Big Boost for Small Spacecraft
The picosat revolution is in full swing. Scientists, engineers, and students alike are all participating in the development of these cheap, small, and increasingly complex spacecraft. Much like the ubiquitous blizzard of smart phone apps that has overtaken the general public in recent years, so too have picosat applications within the space science community. But the technology is still at a nascent stage, and though picosats are advancing technically at an exponential rate each year, they still lack many basic capabilities. For instance, most picosats don’t have propulsion of any kind.
Often a secondary payload, picosats are consistently at the mercy of the primary payloads that they are launched with. There is currently no effective way for picosats to enter into new orbits or complete precise maneuvers while on orbit.
Essentially, picosats are stuck in a limited orbit without the ability to move autonomously in space. This lack of dynamism, in terms of navigation, greatly constricts the kinds of research and functions picosats can perform. Currently, the vast majority of picosats continue to lack engines or propulsion systems, and those propulsion systems that are available are utilizing technology that is expensive or doesn’t work particularly efficiently. Aerospace’s Teresa Moore plans to change that.
“Everybody, not just Aerospace, but everybody in the community is really eager to develop propulsion for picosats because it’s so useful,” says Moore. Aside from the obvious navigational improvements that will come with propulsion, Moore also believes that such systems can directly extend mission life. “As secondary payloads, picosats are usually placed in low earth orbit where there’s a lot of atmospheric drag,” says Moore. “So as a result, you crash and burn in a few months. But if you could use propulsion to make up for that atmospheric drag or move the picosat to a higher orbit, then you could extend your mission and remain useful for a long time.”
Over the past few years, Moore has developed a propulsion system for picosats using electrolysis to generate gas, which is then burned to create thrust. The concept took shape over a number of years, during which Moore became deeply familiar with the multifaceted uses of electrolysis/combustion systems.
In 2005, Moore began work on a Defense Advanced Research Projects Agency project that utilized electrolysis as a means of dynamically reshaping a helicopter blade. By altering the blade’s cross section, the helicopter would hopefully retain a higher weight capacity. The project stalled and was eventually scrapped, leading Moore and her colleagues to apply their research in a new area. Their next electrolysis project involved the development of a system that could detonate mines in an underwater minefield. Moore would work on numerous incarnations of the DARPA project until 2011.
As her work with DARPA was winding down, Moore met with Aerospace’s CubeSat team and discussed the possibility of developing a thruster for their CubeSats. Moore’s main goal was to develop a thruster that improved greatly upon the capabilities of the cold gas thruster. Cold gas thrusters had flown in picosat missions before, but are inherently inefficient. Moore saw a great opportunity for innovation and set out to develop a CubeSat propulsion system of her own. In collaboration with the CubeSat team, Moore wrote an Independent Research and Development (IRAD) proposal and received funding for the project in 2010.
There are many engineering restrictions that come with the development of a CubeSat. First, and most significantly, is the size. Since the CubeSat itself is quite small (a standard 1U cubesat is 10 X 10 X 10 cm), any additions to the satellite must be compact and efficiently designed. Since CubeSats tend to be secondary payloads, they are forbidden from carrying hazardous or high-pressure materials that might compromise the successful deployment of the primary payload. So, when developing her propulsion system, Moore had to produce a design that was incredibly small and free of any chemicals that might be considered hazardous— a very difficult proposition when building a propulsion system.
But Moore’s experience with electrolysis proved to be the solution. With her design, electrons from the CubeSat’s solar panels are pumped through a polymer electrolyte membrane assembly submerged in water, which splits the water into hydrogen and oxygen gas. “CubeSats have tons of restrictions, so if you wanted to launch just a tank of oxygen and a tank of hydrogen, you couldn’t do it,” says Moore. “But, you can use water because water is benign. So, if you launch with pure water you can wait till the CubeSat is on orbit and use electrolysis to turn your water into hydrogen and oxygen. It’s safe to launch and it gives you the high-performance of hydrogen and oxygen combustion.” In addition, Moore’s design only takes up ¼ U, leaving plenty of room on a CubeSat for scientific instruments or supplementary applications.
The initial tests of Moore’s propulsion system have been successful, showing that it can change the velocity of a CubeSat by 50 meters per second (50 m/s delta v), which, based on the thruster carrying about three tablespoons of water, could potentially raise a 1U CubeSat by about 100 km while on orbit. In tests, the electrolysis propulsion system clearly outperforms the cold gas system and meets all of the power specifications for a 1U CubeSat. In addition, the cost of Moore’s design is exceptionally low, totaling between $5,000 and $10,000, while current commercially available cold gas systems cost between $100,000 and $500,000. A few on-orbit electrolysis systems may be coming to market in the next year or so, but they are only appropriate for larger picosats. Moore has developed a remarkable system that is more efficient, smaller, and cheaper than anything on the market today.
Still, more testing is required before Moore’s design gets to take flight. She sees a bright future for propulsion as the evolution of the CubeSat continues to accelerate. “It’s going to be very exciting when we start getting more CubeSats with propulsion on them,” says Moore. “We can make inspector satellites and maybe even repair satellites or refueling satellites or garbage-collecting satellites. There’s a lot of space junk in orbit and it would be really great if you could have a garbage truck satellite that could be used for clean up. There are tons of possibilities.” Moore’s propulsion system and others like it will undoubtedly revolutionize the uses and applications for picosats in the coming years. Propulsion brings autonomy to the small satellite, allowing it to control its own destiny while on orbit— no longer helplessly floating in space.