The objective of the European Green Deal is to reduce net greenhouse gas emissions by at least 55% by 2030 and to achieve climate neutrality by 2050 [1]. Due to a growing global population and increased needs for travelling on the one hand, and progressive bans of short-haul flights by the governments on the other, the need for a more sustainable and fast means of transportation comes in high demand.

The Hyperloop transportation system has emerged as the fifth mode of transportation, offering an energy-efficient, fast alternative for freight and passenger transportation. However, to successfully establish the Hyperloop network, an extensive tube infrastructure would need to be constructed with requirements of being safe, sustainable, and cost-effective. At the time of this project, various tube designs and materials have already been examined and evaluated; given its preliminary stage of development, new design ideas are rapidly emerging. Engineers are faced with two fundamental challenges: firstly, defining safety limits, and secondly, establishing the balance between the safety, environmental footprint, and operational efficiency of hyperloop infrastructure.

The Hyperloop Skeleton tube design is the latest addition to the integral designs that holds great potential in terms of weight efficiency. The aim of this research is to determine the applicability and efficiency of the newly proposed tube design and to evaluate structural performance to imposed loads.
For the design evaluation, the study uses a numerical approach. Skeleton tube design is initially disassembled into individual components, which are analysed separately to identify potential weaknesses of the design as well as to predict their behaviour within the assembly. After that, the study conducts the analysis of the assembly. The initial design lacked rail support design; thus, a design is proposed and implemented in the model for the global analysis. Within the assembly, the research identifies critical sections and design weaknesses. In accordance with this, it proposes and analyses a new ring-to-stringer connection design. Additionally, a comparison study has been conducted with the conventional (plain) tube design, currently used at the European Hyperloop Centre (EHC) [2].

Based on numerical results, the skeleton tube design is conditionally satisfactory in terms of ultimate and serviceability limit states. The design can resist the main load case – vacuum pressure. Nevertheless, the slender components and thin plates make the tube susceptible to plate rupture or penetration if exposed to environmental actions; thus, making the hyperloop system vulnerable to accidental and impact loads. Moreover, the elastic strength capacity of rings, which are primarily in compression and are therefore critical components, is nearly reached. An initiation of local plastic response is observed, yet due to integral design, the stresses distribute among components; thus, it does not progress into a fully plastic response. Based on these findings and considering that dynamic loads are yet to be assessed, it can be projected that a strength capacity will be exceeded in further research.

The proposed steel bracket design for ring-to-stringer connection provides an alternative to welded connections. It improves stress concentrations within the ring, and considering that it is a bolted connection, further contributes to the ease of assembly, maintenance and the demountability aspect. However, the requirement for 288 such connections per 16-meter-long tube section might significantly increase the total cost of skeleton tube design.

Based on the comparison study, it is proven that the conventional tube design performs better in terms of structural performance under the considered loading conditions. However, in a controlled environment with the absence of external actions, the skeleton tube design could efficiently operate. In this case, a material efficiency of 28% can be achieved if the structure supports the rails and the pod, and up to 37.8% if the tube is solely used for vacuum pressure retention.

Emerging technologies, currently in the process of development and yet to demonstrate their contribution to a more sustainable future significantly depend on the performance and success of their initial prototypes and real-world applications. Skeleton design offers a cutting-edge design, which is on the safety – sustainability spectrum drastically leaning to the latter. A secondary protective structure is required for consideration of the skeleton design in the hyperloop application. The design can nonetheless be viable for other applications, which operate in a safe and controlled environment, with the absence of external loads. This research lays a foundation for any further research on the skeleton tube design.