Space is becoming more crowded and as the number of satellites increases, so too does the risk of a collision, highlighting the need for multimode propulsion in spacecraft, according to industry panelists at the 40th Space Symposium.

Multimode means an engine that can operate in a high efficiency/low thrust mode and a high thrust/low efficiency mode. This gives them flexibility to stay in orbit as needed — most spacecraft orbits naturally decay, especially in low earth orbit — but also dodge incoming threats if they emerge.

One Colorado company, Agile Industries, is leading the charge with their Hesperus design. The engine uses hydrazine as a fuel, which CEO Chris Pearson says allows for more immediate introduction because the infrastructure for it already exists.

“We have a gas reactor that turns hydrazine into nitrogen and hydrogen, meaning we can use pure hydrazine for normal (monopropellant) thrusters. … But then we actually use the decomposed gases to get the high efficiency performance,” he said, adding that Agile is currently working with the Space Force. “There’s hardware that’s being put together, and we’re going to be testing in the summer.”

Hydrazine has fueled spacecraft since World War II, so Agile isn’t trying to reinvent the wheel.

Other members of the panel like Phase Four’s Chief Technology Officer Umair Siddiqui admit that hydrazine is great but may not be the fuel of the future.

Towards the end of the panel, Siddiqui said we may one day look back on hydrazine as we see sailboats today: antiquated.

“My bet is the first multimode systems deployed in the new space generation is going to be hydrazine based … (but) that doesn’t mean we shouldn’t explore and continue to develop other propellants,” Siddiqui said. “Industries are testing new green propellants. … And the Air Force Research Lab is continuing to push forward a new green propellant called ascent.”

All of the new propellant concepts have their own pros, cons and cost curves, he said, but they still warrant investment and investigation.

While Pearson may have a natural preference to working with hydrazine, as Agile Industries is building a multimode engine to use it, Adam Dissel, the director of transformation, space propulsion & power systems at L3Harris Technologies is more agnostic in choosing his “favorite” propellant. He joked that if something is a good propellant or oxidizer, it’s probably dangerous.

“We actually at L3Harris are working on some new propellants to bring to the table (and they are) best fit for the mission,” he said. “So I think, really important is to make sure that we deliver on the capabilities that are really needed.”

Dissel supports using both hydrazine as a near term solution while growing and fostering the infrastructure needed for other options. By using older, “heritage” designs and fuels, companies can insert multimode propellant and engines into spacecraft faster, fulfilling the pressing need for it currently.

But the challenges for a business in operation now comes down to “demand signals,” Pearson said. As a business person, he needs to have an idea of how large a certain market is before going in and making investment decisions.

Over the course of the panel, Pearson said speakers touched on seven applications for multimode propulsion, all related to national security, civil and commercial space.

“You need to, in my opinion, do some kind of collaborative trade studies, where you’re really looking at these different applications: Where is that sweet spot?” he said. “I’ve got an idea of how big the market is, I’m very excited about it, otherwise I wouldn’t be up here. But we need to work it out together, and then we can bring this capability to market faster.”