Nanotip Ion Thruster Promises Power Efficiency Boost

It sounds like a NASA pipe dream: a new spacecraft booster that’s up to 40 percent more energy efficient than today’s. Better yet, its fuel costs less than a thousandth and weighs an eighth of its mass. A startup called Orbital Arc claims to be able to manufacture such a propellant.

With this design, “we can go from a thruster that’s about a few inches in diameter and several kilograms to a thruster on a chip that’s about an inch in diameter and has the same thrust, but weighs about an eighth as much,” says company founder Jonathan Huffman.

According to Orbital Arc, the hardware would be small enough to fit on the smallest satellites in low Earth orbit, but would generate enough power for an interplanetary mission. Such an inexpensive boost could bring significant savings to satellite operators hoping to avoid debris, or to mission operators wanting to send probes to distant planets.

The key to these innovations is the combination of cheap, readily available fuel, MEMS microfabrication, and a strong love for science fiction.

Design a better thruster

Thrusters typically work by creating and then expelling plasma, pushing a spacecraft in the opposite direction. Inside the popular Hall thrusters, a magnetic field traps electrons in a tight circular orbit. A noble gas, usually xenon, drifts into a narrow channel where it collides with the circulating charge, stripping away electrons and ionizing them into plasma. A high-voltage electric field then projects the plasma out of the exhaust gases.

Orbital Arc’s technology is a bit different and appeared almost by chance. Huffman was a biotechnology consultant and self-described “science fiction nerd” who, in his spare time, was tasked with designing fictional technology for a futuristic video game. He needed to figure out how planes might maneuver 250 years from now to make the game’s controls realistic, and so he began researching cutting-edge propulsion systems.

He quickly realized a limitation of existing ion thrusters that he believed could be improved over the coming centuries, and (spoiler alert) perhaps sooner: if a mission requires more thrust, its thruster must be heavier. But importantly, “there’s a certain point at which adding more mass to the propellant negates any benefits you can get from extra thrust,” he says. So, to maintain these advantages, thrusters must be small but powerful.

Huffman’s familiarity with biology laboratories gave him an unexpected advantage in propulsion design. Through his work, he discovered nanoscale tips (nozzles that emit ions) used to generate intense electromagnetic fields for biomedical research. They are found in mass spectrometers, instruments that identify unknown chemicals by converting them into ions, accelerating them, and observing how they fly.

He suspected that such a system could be further miniaturized to carry out the ionization process in a propellant. After a year and a half of developing the concept, Huffman was convinced that his idea for a small thruster had potential beyond a video game.

And he was right. Each Orbital Arc thruster has a chip at its heart with millions of micrometer-scale positively charged tips and channels to direct the flow of gas: naphthalene goes in and ions go out.

When the naphthalene molecules pass through the charged tips, the molecules polarize – here it means that the electrons of a molecule cluster together on one of its sides. Due to the uneven field created by the charge, molecules are pulled toward a tip and then become trapped there, unable to escape until they release electrons.

Once they release electrons, “you have an ion that’s at the tip of a very sharp, positively charged object, and it itself is now positively charged. So it’s accelerating,” Huffman says. The repelled ions fly and disperse into space, propelling the spacecraft forward.

An advantage of this design is the energy savings that result from avoiding the internal plasma generation that other thrusters rely on, Huffman says. “Plasmas suffer losses because everything is mixed together in one big soup,” says Huffman. Free electrons in a plasma can recombine with ions to produce neutral atoms “and now I’ve lost the energy I put into making this charged particle. It’s a waste of energy.” Recent calculations show that naphthalene nanotip propellant improves fuel efficiency by 30 to 40 percent, he says.

By avoiding plasmas altogether, the Orbital Arc design is able to capitalize on energy savings, as shown in a recent demonstration. In a recent test, just six Orbital Arc tips were able to generate about three times as much ionic current as a set of 320,000 tips from a group at MIT, Huffman says.

Two and a half years after his “aha” moment (and after “building everything in Excel”), Huffman is the CEO of Orbital Arc, a startup testing four working prototypes of its tiny chips.

The thruster is innovative not only in its size, but also in its fuel. Naphthalene, the main ingredient in mothballs, is a readily available byproduct from oil refineries. The compound may smell bad, but it is safe to handle and extremely cheap, Huffman says, costing about US$1.50 per kilogram, compared to about $3,000 per kilogram for xenon.

Orbital Arc’s use of naphthalene helps reduce product costs, which the company says make up 1 percent of traditional Hall thrusters. “I think it’s credible,” says Jonathan MacArthur, a postdoctoral researcher at Princeton University’s Electric Propulsion and Plasma Dynamics Laboratory. “What remains to be seen is that it’s cheap, but if I put diesel in my gasoline car because it’s on sale, that doesn’t bode well for my car’s engine.” He wants the startup to release data to back up its cost claims — and while they’re at it, data to back up their performance claims, too.

From prototype to flight

So far, at the prototype stage, each chip contains just six tips, made using MEMS manufacturing processes in a clean room at Oak Ridge National Laboratory. But the next step will be to make a full-scale version of the chip in a university lab, Huffman says.

Next, the company will need to build the thruster that surrounds the chip. “It’s a relatively simple device. It’s a valve, it’s a few wires, it’s a few structural components. Very, very simple,” Huffman says. He says he’ll have to integrate all of those parts before going through vibration testing, radiation testing, thermal cycling and other steps before getting flight qualification. “In two years I can probably have a sellable product.”

Huffman thinks Orbital Arc’s first customers would be small teams, like startups or research groups. He is confident that they will be willing to try the new thrusters, despite the risks inherent in new technologies, because of the expected performance at low cost. “Some people just won’t have any choice but to buy it, even if it’s never flown before. If they want to accomplish the mission, they’ll take the risk,” he says.

MacArthur of Princeton is skeptical of this claim. “When choosing a propulsion system, data and heritage are usually key.” It is not clear whether customers are willing to take the risk on a new propellant with no flight history.

Still, some CubeSat-scale missions might be willing to use new, lower-cost thrusters, suggests Oliver Jia-Richards, who studies space propulsion at the University of Michigan. Customers might also be willing to take a chance on Orbital Arc because other startups, like Enpulsion, have recently seen success with their new electric propulsion technology, he says. But “with this kind of thing, there are always risks.”

After targeting small missions, Huffman wants to “build something where we show off a little.” He notes that, until now, no satellite has made a round trip to the Moon after a year in Earth’s orbit without refueling. It depends on funding and better opportunities could arise, “so we’ll see,” he says.

And it doesn’t stop there. “We’re tapping into a mathematical reality,” Huffman says. “If you reduce the dry mass of a spacecraft, you get exponential benefits in its performance because of the way the rocket equation works. You are penalized exponentially for additional dry mass.”

By integrating Orbital Arc’s thrusters, he says, a mission could reduce the mass of the solar panels and power supply because driving them is more energy efficient, reduce the mass of the tank because naphthalene does not require a pressure vessel unlike xenon, and reduce the mass of the thruster itself. With these savings, “you go from one-way science missions to Mars to two-way human missions to Jupiter without resupply,” Huffman says.

So even though the thruster is the first step in Orbital Arc, Huffman envisions an ultra-light space bus next, arriving well before the distant video game era that inspired it.