CMU Robotics Team Helps Northrop Grumman Dock With and Refuel Satellites

A peg-in-hole problem 22,000 miles up

Placing a peg in a hole is a simple task for a child, but it becomes far harder when the peg and the hole are more than 22,000 miles above Earth. According to Carnegie Mellon University, perfecting that capability is central to a first-of-its-kind spacecraft designed to refuel and otherwise service satellites in geosynchronous orbit.

A Robotics Institute team led by professor Howie Choset is providing expertise to Northrop Grumman. Choset said thousands of satellites now orbiting Earth are not functional, most often because they ran out of fuel. "There's been this paradigm in aerospace, 'launch once, use once,'" said Choset, the Kavčić-Moura Professor of Computer Science in the RI. "Now we're poised to break that paradigm, so a satellite could remain in service indefinitely."

A probe that docks through the engine nozzle

Northrop Grumman operates several servicing spacecraft. The MRV is about the size of a short school bus. Andrew Kwas, senior fellow at Northrop Grumman Space, said Choset and his students worked with the company's experts on a new technique that lets the MRV safely approach and dock with another satellite.

Kwas said satellites previously lacked an easy way to be grappled and held with a tight connection that allows refueling through a separate port and keeps them stable for robotic operations. "The new technique uses a probe that inserts into the nozzle of the target satellite's liquid apogee engine to dock the spacecraft together," he said. The liquid apogee engine is typically a spacecraft's main engine.

The Robotics Institute provided analytical and modeling support for the procedure. "CMU and Northrop Grumman worked together to assess all contingencies for several years before the first successful mission," Kwas said. "Now the process has been repeatedly performed on orbit several times."

Refueling has been the main use for the servicing spacecraft so far. With the MRV, Kwas said, the company also has the option to perform repairs, replace parts or carry out upgrades in orbit, including dislodging or repairing a solar panel, which he called another failure point on satellites.

Force feedback and a space-simulating testbed

Aligning a peg with a hole requires not only visual but also touch, or haptic, feedback, a classic challenge in robotics. Choset drew on earlier peg-in-the-hole projects at the RI. He is widely known for snake-like robots, including ones his team recently used in Venezuela to look for survivors in the rubble of the June earthquake there.

Years ago, his then-student David Rollinson devised a "series elastic actuator," a motor that can measure the amount of force it applies. Rollinson is now chief technology officer of HEBI Robotics, a company Choset founded in 2014. "It's just like your muscles: When you press on a table, your muscles deliver force, but they also feel the reaction force created by pressing on the table," Choset said.

Choset's group also built a robotic testbed called a holodeck, which uses two robotic arms on a rail to simulate how robots behave in the weightlessness of space. Kwas said the team "showed us early on that they had the skills to perform the dynamic space operations necessary to refine our design."

What it could mean for space operations

Choset suggested that solving the peg-in-hole problem could have major implications. "You solve that and you can assemble structures in space. You can maintain structures in space," he said. He added that the capability could make it possible to build spacecraft in orbit rather than on Earth, removing the need for them to survive the stresses of launch.

The collaboration has also brought Choset new opportunities, including two Space University Research Initiative grants from the U.S. Space Command. One of them supports the work with Northrop Grumman.