Hyperloop Academic Research: A Systematic Review and a Taxonomy of Issues

: Hyperloop is a proposed very high-speed ground transportation system for both passenger and freight that has the potential to be revolutionary, and which has attracted much attention in the last few years. The concept was introduced in its modern form relatively recently, yet substantial progress has been made in the past years, with research and development taking place globally, from several Hyperloop companies and academics. This study examined the status of Hyperloop development and identiﬁed issues and challenges by means of a systematic review that analyzed 161 documents from the Scopus database on Hyperloop since 2014. Following that, a taxonomy of topics from scientiﬁc research was built under different physical and operational clusters. The ﬁndings could be of help to transportation academics and professionals who are interested in the developments in the ﬁeld, and form the basis for policy decisions for the future implementation of Hyperloop.


Introduction
Mobility and transportation are among the most essential and important services to society. They encompass interconnected systems that are intended to cover the demand for mobility of people and goods. Transportation systems are intrinsically complex, including elements, both physical and organizational, that interact with and influence each other directly and indirectly, frequently in a nonlinear manner, and with the occurrence of feedback loops. [1]. According to this perspective, the transportation system is essentially a highly dynamic complex, large-scale, interconnected, open, socio-technical (CLIOS) system [2]. Nevertheless, present-day transportation modes (i.e., rail, road, air and waterborne transportation) are based on consolidated concepts, and improvements over the years have been essentially evolutionary, focusing on delivering a safe, efficient, reliable and accessible transportation system.
In the last decade, several transportation concepts and technologies have been identified as very promising. The impact of disruptive transportation technologies, i.e., those technologies with the potential to create disruptive innovation at industry and society level [3], has been an important area of research and development. In the transportation sector, information and communication technologies (ICT) and the Internet of Things (IoT) are bringing a revolution to the sector, with the advent of connected and automated road mobility being a notable example [4].
Hyperloop is one of those very promising and possibly disruptive future transportation technologies. Its development has received extensive media coverage over the last years following the Hyperloop Alpha white paper by Elon Musk published in 2013 [5]. Hyperloop consists of a system of tubes where vehicles (pods) travel at high speed (the original concept claims a top speed of 1220 km/h) in a low-pressure environment. Other than speed, Hyperloop's main advantage is that the partial vacuum lowers the air resistance (drag), thus, consuming less energy during acceleration and cruise [6]. An initial feasibility study published already in 2016 identifies research topics related to Hyperloop technologies [7].
After the white paper and the initial hype, several companies in the US brought together engineers and venture capital money to perform research and development and make Hyperloop a reality [8]. Later on, the same companies expanded to Europe, and other Europe-based companies engaged in similar activities [9], including the planning and development of Hyperloop test sites.
Furthermore, recent developments regarding the need for standardizationin Europe and the US highlight the interest in the regulation of Hyperloop. In Europe, the "Sustainable and Smart Mobility Strategy" was presented in December 2020 by the European Commission and the accompanying action plan of initiatives will guide its work for the next four years. Among the objectives of this plan is to "assess the need for regulatory actions to ensure safety and security of new technologies and concepts such as Hyperloop" [10]. Before that, a new Joint Technical Committee (TC), CEN/CLC/JTC 20, was launched by the European Committee for Standardization (CEN) and the European Committee for Electrotechnical Standardization (CENELEC) to address the need for the standardization of Hyperloop systems [11]. A year before, in 2019, the U.S. Department of Transportation (DOT) created the Non-Traditional and Emerging Transportation Technology (NETT) Council, an internal body with the objective of identifying and resolving gaps, either legal or regulatory, that may obstruct the deployment of Hyperloop, among other new technologies [12]. In January 2021, the NETT Council presented the "Hyperloop Standards Desk Review" with the scope of assessing the status of Hyperloop standardization activities, developing a foundation for future Hyperloop standardization efforts, and consequently, paving the way towards the development of a preliminary framework of Hyperloop system components and associated regulations and voluntary technical standards [13].
The dynamics of the technology and the progress made toward future Hyperloop deployment in Europe is highlighted by a recent mapping of activities in the industry and European institutions [14]. Nevertheless, to test the safety, efficiency and reliability of Hyperloop in the field, beyond research and development (R&D), a long enough, full-scale prototype track is necessary.
Beyond the US and Europe, in China and Korea, as patent activity shows, there is substantial R&D from CRRC Yangtze Co., the Korea Railroad Research Institute (KRRI) and the Korea Institute of Construction Technology (KICT) [14,15].
Considering the above, this study examines the status of Hyperloop scientific developments, identifying issues and challenges. It is based on initial considerations developed in [14]. Compared to that previous study, a systematic review was performed, and the fields of research were explicitly identified. Consequently, a taxonomy of scientific research issues was developed by analyzing all Hyperloop research in the literature, using the methodology developed by the European Commission's Transport Research and Innovation Monitoring and Information System (TRIMIS) [16]. Accordingly, the literature was organized in relevant clusters and for each cluster combination, the issues were identified as lower-level items in the taxonomy.
The findings could be of help to transportation academics and professionals who are interested in developments in the field, and form the basis for policy decisions for the future implementation of Hyperloop.
The paper consists of the following parts: after the introduction, the next section discusses the materials and methods used in this study, drawing from the Scopus database and a physical system decomposed into several clusters. Section 3 provides the results from the analyses grouped under the different clusters. Section 4 provides an initial taxonomy based on the performed analysis and a brief discussion. Section 5 provides the conclusions.

