1 Introduction

The increasing trend in the development of new advanced technologies, in different engineering fields, is pushing even more the humankind to reach the edge of space. In this context, not only innovative earth-to-orbit transportation systems have been recently developed, but there is also an increasing demand for sub-orbital transportation systems [1]. In fact, the capability of carrying passengers up to a meaningful target altitude is seen as a promising near-future routine service able to guarantee a high level of profits to allow microgravity experience and an amazing view of our planet. From a technical point of view, the development of a sub-orbital transportation system is regarded as an intermediate and necessary step of a longer time vision roadmap aimed at developing reusable earth-to-orbit transportation systems (from a “space” point of view) or at designing a hypersonic spaceplane (from an “aeronautical” point of view).

In this context, it is extremely important to develop an accurate design methodology able to understand and discover the high-level qualitative stakeholders’ needs and desirables and to think about all the possible mission concepts able to accomplish these goals. The very fast technological development and the extremely wide number of competitors require the capability of proposing different mission concept options and a proper trade-off strategy to the customers. This paper tries to solve this problem suggesting an innovative conceptual design methodology able to guide the developers from the first stakeholders’ analyses to the actual production phase. In particular, after a theoretical description of the proposed methodology and of the selected exploitable tools (Sect. 2), this approach is applied to define, prune, and select a set of mission concepts aimed at providing a sub-orbital flight service (Sect. 3). In particular, in accordance with the stakeholders desires, the mission should guarantee at least a certain number of flight participants at a time to reach a target altitude (100 km), with a spacecraft able to perform vertical take-off and landing. As the reader can easily figure out, this very demanding requirement will strongly affect and lead the development of the vehicle and will impact on the design of the overall mission. At the end of the section, an overview of the reference case study is presented. In this context, some additional details are provided with the help of CAD drawings.

2 Conceptual design methodology

This section deals with the description of a conceptual design methodology allowing engineers to take high-level decisions trying to satisfy the stakeholders’ needs in the best way.

In preliminary design phases, where decisions have a deep impact on the mission from several perspectives, it is very important to persuade the customers, clearly showing the technological limits or conflicts among their requirements.

The analysis of the stakeholders’ needs is the basic brick of the methodology, which could be roughly divided in two different parts:

  • The mission analysis and design,

  • The systems design and sizing.

The following sub-sections describe step-by-step the proposed advancements in the conceptual design methodology with respect to previous works of the authors [2, 3]. Moreover, the paper aims at suggesting tools that could be usefully employed to enhance the automation level of the proposed approach.

Figure 1 summarises the fundamental steps of the methodology proposed in this paper.

Fig. 1
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Conceptual design methodology overview

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2.1 Stakeholders and other preliminary analysis

The first step of the propose methodology consists in defining and identifying the stakeholders (i.e., all those people that could be interested in the project). It is important to understand their role (funders, users, customers, etc.) their needs and their desirables. It is also important to sense and evaluate the different levels of importance of their requests. In fact, it is a very good practice to distinguish since the beginning, the constraints of the project and the other needs that could be partially satisfied or negotiated in a close cooperation between stakeholders and designers.

Following the NASA guidelines for classification [4], stakeholders can be grouped in:

  • Sponsors: those associations or private who establish mission statement and fix bounds on schedule and funds availability;

  • Operators: in charge of controlling and maintaining space and ground assets. Typically, they consist of engineering organizations;

  • End-users: those people that receive and use space mission’s products and capabilities. They are usually scientists or engineers;

  • Customers: differ from the previous category, because they are users who pay fees to utilize a specific space mission’s product or service.

Moreover, in this preliminary phase, it is also very important to understand the operative environment, in which the mission should be carried out, or the different geographical origins of the stakeholders and of the partners involved in the project. This analysis is focused on identifying the regulatory entities, laws, and suggestions that should be considered as a fundamental source of requirements and constraints. Nowadays, some latest issues of aeronautical regulations are so precise and detailed that can also be considered as good guide and leading methodology for a high-level sizing attempt.

In this top-level design context, it is convenient to take a look at current market trends and future forecasting. In addition to the very useful overview of the main competitors or possible additional or alternative stakeholders, the market analysis gives an overview of past, present, and future projects from which take inspirations.

