Turboelectric Propulsion - Superconducting or Not?
1:27 AM on Mar 24, 2012
When NASA talks about turboelectric distributed propulsion, it talks in terms of superconducting generators and motors in a hybrid wing-body concept aircraft it calls the N3-X. But there are other ways of approaching all-electric propulsion.
Empirical Systems Aerospace (ESAero), a small advanced-design house based in Pismo Beach, California, assumed superconducting technology when it produced a concept (below) for a 2030-35 timeframe turboelectric-powered 150-seat airliner, the ECO-150.
Graphics: ESAero, via NASA
Designed to meet NASA's N+3-generation goals, which include a 60% reduction in energy consumption relative to the CFM56-powered 737-800, the ECO-150 features electrically driven fans embedded in the inboard wing sections, powered by mid-span turbogenerators supported by bracing struts attached to the tails. There are eight fan propulsors per side.
High-efficiency superconducting generators and motors require cryogenic cooling to work, and ESAero president Andy Gibson says the company did not have enough information on cryocoolers so it used liquid hydrogen to cool the electrical systems. Hydrogen is contained in tanks along the top of the fuselage and, after cooling the superconducting systems, is burned in the turbogenerators.
The required superconducting technology does not exist today but ESAero, continuing internal studies into 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.
To the evident surprise of both ESAero and NASA, the N+2 ECO-150 (above) 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.
Ben Schiltgen, Gibson's business partner and architect of the propulsion system, says 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 Technology (LEAPTECH) workshop was held in Dayton, Ohio, in January, where NASA and the US Air Force, Navy and Army shared their ideas in an effort to identify opportunities for collaboration - including the potential for a dual-use demonstrator.
Under its N+2 study contract from NASA, ESAero has also produced a dual-use commercial-airliner/military-transport concept. The two variants of this design have essentially the same fuselage - housing 130 seats as an airliner and 52,000lb of payload as an airlifter. The wing is scaled up 20%-plus for the military version (below).
Although ESAero believes 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.
NOTE: Post corrected to identify NASA Ames as the sponsor of ESAero's N+2 and dual-use concept work.