94 Focus | Fuel systems Modern rigid tanks do however have some technological and operational advantages that make themworth considering for a fuel-powered UAV. One option is a tank made from a rotomoulded, cross-linked polyethylene, which is so strong that (as per military standards) it can be test-dropped from 20 mwhile fully fuelled with 60 US gallons (227.12 litres) onto a steel pad without losing any fuel. The tank might become deformed, but it will not crack. Its manufacture is highly repeatable and not very labour-intensive, making it a good option for bulk-produced UAVs for missions where crashing might be a major concern, such as defence forces, energy infrastructure monitoring or disaster relief applications. Reinforced fabric bladders are often assembled by cutting their different components with tabs, which are then folded over each other and seamed. Film bladders however can be produced using some newer approaches, such as having a cylindrical bladder with a single long tab for each flat surface on either end of the tube section. Having one long tab per end (rather than several, as in the usual method) reduces the number of potential leak paths and hence failure points. A small number of fuel storage suppliers have even developed solutions for protection against ballistics impacts – clearly an ever-present concern for defence UAVs on front lines or those operating near potentially explosive industrial environments. Rather than actually armouring the tanks, which would present a weight concern, the most promising technology is a self-sealing solution composed of a two-stage reactionary material. When a projectile penetrates the bladder, the rubber first ‘wants’ to mechanically return to its original shape, stretching inward towards the gap. Second, a chemical reaction occurs as the fuel makes contact with the uncured rubber revealed by the ballistic wound. This reaction causes the uncured rubber to swell, and in the direction of the hole, thereby closing it. First popularised in military aircraft and civilian armoured vehicles, this solution is not intended to clear a UAV (or other vehicle) for continued, uninterrupted operation but to prevent leaks and explosions of fuel, and potentially ensure that the vehicle can be approached and carried away, or launched to perform just a short flight to a safer location. Fuel level sensors A well-functioning fuel tank or bladder also depends on careful selection of the level sensors for tracking the availability of fuel, to gauge the remaining endurance or spot any problems with fuel storage. There are a few different types of fuel level sensor, including ultrasonic, reed switch and resistive float sensors, and each has its drawbacks. Ultrasonic sensors for instance typically have a ‘dead band’ close to the sensor and can hence fail to measure liquid if it sloshes about. Reed switch sensors meanwhile tend to measure fuel levels with a poor, coarse resolution compared to others, and resistive float sensors are unsuitable for high-vibration applications. Capacitive level sensors have overtaken the other types now though. They operate by measuring the June/July 2023 | Uncrewed Systems Technology Positive-displacement pumps scale down to UAVappropriate sizes more easily than other types, with minimal leakage and good self-priming (Courtesy of Currawong) The most promising fuel tank technology is a self-sealing solution. It is not intended to clear a UAV for continued operation but to prevent explosions
RkJQdWJsaXNoZXIy MjI2Mzk4