Once these top-level analyses have been performed, the main objectives of the project could be defined, identifying the main goals of the mission and clearly stating them in a structured and precise way: the mission statement. According to the NASA guidelines, the mission statement is a concise and precise statement able to describe the aim of the mission. From a deep analysis of this statement and of information obtained from the previous analyses, it is possible to write down a preliminary list of top-level mission requirements. Please notice that these steps are the starting point for developing different mission concept options, selecting the best ones and performing a first system and sub-systems sizing.

2.2 Mission concept options generation and prioritization

Once the main objectives of the mission have been clarified, the developers should elaborate different ideas to accomplish the mission. Nowadays, the fast technological evolution and the even higher computational capabilities can allow considering and manage a very high number of options. One of the main benefits of these innovations is the possibility of postponing the trade-off later on in the projects, when more accurate data could be available.

This sub-section gives suggestions on how to manage the very first brainstorming activities, supporting the generation of mission concept alternatives. The proposed methodology starts from a functional view of the mission that allows identifying the different capabilities that the elements of the system of systems should guarantee. Then, looking at the existing reference missions, but also considering possible future near time evolutions, the developers should identify all the possible elements able to accomplish the defined functionalities. To carry out these two steps, typical tools developed and used for the functional analysis can be exploited. In particular, the functional tree can allow defining the main functions the mission shall perform and a function/product matrix could help to structurally define the variety of elements able to accomplish the previously deduced functions (Fig. 2).

Fig. 2
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Sketch of the tools of the functional analysis that can be used to derive mission elements

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2.2.1 Functional tree

A functional tree expresses the functions to be performed for the execution of the mission. The functional tree allows splitting the higher level and complex functions, which stem from the mission objectives, into lower level functions, through a typical breakdown process, eventually allowing the identification of the basic functions that have to be performed by the future product. Therefore, starting from the so-called top-level functions, the functional tree generates various branches, moving from the most complex functions to the basic functions, i.e., those functions at the bottom of the tree that cannot be split any further. The basic functions help defining the functional requirements of the future product, as each basic function can be rewritten as a functional requirement.

2.2.2 Function/product matrix

It allows identifying the elements or building blocks needed to accomplish the functions. Specifically, the matrix’s rows contain the basic functions coming from the functional tree, while the columns report the products, i.e., the space mission elements capable of performing those functions. Starting from the analysis of the first basic functions, new elements progressively fill in the columns. Eventually, all basic products are determined. As a result, the elements to be involved in the missions are identified, by mapping all basic functions to products.

Then, it is important to group and combine the elements to derive the different mission concept options. During this process, it is also important to evaluate how well each of the different options of each single function is able to accomplish the function itself and which is its relation to all the other functions of the mission. To increase the level of autonomy of the process, the authors suggest to use the quality function deployment (QFD) tool, also known as house-of-quality.

2.2.3 Quality function deployment tool

The quality function deployment tool is a very useful design method to transform qualitative user demands into quantitative parameters, to deploy the functions forming quality and to deploy methods to achieve the design quality into sub-systems and component parts, and, ultimately, to specific elements of the manufacturing process, as described by Akao [5]. From its first theorization, this method has been applied in very different domains [6]. In particular, its become widespread exploited in many design applications, not only at top-level, but also at system and sub-system or equipment levels.

From the graphical point of view, the QFD tool is very similar to a sort of house (in fact, it is very well known has house-of-quality), with external walls, bases, and a roof.

The basic house-of-quality consists of the following parts:

  • Rows definitions,

  • Rows weighting factors,

  • Columns definitions,

  • Interaction matrix,

  • Relationship matrix,

  • Scores or prioritization.

Additional weighting rules or additional compartments to deduce more characteristics could be added.

It is worth to notice that the QFD has not been developed to be used as a stand-alone graphic, but its better exploitation could be obtained within a QFD tool-chain that allows obtaining suggestions for engineering parameters, starting from the top-level market analysis.

The usual sequence of QFDs, covering the overall product life cycle, is reported in Fig. 3.

