aerospace Article Constrained Urban Airspace Design for Large-Scale Drone-Based Delivery Traffic Malik Doole 1, *, Joost Ellerbroek 1 , Victor L. Knoop 2 and Jacco M. Hoekstra 1   Citation: Doole, M.; Ellerbroek, J.; Knoop, V.L.; Hoekstra, J.M. Constrained Urban Airspace Design for Large-Scale Drone-Based Delivery Traffic. Aerospace 2021, 8, 38. https:// doi.org/10.3390/aerospace8020038 Received: 11 December 2020 Accepted: 25 January 2021 Published: 1 February 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- ms in published maps and institutio- nal affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Control and Simulation, Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands; J.Ellerbroek@tudelft.nl (J.E.); J.M.Hoekstra@tudelft.nl (J.M.H.) 2 Transport and Planning, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands; V.L.Knoop@tudelft.nl * Correspondence: M.M.Doole@tudelft.nl Abstract: Large-scale adoption of drone-based delivery in urban areas promise societal benefits with respect to emissions and on-ground traffic congestion, as well as potential cost savings for drone-based logistic companies. However, for this to materialise, the ability of accommodating high volumes of drone traffic in an urban airspace is one of the biggest challenges. For unconstrained airspace, it has been shown that traffic alignment and segmentation can be used to mitigate conflict probability. The current study investigates the application of these principles to a highly constrained airspace. We propose two urban airspace concepts, applying road-based analogies of two-way and one-way streets by imposing horizontal structure. Both of the airspace concepts employ heading- altitude rules to vertically segment cruising traffic according to their travel direction. These airspace configurations also feature transition altitudes to accommodate turning flights that need to decrease the flight speed in order to make safe turns at intersections. While using fast-time simulation experiments, the performance of these airspace concepts is compared and evaluated for multiple traffic demand densities in terms of safety, stability, and efficiency. The results reveal that an effective way to structure drone traffic in a constrained urban area is to have vertically segmented altitude layers with respect to travel direction as well as horizontal constraints imposed to the flow of traffic. The study also makes recommendations for areas of future research, which are aimed at supporting dynamic traffic demand patterns. Keywords: urban airspace design; constrained airspace; UTM; U-Space; drone delivery; urban air mobility; smart cities 1. Introduction The current advancement in unmanned aerial vehicles, which is commonly referred to as drones, has potential applications in agriculture, research, inspection, health-care, urban air mobility [1–7], and logistics, especially in the transport of small express packages of consumer goods and fast-food meals within cities [8–11]. Recently, this demand has been exemplified by commercial logistics companies conducting drone delivery test flights in dense urban areas [12,13]. One reason for this interest is the potential environmental benefit [14] and the attractive economics of drone-based delivery [15,16]. However, if the large-scale adoption of drone-based delivery does begin to unfold, safely organising such traffic in the low altitude urban airspace, which is highly constrained by existing street networks and buildings, will be one of the main challenges to overcome. In order to cope with the future demand for drone-based services, previous research, such as the Metropolis project, has demonstrated that vertically segmenting the airspace in order to separate cruising traffic with respect to travel directions at different altitudes, leads to high levels of safety [17,18]. The study revealed that two factors, segmentation of traffic and the reduction of relative velocities (i.e., alignment of traffic), between cruising traffic at the same altitude were the main contributors to lowering the conflict probability and, thus, Aerospace 2021, 8, 38. https://doi.org/10.3390/aerospace8020038 https://www.mdpi.com/journal/aerospace