano points to weather and climate monitoring as one potential application.
“Say a storm is coming and you want to deploy a constellation of small satellites to monitor one location,” he says. “Depending on the nature of the observation, you can choose to send quickly or slowly. And the only way to do that is if you have two propulsion systems, and now you can.”
This research was supported in part by NASA.
