Unmanned Systems Technology Dec/Jan 2020 | Phoenix UAS | Sonar focus | Construction insight | InterGeo 2019 | Supacat ATMP | Adelan fuel cell | Oregon tour | DSEI 2019 | Copperstone Helix | Power management focus

cheap, almost disposable, alternative. Second, the reduced need for large batteries and arrays of solar cells means the payload fraction (the ratio of payload to the all-up mass of the aircraft) should be far higher than with alternative aircraft. Initial conceptual designs of a possible next generation of the Phoenix concept have shown that lifting a payload of 100 kg to an altitude of 20 km is feasible, something that is impossible with current working alternatives. Third, the exclusion of motors and propellers means the Phoenix concept is mechanically and electrically simpler than equivalent aircraft. That is important 34 First, if we consider the requirement for a high-altitude pseudo satellite, there are several existing aircraft that are designed to perform that role. All of them, however, need significant arrays of solar cells and large batteries to meet the demands of their electric propulsion systems. That adds considerable weight, and also massively increases the cost of the aircraft. For the Phoenix prototype, there was enough power for the propulsion and control systems from small arrays of solar cells on the wings and horizontal tail only – none on the fuselage envelope. That reduced the cost of the aircraft by at least an order of magnitude, providing a for a long-endurance aircraft, as the elegance and simplicity of variable- buoyancy propulsion reduces the probability of mechanical and/or electrical/electronic failures terminating its mission prematurely. The conclusion of the successful flight trials was celebrated with a well-earned glass of champagne, and the task of developing a larger and more capable follow-on vehicle now begins. During the course of the project, one of the members of the consortium got married and another welcomed their first child, but as a team we also created a novel professional partnership and oversaw the birth of a new type of aircraft. December/January 2020 | Unmanned Systems Technology Dossier | Phoenix UAS The propulsion concept behind the Phoenix has been around for a long time, with a patent for it applied to an airship being granted to a Solomon Andrews of New Jersey in the US on July 5, 1864 (US Patent 43,449). Andrews’ proposal for his Aereon suggested that the airship be made buoyant by using hydrogen as a lift gas, and it would therefore ascend. The airship’s buoyancy could be reduced by venting some of the hydrogen to the atmosphere, thus making it heavier than air, and it would consequently descend. A return to lighter-than-air buoyancy would then be achieved by discarding ballast carried aloft in a gondola suspended beneath the airship. Variations in the location of the centre of gravity (CoG) of the airship were achieved by the ‘pilot’ walking to and fro along the length of the gondola, which would result in changes in the airship’s attitude. Walking to the front moved the CoG ahead of the centre of buoyancy, so the airship’s nose would pitch down; walking to the back would move the CoG aft and the nose of the airship would pitch up. Andrews suggested that combining variable buoyancy with these variations in attitude would result in a sinusoidal (porpoise-like) flightpath. When ascending, the nose would be raised; when descending it would be lowered. In doing so, the buoyancy (when lighter than air) and the weight (when heavier than air) have a component along the flightpath that provides the thrust force, as shown in Fig. 1 on page 23. The limitation to Andrews’ proposal was that it had a finite flight duration, concluding when the airship had insufficient lift gas or ballast to achieve the transition to lighter-than-air flight, but the philosophy of the idea was sound. The key to unlocking this idea and creating a useful aircraft is thus the ability to achieve buoyancy variations in a sustainable manner. A variation on this mode of propulsion has been demonstrated successfully in underwater ROVs. Rather than accomplish buoyancy variations through changes in mass, these ‘gliders’ vary their displacement (volume) by repeated expansion and contraction of flexible bladders on the craft using, for example, compressed air carried internally. They have been used as long-endurance survey vehicles that porpoise through the water collecting data; they surface periodically to upload that data before submerging again and continuing on their autopilot-controlled journey. The ROVs are assisted by the fact that water is nearly 1000 times denser than air, so the volume change needed to attain the necessary changes in buoyancy is relatively small. Various incarnations of the variable-buoyancy concept have appeared in the intervening years, but no viable prototype has been flown. For the concept to be useful, it is necessary not only to demonstrate in practice the validity of the philosophical propulsion concept in air, but to do so with a ‘system’ that can take advantage properly of the possibilities it offers. That is, unlike Andrews’ limited-endurance version, a practical aircraft would need to achieve variations in buoyancy through a sustainable mechanism that allows the simplicity of its propulsion to be exploited for ultra-long endurance applications, in a manner similar to the ROV gliders. The propulsion concept