Miniature satellites use water as propellant
Low-cost “microsatellites” and “nanosatellites” far smaller than conventional spacecraft, have become increasingly prevalent. Thousands of the miniature satellites might be launched to perform a variety of tasks, from high-resolution imaging and internet services, to disaster response, environmental monitoring and military surveillance.
“They offer an opportunity for new missions, such as constellation flying and exploration that their larger counterparts cannot economically achieve,” said Alina Alexeenko, a professor in Purdue University’s School of Aeronautics and Astronautics.
However, to achieve their full potential, CubeSats will require micropropulsion devices to deliver precise low-thrust “impulse bits” for scientific, commercial and military space applications.
The new system, called a Film-Evaporation MEMS Tunable Array, or FEMTA thruster, 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.
The research paper was authored by graduate student Katherine Fowee; undergraduate students Steven Pugia, Ryan Clay, Matthew Fuehne and Margaret Linker; postdoctoral research associate Anthony Cofer; and Alexeenko.
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.
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 IoT.
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.
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