Cylindrical lattice structures for satellite applications

ATG Innovation Ltd. (Irish subsidiary of ATG Europe) is leading the industrialisation of this technology.

Satellite central tube (SCT) structures, which form the structural backbone of satellites, currently use a CFRP-based sandwich structure with a perforated aluminium honeycomb core. These structures require significant reinforcement for attachments, as well as having long and complex manufacturing processes. This has led to the search for alternative designs and concepts for SCTs in order to reduce lead times and improve the structure’s functionality. To this end, CFRP lattice grid structures consisting of a regular and symmetric system of hoop and helical ribs were highlighted as a prospective replacement for current SCT structures for a number of reasons, includ­ing an optimised design architecture, a one-shot manufacturing process and mass efficiency.

How it’s made

Composite lattice structures are a family of structural architectures that utilise a continuous-fibre composite material in a regularly-intersecting grid pattern of ribs.

They offer weight saving capabilities with increased structural functionality as the configuration can be tailored to accommodate different load distributions, while also allowing for the inclusion of patches directly in the manu­facturing process to form local attachment points.

This design architecture exploits several intrinsic values in fibre-reinforced polymer (FRP) materials, with multiple benefits compared to standard/isotropic struc­tures, including:

• The ribs feature unidirectional fibre materialisation, giving an optimal strength and stiffness in their lengthwise direction for a low weight.
• The combination of several of these unidirectional fibre ribs, placed at intelli­gently chosen angles with respect to each other, gives a mix of optimal global and local load-carrying capacity, plus the ribs stabilise each other.

• Local attachment points can be optimized in terms of mass and positioning due to the flexibility afforded by integrating FRP patches directly into the lattice structure.

• The ribs can be transitioned to a mono­lithic laminate at one or both ends of the cylinder, which facilitates the joining of the cylinder to adjacent structures.

Innovative manufacture

Typically, the bulk of an FRP structure is largely additively manufactured (as opposed to milling away material out of a metallic block). Specifically, this technol­ogy features a one-shot process where the whole cylindrical composite part, which has significant 3D complexity as opposed to a flat part, is additively manufactured in a single process step. The cur­rent process includes manually laying up prepreg tows onto a mandrel, directly in­tegrating the local attachment patches and curing in an autoclave in one-shot, which leads to a high material usage efficiency and a low manufacturing time and cost.

This manufacturing method offers several benefits, such as the higher obtainable fi­bre volume fraction of ~60%, resulting in a higher specific performance, and reducing the number of costly and time-consuming post-processing operations typically associated with sandwich panel structures. These benefits are achieved while also maintaining or surpassing the level of performance (weight, stiffness, strength) of existing structural solutions.

The use of unidirectional fibre ribs to form a lattice structure also allows for easy ac­cess and viewing inside the manufactured cylinder, which could facilitate systems in­tegration in a production programme with electrical cabling, fuel pipes and so on. A lattice structure may mean that no person needs to go inside the cylinder during systems integration and that, if someone must go inside, then there is plenty of light and ventilation compared to a typical sandwich panel structure. This is beneficial from a health and safety point of view, while potentially allowing for a decrease in time taken to perform assembly and integration activities due to better lighting and access to the structure.

All manufacturing activities are performed by ÉireComposites Teo at their facilities in Galway, Ireland.

Test campaign

An extensive test campaign was undertak¬en to validate the manufacturing process developed for the production of SCT lattice structures. Initially, coupon-level tests were performed for material property characterisation and to define A-basis material property values, which were used in all subsequent finite element (FE) analyses. Then, a flat panel of the lattice structure was manufactured for element-level tests whose results were fed into the design and analysis of the more complex cylindrical structures. These element-level tests included ribs tested for compressive strength and modulus, as well as flexural strength and modulus, and nodes tested for compressive strength and flexural strength, with results show¬ing good correlation with FE predictions. The outputs of these activities were fed into the analysis of a full-scale SCT lattice cylinder.
An intermediate-scale cylindrical sample of 0.5m diameter was manufactured using the developed process and served as a demonstrator to uncover and solve the manufacturing difficulties and unforeseen effects of a cylindrical part compared to a flat panel. The complexity of this part was high, with multiple attachment regions and types, which also allowed for drilling trials in a curved structure.

Finally, taking into account all of the lessons learnt during the previous steps, a full-scale 1.5m-diameter lattice cylinder was designed, manufactured and tested under representative satellite design loads to assess its load-bearing capabilities. The design and manufacture included assembly with end zone attachments suitable for interfacing with a large-scale test rig . The tests included local attachment loading from a shear web, global compression and combined compression plus bending in order to reach the failure load of the structure. This cylinder weighed less than 10 kg and was easily capable of withstanding the 175 kN design compression load, ultimately fail­ing at a line load of 184 kN/m, equivalent to an overall compression load of 870 kN.

Coupon and element-level testing was performed by Composites Testing Laboratory (CTL) at their facilities in Galway, Ireland. The full-scale testing activities were conducted in conjunction with NUIG and their heavy structures lab, also in Galway, Ireland.

Mass efficiency

This technology employs an efficient ar­chitecture as unidirectional composite ribs are placed only where they are needed, thus optimizing the mass of the structure. As the cost of launching into space is €10,000 per kg today, mass efficiency is extremely important in spacecraft structures. A feasi­bility study performed by ATG Innovation showed that lattice structures for a typical satellite central tube design can offer up to 25% mass saving when compared with a standard CFRP and aluminium honeycomb sandwich panel design. In addition, there is less material waste due to the tailored design and additive manu­facturing nature of this technology, with access ports and cut-outs being seamlessly integrated into the structure during the manufacturing process.

Other applications

A number of potential applications have been identified in a variety of different markets including manned aviation (fu­selages, equipment frames), unmanned aviation (large drones) and renewable energy (wind turbine structures). The lattice technology can also be combined with a skin to create a grid-stiffened architecture to offer a highly-efficient structural solution for other spacecraft applications such as launcher structures. This application is already being inves­tigated by ATG Europe with promising initial results.