Fig. 3
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QFD typical tool chain covering all the development and production phases

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The tool chain proposed in this paper and described in this and in the following sub-sections aims at providing as main output a series of prioritized mission concept options able to satisfy the top-level mission requirements. It is up to the engineers the definition of the number of options to select for the follow-on of the process. Depending on the number of personnel, budget, and time schedule, it would be convenient to carry on at least two or three different mission concepts. This could be a conservative approach, preserving from unexpected changings at geo-political, management, or economical levels. Moreover, carrying on the procedure, it is possible to obtain a list of enabling technologies from which it is possible to define development roadmaps [7–9].

The first use of the QFD in our methodology aims at discovering the importance of each top-level mission building blocks in an aerospace mission. This result can be obtained using the QFD, as outlined in Fig. 3. The rows contain the list of top-level requirements mainly obtained during the stakeholder analysis (Fig. 4). The columns contain the primary building blocks of the mission, obtained by the joint exploitation of functional tree and functions/products matrix, stopped at first level. The scores obtained in output give the designer an overview of the relative importance of each building block for the considered mission. This is very important not only from a pure technical point of view, but also from a managerial perspective. In fact, the building block with the highest score should be in-depth analysed, and additional efforts should be devoted to its development, in terms of personnel, resources, or budget, because it is the mission component, on which the customer requirements will have the major impact.

Fig. 4
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QFD initialization

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The starting point is the requirement weighting process. This activity is a direct consequence of the analyses carried out at the very beginning allowing the elicitation of the first draft list of mission requirements. Depending on the wishes of the stakeholders, the deduced requirements can have different levels of importance. In this paper, the authors suggested to weight the requirements from 1 to 10, where the maximum score is assigned to constraints and the minimum is related to low impact nice-to-have. Besides the foreseeable negligible impact of some requirements, it is useful to take them into account, because of their direct impact on some mission elements or on the overall configuration. The same reasoning is also valid for the other top-level requirements coming from other sources, such as regulations or geo-political aspects.

The following step is the definition of the impact of the building blocks on the requirements satisfaction (i.e.: “How well this element is able to fulfil the requirement?”). Several strategies could be used at this purpose. In this case, the authors propose a modified version of the classical QFD tool that, in the original version, suggested filling in the matrix customer needs/products with:

  • “0” in case, the requirement is not affecting the product design,

  • “3” in case, the requirement is moderately affecting the product design,

  • “9” in case, the requirement is strongly affecting the product design.

The authors suggest an extension of the ranking rules embracing the possibility that a one or more defined mission elements could be in contrast with some requirements. To take it into account, the authors propose to add:

  • “−3” in case, the requirement is moderately against the product design,

  • “−9” in case, the requirement is strongly against the product design.

Moreover, a requirement with a weight greater or equal to 8 cannot admit elements with negative influence score on it. If it happens, the related element should unavoidably be erased from the list of options. Once the scoring process has been concluded, it is possible to rank the elements inserted in the columns. This is obtained applying the following equation:

$$S_{{{\text{BB}}_{j} }} = \mathop \sum \limits_{i = 1}^{{n_{\text{req}} }} \left[ {(w_{\text{req}} )_{i} \times (w_{\text{rel}} )_{ij} } \right]$$

where: \(i\) is the requirements index; \(j\) is the building blocks index; \(S_{{{\text{BB}}_{j} }}\) represents the score related to the j-th building block; \((w_{\text{req}} )_{i}\) is the weighting factor assigned to the i-th requirement; and \((w_{\text{rel}} )_{ij}\) is the weighting factor assigned within the relation matrix.

Then, a second QFD matrix could be used to prioritize the mission elements options. Indeed, each building block has to be considered as a collection of interconnected elements. At top level, it is important to consider all the possible options for the elements of a mission. To this purpose, the methodology has been applied to prioritize the mission elements. To perform this activity in a logical and structured way, the authors propose to build several QFDs, one per each original function of the functional tree and use a combination algorithm later on, to generate the different mission concept options.

Applying the same above-described methodology, the mission elements prioritization could be obtained applying the following equation:

$$(S_{\text{EO}} )_{lm} = \mathop \sum \limits_{i = 1}^{{n_{\text{req}} }} \left[ {(w_{\text{req}} )_{i} \times (w_{\text{rel}} )_{il} } \right]$$

where: \(i\) is the requirements index; \(l\) is the element options index; \((S_{\text{EO}} )_{lm}\) represents the score related to the l-th element option able to accomplish the m-th mission function; \((w_{\text{req}} )_{i}\) is the weighting factor assigned to the i-th requirement; and \((w_{\text{rel}} )_{il}\) is the weighting factor assigned within the relation matrix.

