NASA/TM-2014-XXXXXX System-of-Systems Considerations in the Notional Development of a Metropolitan Aerial Transportation System: Implications as to the Identification of Enabling Technologies and Reference Designs for Extreme Short Haul VTOL Vehicles with Electric Propulsion Juan J. Alonso Heather M. Arneson John E. Melton Michael Vegh Cedric Walker Larry A. Young 1 Table of Contents Executive Summary ................................................................................................ 4 Introduction ............................................................................................................. 8 Objective of Current Study and Its Relationship to Earlier Studies ..................... 15 Vehicle Fleet and Fleet Introduction Profiles ....................................................... 16 Network Models.................................................................................................... 31 BaySim Metro/Regional Aerial Transportation System Modeling ................... 31 Integration of Hopper Flights into San Francisco Bay Area Airspace ............ 66 Potential Business Models .................................................................................... 69 Notional Implementation Plan Scenarios and Future State Iterative “Forecasting” and “Backcasting” ................................................................................................. 78 Station Conceptual Design and Operations Considerations ................................. 80 Alternative Vehicle Concepts ............................................................................... 91 Concept Applicability to Other Regional Transportation Networks .................. 108 Technology Roadmap ......................................................................................... 111 Integration with Current NASA Rotorcraft Research Efforts ............................. 136 Concluding Remarks ........................................................................................... 138 Acknowledgments............................................................................................... 140 References ........................................................................................................... 141 Appendix A – CFD Predictions of Hopper Vehicle(s) and Associated Rotor Wake Interactions .......................................................................................................... 152 Appendix B – Acoustic Noise Source Estimates ................................................ 163 Appendix C – IMPLEMENT Analysis ............................................................... 173 Appendix D – BaySim Simulation Analysis ...................................................... 175 Appendix E – Fleet Assignment/Optimization Transportation Network Analysis ............................................................................................................................. 201 Appendix F – FACET Metro/Regional Airspace Analysis ................................ 204 Appendix G – Updates to Hopper Conceptual Designs and Refined Design Analysis............................................................................................................... 212 2 Nomenclature & Acronyms ARMD NASA Aeronautics Research Mission Directorate ATM Air Traffic Management BVI Blade vortex interaction Rotor power coefficient, nondim. Rotor thrust coefficient, nondim. FACET Future ATM concept evaluation tool FAP NASA Fundamental Aeronautics Program fDist Flight distance in statute miles (parameter used in the BaySim software tool) FM Rotor hover figure-of-merit HIGE Hover in-ground-effect HOGE Hover out-of-ground-effect HSI High speed impulsive (noise) ⁄ Vehicle hover-in-ground-effect; ratio of height above ground to rotor radius iPX Number of passengers on a flight (parameter used in the BaySim software tool) ⁄ Vehicle lift-over-drag ratio LOS Loss-of-separation NextGen Next Generation Air Transportation System OASPL Overall Acoustic Sound Pressure Level, dB OEI One engine inoperative PAX Number of passengers onboard vehicle NAS National airspace system SOA State-of-the-art VTOL Vertical takeoff and landing ⁄ Vehicle hover-in-ground-effect; ratio of lateral separation (to referenced object) to rotor radius 3 Executive Summary There are substantial future challenges as related to sustaining and improving efficient, cost-effective, and environmentally friendly transportation options for urban regions. Over the past several decades there has been a worldwide trend towards increasing urbanization of society. Accompanying this urbanization are increasing surface transportation infrastructure costs and, despite public infrastructure investments, increasing surface transportation “gridlock.” In addition to this global urbanization trend, there has been a substantial increase in concern regarding energy sustainability, fossil fuel emissions, and the potential implications of global climate change. A recently completed study investigated the feasibility of an aviation solution for future urban transportation. Such an aerial transportation system could ideally tackle some of the above noted concerns related to urbanization, transportation gridlock, and fossil fuel emissions. Additionally, a metro/regional aerial transportation system could provide enhanced transportation flexibility to accommodate extraordinary events such as surface (rail/road) transportation network disruptions and emergency/disaster relief responses. The goal of this study effort was to develop an integrated system simulation that incorporated models of aircraft, vertiport stations, fleet operations, and airspace management technologies to determine the feasibility of using electric- propulsion, vertical takeoff and landing vehicles to serve a metro-regional transportation system. The technical challenges of developing a metro-regional aerial transportation system based on extremely short haul vertical lift vehicles, aka Hoppers, was examined through these simulations; Fig. 1. 