Although maneuvering nanosatellites in space is a complex procedure, a new micro-propulsion method features the simplest of ingredients: water.
The system from Purdue University uses tiny nozzles to release precise bursts of water vapor. The development supports future efforts to steer the miniature “CubeSats” in space.
A Compact Idea
Nanosatellites, which NASA specifies as having a mass of 1 to 10 kilograms, have been increasingly employed to perform a variety of tasks, including high-resolution imaging, Internet services, environmental monitoring, and military surveillance.
Many missions require a particular instrument, such as a camera, spectrometer, or antenna, to be pointed in a specific direction. Purdue’s miniature propulsion system allows a CubeSat, a type of nanosatellite used as a platform for space research, to rotate 360 degrees and stop at any given angle.
“The main benefit this system offers is its compactness,” said postdoctoral research associate Anthony Cofer, who spoke to Tech Briefs and helped to develop the technology. “A complete thruster with 1 gram of propellant can be made as small as 2 cubic centimeters, or a little larger than a sugar cube.”
Many of the current propulsion options being explored for CubeSats are scaled-down versions of larger systems. On the micro-scale, certain physical effects such as surface tension, viscosity, and hydrophobicity become so dominant that the systems become inefficient and unreliable.
“We wanted to try to use these effects instead of fighting them,” said Cofer.
The new technology, called a Film-Evaporation MEMS Tunable Array (FEMTA) thruster, uses tiny capillaries to handle the flow of water. The microscale nozzles are manufactured on silicon wafers via nanofabrication techniques.
Pure silicon’s water-repelling qualities and high surface tension combine to produce a thermal valve with no moving parts. Because the capillaries feature a tiny, 10-micrometer width, the surface tension keeps the fluid from flowing out, even in the vacuum of space.
Tiny heaters, located near the ends of the capillaries, trigger vacuum boiling. The result: water vapor and thrust at lower-than-normal temperatures.
The system allows the capillaries to act as valves that turn on and off with heat activation. The thruster technology is similar to an inkjet printer, said the researchers, which uses heaters to push out droplets of ink.
A ‘Micro’ Test Run
CubeSats generally consist of several units, each measuring 100 cubic centimeters. The Purdue researchers integrated four FEMTA thrusters, filled with approximately one teaspoon of water, into a one-unit CubeSat prototype. The system was then tested in Purdue’s High Vacuum Facility’s large vacuum chamber.
The thruster demonstrated a thrust-to-power ratio of 230 micronewtons per watt. A 1U CubeSat requires about 60 micronewtons to spin 180 degrees in a minute, said Cofer.
The 500-gram prototype contained electronics and an inertial measurement unit (IMU) sensor to monitor the thruster's performance. The IMU sensor handled 10 separate measurements needed to steer the satellite, and an onboard computer wirelessly received signals to fire the thruster and transmit motion data.
The ultra-purified water used by the Purdue-developed system offers efficiency benefits compared to current propulsion options, according to the engineers.
“Water is the very definition of ‘green’ propellant, which eliminates costly equipment and procedures needed to deal with toxic and combustible propellants,” said Cofer. “It is also stored at low pressure so bulky propellant containment systems are not needed like with cold gas.”
There have been substantial improvements made in micro-propulsion technologies, but further reductions in mass, volume, and power are necessary for integration with small spacecraft.
“We are hoping to fit the specifications of our design with a specific space mission,” said Cofer.
NASA's Small Spacecraft Technology Program has selected 13 university teams to collaborate on the demonstration of new technologies and capabilities for the miniature satellites. The technology is an exciting development to NASA as the agency seeks to improve CubeSat capabilities in a number of systems and subsystems, including propulsion.
"Green propulsion, like FEMTA and other propulsion systems, will enable the advancements of CubeSats’ capabilities and their abilities to conduct science missions in the future,” Roger Hunter, NASA's Small Spacecraft Technology Program manager, told Tech Briefs.
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