Issue 41 Unmanned Systems Technology December/January 2022 PteroDynamics X-P4 l Sense & avoid l 4Front Robotics Cricket l Autonomous transport l NWFC-1500 fuel cell l DroneX report l OceanScout I Composites I DSEI 2021 report

83 into a high-quality composite part with quasi-isotropic fabric, it might be called isotropic: having the same mechanical properties in every direction, as most metals and alloys do. That may not always be necessary though. For instance, unidirectional fibre can be laid precisely as needed for a part’s load case (as in tubing, spars or struts) by an experienced composites engineer, hence that part should transmit loads essentially as needed across its lifetime of use, without incurring the material costs of an isotropic part. However, inserting more fibre into a material relative to the amount of resin (or any other matrix material) is widely desirable these days, since the fibres add reinforcement while the matrix mainly just binds and adds weight. Increasing the fibre volume fraction (Vf) – the ratio of fibre to matrix by volume – is therefore much of the focus of composite suppliers’ r&d. On the topic of material inputs, it is also worth mentioning some newer fibre types that have gained in prominence in recent years. Basalt composite, for example, has been used to great effect in the hull of at least one high-end USV, as well as by VW, Bentley and other OEMs in the automotive and motorsport markets. In principle, basalt is an ideal substance for fibre reinforcement. It exhibits a high elastic modulus as well as good heat resistance and acoustic damping, with a tensile strength ranging from 1500 to 4000 MPa and safe operating temperatures of up to 800 C. It has a melting point of about 1400 C and performs consistently while immersed in a range of fluids. It also provides a noteworthy bridge (in performance terms) between carbon and glass fibres. Carbon fibres remain the most widely used in unmanned vehicles thanks to their high thermal and mechanical stability, although they largely remain the most expensive. Glass fibres are less pricey but have comparatively lower thermal and mechanical resilience. Basalt fibres meanwhile cost around the same as glass fibres while exhibiting mechanical and thermal properties closer to those of carbon fibres. Mica-reinforced plastics are also being used more widely, being combined with polyesters and polypropylenes for automotive parts such as fenders and AC heater housings, as well as aerospace components such as stabilisers and fairings. Their high heat dimensional stability, low warpage, resistance to many harmful fluids, and other properties, make them popular in these applications, as well as in cable jackets for high-temperature and chemically hazardous installations. As far as matrix materials are concerned, epoxy resins remain the most widely used, although other plastics such as polyamides, urethanes and acrylics are also well-established. Metal matrix composites using aluminium, titanium or magnesium, as well as ceramic matrix composites based on silicon carbide, alumina or mullite, continue to advance and gain traction, particularly for very high heat- or strength-critical applications. Recent years have seen the rise of new matrix materials such as cyanate esters, which have shown excellent thermal Composites | Focus Unmanned Systems Technology | December/January 2022 High-end composites companies will often improve upon received parts, for example replacing cut or drilled holes with moulded ones (Courtesy of Real Carbon) Filament winding can produce high-quality axisymmetric composite parts, such as tubes, pressure vessels and even rotors and transmission shafts (Courtesy of Lentus Composites)