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Aircraft range at maximum gross weight – Given that the total empty weight of the aircraft will likely grow with these modifications, there will likely be less mass available for fuel. However, the increased efficiency of the power system will require less fuel for a given range.

 Cruise speed – Though speed is not a primary factor, potential retrofit concepts are more attractive if they enable users to get to their destinations at faster speeds. Hence, faster aircraft were considered more favorable, in addition to the other decision criteria.

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 Safety of flight impact – This refers to potential hazards introduced to the aircraft and crew associated with integration and flight of the new power and propulsion architecture. Some demonstrator and integration choices (single vs. multi-engine, equipment required in crew compartment aft of crew, etc.) may adversely affect safety of flight.

 Public perception (“wow” factor) – A driving assumption to this research is to demonstrate performance and utility that will be appealing to early adopters. Concepts that score higher here are all- encompassing, full flight demonstrators with compelling performance, particularly associated with the initial target market.

B. Power System Development The power system is the key technology component of this demonstration. It must enable high efficiency from

traditional infrastructure-friendly hydrocarbon fuels to meet the program goals. The Boeing Co., a partner to this demonstrator investigation, has been developing a regenerative SOFC-enabled flight-weight power system under the DARPA Vulture program that is lighter and more efficient than current fuel cell technology. It is well-suited for application to hydrocarbon fuels, and yields additional mass savings and efficiency increases when used in non- regenerative applications. A companion paper [23] details the technologies and trades associated with the hybrid SOFC power system for this proposed demonstrator program.

To quantify power system scaling, Boeing conducted a study to provide volume, mass, and efficiency metrics associated with different required power levels. The SOFC portion of the hybrid system could be sized to provide takeoff, climb, or cruise power, with the balance of power coming from an internal battery (hence the “hybrid” nomenclature). As many of the light aircraft considered for the demonstrator baseline use a limited set of approved gasoline-powered aviation engines, the basic requirements for the power system output were anchored to the performance of these engines. Arguably, there are three major power classes of normally-aspirated aviation engines at this scale, in terms of maximum brake horsepower produced: 100hp (e.g., Rotax 912S [24]), 180hp (e.g., Lycoming O-360-A4M [25]), and 310hp (e.g. Continental IO-550-N [26]). These power ratings represent maximum output on a sea level standard day, and do not consider drops in output to the propeller due to accessories (alternator, propeller governor, vacuum pump, etc.). These maximum power levels are typically only realized during takeoff or contingency conditions; instead, cruise flight for most of these aircraft takes place between 55-75% of rated power. The power levels for the hybrid system were therefore sized to these reduced power levels, which included a 10% reduction to account for the elimination of shaft-driven accessories and a “flat rating” of approximately 2,000 feet in density altitude. The power requirements after these reductions are given in Table 1.

Table 1: Power system sizing requirements for primary propulsion.

Replacement Class Rotax 912S (Nominally 73.5 kW)

Lycoming O-360-A4M (Nominally 134.3 kW)

Continental IO-550N (Nominally 231 kW)

Takeoff (2-5min), kW 66 121 208 Cruise Climb (10+ min), kW 56 103 176 Cruise (indefinite), kW 40 79 135

In addition to the amount of power needed for propulsion, each aircraft would need approximately 1.2-2.4 kW to

drive electrical accessories, including avionics, electrical actuators, and propeller governors (if installed). These systems would likely be at a much lower voltage than the propulsion system, but would still be charged from the hybrid SOFC-electric powertrain.

C. Electric Powertrain Development Electric powertrains are an area of ongoing research both in and outside of NASA, and have advanced to a

reasonable level of maturity. To keep risk and cost reasonable, the demonstrator will use a suitable Commercial Off- The-Shelf (COTS) or near-COTS solution for the electric motors, drives, gearboxes, cooling system, and propeller. Aircraft propeller torque and speed requirements at takeoff and cruise are important for system sizing, as well as any

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