4 Figure 1. One Possible Future for Metro/Regional Transportation: an Aerial Public Transit System using Rotorcraft (Background Image Courtesy of Google Earth) There were three key accomplishments to this study. First, an expanded vehicle sizing design space was examined for rotary-wing vehicles incorporating electric propulsion. This design space included not only modeling emerging advanced battery technologies but hydrogen-fuel-cell systems and hybrid (turboshaft engine and battery/fuel-cell-driven electric motors) propulsion systems. Additionally, the design space was influenced by insights gained during the study as to mission profile requirements particularly vehicle range requirements (25-100 nautical miles). The expanded design space was explored with a new rotorcraft-sizing software tool. Second, a considerable effort was expended on developing the tools and analysis to support investigation of near- to far-term evolution of the notional aerial transportation network – from simple three-node networks to large multi-node systems. The ability to model/analyze this network evolution begins to capture the nuances of a more realistic potential implementation of such a metro/regional aerial transportation system. The primary simulation tool used for this investigation was the BaySim software tool, which was developed as a part of this study. BaySim incorporated public domain San Francisco Bay Area population data from Government census/zip code databases (which is readily extensible to other metropolitan areas). BaySim also included alternate network topologies, the Dijkstra optimization algorithm, and air traffic management integration via “speed tables” (time-of-day flight path dynamic routing). And finally, third, a number of operational (and miscellaneous 5 related) issues were explored in a very preliminary sense using computational fluid dynamics, acoustics analysis, and the analysis of air traffic management issues resulting from interaction of the Hopper fleet with commercial air transport and general aviation aircraft using the well-known NASA-developed tool FACET. Altogether, the study revealed that a mid-term (next fifteen to twenty years) solution for electric vertical lift vehicles supporting the Hopper metro/regional aerial transportation system mission might be feasible. Specifically, relatively near-term battery/hybrid (i.e. turboshaft with battery or fuel-cell)/fuel-cell technologies can make these short-range vehicles realizable perhaps within the next ten years. Near-term battery technology (500-600 W-h/kg at reasonable power densities) will satisfy power and energy requirements for these short-range vehicles. To successfully develop rotorcraft with electric-propulsion-capability (or any other type of aircraft) it is not simply a question of waiting until the power/energy-storage-device industry produces a sufficiently advanced battery (or fuel-cell or hybrid system) in terms of specific power and specific energy capability. Clearly, when considering that Hopper battery packs will be an order- of-magnitude greater in size/capacity as compared to current production automotive electric vehicle batteries, there are aviation-unique aspects to electric- propulsion system development. With regards to the foundational concept of such vehicles supporting a mission application targeted around metro/regional aerial transportation, large numbers of medium- to large-aircraft are required to serve assumed passenger levels (which ranged in the study from 5,000 to 30,000 passengers per day). Flying and VTOL are energy intensive transportation modes. While originally conceived as "light infrastructure," i.e. no new roads or rail lines, Hopper station power requirements and real estate footprints of ramps and terminals will be nontrivial when large numbers of aircraft are involved. Two examples of this secondary but essential infrastructure are: trucks will be required if charged batteries are to be delivered to stations (and discharged batteries removed for offsite recharging) and/or high-power electrical transmission must be provided to stations if charging is onsite. Nonetheless, the simulations and schedule optimization analysis performed during this study support the possibility of Hopper-type vehicles carrying a substantial daily passenger load within the San Francisco Bay Area (and, by extension, other metropolitan areas). Other operational considerations for operating a large fleet of medium to heavy vertical lift vehicles performing sustained frequent over-flights over the urban environment include the issue of noise and emissions. From an emissions 6 standpoint, rotorcraft with electric-propulsion are more environmentally benign that turboshaft-engined rotorcraft. However, though the vehicles conceptually designed in this study were required to operate at much lower tip speeds and disk loading than conventional helicopters, rotorcraft noise reduction will still be an important technological challenge. A very preliminary set of acoustic predictions was made during this study. Other important operational considerations include rotor wake interactions in the proximity of vertiports (aka Hopper network stations) as well as airspace management issues inherent in attempting to notionally interject a large fleet of aircraft into an already complex, congested airspace. Emerging technological and procedural advances, currently being proposed under the Next Generation Air Transportation System (NextGen), could be leveraged to