Building a Water-Powered CubeSat
Jeff Bezos’ infamous statement made last year that “there’s not that much interesting about CubeSats” is starting to look increasingly thread bare as a new type of micro-propulsion system, making use of tiny nozzles to release precise bursts of water vapor to manoeuvre the spacecraft, has now been developed.
Created almost twenty years ago the CubeSat standard has lowered the barrier to entry to the point where you can put your own satellite into orbit for not much more than the price of a high end car. The 10 cm × 10 cm × 10 cm cubes have become so common that you can buy most of the structural and flight components of your satellite straight off the shelf.
So far more than 700 CubeSats have made their way to orbit, and the predicted sharp rise in the number of satellites needing to be launched over the next few years which has triggered a smallsat launcher war with a new generation of small launchers being built and tested.
However as the numbers of CubeSats increase, so do worries around orbital debris. Most of small satellites are not equipped with thrusters, which would give them a way to move out of the way of other satellites to prevent collision, or to de-orbit themselves at end-of-life. According to NASA, “At higher altitudes, all CubeSats display longer on-orbit lifetimes and non-compliant residence times. Lower altitude deployments, such as those from the ISS, are projected to be compliant.” In other words, CubeSats are lingering in orbit too long, and becoming potential hazards to navigation.
While some CubeSats have launched with thrusters onboard — low ∆v pulsed plasma thrusters intended for CubeSats can even be bought off the shelf — they’re expensive, especially compared to the relatively low initial cost of the CubeSat itself. Most small satellites therefore make due with spin or gyroscopic stabilisation, and forgo station keeping or manoeuvring thrusters.
While this lack is partially due budget, and to the size and weight limitations of the CubeSat form factor, there are also issues around launch. With most CubeSats riding as secondary payloads, or being deployed from the ISS, having possibly explosive propellants onboard is just not practical.
This has led to people to look at alternatives, and one of these is water power. The NASA Cube Quest Challenge may even see a water-powered CubeSat from Cornell University launch as payload on the EM-1 shakedown flight of the Space Launch System into cislunar orbit.
However new work at Purdue University by a team—consisting of research student Katherine Fowee, postdoctoral researcher Anthony Cofe, and four undergraduate students; Steven Pugia, Ryan Clay, Matthew Fuehne, and Margaret Linker—led by Alina Alexeenko, a professor at the School of Aeronautics and Astronautics, into MEMS based water thrusters have crammed the technology into even smaller spaces. Opening up the possibility that even the smallest 1U CubeSats could in future be equipped with thrusters.
“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.”
Like many of the new generation of CubeSats the test satellite was technologies we as makers will find rather familiar, like the Raspberry Pi.
The test satellite was built around a Raspberry Pi and even uses several of the distinctive blue Adafruit boards — a PowerBoost board for battery management, along with a 9-DOF inertial measurement sensor board for orientation. “[The] undergraduate students integrated all the [Internet of Things] technologies, which, frankly, they know more about than I do,” said Alexeenko.
Other than the frame most of the parts might well be found in your own workshop, and you can 3D print your own cubesat frame.
The prototype, weighing in at a hefty 2.8 kg (about 6 lb) uses the inertial measurement unit to monitor the performance of the thruster system, which rotates the satellite using short-lived bursts of water vapor.
The model was tested in Purdue’s High Vacuum Facility’s large vacuum chamber. Suspended on thin thread inside the vacuum chamber, the thruster demonstrated a thrust-to-power ratio of 230 µN per W for impulses lasting 80 s. “This is a very low power,” said Alexeenko, “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.”
That’s going to be needed as CubeSats are increasingly being used in more ambitious missions. The Planetary Society has already launched a light sail prototype—based around a 3U CubeSat—into orbit, and after a partial successful mission, a second light sail mission is almost ready for launch. The InSight Mars Lander mission which is scheduled to launch next year will carry the first CubeSats into Mars orbit where they’ll be ejected before decent to serve as orbital communication relays for the lander.
Despite launch failures, and other setbacks in orbit, CubeSats have offered an invaluable stepping stone to Low Earth Orbit at a fraction of the price of traditional satellites and everyone from University teams, to small companies, and even elementary schools have managed to launch hardware into orbit. Just by being cheap, they have driven innovation, and created a market that could see clusters of thousands of satellites launched to make up orbital constellations to provide an Internet connection anywhere on the planet.