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Purdue University graduate student Katherine Fowee and postdoctoral research associate Anthony Cofer work on a new micropropulsion system for miniature satellites, or CubeSats.

CubeSat prototype to use water as propellant

Conventional satellites are about the size of a school bus, weigh thousands of pounds, and sometimes cost hundreds of millions of dollars. They also require specialized electronics that can withstand the harsh conditions of space. These are just a few reasons why microsatellites and nanosatellites, or CubeSats, have increasingly become the satellites of choice.

For example, thousands of CubeSats can be launched as a constellation to perform a variety of tasks, from high-resolution imaging and Internet services, to disaster response, environmental monitoring, and military surveillance. And CubeSats can be built with off-the-shelf components, thus making them not only relatively inexpensive, but essentially disposable, minimizing the monetary and mission impact of losing individual satellites.

However, there is still work to be done. To achieve their full potential, CubeSats will require micropropulsion devices to deliver precise low-thrust “impulse bits” for scientific, commercial, and military space applications.

“There have been substantial improvements made in micropropulsion technologies, but further reductions in mass, volume, and power are necessary for integration with small spacecraft,” said Alina Alexeenko, Professor, Purdue University’s School of Aeronautics and Astronautics. She is leading a team—a unique mix of undergraduate, graduate, and PhD students—in an R&D effort for a new micropropulsion system that uses ultra-purified water.

“Water is thought to be abundant on the Martian moon Phobos, making it potentially a huge gas station in space,” she said. “Water is also a very clean propellant, reducing risk of contamination of sensitive instruments by the backflow from thruster plumes.

As she describes the new system, called a Film-Evaporation MEMS Tunable Array (FEMTA) thruster, it uses capillaries small enough to harness the microscopic properties of water. Because the capillaries are only about 10 µm 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. (To watch a video about the technology, click here.)

CubeSats are made up of several units, each measuring 10-cm³. 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 kg, 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.

The FEMTA technology is a MEMS (micro-electromechanical system), which are tiny machines that contain components measured on the scale of microns. The thruster demonstrated a thrust-to-power ratio of 230 mN/W for impulses lasting 80 s.

“This is a very low power,” Alexeenko said. “We demonstrate that one 180º 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 inertial measurement unit (IMU) 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.

A patent application for the concept has been filed through the Purdue Research Foundation’s Office of Technology Commercialization. The nozzles for the system were fabricated in the Scifres Nanofabrication Laboratory in the Birck Nanotechnology Center in Purdue’s Discovery Park.

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