Dr Geoffrey Knott

Space data services provide the largest market value to the global space industry. With the increasing use of small satellites that lower costs and lead times, the entrepreneurial space age has begun. However, advances in technology miniaturization are required to improve small satellite capabilities, which are limited by small volumes and low power consumption. This paper presents a deployable antenna for small satellites capable of achieving high-gain radiation performance despite being ultra-compact. The antenna is a helically curved boom that is deployed from a coil. The boom is an open slit tube. A ground plane comprised of four metallic booms supporting a sparse mesh is deployed by stored strain energy. A prototype of the antenna system has been built to test and validate the deployer mechanism, deployment strategy, and dimensional stability of the helical antenna and ground plane. The architecture builds on proven space technology, specifically the deployer mechanism of the InflateSail de-orbiting drag sail that successfully demonstrated low-Earth orbit space-debris removal in 2017. In this work, the deployer unrolls the helical boom whilst the sail itself is repurposed to boost the radiation performance of the helical antenna.

An ultra-compact deployable helical antenna is presented, designed to enhance space-based reception of Automatic Identification System signals for maritime surveillance. The radio frequency performance (i.e. peak gain and directionality) is simulated at 162 MHz using ANSYS High Frequency Structure Simulator and evaluated over a range [0.5–8] of helical turns. Established and commercially available omnidirectional antennas suffer interference caused by the large number of incoming signals. A 7-turn helix with planar ground plane is proposed as a compact directional-antenna solution, which produces a peak gain of 11.21±0.14 dBi and half-power beam width of 46.5±0.5 degrees. Manufacturing the helical structure using bistable composite enables uniquely high packaging efficiencies. The helix has a deployed axial length of 3.22 m, a diameter of 58 cm, and a stowed (i.e. coiled) height and diameter of 5 cm — the stowed-to-deployed volume ratio is approximately 1:9,800 (0.01%). The use of ultra-thin and lightweight composite results in an estimated mass of 163 grams. The structural stability (i.e. natural vibration frequency) is also investigated to evaluate the risk an unstable deployed antenna may have on the radio frequency performance. The first vibration mode of the 7-turn helix is at 0.032 Hz indicating the need for additional stiffening.

Bistability in doubly curved and twisted (helical) composite slit tubes is investigated for the first time. This work establishes a natural extension in this area which has been focused on straight and until more recently, doubly curved (toroidal) tubes with positive Gaussian curvature. The model developed introduces longitudinal and transverse curvature, and twist into strips of laminated composite material. The composite is engineered to be bistable and the second stable state determined via strain energy minimisation using the Rayleigh-Ritz method. The strain energy is formulated as a function of curvature strains, longitudinal stretching and a variable middle ply fibre angle of the laminate. The second stable state forms a compact and untwisted cylindrical coil with the latter engineered by tailoring the middle ply fibre angle. A new manufacturing process capable of producing helically curved tubes using glass-fibre/polypropylene-matrix composite is presented to verify the hypothesis of this work. An untwisted coil enables the efficient stowage and deployment of new forms of bistable composite tube which adhere to similar form factors as straight and toroidal ones. By embedding electrical conductors, helical bistable composites enable new lightweight, compact and multifunctional structures for communication and sensing applications.

Communications present a major bottleneck for small-satellite functionality given their extremely small volumes and low power. This work addresses this gap by presenting an ultra-compact, high-gain deployable helical antenna designed for space-based reception of Automatic Identification System signals at 162 MHz for maritime surveillance. The radio frequency characteristics of helically curved ribbons are investigated and optimized through a parametric study of the helical and ground plane geometry. Square, planar ground planes of various size and thickness, and a range of helical ribbon widths are studied. Both are modeled as perfect electrical conductors using ANSYS High Frequency Structure Simulator. Simulation results indicate that the addition of a ground plane centered and positioned at the base of the helical antenna element: 1) reduces back lobe radiation and 2) enables optimization of the radiative performance through adjusting the antenna geometry i.e. the peak gain may be increased by 3.5% (on average) for each additional helical turn — 1-8 helical turns are simulated. The half-power beam width may also be improved indefinitely by adding more helical turns. The most focused beam presented, 40 deg, is produced by an 8-turn helix, which is 58 cm in diameter and has an axial length of 3.68 m. Two ground plane sizes are considered, with the largest, which is four times larger in area, producing 5% higher peak gain. Conversely, the ground plane size had negligible effect on the half-power beam width in long helices (i.e. >3 helical turns). Increasing the helical ribbon width in steps of 10 mm was found to improve the peak gain by 8% on average in long helices.

