Purdue University graduate student Katherine Fowee and postdoctoral research associate Anthony Cofer work on a new micropropulsion system for miniature satellites called CubeSats. (Image courtesy of Purdue University/Erin Easterling.)
A research team comprised of undergraduate and graduate engineering students at Purdue University has developed a new type of micropropulsion system for CubeSats, using an innovative design of tiny nozzles that release precise bursts of water vapor to maneuver the spacecraft.
Low-cost “microsatellites” and “nanosatellites” are far smaller than conventional spacecraft, and have gained popularity in scientific and educational arenas due to their affordability and flexibility. For example, thousands of these miniature satellites could be launched to perform a variety of tasks either singly or part of a network, from high-resolution imaging and internet services, to disaster response, environmental monitoring and military surveillance.
However, to achieve their full potential, CubeSats will require micropropulsion devices capable of delivering precise low-thrust “impulse bits” for these scientific, commercial and military space applications.
This is where the student research team stepped in. Led by Alina Alexeenko, a professor in Purdue University’s School of Aeronautics and Astronautics, the research was conducted by graduate student Katherine Fowee; undergraduate students Steven Pugia, Ryan Clay, Matthew Fuehne and Margaret Linker; and postdoctoral research associate Anthony Cofer.
“It’s very unusual for undergraduate students to have such a prominent role in advanced research like this,” Alexeenko said.
The students performed the research as part of a propulsion design course.
The research team has worked to develop a new micropropulsion system that uses ultra-purified water.
“Water is thought to be abundant on the Martian moon Phobos,” Alexeenko said. “Making it potentially a huge gas station in space. Water is also a very clean propellant, reducing risk of contamination of sensitive instruments by the backflow from thruster plumes.”
The new system is called a Film-Evaporation MEMS Tunable Array, or FEMTA thruster. The thruster system uses capillaries small enough to harness the microscopic properties of water. Because the capillaries are only about 10 micrometers in diameter, the surface tension of the fluid keeps it from flowing out, even in the vacuum of space. Activating small heaters located near the ends of the capillaries creates water vapor and provides thrust. In this way, the capillaries become valves that can be turned on and off by activating the heaters. The technology is similar to an inkjet printer, which uses heaters to push out droplets of ink.
This tiny engine is part of the new micropropulsion system. (Image courtesy of Purdue University photo/Erin Easterling.)
Most CubeSats are made up of several units, each measuring 10-centimeters cubed. In the Purdue research, four FEMTA thrusters loaded with about a teaspoon of water were integrated into a one-unit CubeSat prototype and tested in a vacuum.
The prototype, which weighs 2.8 kilograms, or about six pounds, contained electronics and an inertial measurement unit sensor to monitor the performance of the thruster system, which rotates the satellite using short-lived bursts of water vapor.
Typical satellites are about the size of a school bus, weigh thousands of pounds and sometimes cost hundreds of millions of dollars. And while conventional satellites require specialized electronics that can withstand the harsh conditions of space, CubeSats can be built with low-cost, off-the-shelf components. Constellations of many inexpensive, disposable satellites might be launched, minimizing the impact of losing individual satellites.
However, improvements are needed in micropropulsion systems to mobilize and precisely control the satellites.
“There have been substantial improvements made in micropropulsion technologies, but further reductions in mass, volume, and power are necessary for integration with small spacecraft,” Alexeenko said.
The FEMTA technology is a micro-electromechanical system, or a MEMS, which are tiny machines that contain components measured on the scale of microns, or millionths of a meter. The thruster demonstrated a thrust-to-power ratio of 230 micronewtons per watt for impulses lasting 80 seconds.
“This is a very low power,” Alexeenko said. “We demonstrate that one 180-degree rotation can be performed in less than a minute and requires less than a quarter watt, showing that FEMTA is a viable method for attitude control of CubeSats.”
The FEMTA thrusters are microscale nozzles manufactured on silicon wafers using nanofabrication techniques common in industry. The model was tested in Purdue’s High Vacuum Facility’s large vacuum chamber.
Inside the CubeSat. (Image courtesy of Purdue University/Erin Easterling.)
Although the researchers used four thrusters, which allow the satellite to rotate on a single axis, a fully functional satellite would require 12 thrusters for 3-axis rotation.
The team built the system with inexpensive, commercially available devices that are integral for the Internet of Things (IoT), an emerging phenomenon in which many everyday objects such as appliances and cars have their own internet addresses and can communicate.
“These undergraduate students integrated all the IOT technologies, which, frankly, they know more about than I do,” Alexeenko added.
The inertial measurement unit handles 10 different types of measurements needed to maneuver and control the satellite. An onboard computer wirelessly receives signals to fire the thruster and transmits motion data using this IMU chip.
“What we really want to do next is integrate our system into a satellite for an actual space mission,” she said.
The research involved a collaboration with NASA’s Goddard Space Flight Center through the space agency’s SmallSat Technology Partnership program, which provided critical funding since the concept inception in 2013.
To learn more, visit the Purdue College of Engineering School of Aeronautics and Astronautics website.