Bioinspired propulsion

In contrast, Lozano’s microthruster design adds little to a satellite’s overall weight. The microchip is composed of several layers of porous metal, the top layer of which is textured with 500 evenly spaced metallic tips. The bottom of the chip contains a small reservoir of liquid — a “liquid plasma” of free-floating ions that is key to the operation of the device.

To explain how the thruster works, Lozano invokes the analogy of a tree: Water from the ground is pulled up a tree through a succession of smaller and smaller pores, first in the roots, then up the trunk, and finally through the leaves, where sunshine evaporates the water as gas. Lozano’s microthruster works by a similar capillary action: Each layer of metal contains smaller and smaller pores, which passively suck the ionic liquid up through the chip, to the tops of the metallic tips.

The group engineered a gold-coated plate over the chip, then applied a voltage, generating an electric field between the plate and the thruster’s tips. In response, beams of ions escaped the tips, creating a thrust. The researchers found that an array of 500 tips produces 50 micronewtons of force — an amount of thrust that, on Earth, could only support a small shred of paper. But in zero-gravity space, this tiny force would be enough to propel a two-pound satellite.

Lozano and co-author Dan Courtney also found that very small increases in voltage generated a big increase in force among the thruster’s 500 tips, a promising result in terms of energy efficiency.

“It means you have a lot of control with your voltage,” Lozano says. “You don’t have to increase a lot of voltage to attain higher current. It’s a very small, modest increase.”

Timothy Graves, manager of electric propulsion and plasma science at Aerospace Corp. in El Segundo, Calif., says the microthruster design stands out among satellite propellant systems for its size and low power consumption.

“Normally, propulsion systems have significant infrastructure associated with propellant feed lines, valves [and] complex power conditioning systems,” says Graves, who was not involved in the research. “Additionally, the postage-stamp size of this thruster makes it easy to implement in comparison to other, larger propulsion systems.”

The researchers envision a small satellite with several microthrusters, possibly oriented in different directions. When the satellite needs to propel out of orbit, onboard solar panels would temporarily activate the thrusters. In the future, Lozano predicts, microthrusters may even be used to power much larger satellites: Flat panels lined with multiple thrusters could propel a satellite through space, switching directions much like a rudder, or the tail of a fish.

“Just like solar panels you can aim at the sun, you can point the thrusters in any direction you want, and then thrust,” Lozano says. “That gives you a lot of flexibility. That’s pretty cool.”