Why a superconducting motor?
To understand why a superconducting motor is such an enticing concept for electrified flight, it's helpful first to understand the use of carbon-neutral liquefied natural gas and hydrogen as alternatives to conventional jet fuel.
Those fuels have low energy density, meaning it takes large volumes to provide the necessary power – so large, in fact, they have to be chilled to extremely cold temperatures and condensed into liquid form to fit into the fuel tank.
And that's where superconductivity – the phenomenon of electricity flowing through a conductor with no resistance – comes in. Almost as if by coincidence, the cryogenic temperatures that make fuel liquefication possible can also unlock superconducting properties in certain materials – materials that can be used, for example, to build motors.
That made the experts at the Raytheon Technologies Research Center think: Why settle for just one huge improvement – clean aircraft fuel – when we can also get near-perfect electric power efficiency in our propulsion system?
The potential was too great to pass up.
"The superconducting system – if this is on the table, why not use it?" Kshirsagar said. "We're not just burning the fuel – you've got to go beyond that."
How it would work
Just like a hybrid electric car, a hybrid electric plane would have both an engine and a motor – and probably several of each. The engines would burn fuel and use the heat to create mechanical power, then send it to the motors in electric form by way of generators.
So far, that's pretty ordinary. But once you bring in the concept of superconductivity, that's where the Raytheon Technologies Research Center’s design for ARPA-E gets creative.
In an ideal situation, the engineers would simply use cryocooled fuel to refrigerate the motor parts to their superconducting state. Just one problem: The fuel is flammable, so using it as a direct coolant would pose safety risks.
Until the engineers can address that concern, they've come up with a workaround. The research center's design calls for a thermal management system to refrigerate helium to -423 degrees Fahrenheit, also known as 20 Kelvin, then run it through a loop that would chill the motor stators, rotors and terminals to their superconducting state. The fuel would double as a heat sink – a place for the thermal management system to dump the warmth the helium draws from the motor parts.
In the middle of the design, driving each motor, would be a power converter made with gallium nitride – the same material that helps Raytheon Missiles & Defense build powerful, energy-efficient radars and other sensors.
Overall, the project aims to build an early version of a motor that, once scaled up, would power single-aisle aircraft during takeoff and ascent – the two phases of flight that require the most energy. The motor and motor drive combined would have an energy-conversion efficiency greater than 99 percent.
"The efficiencies of these (superconducting) motors … you can't beat that with anything else," Roelofs said.