U.S. AFRL Explores Swappable Modular Propulsion For Small Satellites

By Garrett Reim

Aviation Week

Nov. 1, 2022

Story link: https://bit.ly/ModPropAviationWeek

The U.S. Air Force Research Laboratory is exploring modular propulsion for small satellites, with the aim of demonstrating the concept on orbit within a few years.

The research laboratory wants to establish a standardized interface between a satellite bus and a modular propulsion unit, Justin Koo, technology advisor for the in-space propulsion branch of the Air Force Research Laboratory (AFRL) Aerospace Systems Directorate, told Aviation Week at the American Institute of Aeronautics and Astronautics’ Ascend conference in Las Vegas and in a subsequent interview.

The AFRL envisions that a standardized interface would allow different propulsion products to be integrated more easily into a satellite bus, speeding up upgrades and giving innovative new entrants to the industry the ability to connect their propulsion to satellites.

“We want new propulsion vendors to come into the market,” he said. “Unfortunately, for propulsion you have to get to flight demonstration to really become a credible option. We view [modular propulsion] as an incredibly powerful tool to get new propulsion technologies and to improve propulsion technologies by getting to orbit faster.”

A standardized interface could also enable parallel fabrication of a satellite bus and modular propulsion unit. That would make manufacturing and testing of the components more efficient, the AFRL expects.

The modular propulsion effort, which the AFRL calls ModProp, is planned to be a standardized interface owned and maintained by the government. The interface, it says, would be made of mechanical, electrical and software components and compatible with electric as well as chemical propulsion systems. The modular propulsion unit would contain tanks, valves, thrusters, a refueling capability and a digital control and interface unit.

The digital control and interface unit is likely to become increasingly valuable as the amount of data a spacecraft generates increases, Koo said.

“We are going to get more feedback from our propulsion units as we start to move to a better state of health monitoring and eventually [to] things like digital twins,” he said. “That could be a fairly large data stream. We want to put that through a digital pipe.”

As planned, no fluids would pass between the main bus and the modular propulsion unit. The bus would retain the satellite’s mission payload, power source, flight computer and communications system.

The AFRL wants the interface to be as simple as possible to appeal to as many satellite and propulsion manufacturers as possible, Koo said. The research laboratory is asking for feedback from four satellite and propulsion manufacturers on its draft interface and is in discussions to add more partners to its effort.

The ModProp standard has similarities to the AFRL’s Evolved Expendable Launch Vehicle (EELV) Secondary Payload Adapter (ESPA), a ring for attaching secondary payloads to orbital launch vehicles that has become widely used as a standard across military and commercial space missions. “We took a huge amount of inspiration from ESPA,” Koo said. “The simple publication of a standard and a few demonstration missions on ESPA have really opened up a whole class of vehicle.”

Because a modular interface could become a well-known commodity, it could lend itself to high-fidelity digital engineering models that could allow manufacturers to virtually test how different combinations of propulsion units and buses might perform.

“When you actually mate the two pieces together, you’ve already chased down some of the demons,” Koo said. “The integration process is streamlined significantly if we already kind of know what the behavior ought to be.”

The AFRL envisions a second generation of the modular interface eventually, which would enable on-orbit propulsion-unit replacement. As satellites are refueled in orbit and use that fuel to maneuver more, the research laboratory expects that worn-out propulsion units would have to be replaced on-orbit. Engine replacement also could allow spacecraft to receive upgraded propulsion capabilities.

“We’re very closely plugged into what is going on with [in-space servicing assembly and manufacturing],” said Koo, noting that the design of an interface for in-space replacement of a modular propulsion unit has not been determined. “Space robotics will likely play a very large role in being able to take a [Defense Department] spacecraft and completely upgrade that propulsion capability.”