87 relative to the ground plane, the amount of launching power (typically measured in kilojoules) the catapult is designed to produce and the range of environmental temperatures it is designed to work in. Each specification can then depend on numerous factors of the UAV and its mission. The right launch angle, for example, can be linked to the mission’s operating altitude and the capacity for the aircraft to carry sufficient fuel to power climbing as well as the subsequent endurance needed. Launch power is tied closely to the weight of the UAV and its users’ desired speed at the point of release from the catapult. A 2 kj launcher should work optimally for deploying a 12 kg UAV at around 23 m/s, whereas an 18 kg UAV using something weighing around 4 kj would get about 26 m/s at launch. In both cases, a somewhat heavier UAV would still probably work with such catapults, with the tradeoff of a slightly slower launch speed, whereas a slightly lighter UAV would get a faster one, but potentially risk damaging the aircraft due to the force with which it is launched. For a wide range of working temperatures, pneumatics work well, but to build a small catapult either metal springs or elastic slings are typically used as a source of potential energy for launching. These come with different environmental considerations: metallic springs tend to function far better than elastic in very cold environments, particularly as traditional elastics could not be certified to work in -20 C (losing launch power due to thermal contraction). However, a couple of suppliers have recently come out with the ability to provide elastic that retains its operative qualities down to -20 C. As a result, lightweight, elastic-based catapults weighing 40-60 kg are proliferating throughout the UAV industry, with their transportability enabling more survey work with small, cost-effective, non-VTOL, fixed-wing aircraft, and teams of no more than two people. Weight is a major concern, especially for infrastructure owners who want to be able to transport everything needed for a day’s survey in a single, small road vehicle. The elastic needed to launch 10-20 kg UAVs tends to weigh around 5 kg, whereas an equivalent metal spring is likely to be 25-35 kg, thus potentially doubling the weight of an existing elastic-powered catapult. The mechanical design of the catapult bears close optimisation, particularly to ensure the elastic (or other launch material) can stretch to the exact length needed, as the relationship between launching force and length of extension is not linear: stretching a sling to exactly 90% of its optimal length, for instance, will give considerably less than 90% of its optimal launching energy. For a 2 kj launcher, the elastic must be able to stretch to a length of about 650 mm, whereas for 4 kj it is likely to be stretched to 1100 mm. A company that skimps on such length to provide a more compact and thus outwardly convenient catapult will find its solution fails to give the degree of ‘push’ during takeoff that its end-users might like. The more force imparted by the energetic device inside the catapult, the more important it is to build it with sufficient housing and ruggedisation. Over time, some cracking in the metal or elastic is inevitable, raising the risk of breakage and debris flying out, which poses an injury hazard, so extra weight in ruggedness can be vital for the safety case. Aluminium remains a predominant choice for catapults for its high tensile strength relative to weight. Launch & recovery | Focus The more force imparted by the energetic device inside the catapult, the more important it is to build it with sufficient housing and ruggedisation Uncrewed Systems Technology | December/January 2025 Elastic-powered launch catapults are gaining popularity for their light weight and material advancements, enabling them to function in sub-freezing temperatures (Image courtesy of ElevonX)
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