Humans have been using rocket propulsion for almost a millennium, starting with Chinese rockets and “fire arrows” in the 13th century and continuing to the modern era's powerful Space Shuttle and Falcon rockets. For most of that history, rockets have been chemically fueled, but in the past century scientists and engineers have also experimented with electric rockets, also known as ion engines or ion propulsion systems.

Electrospray thruster chips (in gold) arranged in a propulsion array on a satellite (artist's concept). Illustration credit: Zina Deretsky

Rather than using chemical reactions to create heat and accelerate a propellant, electric rockets use electromagnetic or electrostatic fields acting on charged ions of propellant, speeding them up and shooting them out, away from the vehicle, producing thrust. The electrical energy to generate these fields comes from the sun, from batteries, or both.

Ion engines might sound like something you'd find on the Starship Enterprise, but in fact they've emerged as a practical solution for in-space maneuvers. NASA's Deep Space 1, launched in 1998, demonstrated the sustained use of an ion thruster in space. The most recent version of Boeing's 702 satellite bus uses an all-electric propulsion system for orbit transfer and maneuvering the satellite once it is in orbit (prior versions used hybrid chemical and electric engines).

Electric rocket engines are far less powerful than chemical rockets, so they can't be used to launch rockets into space. But once in orbit, they have some big advantages: They're far more efficient, per unit of propellant mass, than chemical rockets. And because they rely on electricity, they can be powered by solar panels.

Accion Systems, Inc. has developed an electric rocket system, based on electrospray propulsion technology, that can be made to work at a far smaller scale than previous ion engines. It's also cheaper and easier to manufacture these thrusters in large numbers. That makes them well-suited to deployment on small satellites and nanosatellites, where these engines can help maintain orbit, change a satellite's orientation, or even move it to a different orbit with great efficiency.

Electrospray Propulsion: How It Works

Electrospray thruster chips are small enough to fit on a fingertip and have no moving parts. (Photo: Accion Systems, Inc.)
How the TILE system generates thrust. (Illustration: Zina Deretsky)

Research into electric rockets began in earnest in the mid-20th century as part of the space race. Eventually, two main types emerged. In the Soviet Union, “Hall thrusters” saw some limited operational use in the 20th century. In the United States, “gridded ion engines” were a subject of experimentation but were not widely deployed until recently, as part of Boeing's Xenon Ion Propulsion System (XIPS), the electric propulsion system used on the company's Boeing 702 satellite bus.

A third type of electric rocket technology, electrospray propulsion, didn't proceed past the experimental stage in the 1960s and 1970s. However, research in the past decade by Paulo Lozano at MIT, assisted by Natalya Brikner and Louis Perna, pushed electrospray from theory into reality. (Perna and Brikner were graduate students under Lozano's supervision and are the cofounders of Accion Systems.)

The basic idea with electrospray propulsion is that you start with a conductive liquid and expose it to a strong electric field. That field brings charged ions to the surface, deforming the surface of the liquid and pulling it up and away from the rest of the liquid. As the liquid deforms, it extends into stronger parts of the electric field, deforming it still further, and so on. Eventually the field pulls a tiny droplet (or even a single ion) off the very tip of the deformation, accelerating it out and away, generating thrust.

Recent Innovations

Building on the MIT research, Accion has advanced electrospray propulsion in several ways. First, it uses a conductive liquid as a propellant – a compound that's liquid at room temperature and which contains two different molecules (one positively charged and one negatively charged). Because this propellant already contains charged ions it doesn't need to be ionized, which is why the engineers dub it “plasma in a bottle.” By contrast, other ion engines require an ionization step prior to accelerating the ionized propellant.

Second, the propellant is stored within a porous material, which brings the liquid into the thruster's electric field through an array of sharp microstructures on its surface. The porous material acts as a wick, drawing the propellant out of a reservoir and into the thruster. The microstructures pre-deform the liquid, so the electric field need only operate at the tips, pulling the liquid out still further and extracting it a few ions or molecules at a time.

Third, the extractor (the part that generates the electric field) is made with micro-emitter holes that line up with the microstructures on the porous material. Its field is designed to extract ions from each tip separately. Altogether, a single thruster chip is about the size of a U.S. penny, but contains hundreds of emitters, all firing in the same direction.

It's not very powerful: Each pennysized chip generates only a few tens of micronewtons of force—approximately equal to the force exerted by a mosquito's wings. But the technology has substantial advantages that can make up for its low level of force.

Electrospray Propulsion: Pros and Cons

Accion's chips can be manufactured with techniques used to manufacture silicon chips and MEMS devices. (Photo: Accion Systems, Inc.)

Ion engines, including electrospray propulsion systems, have excellent specific impulse—a measure of propellant mass efficiency. So while their thrust is weak, they use propellant extremely efficiently. That means they are well suited to applications where the time required to accelerate is not as important as the overall mass of the satellite.

For unmanned satellites, that tradeoff is easy to make. More efficient propulsion means you don't need to devote as much of the satellite's mass to propellant, leaving more room for the payload—or making the overall satellite less massive and therefore cheaper to launch. Meanwhile, if it takes days or weeks to complete a maneuver, that's not a problem for the satellite's computers: They can just wait patiently.

Because the propulsion system can act on the conductive liquid directly, power is not needed to ionize the propellant.

What's more, this liquid is extremely non-volatile, which means it's relatively safe to handle. It emits no dangerous vapors. If you spill some of it on a surface, it will just sit there, without evaporating, for years.

It doesn't even evaporate in the vacuum of space, which leads to another benefit — because the propellant has such a low evaporation rate and is liquid at a wide range of temperatures, it doesn't need to be compressed, as do the gas propellants used by other ion propulsion systems. That eliminates the need for bulky, massive, thick-walled compression tanks.

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