Micro Ion Thrusters: The newest little idea for nanosat micro rockets

L. Brad King's prototype of a ferrofluid ion thruster. When subjected to voltage, the points of the crown arise from a ring-shaped trench circling a one-inch block of aluminum. 

Credit: L. Brad King

Nanosatellites are smartphone-sized gadgets that can perform simple, yet valuable, space missions.

Dozens of these little spacecraft are now tirelessly orbiting the earth performing valuable missions for NASA, the Department of Defense and even private companies.

Nano-satellites borrow many of their components from terrestrial gadgets: miniaturized cameras, wireless radios and GPS receivers that have been perfected for hand-held devices are also perfect for spacecraft.

However, according to Michigan Technological University's Lyon Brad King, there is at least one technology need that is unique to space: "Even the best smartphones don't have miniaturized rocket engines, so we need to develop them from scratch."

Miniature rockets aren't needed to launch a nano-satellite from Earth. The small vehicles can hitchhike with a regular rocket that is going that way anyway.

But because they are hitchhikers, these nano-satellites don't always get dropped off in their preferred location.

Once in space, a nano-satellite might need some type of propulsion to move it from its drop-off point into its desired orbit. This is where the micro rocket engine comes in.

For the last few years, researchers around the world have been trying to build such rockets using microscopic hollow needles to electrically spray thin jets of fluid, which push the spacecraft in the opposite direction.

The fluid propellant is a special chemical known as an ionic liquid. A single thruster needle is finer than a human hair, less than one millimeter long and produces a thrust force equivalent to the weight of a few grains of sand.

A few hundred of these needles fit in a postage-stamp-size package and produce enough thrust to maneuver a nano-satellite.

Lyon Brad King

These new electrospray thrusters face some design challenges, however. "Because they are so small and intricate, they are expensive to make, and the needles are fragile," says King, the 'Ron and Elaine Starr' Professor of Mechanical Engineering-Engineering Mechanics.

"They are easily destroyed either by a careless bump or an electrical arc when they're running."

To get around the problem, King and his team have developed an elegant strategy: eliminate the expensive and tedious micro-fabrication required to make the needles by letting Mother Nature take care of the assembly.

"We're working with a unique type of liquid called a ferro-fluid that naturally forms a stationary pattern of sharp tips in the liquid surface," he says.

"Each tip in this self-assembling structure can spray a jet of fluid just like a micro-needle, so we don't actually have to make any needles."

Ferro-fluids have been around since the 1960s. They are made of tiny magnetic particles suspended in a solvent that moves when magnetic force is applied.

King illustrates with a tiny container holding a ferro-fluid made of kerosene and iron dust. The fluid lies flat until he puts a magnet beneath it.

Then suddenly, the liquid forms a regular series of peaks reminiscent of a mountain range.

These peaks remain perfectly stable despite vigorous shaking and even turning the container upside down. It is, nonetheless, completely liquid, as a finger-tip touch proves undeniably.

When the magnet is removed, the liquid relaxes to a perfectly flat surface.

King's team was trying to make an ionic liquid that behaved like a ferro-fluid when they learned about a research team at the University of Sydney that was already making these substances.

The Sydney team was using magnetic nanoparticles made by the life-sciences company Sirtex, which are used to treat liver cancer. "They sent us a sample, and we've used it to develop a thruster," King said.

"Now we have a nice collaboration going. It's amazing that the same technology used to treat cancer can also function as a micro rocket for spacecraft."

King's first thruster is made of a one-inch block of aluminum containing a small ring of the special fluid. When a magnet is placed beneath the block, the liquid forms a tiny, five-tipped crown.

Their thruster isn't ready to push a satellite around in orbit just yet. "First we have to really understand what is happening on a microscopic level, and then develop a larger prototype based on what we learn," King said.

"We're not quite there yet; we can't build a person out of liquid, like the notorious villain from the Terminator movies. But we're pretty sure we can build a rocket engine."