The values obtained could be used to prioritize the options for each element. If the process is carried out for each function that the mission shall perform, the engineers can have several rankings, one for each function. The following step implies the combination of the elements to create mission concept options. This activity can be automatically performed making all the existing combinations, sorting one element per list.

Remembering that each element has been previously scored, the score related to each derived mission concept is a linear combination of the scores obtained in the previous steps, as stated by the following equation:

$$({\text{MC}})_{k} = \mathop \sum \limits_{p = 1}^{{n_{\text{ele}} }} \left[ {(S_{\text{EO}} )_{p} } \right]$$

where: \(k\) is the mission concept index; \(p\) is the element options index; and \((S_{\text{EO}} )_{p}\) represents the score related to the l-th element option able to accomplish the m-th mission function;

The number of possible combination will be exactly foreseen, since the beginning using the following equation:

$$n_{\text{MC}} = \mathop \prod \limits_{q = 1}^{{n_{\text{fun}} }} (n_{\text{eo}} )_{q}$$

where: \(n_{\text{MC}}\) is the maximum number of mission concept options; \(n_{\text{eo}}\) is the overall number of element options; and \(n_{\text{fun}}\) is the number of functions (i.e., the groups from which element options should be taken) (Figs. 5, 6, 7).

Fig. 5
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From the functions identification to the mission elements prioritization

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Fig. 6
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From the mission elements prioritization to the mission concept proposal

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Fig. 7
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Mission concept options prioritization

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2.3 Mission concept characterization

The mission concepts derived exploiting the QFD technique are simple combinations of elements like puzzles. It is clear that an additional characterization is required, because a system is not only defined by the elements themselves, but also by their mutual connections. In particular, to discover the relationships among the elements, the authors propose to start from a functional point of view to reach a physical and operative perspective. At this purpose, several tools of the functional analysis could be employed. In particular, the product tree, block diagrams, and functional flow block diagrams are suggested. Please notice that is convenient to apply this and the following steps of the methodology, only at the mission concept options selected as baselines because this approach can avoid worthless waste of time and money.

2.3.1 Product tree

Product tree can be obtained grouping together the elements identified in the function/product matrix. Unlike the functional tree, which has a typical top-down approach, the development of the product tree follows a straightforward bottom-up process. Like in the functional tree, also in this case, it is extremely important to clearly define the level of decomposition at which each product belongs to.

2.3.2 Block diagrams

Block diagrams represent the building blocks linked through point-to-point connections. The block diagram provides the designer with further information, if compared to the connection matrix, about the links’ directionality. Moreover, it gives evidence of the type of connection (e.g., mechanical, electrical, etc.). From these diagrams, configuration requirements can be refined and interface requirements can be derived.

2.3.3 Functional flow block diagrams

Functional flow block diagrams (FFBD) allow defining the different operations, the system shall perform, and the different phases and operative modes. FFBDs specifically depict each functional event (represented by a block) occurring, following the preceding function. Some functions may be performed in parallel, or alternative paths may be taken. The FFBD network shows the logical sequence of “what” must happen; it does not ascribe time duration to functions or between functions. The FFBDs are function oriented, not equipment oriented.

Moreover, it is convenient to sketch the so-called concept of operations.

2.3.4 Concept of operations

Complementary, to derive possible mission concepts, it is also important to describe the systems from an operative point of view. At this first level, the concept of operations consists in hypothesizing the general way of working of the systems, including evaluations of mission phases, operation timelines, operational scenarios, end-to-end communications strategy, command and data architecture, operational facilities, integrated logistic support, and critical events. In fact, according to NASA Handbook [4], the ConOps is an important component in capturing stakeholder expectations, requirements, and the architecture of the project. It stimulates the development of the requirements and architecture related to the user elements of the system. It serves as the basis for subsequent definition documents, such as the operations plan, launch and early orbit plan, and operations handbook, and provides the foundation for the long-range operational planning activities, such as operational facilities, staffing, and network scheduling.

