Around the Fall of 2019, two of my best friends from college asked if I was interested in joining a project they were starting. Their goal was to design a high endurance fixed-wing UAV for fire surveillance. I was immediately interested because I was working a seasonal job at Lowe’s and was looking for something to keep my mind active. I was developing the PAC program during that time, which is still an ongoing project, however the idea of working on something more ambitious was intriguing.
I agreed to join and was added to the chat. However, as a team we never made it past the planning stage due to conflicting schedules and the demands of their jobs. They both had engineering jobs, whereas I just worked at a warehouse with no responsibilities once I went home. During the couple weeks we were discussing the project, I was brainstorming ideas to set our project apart from the others in the market. The endurance of an surveillance aircraft is one of the main performance parameters and is a frequently cited metric in comparing competing designs.
While considering options to increase the endurance of a battery powered aircraft, a thought struck me, “why not just change the battery in-flight?” The idea was intriguing. With the capability of physically swapping the battery while flying, the aircraft would never to land, would not have to stay on station for an extended duration to recharge, and would not suffer from decreasing battery range due to frequent recharges.
The reason for these points is as follows: by swapping the battery instead of recharging, the time on station to perform the maneuver is only limited by the mechanical switching of the batteries, not recharge time. Further, by continually swapping the batteries out, any pack with decreasing efficiency due to continuous recharges, can just be removed from the pool of batteries, eliminating the need to land the aircraft for a new pack. Finally, due to the speed of swapping the batteries, a network of surveillance aircraft could be operated all serviced by the same supply aircraft.
With this concept in mind I started researching possible existing designs to see if anyone had already tried a similar approach. To the best of my knowledge, the only similar design is recharging by trailing a drogue.
UAV Docking for Autonomous Aerial Refueling
While this approach works, it has a number of inherent disadvantages; the biggest is due to the extended time on station to recharge the battery. This time limits the number of possible aircraft to service during a flight. Also any degradation in the battery necessitates landing the aircraft to switch the battery pack. Obviously an aircraft that has many moving parts increases the potential for failure. However, limiting the endurance of an aircraft to the number of docking maneuvers to perform rather then the charge capacity of a battery allows for an exponentially longer flight time.