High Altitude Platform Design Optimization

Abstract

High Altitude Platforms (HAP) or Pseudo Satellites are unmanned aerial vehicles (UAV) which perform long endurance tasks in the field of observation or communication. Their operational altitude is above the civil airspace, starting at 15 km up to more than 20 km. The German Aerospace Center (DLR) is currently con-ducting a study on High Altitude Platforms and their applications. 14 DLR institutes are involved. This work investigates details about a control based optimization in early stages of the design. For search of a suitable HAP configuration, it is evident to investigate a 24 h simulation of the platform within a desired mission scenario. Most critical output is the change of battery charge after a one full simulated mission day. Thus, a design optimization framework is executed with goal of reaching a change it battery charge greater zero for the 24 h cycle. Within this optimisation, vertical and lateral flight profiles and their impact on the platform configuration are investigated. Firstly, a vertical flight profile is introduced into a prelimi-nary design and simulation framework. The altitude change shall minimize the amount of excess solar energy that cannot be stored for flight times with insufficient solar radiation input. The maximum storable energy of a platform is in general constrained by the capacity of the batteries and the maximum reachable altitude due to maximum engine power and aircraft configuration. Besides storing additional energy in potential energy, descent during night saves additional energy as flying on lower altitudes is more efficient and therefore allows further downsizing of the battery system. With the possibility of adjusting different parameters for the profile generation, an optimization can be started which either maximizes the total increase in battery state of charge of a 24 hour flight cycle, or scales a configuration iteratively to obtain a desired delta in battery charge for a 24 h simulation. The vertical profile of the aircraft is subject to a total ener-gy control system (TECS). It is desired to transform the available excess power which cannot be stored into batteries, into potential energy. This is limited by the maximum power intake of the engines. In the phase when the available solar energy is not sufficient for propelling the HAP on the reached ceiling altitude, propulsive energy is reduced to the amount of gained solar energy, resulting in a negative flight path angle. When reaching a minimal mission altitude, the platform returns to level flight and propulsion energy is obtained from the battery system. Results show that investigated combinations of platform configuration and vertical profile have a positive energy balance for the 24 hour flight cycle and are superior to platforms operating at one stationary altitude. In further analysis, lateral flight profiles which shall optimize the solar energy intake will be taken into account. The early design stage also includes mean wind speeds into the design. As a result, using control strategies already in the preliminary design adds parameters for optimization process with the benefit of receiving an already mission capable platform.