It is clear that an extremely high level of uncertainty characterizes this preliminary design stage. To mitigate this problem, an ad-hoc tool has been built to simulate the mission, starting from a limited set of inputs. The results, although characterized by a high degree of approximation, can allow estimating the very first quantitative data, and they can be exploited to evaluate figures of merit in the trade-off analysis.

2.4 Systems and sub-systems design

Once the best mission concept has been selected, each segment, i.e., elements playing a role in the system of systems, should be analysed in detail.

To select a design baseline for the following stages of the design and development activities, it is necessary to consider each system identified with the previous analysis and perform a first sizing attempt. Considering the peculiarity of a mission involving trans-atmospheric trajectories, flight segment should be in-depth analysed and a special attention should be devoted to the on-ground infrastructures required to support the operations. In this way, it is possible to make comparisons among different mission concepts and architecture on quantitative bases. These evaluations shall be repeated all along the design process at different levels until the design of each single piece that shall be built or bought. It is clear that a continuous updating of requirements, constraints, and design specifications shall support this activity. In particular, new branch of model based system engineering (MBSE) [10] developed tools able to automatically keep all the specifications updated during the overall lifecycle. This is considered a powerful capability, because it guarantees the traceability of products and requirements and also a full historical background of all the changes performed during the project.

3 Conceptual design of a crewed space transportation system

This section deals the application of the previously defined methodology to a peculiar reference case study, in which Politecnico di Torino has been involved with Thales Alenia Space Italy, leaded by Altec Space. In particular, the methodology sketched in Sect. 2 is here detailed and well explained, thanks to the help of a real application.

3.1 Mission statement and mission objectives

The reference case study proposed in this paper deals with the conceptual design of a spacecraft aimed at parabolic flights with the special capability of being able to perform a vertical take-off and landing (VTOL). As it can be seen in the following sub-section, dedicated at the stakeholder analysis, VTOL capabilities and other peculiar requests directly come from a certain group of people and will deeply affect the design of the overall mission and related systems and sub-systems.

For the sake of clarity, the mission statement, a concise and precise phrase, describing the objectives of the mission, its principal goals proposing a way to fulfil the aim of the mission, is here reported.

“The mission shall allow regular flight services to enable 4 flight participants at a time to reach 100 km to experience a period of microgravity and an amazing view of the Earth. The spacecraft shall perform a vertical take-off from a sea-based or land-based platform and a vertical landing on the same site. Moreover, the additional capability to perform an un-crewed mission shall be considered”

Please, notice that within the statement, there is a clear indication about the minimum number of flight participant to accommodate. To be conservative, the designers considered:

  • 1 pilot,

  • 1 co-pilot (or scientist devoted to carry out experiments), and

  • 2 passengers.

Considering the results of the very preliminary sizing, the reader will see that the consistent margins considered during the first design activities will allow accommodating two more passengers within the hypothesized cabin envelope, with negligible impact on the overall architecture.

A list of top-level mission requirements has been derived. It is important to notice that proper ID codes associated to each requirement have been introduced allowing tracing them all along the project. Moreover, the requirement ID will permit to find out the source (or in this case the stakeholder) to which it is related.

3.2 Stakeholders’ analysis

As it has been stated in Sect. 2.1, the main goal of the project was to find a way to accomplish the needs expressed by a group of identified interested people. Table 1 summarises the results of the stakeholders’ analysis carried out following the procedure suggested in [4] and indicates a series of desirables. It is clear that the main goal of the designers will be the proposal of mission concept options able to make a limited number of passengers to enjoy parabolic flights.

Table 1 Stakeholders identification
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As it could be easily understood, the main difficulties of the project are related to the constraints about the take-off and landing strategies.

Moreover, as it has been expressed previously, a detailed analysis of the geo-political aspects, in which the mission is supposed to be carried out, is mandatory to consider pressing constraints or to foresee additional hidden positive consequences of the mission. In this specific case, stakeholders came from Malaysia and their desirables are strongly supported by the government and ministerial entities. An overview of the current political and economical situation has been performed, and a list of technological topics, on which they are focusing on, has been obtained. This is the reason for which topics, such as electric propulsion or high-tech cabin design, have been selected for further evaluations, and have been considered during the system design phase. In particular, the possibility of exploiting electric propulsion has been considered for attitude controls at higher attitudes.

