CubeSats are one of the cheapest, most efficient ways to get to space. Each CubeSat unit measures just 10 centimeters on a side, which is usually enough room for solar panels, communications equipment, and a small science payload. It isn’t enough room for an engine, and generally, most CubeSats are dumped into orbit and left to fend for themselves, tumbling aimlessly until drag pulls them to earth after a few months or so. This makes them cheap for a spacecraft (usually a little over $100,000 each including launch costs), but places rather severe limits on what they’re able to accomplish.
In 2013, NASA funded three different groups to develop small, highly efficient propulsion systems specifically designed to enable spacecraft like CubeSats to orient themselves, maneuver, and even change their own orbits. The propulsion technology that NASA is interested in is called ion electrospray, and MIT’s prototype is a modular, eight-thruster unit just 21 millimeters thick that can change the velocity of a CubeSat by a staggering 100 meters per second.
Fundamentally, electrospray propulsion works like any other rocket engine: stuff comes flying out of the back of the engine, and the spacecraft that the engine is attached to gets pushed forward. In an electrospray thruster, the propellant is a nontoxic ionic liquid comprised of heavy charged particles of salt, both positive and negative. The business end of the thruster is a metallic chip that is charged to 1,000 volts, pulling ions out of the liquid and accelerating them to very high velocities. A single thruster generates 15 micronewtons of throttleable thrust, and an array of eight (taking up one face of a CubeSat) brings that up to 120 μN. That’s about the force that it takes to move a piece of paper on Earth, but in space, it’s easily enough to push a CubeSat around, given enough time.
As long as you’re not in a rush, an electrospray thruster is extremely efficient: MIT’s version, called S-iEPS (Scalable ion Electrospray Propulsion System) has a specific impulse of 1,500 – 2,000 seconds and an overall efficiency of 70 percent, and it’ll happily operate for hundreds of hours as long as you keep feeding it propellant and about 5 watts of electricity. Each thruster unit is modular, and the system as a whole is easily scalable: add more modules and you get more thrust, with a minimum of extra integration infrastructure.
Electrospray is, in principle, a similar method of operation to an ion thruster or Hall thruster, except that electrospray thrusters are far less complicated: they don’t need to use a plasma as a propellant source, and they’re extraordinarily simple to construct, having no moving parts. The propellant is a stable liquid that flows into the thruster via capillary action, so you don’t need pressure vessels or pumps, and the thruster itself is fabricated like an electronic chip. This all results in a relatively inexpensive, mass producible thruster with a high thrust to mass ratio, ideal for the CubeSat application.
In practice, the S-iEPS thrusters are mounted in a grid at one end of a CubeSat. Each thruster module is self-contained, with about a gram of fuel inside. Depending on what you want the CubeSat to do, you can mount some of the thrusters sideways to control orientation, or add additional modules elsewhere on the CubeSat to do this. The thrusters harvest electricity from solar panels on the surface of CubeSat to power themselves, and each thruster will alternate its polarity every ten seconds or so, allowing it to use both the positive and negative ions in the fuel.
A basic CubeSat is one 10 cm x 10 cm x 10 cm cube, but you can also stack a bunch of CubeSats together into larger units of 2U (20 cm long), 3U (30 cm long), and so forth. A 6U CubeSat, a spacecraft just 60cm long, could leverage S-iEPS thrusters to get itself to the moon. And once you’re able to escape from Earth orbit, the entire solar system is out there to be explored, opening up possibilites for interplanetary missions that are an order of magnitude more affordable than they are right now.
Graduates from MIT’s Space Propulsion Lab have founded a company, Accion Systems, to commercialize the S-iEPS thruster. A prototype is scheduled to fly on a 1U CubeSat this week. In the meantime, Accion has already started development on the next generation S-iEPS thruster, which the company says will be powerful enough to enable interplanetary transfers for satellites up to 150 kilograms.