facilitate the realization of the Hopper concept. Further, the challenges in integrating Hopper flights and Bay Area commercial traffic that can be partially tackled with Hopper aircraft time-of-day dynamic routing in addition to NextGen technologies. This study directly addressed NASA strategic goals to advance aeronautics research for societal benefit. Transportation is a first-order driver to the economy; a cost-effective and adaptive metro/regional aerial transportation system would have a first-order effect on regional economies and direct economic benefit to the Nation. The immediate benefit of the Hopper study, though, is in providing rotorcraft researchers with a novel electric aerial vehicle reference design and a notional mission/application that can potentially act as an aid in defining future technology development roadmaps for NASA. In this regards, a considerable body of NASA rotorcraft research (most of which focused on conventional rotor configurations) potentially awaits transitioning from the lab, wind tunnel, simulator, etc. to flight demonstration and production. Such research into active rotor control, active flow control, active structures and many more technologies would transform a vertical lift vehicle, in its most represented form, the helicopter, from an aircraft with electric-propulsion to an “electric rotorcraft” – a vehicle that is intrinsically “wired” to drive not only electric motors but a suite of electrically/actively controlled actuators and devices. Such a Hopper-inspired electric rotorcraft could one day potentially result in a vehicle with improved performance and passenger/community friendliness. 7 Introduction There are substantial future challenges as related to sustaining and improving efficient, cost-effective, and environmentally friendly transportation options for urban regions. Over the past several decades there has been a worldwide trend towards increasing urbanization of society. Accompanying this urbanization is increasing surface transportation infrastructure costs and, despite such public infrastructure investments, increasing surface transportation “gridlock.” In addition to this global urbanization trend, there has been a substantial increase in the concern regarding energy sustainability, fossil fuel emissions, and the potential implications of global climate change (Ref. 121). This study -- and an earlier companion study (Ref. 1) -- investigates the feasibility of an aviation solution for future urban transportation. Such an aerial transportation system could ideally tackle some of these concerns related to urbanization, transportation gridlock, and fossil fuel emissions. Specifically, this study documents the results of a conceptual design and systems analysis investigation into a notional metropolitan aerial transportation system. The goal of the earlier Phase I effort (Ref. 1) was to develop an integrated system simulation that incorporated models of compatibly designed aircraft, stations, fleet operations, and airspace to determine the feasibility of using electric, short or vertical takeoff and landing vehicles to serve a metro- regional transportation system. A baseline system simulation was achieved. It incorporated a newly developed discrete event simulator, modified network optimization algorithms, new electric propulsion modules in NASA’s premier rotorcraft design tool, NDARC (Refs. 2 and 118), and database expansion of NASA’s premier airspace simulation software (FACET). Key findings from Phase I were 1) that aircraft designed for extreme short-haul (defined as less than 100 nautical miles per flight leg) could be utilized to serve tens of thousands of daily commuters in a metropolitan area, 2) that aircraft designs close using conventional propulsion and today’s technology 3) that aircraft designs using electric propulsion will be possible in 15 years with larger vehicles possible in 30 years, and 4) the aircraft would likely need to fly below 5kft to minimize airspace conflict. The results from both the Phase I and Phase II efforts provide insight into the potential of electric aircraft to serve a unique urban transportation role. Figure 2a summarizes the Hopper stations/network studied in Phase I; Fig. 2b illustrates representative Phase I BaySim discrete event simulation results of the Hopper vehicles/network as a function of time-of-day. Note that in the Phase I 8 effort, aircraft could travel from one station/node to any other node in the network. (a) (b) Figure 2 – (a) Phase I Hopper stations/network and (b) representative time-of-day snapshot of Phase I BaySim output 9 Figure 3 illustrates a representative snapshot, using the FACET airspace analysis tool during the Phase I study, of the estimated loss-of-separation events – and their spatial distribution throughout the San Francisco Bay Area airspace – resulting from the introduction of the notional fleet of Hopper aircraft. These projected airspace conflicts presented a key technological challenge for the Hopper metro/regional aerial transportation system concept. It was clear from these and similar results that innovative airspace management technologies and techniques would likely need to be implemented in order to support the introduction of a large number of addition aircraft into heavily congested airspace such as that in the Bay Area. Further, this airspace management challenge, though identified in the context of the Hopper concept, would likely be manifested with other alternate aeronautics innovations such as small UAS (unmanned aerial systems) platforms, advanced GA (General Aviation) aircraft, and personal air vehicles (PAV). Figure 3. Phase I FACET Loss-of-Separation Assessments (gold circles represent individual unique loss of separation locations; Ref. 1) 10