Small-scale satellites show a lot promise, but unless they have equally small-scale thrusters they’re pretty limited in what they can do. Unfortunately conventional thrusters are heavy and take up a lot of valuable space, but a penny-sized rocket engine developed at MIT holds the prospect of not only increasing the capabilities of miniature satellites, but of combating space junk as well.
Cubesats are a class of nanosatellites. Unlike most satellites, which weigh in at several tons and can be as big as a bus, cubesats are, as the name implies, tiny cubes only four inches (10.16 cm) on a side and weighing about three pounds (1.36 kg). They’re attractive to space engineers and scientist because they’re small enough to be launched as hitchhikers with larger payloads and they can be used either singly, flown in formation or docked together like building blocks to form a bigger satellite.
The only problem with cubesats is that space in them is limited and so they tend to economize on things like rocket thrusters for attitude control. Nanosatellite engines do exist, but they are complicated and large in comparison to the tiny satellite, which means that installing them means some severe trade offs in terms of payload and capability.
The MIT team led by associate professor of aeronautics and astronautics Paulo Lozano is tackling this problem with the development of a rocket thruster that’s about the size and shape of a sugar cube. Its upper face is made up of 500 microscopic tips, each a tiny thruster in its own right. The purpose of this thruster is to greatly simplify the design and hence save space and weight. Whether burning fuel or using cold gas, conventional thrusters need valves, tanks, pipes, venturis and other components that take up weight and space. The MIT microthrusters are not only smaller, they’re mechanically very simple, yet technologically very sophisticated.
Each thruster is made of a reservoir of fuel at the bottom of the cube that Lozano describes as a “‘liquid plasma’ of free-floating ions.” Above this reservoir are layers of porous metal. Each pore leads into more numerous, smaller pores on the next level like the spreading branches of a tree. By the time they reach the top layer, there are 500 pores that make up the thruster tips. On the very top is a perforated gold plate, which acts as a cathode. When the plate is electrified, it charges the fuel drops in the thruster tips, turning them into charged ions and expelling them at high speed to generate thrust. In essence, the thruster is a tiny ion drive.
The clever bit about this whole arrangement is that the ions shooting out of the thruster generate a capillary effect in the system of pores. Like sap being drawn up a tree, the capillary pressure draws the fuel to the thruster tips without any mechanical pumps or valves.
To test the thrusters, the MIT team had to employ a novel apparatus. They mounted the thrusters on a mockup satellite, suspended it in space using counterbalanced magnets and then placed it in a vacuum chamber. Part of the reason for doing this is that the thrust of the engine isn’t much - only about 50 micronewtons, but for a tiny satellite in freefall, it’s enough for practical attitude control and changing orbits.
The reason why Lozano and his team are developing these thrusters is more than just expanding the capabilities of cubesats. The current technology means that even cubesats with conventional thrusters will have a very limited life. If they’re launched into low-earth orbit, they will end up passively circling the globe until their orbits decay and they burn up in the atmosphere. However, if they’re put into a higher orbit, they are destined to become hazardous space junk.
“These satellites could stay in space forever as trash,” said Lozano.“This trash could collide with other satellites. … You could basically stop the Space Age with just a handful of collisions.”
The hope is that these thrusters could be used to guide the cubesats into the atmosphere at the end of their missions. The team also notes that small increases in electrical voltage produces a disproportionate increase in thrust. This means that in the future cubesats could be sent to rendezvous with dead conventional satellites and herd them into reentry like fleets of miniature space tugs. In addition, Lozano also believes that these thrusters could be formed into large panels that could be used by larger satellites to tack across the heavens.
Paulo Lozano discusses the microthruster technology in the MIT video below.