It will take some extreme measures to reduce fuel burn to more than 70% below that of today's GE90-powered Boeing 777-200LR - NASA's goal for an airliner entering service after 2030-35. So how about an all-composite, laminar-flow, hybrid wing-body (HWB) airframe with turboelectric distributed propulsion?Graphics: NASA
Meet NASA's N3-X. This is the latest and most advanced evolution of NASA's HWB (a non-proprietary interpretation of Boeing's blended wing-body (BWB) configuration) - starting with the SAX-40 designed by the Cambridge-MIT Institute under the Silent Aircraft Initiative
and evolving by way of the 2025-timeframe N2A and N2B configurations being windtunnel-tested by NASA.
What sets the N3-X apart is its turboelectric distributed propulsion - TeDP - which decouples the device producing propulsion from the device producing power. So where the core of a high-bypass turbofan generates the power that drives the fan that produces the thrust, in TeDP two wingtip-mounted turboshafts drive superconducting generators to produce the electrical power that drives 15 superconducting motor-driven propulsors embedded in the fuselage.
The key word here is superconducting, which implies supercooling of the generators, wiring and motors. NASA is looking at two cooling schemes: cryo-cooled and liquid-hydrogen cooled. In the first approach, cryocoolers running on jet fuel would produce the low temperatures required to achieve superconductivity. In the other, the aircraft would carry LH2 tanks to provide cryogenic cooling.
The cryocooled approach would use the same fuel for power generation and cooling. The other way would require jet fuel for power generation and LH2 for cooling, but fortuitously there is plenty room in the HWB for extra cyrogenic tanks (shown below as the three lighter-green cylinders in the rear of the centerbody).
NASA's N3-X design is for a 300-passenger airliner capable of cruising at Mach 0.84 and flying 7,500nm. Wing span is 134ft. The 15 electrically driven propulsors have fan diameters of 43in and are spaced 5in apart, breathing through a "mail slot" inlet 25in high and 60ft long and producing a combined 90,000lb of take-off thrust . Together the two 30,000shp-plus wingtip generators would produce more than 45MW of electrical power.
While preliminary estimates indicate N3-X could meet NASA's tough N+3 goal of reducing fuel burn more than 70% from the 777-200LR, the agency cautions further detailed design analysis is needed. One reason is the sensitivity of results to inlet conditions, which demands a fully integrated airframe/engine analysis from the outset.