In recent years extremely small satellites have been developed in response to trends in the space industry to achieve more for less cost. Extremely lightweight and efficiently packaged deployable structures are essential for achieving large-scale applications including communication antennas, solar arrays, and in recent years, deorbiting drag-sails. This thesis is motivated for developing novel deployable helical antennas for space-based maritime surveillance. The helical antenna technology provides packaging efficiency and radio frequency characteristics superior to the latest efforts of international research groups. To achieve this, the research presented focuses on developing the proven bistable composite slit tube (BCST) deployable technology. These are open-section tubular structures which can be deployed and rolled up into a compact coil, analogous to a tape measure, but do not require constraint to remain stowed. This behaviour is referred to as bistability and enables lightweight and relatively simple deployable structures for spacecraft applications. New forms of BCST are modelled through the introduction of additional curvatures, manufactured and described in this work with two new subcategorisations established: toroidal and helical. Toroidal BCSTs are doubly curved with both principal curvatures initially non-zero in the deployed stress-free state. Helical BCSTs are doubly curved and twisted out-of-plane. Investigations into the effects of geometrical parameters and laminated composite material properties on the bistable coils of both types are presented. The results provide an understanding of bistable behaviour in new forms of BCST previously neglected in the literature, which is almost exclusively focused on straight forms. As a consequence of this research, new deployable structure technologies are envisaged in the areas of compact terrestrial shelters and small satellite communications.

An investigation into the bistability of positively curved laminated composite slit tubes is presented, establishing a natural extension in this area that has previously been focused on straight tubes. Curved slit tubes are modeled as the surface segments of a torus. The design space is explored through a parametric study to investigate the effect on the second stable state, representing a small coil. This includes the effects of longitudinal curvature, cross-section subtending angles, nonuniform transverse curvature, and spatially varying laminate properties. The second equilibrium state is determined through strain energy minimization using the Rayleigh–Ritz method. To verify the model, samples are manufactured from glass-fiber braid and polypropylene resin. This investigation finds 1) the initial curvature along the length of the tube has little effect on coil radius, however, the coil has a distinct barrel shape; 2) highly enclosed and 3) highly curved cross-sections result in higher edge strains of the second equilibrium, enabling identification of practical bistable tubes; and 4) conversely, the greater the initial curvature along the length of the tube, the lower the second equilibrium strain.

A deployment solution for a parabolic sail structure for solar photon thrusters (SPTs) is presented. SPTs decouple the function of collection and reflection of light, achieving many advantages over flat solar sails. Although recent and increasingly realistic studies have concluded SPTs an unattractive option, the motivation behind this work is to progress the novel SPT concepts by resolving two problems identified: presenting a feasible solution for deployment and maintaining tight control over the collector shape; and addressing the space durability of carbon-fibre reinforced epoxy-resin composites for long duration solar sailing missions. Laterally curved bistable reeled composites were manufactured in such a way that their beneficial structural properties and bistable behaviour have been complimented with improved environmental resistance. This was achieved by implementing a cycloaliphatic based coating system reinforced with silicon nano-additive. The effect of curvature and additive on the natural frequency were investigated. In addition, response to vacuum outgassing, UV resistance, surface degradation due to atmospheric oxygen and thermal stability were investigated and improved.

The vibration characteristics of cantilevered straight and curved carbon/epoxy bistable reeled composites (BRCs) have been investigated. The tube length, cross-section radius, subtending angle, longitudinal curvature and number of plies-design parameters were investigated for their effects on the vibration modes. The boom length affects the frequency the most, which is found to be inversely proportional to the square of boom length, in addition to ABAQUS simulation results showing that frequency is proportional to curvature. Short, three-ply carbon/epoxy samples were manufactured and tested. A regime change from short (48.5cm) to slender (≈150cm) tubes was observed, signified by curved tubes exhibiting higher vibration modes in a particular plane than the straight ones in simulation-highlighting the scalability of curved BRC applications. Recommendations for the upcoming CleanSpace One, EPFL space mission which uses curved tubes for its capture mechanism, are discussed. Dynamic stability analysis was performed by simulating increasing rotary accelerations, causing the cantilevered BRCs attached to a spacecraft to rotate. A failure point derived from the Budiansky-Hutchinson criterion was developed to determine the maximum rotation acceleration-the critical value by which the tube loses stability.

The bistability of a toroidal slit tube is modeled using the Rayleigh-Ritz method. Approximate explicit expressions for the original stable deployed geometry, and the deformed stowed geometry are used to derive forms for the bending and stretching strain energy. The surface of a torus has varying Gaussian curvature, requiring a new approach to the modeling and analysis of the stable configurations. A comparative study with established straight-BRC models was conducted from which the doubly curved-BRC model presented here predicts second stable state coil radii with 96.25% agreement.