ESAero,studied hybrid propulsion systems, became convinced that conventional, non-superconducting electrical systems could be made to work in a large aircraft. It was funded by NASA Ames to take the ECO-150 concept and rework it around ambient-temperature generator and motor technology available to meet NASA’s 2020-25 timeframe N+2 goals (40% less fuel used, lower emissions and lower noise).
To the evident surprise of both ESAero and NASA, the N+2 ECO-150 design closed – met its requirements – despite having a significantly heavier turboelectric distributed-propulsion system using technology available today in industries outside aerospace. “Our main interest was could we even get the aircraft to close, and the answer is yes,” says Gibson.
“This is our first shot at getting the aircraft to close, and performance is about equal to a CFM56-powered 737-700,” he says. Without the benefit of high-efficiency superconducting motors and generators, the propulsors are significantly larger (below, superconducting on the right and non-superconducting on the left). Gibson says ESAero might redo the N+2 ECO-150 design and increase fan diameter, which would allow the motors to be shorter.
Past studies of hybrid turboelectric power concluded it would take electric-motor power densities of 10hp/lb to make a design work. Current technology is around 4-5hp/lb. “Technology is not even close to 10hp/lb, but it appears we do not need that kind of power to close an aircraft,” he says. The N+2 ECO-150 has generators and motors in the 2.4-4.5lb/hp range.
The result of all this work is growing military interest in turboelectric propulsion – superconducting and non-superconducting – and a Large Electric Aircraft Propulsion
The technology for such an aircraft is available, it is outside aerospace and needs to be scaled up. “The motors are about an order of magnitude larger than exist today,” says Schiltgen. Even using today’s non-superconducting technology, the time needed to scale up the motors, make sure they work at altitude and find ways to dissipate the heat they generate, would put a turboelectric-powered aircraft out into the 2025 timeframe, he believes.
While NASA believes ambient-temperature turboelectric propulsion could be used in a demonstrator aircraft, it continues to pursue cyrogenic superconducting technology to get the power density and energy efficiency it is seeking. To that end, it has awarded contracts to Rolls-Royce Liberty Works to design a 50MW-class propulsive electric grid; Advanced Magnet Lab for a fully superconducting motor/generator; Creare for a flight-weight cryocooler; and MTECH Laboratories for a cryogenic inverter/rectifier.