Materials and Methods
The methodology presented in this section focuses on capturing research findings, aiming at the identification of trends, and consequently, building a taxonomy of issues. The Scopus database, which has scrupulous indexing rules, was used as a source.
For the analysis, the following steps were taken: • A search using specific keywords ("Hyperloop" or "tube transport" or "vactrain") was carried out, in the abstract, title, or keywords. Results were limited to those published after 2013 (when the modern concept of Hyperloop was introduced), and documents from health sciences were excluded due to the lexical ambiguity of "Hyperloop transport" term. The exact query used was: TITLE-ABS-KEY ("Hyperloop" OR "tube transport*" or "vactrain") AND PUBYEAR > 2013 and not SUBJAREA (MEDI OR NURS OR VETE OR DENT OR HEAL). This search performed in June 2021 resulted in 229 documents.

•
An additional manual filtering of the documents one-by-one, on the basis of their title or abstract limited, resulted in 161 documents. The aim of this filtering was to eliminate those documents that were not relevant to the field due to lexical ambiguity and those that simply outlined Hyperloop-related aspects. This left 96 articles, 57 conference papers, three reviews, three notes, one letter and one book chapter. Figure 1 shows the distribution of the documents over the considered time period.

Materials and Methods
The methodology presented in this section focuses on capturing research findings, aiming at the identification of trends, and consequently, building a taxonomy of issues. The Scopus database, which has scrupulous indexing rules, was used as a source.
For the analysis, the following steps were taken: • A search using specific keywords ("Hyperloop" or "tube transport" or "vactrain") was carried out, in the abstract, title, or keywords. Results were limited to those published after 2013 (when the modern concept of Hyperloop was introduced), and documents from health sciences were excluded due to the lexical ambiguity of "Hyperloop transport" term. The exact query used was: TITLE-ABS-KEY ("Hyperloop" OR "tube transport*" or "vactrain") AND PUBYEAR > 2013 and not SUBJAREA (MEDI OR NURS OR VETE OR DENT OR HEAL). This search performed in June 2021 resulted in 229 documents.

•
An additional manual filtering of the documents one-by-one, on the basis of their title or abstract limited, resulted in 161 documents. The aim of this filtering was to eliminate those documents that were not relevant to the field due to lexical ambiguity and those that simply outlined Hyperloop-related aspects. This left 96 articles, 57 conference papers, three reviews, three notes, one letter and one book chapter. Figure 1 shows the distribution of the documents over the considered time period.  After this step, an analysis of all abstracts (and in case of doubt, of the full paper) took place, and the research was quantitatively assessed, focusing on several clusters. Inspired by the decomposition approach from [14], this was done by means of a system approach, breaking the Hyperloop system into five physical parts ( Figure 2). These parts cover the entire hyperloop system, and outline interacting subsystems.  Figure 1 shows an overview of the results, which are destined to increase in 2021. After this step, an analysis of all abstracts (and in case of doubt, of the full paper) took place, and the research was quantitatively assessed, focusing on several clusters. Inspired by the decomposition approach from [14], this was done by means of a system approach, breaking the Hyperloop system into five physical parts ( Figure 2). These parts cover the entire hyperloop system, and outline interacting subsystems. The five physical clusters are: • Hyperloop as a system: this includes research that encompasses the entire system and that cannot be considered under other disaggregated levels. Examples may include efficiency and energy studies of the system in operation. • Substructure (including foundations and bridge work): focuses mostly on structural engineering design for the supporting structure. • Tube: considers aspects related to the tube structure.

