Small satellite propulsion begins to prove itself

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Luis Gomes was fed up with propulsion promises.

As chief technology officer for Surrey Satellite Technology Ltd. in 2018, Gomes told his assistant to halt all meetings with spacecraft propulsion companies.

“Two companies a week were coming in with the same designs and talking to me about the same things,” said Gomes, now CEO of Sweden’s AAC Clyde Space. “They were just trying to get potential customers so they could go back to their investors and get money to develop it. I told them, ‘I will wait until prove your technology and then you can come back to me.’”

Three years later, Gomes along with the rest of the small satellite community is starting to see that proof.

“There are advancements being made across the waterfront,” Bruce Yost, director of the NASA Small Spacecraft Systems Virtual Institute, said Aug. 10 at the virtual Small Satellite Conference.

Five years ago, cubesats and microsatellites had few inexpensive propulsion options. In the last couple of years, more than a dozen companies have reported successful firing of new satellites thrusters, among them: Enpulsion of Austria; Germany’s Morpheus Space; French firms Exotrail and ThrustMe; Denmark’s GomSpace; D-Orbit and T4i of Italy; Dawn Aerospace of the Netherlands; NanoAvionics of Lithuania; and U.S. companies Apollo Fusion, Bradford Space, Momentus, Phase Four, Rocket Lab, Stellar Exploration and Tethers Unlimited. (Momentus thruster performance claims were later questioned by the U.S. Securities and Exchange Commission.)

Still, it’s too soon to say much about the performance of many of the new cubesat and small satellite thrusters.

“There are a lot of missions that recently launched or are known to be coming in the next year or so,” said Gabriel Benavides, a researcher and engineer in the NASA Glenn Research Center’s Electric Propulsion Systems branch. “At the moment, we still have fairly limited flight data on these systems.”

MORE TESTING COMING

The first thrusters from Accion Systems and Benchmark Space Systems reached orbit June 30 on satellites launched as rideshares on the SpaceX Transporter-2 flight. The two companies do not yet have flight data to share.

“These are paying customers with operational missions,” said Benchmark CEO Ryan McDevitt. “They are going to use it when they need it over the next weeks and months.”

Even when customers begin firing thrusters, it can take years to observe the full range of propulsive capabilities including initial orbit maneuvers, long-term stationkeeping and deorbit at the conclusion of missions.

How important are flight tests for thrusters? Experts offer a range of opinions.

Extensive ground-based testing can verify 80 to 85 percent of thruster performance and behavior, said Natalya Bailey, Accion Systems co-founder and CTO. In-orbit demonstrations verify that last 15 to 20 percent and show that thrusters work as designed, she added.

Umair Siddiqui, Phase Four CTO, saw that firsthand. Despite his conviction that Phase Four’s Maxwell engine would work in orbit exactly as it had in extensive testing on the ground, he felt intense relief when he saw proof earlier this year.

“I had the most unexpected visceral reaction,” Siddiqui said. “In five years, we went from a figment of imagination to a unit that’s serving mission needs for customers in space. That all was realized in a feeling.”

NASA considers propulsion systems with space flight heritage to be lower risk than systems tested exclusively on the ground, making the technology with flight heritage more likely to win space agency funding and to be included in missions, Yost said.

Certain technologies, though, can be adequately tested on the ground.

For some chemical propulsion and heritage electric propulsion systems, “we’ve developed a lot of good ground test facilities and methodologies,” Benavides said. In those cases, “it’s logical to do as much testing as you can feasibly within your budget on the ground to reduce the cost,” he added.

JUST OPEN A VALVE

When companies report thruster firings in orbit, it’s important to understand exactly what they have shown.

“If you open a valve with a liquid to outgas, it will provide some thrust,” said Tomas Svitek, Stellar Exploration president. “That does not automatically imply that this is a useful propulsion system.”

Brennan Bryant, Stellar Exploration design engineer, is shown with the Stellar Exploration propulsion module that helped propel an EchoStar nanosatellite to secure its claim to a global S-band slot. Credit: Stellar Exploration

Even if a thruster produces a specific change in velocity for one cubesat or small satellite in low Earth orbit, it may mean that the physics involved is sound but it does not necessarily mean future thrusters produced by the company will work equally well.

“Propulsion systems need to be considered in the context of the anticipated mission,” Benavides said. “A lot of missions get into trouble when they assume substantial similarity between their mission and other missions, which may or may not exist. Then, late in their mission development they find that there are issues with propulsion that they didn’t perceive.”

“I strongly encourage any mission considering one or more propulsion technologies to do a deep dive and understand the technology readiness level of any propulsion system within the context of your mission and your mission’s requirements,” Benavides said. “Do that early and it’ll save a lot of costs and headaches in the long run.”

TECHNICAL READINESS

Propulsion is often a spacecraft’s most expensive and complex system.

“If you want to buy low-power processors, you can get them from huge terrestrial markets,” said Brad King, Orbion Space Technology CEO and founder. “If you want to buy GPS receivers, you can get them from terrestrial markets. But the iPhone doesn’t yet have a thruster,” he said, making propulsion systems “one of the last remaining purely space technologies.”

Since propulsion can claim a large share of a satellite budget, some companies are eager to find inexpensive options.

“Everyone wants fast, cheap and risky until something happens,” said Alexander Reissner, Enpulsion CEO and founder.

To evaluate risk, customers often ask propulsion suppliers about the technical readiness level (TRL) of thrusters.

Some companies fire thrusters in space for the first time and begin “shouting around from the rooftops ‘I’m TRL 9 because I’m working in space,’” Reissner said.

Technology does not earn the moniker TRL 9, however, until it completes a full mission. If it’s a thruster-demonstration mission, that’s far easier to complete than a multiyear Earth-observation or communications mission.

“Putting your product in orbit and turning it on for an hour does nothing to prove that you can meet a 5,000-hour lifetime,” King said.

This article originally appeared in the August 2021 issue of SpaceNews magazine.