•
Tube pod interface: focuses on research on the interface between the tube and the pod. Examples may include aerodynamic phenomena as a consequence of the pressure variation. • Pod: focuses on aspects related to the pod (e.g., levitation, suspension, powertrain, electronics) In addition, five horizontal (operational) clusters (energy, operations, communications, aerodynamics, safety) were considered.
It should be noted that this decomposition (into five physical and five horizontal clusters) while meaningful, is not the only one possible. In fact, in a design process it is impossible to decompose a system uniquely [17]. Nevertheless, this provides a rather generic and complete higher-level decomposition, which can be further broken down into lower hierarchies. For example, the "pod" cluster can be further decomposed into subclusters, covering the powertrain, the levitation and suspension blocks, etc. Likewise, the horizontal clusters can be further elaborated to cover additional operations. In this sense, the decomposition is scalable and provides the starting point for adding more elaborated layers of detail.
These clusters, although developed independently for this study, also encompass and are aligned with the priority work areas identified by the CEN/CENELEC TC on Hyperloop standardization, which include pressures of operation, door sealing, vehicle-tube interface, communication protocols and emergency evacuation [13].
Sections 3.1 to 3.5 present the results for the five physical clusters. In the analyses, each paper is also linked to one of the five horizonal clusters. Finally, Sections 3.6 and 3.7 present an overview of research involving general discussions and Hyperloop network developments. These last two, are not linked to the physical clusters since they focus on discussion rather than on the development of specific technologies. The five physical clusters are: • Hyperloop as a system: this includes research that encompasses the entire system and that cannot be considered under other disaggregated levels. Examples may include efficiency and energy studies of the system in operation. • Substructure (including foundations and bridge work): focuses mostly on structural engineering design for the supporting structure. • Tube: considers aspects related to the tube structure. • Tube pod interface: focuses on research on the interface between the tube and the pod. Examples may include aerodynamic phenomena as a consequence of the pressure variation. • Pod: focuses on aspects related to the pod (e.g., levitation, suspension, powertrain, electronics) In addition, five horizontal (operational) clusters (energy, operations, communications, aerodynamics, safety) were considered.
It should be noted that this decomposition (into five physical and five horizontal clusters) while meaningful, is not the only one possible. In fact, in a design process it is impossible to decompose a system uniquely [17]. Nevertheless, this provides a rather generic and complete higher-level decomposition, which can be further broken down into lower hierarchies. For example, the "pod" cluster can be further decomposed into subclusters, covering the powertrain, the levitation and suspension blocks, etc. Likewise, the horizontal clusters can be further elaborated to cover additional operations. In this sense, the decomposition is scalable and provides the starting point for adding more elaborated layers of detail.
These clusters, although developed independently for this study, also encompass and are aligned with the priority work areas identified by the CEN/CENELEC TC on Hyperloop standardization, which include pressures of operation, door sealing, vehicletube interface, communication protocols and emergency evacuation [13].
Sections 3.1-3.5 present the results for the five physical clusters. In the analyses, each paper is also linked to one of the five horizonal clusters. Finally, Sections 3.6 and 3.7 present an overview of research involving general discussions and Hyperloop network developments. These last two, are not linked to the physical clusters since they focus on discussion rather than on the development of specific technologies.

Research on the Hyperloop System
This section focuses on scientific research documents dealing with the Hyperloop system in general. Thirty-two papers were identified from the analysis.
An overview of the issues identified in the scientific literature under the five utility clusters is provided in Table 1. Table 1. Issues identified in research on the Hyperloop system.

Authors
Year Tavsanoglu et al. [18] 2021 Pod to ground wireless communication X Fernández Gago and Collado Perez-Seoane [19] 2021 Geometric design and linear infrastructure planning X Huang et al. [20] 2021 Optical wireless communication system X Tbaileh et al. [21] 2021 Power requirements and impact on the electricity grid X Han et al. [22] 2020 Wireless network architecture X Brown et al. [23] 2020 Short-range communication X Eichelberger et al. [24] 2020 Scheduling X Zhang et al. [25] 2020 Pod to ground wireless communication X Qiu et al. [26] 2020 Pod to ground wireless communication X Janić [27] 2020 Energy consumption and CO 2 emissions X Lafoz et al. [28] 2020 Energy Storage Systems X Zhang et al. [29] 2020 Pod to ground wireless communication X Khan [30] 2020 Overall system development X Narayan S. [31] 2020 Solar panel power X Bempah et al. [32] 2019 Photovoltaic panel configurations for tube X Huang et al. [33] 2019 Lateral drift under different low pressures X Jin et al. [34] 2019 Dynamic characteristics under low-pressure X Thakur et al. [35] 2019 Braking and deceleration X Kim and Rho [36] 2019 Support facility and pods X Dudnikov [37] 2019 Network operations X Allen et al. [38] 2019 Pod to ground wireless communication X Sutton [39] 2019 Process safety and generic safety cases X Kauzinyte et al. [40] 2019 Simulation with aerodynamic constraints X Deng et al. [41] 2018 System simulation X Nikolaev et al. [42] 2018 Electric and software system X Deng et al. [43] 2017 System simulation X Janzen [44] 2017 Dynamic characteristics under low-pressure X Kwon et al. [45] 2017 Photovoltaic panel configurations for tube X Ali et al. [46] 2017 Handover algorithm X Decker et al. [47] 2017 Conceptual feasibility study X Zhou et al. [48] 2016 Energy consumption X Brusyanin and Vikharev [49] 2014 Conceptual functional safety assessment X

Research on Hyperloop Substructure
This section focuses on scientific research documents dealing with the Hyperloop substructure. Eight papers were identified from the analysis.
An overview of the issues identified regarding Hyperloop substructure, under the five utility clusters, is provided in Table 2.

Research on Hyperloop Tube Structure
This section focuses on scientific research documents dealing with the Hyperloop tube structure. Seven papers were identified from the analysis.
An overview of the issues identified in regard to Hyperloop tube structure, under the five utility clusters, is provided in Table 3. As can be seen, the principal topic of research is the airtightness of concrete tubes. Table 3. Issues identified in research on Hyperloop tube structure.

Research on Hyperloop Tube-Pod Interface
This section focuses on scientific research documents dealing with the Hyperloop tube-interface. Forty-eight papers were identified from the analysis.
An overview of the issues identified regarding the Hyperloop tube-pod interface, under the five utility clusters, is provided in Table 4. Table 4. Issues identified in research on Hyperloop tube-pod interface.

Authors
Year

Bose and Viswanathan [65] 2021
Piston effect mitigation using airfoils X Lluesma-R. et al. [66] 2021 Use of compressor to mitigate aerodynamic drag X Zhou et al. [67] 2021 Radial gap and flow field X Hu et al. [68] 2021 Cross passage and flow field X Lluesma-R. et al. [69] 2021 Drag coefficient effect on the aerodynamic performance X Vakulenko et al. [70] 2021 Effect of external air exchange system X Uddin et al. [71] 2021 Drag-based aerodynamic braking X Huang et al. [72] 2020 Transient pressure on the tube X Galluzzi et al. [73] 2020 Stabilization of electrodynamic levitation systems X Nick and Sato [74] 2020 Pod structure aerodynamic optimization X Le et al. [75] 2020 Aerodynamic drag and pressure waves X Wang et al. [76] 2020 Blockage ratio and aerodynamic drag X Ma et al. [77] 2020 Air pressure and aerodynamic drag X Chen et al. [78] 2020 Structural mechanics properties of tube-wall X Jia et al. [79] 2020 Heat recycle duct and energy accumulation X Table 4. Cont.

Research on Hyperloop Pod
This section focuses on scientific research documents dealing with the Hyperloop pod. Twenty-seven papers were identified from the analysis.
An overview of the issues identified regarding the Hyperloop pod, under the five utility clusters, is provided in Table 5. Table 5. Issues identified in research on Hyperloop pod.

Discussion Papers on Hyperloop
This section focuses on scientific research documents that focus on general discussions. Thirty papers were identified from the analysis. Table 6 provides an overview of the topics discussed. Table 6. General discussion papers.

Research on Hyperloop Networks
This section focuses on scientific research documents that focus on the development of Hyperloop networks. Ten papers were identified from the analysis. Table 7 provides an overview of the topics discussed. Table 7. Network papers.

Authors Year Issue
Merchant and Chankov [168] 2020 Scenario analysis in Europe Neef et al. [169] 2020 Scenario analysis on infrastructure networks Bertolotti and Occa [170] 2020 Agent-based model of supply chain system Rajendran and Harper [171] 2020 Define, Measure, Analyze, Design, and Verify (DMADV) approach Cho [172] 2019 Implications at local level Pfoser et al. [173] 2018 Hyperloop and synchromodality Voltes-Dorta and Becker [174] 2018 Implications at local level Markvica et al. [175] 2018 Hyperloop impact in Europe Schodl et al. [176] 2018 Large scale regional impact Werner et al. [177] 2016 Implications at local level (cargo) The relationship between vertical and decomposition clusters in the documents is shown in the chord diagram of As can be seen, and with regard to the physical decomposition, the majority of research focuses on the pod-tube interface and aerodynamics (29 documents) and the pod and operations (21 documents). Communication technologies were researched in nine documents at a system level. The 21 documents focusing explicitly on safety issues, cover all horizontal areas. As can be seen, and with regard to the physical decomposition, the majority of research focuses on the pod-tube interface and aerodynamics (29 documents) and the pod and operations (21 documents). Communication technologies were researched in nine documents at a system level. The 21 documents focusing explicitly on safety issues, cover all horizontal areas.

Initial Taxonomy of Issues
The next step was to build a preliminary taxonomy of research topics. As explained in Section 3, all papers were read and grouped under the different clusters. Each paper was also flagged for the respective research issues. Table 8 aggregates the findings from the 161 documents. For the utility clusters, an overview of the emerging issues is reported, while for the physical and generic clusters, the research issues are reported in detail, aggregating the identified issues from Section 3. It should be noted that the obtained taxonomy is not unique, and further readings could identify additional elements. A variety of researched topics emerges from Table 8. The Hyperloop as a system cluster (A) includes a lot of research on different operational aspects, in particular communications. In fact, this aspect appears to be challenging at very high speeds in tunnel structures. Some other aspects related to the geometric design and the linear infrastructure development are also covered in this cluster in an analytical manner.
The Hyperloop substructure cluster (B) includes a great deal of research from the fields of structural and bridge engineering. The major difference is the dynamic loads imposed by the Hyperloop pods, which influence the design of substructure and need to be accounted for.
Some research deficiencies were identified. This is the case for research focusing on the Hyperloop tube cluster (C), and consequently, on infrastructure. Considering that infrastructure costs are high (especially for a new system) the lack of research in this area (e.g., materials, tube thickness) is visible.
At the same time, Hyperloop tube-pod interface cluster (D) research focuses on a variety of issues linked in particular to aerodynamic performance under low pressure.
Research focusing on the Hyperloop pod cluster (E) covers many aspects that are linked to the powertrain, suspension, magnetic levitation and guidance. A number of similarities with high-speed rail and (especially) magnetic levitation (Maglev) trains are apparent, something that may lead to research spillovers from the two transport modes.
Finally, the rather high number of discussion papers and those related to Hyperloop networks highlight the overall interest in Hyperloop as a transport mode.

Conclusions
Hyperloop is a proposed very high-speed ground transportation system that has great potential for the decarbonization of transportation, and it has received a great deal of attention from transportation academics. This study aimed to provide a baseline with regard to the topics and challenges identified in the scientific research, for the effective testing and deployment of Hyperloop. The presentation of the issues follows a structured methodology, and provides insights for future research. In particular, the adopted clustering is scalable, and consequently, more detailed sub-clusters could be easily identified. The performed extensive literature review, to the authors' knowledge, is the most complete of its kind.
As discussed in the previous section, based on the detailed findings and the taxonomy of issues identified under the overarching clusters, there is vast interest from the research community on this topic.
These findings could play an important role in providing input to ongoing Hyperloop standardization processes by looking into the different approaches for solving specific issues. The findings also complement proprietary technologies developed by Hyperloop promoters, since in many cases, academic research on the same topics is independent. Therefore, it can provide a fresh perspective since academic research follows different paths of knowledge compared to industry. This is more evident in specific clusters (e.g., substructure and tube) where structural engineering approaches are implemented, relying on the long-standing expertise of researchers in the specific field.
Another possible use that emerges is the opportunity to compare the taxonomy with research issues in legacy systems, e.g., high speed rail. In this way, it is possible to quickly check (a) similarities in the research in the two systems, and consequently, possible research spillovers, and (b) research issues not yet explored. The results from such an exercise could provide valuable input to standardization and certification bodies.
The findings could ignite policy initiatives focusing on future decisions regarding the Hyperloop. For this process to succeed, the continuous identification and assessment of issues will be necessary, including challenges beyond technology (e.g., social aspects, project financing), which will help to make the demonstration and deployment of Hyperloop possible. Outside policymaking, this paper helps academics and professionals who are interested in the development of Hyperloop technologies by providing digested information on scientific developments in this area.
Future research could focus on expanding this taxonomy to cover other domains of knowledge, in particular, intellectual property applications from Hyperloop promoters and nationally funded research.