From the point of view of the market, it is important to look at similar initiatives aimed at providing parabolic flight capabilities all around the world. US, for example, accounts several projects at different development phases for human transportation systems. It is also important to notice that many of these initiatives consider the parabolic mission as a demo-mission, having the on-orbit transportation or the sub-orbital hypersonic profiles as final goals. Moreover, considering the very demanding take-off and landing strategies, it is important to take a look at existing or under-development technical solutions to carry them out.

3.3 How to derive mission concept alternatives from stakeholder analysis

From the mission statement and the stakeholder analysis, it is possible to derive one or more top-level functions, expressing what the mission (intended as a group of elements and related relationships) shall do. Starting from these top-level functions, it is possible to start the sub-function elicitation process.

The following pictures give a glance of the way, in which the standardized functional analysis has been implemented for this application. In particular, Fig. 8 shows a functional tree developed for the first analysis level. Within the same level of detail, it is necessary to move from a functional to a physical view of the project, and following the suggestions proposed in [3], a function/product matrix has been built, and it is reported in Fig. 9. This matrix allows connecting each single function to possible elements (hardware) able to perform it. It is convenient to notice that the crosses mean that the considered product could be theoretically able to perform the considered function. For example, depending on the fact that at this level of details, we have not yet decided whether the transportation system will be a single stage or a multi-stage, both the launch segment and the space segment could be in charge of reaching the target altitude. On the other hand, it is clear that tourists will be hosted for sure in the space segment as well as the ground segment will perform all those activities related to mission support.

Fig. 8
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Top-level functional tree for the reference parabolic mission

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Fig. 9
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Functions/products matrix for the reference parabolic mission

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The exploitation of these two tools typical of the functional analysis serves to build the bases for the application of the QFD tool and the whole QFD tool-chain aimed at obtaining the highest possible number of mission concepts, because from the connection matrix, it is possible to obtain the columns of the first QFD matrix. Indeed, the rows contain the stakeholder requirements (Fig. 5).

Figure 10 shows the initialization of the QFD matrix with the selection of the most important stakeholders’ requirements and with the assignation of the weighting factors. Subsequently, the QFD has been exploited with the aim of prioritize the different mission segments.

Fig. 10
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QFD initialization for the reference case study

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In particular, Figs. 10 and 11 show a particular variation of the traditional house-of-quality, because the element contained within the columns are not only the three segments deduced from the functional analysis, but they are presented with some related engineering features or qualities conferring a higher detail level to the elements.

Fig. 11
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Example of QFD for the reference case study: building blocks main features prioritization

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Figures 10, 11, 12, 13, and 14 show the QFD obtained for the specific case study following the process sketched in Fig. 5. The exploitation of this QFD chain allows the definition of the main element options for each identified building blocks.

Fig. 12
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Example of QFD for the reference case study: options for operations prioritization

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Fig. 13
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Example of QFD for the reference case study: options for launcher prioritization

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Fig. 14
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Example of QFD for the reference case study: options for vehicle prioritization

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For the sake of clarity, consider that the weighting factors of the relationship matrix have been assigned following the legend in Fig. 4. Considering that the design here reported is at the very beginning of the product development cycle, it is important to notice that it is not possible to associate all the parameters with mathematical evaluations, but some of them remain qualitative assumptions.

Nevertheless, these assumptions are not so fantastic and will have to be confirmed at later stages of development and analysis. For example, considering the need of benefitting of a proper view of the earth has been considered of high importance for the design of the spacecraft, but not of extreme importance (indeed, a weight of 6 has been assigned, instead of 9). The main reason for this choice was that level 9 has been assigned only in those cases, in which the need is so oppressive that the designers can envisage only one way to carry it out, meaning that this need is impacting and strongly affecting the system design. In this case, you can guarantee a proper view of the earth in different ways, for example, you can enlarge your glass surface (with related structural drawbacks) or exploit innovative technologies like O-LED panels and external cameras, able to make passengers feel an immersion in the external environment (with less structural drawbacks but higher power consumption requirement).

Table 2 presents the rationale used to fill in the QFD presented in Fig. 11.

Table 2 QFD weightings rationale (proper view of the earth and routine services)
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Moving from Fig. 11 to 12, it is important to see that the item “staging strategy” is no more present. This is a clear consequence of stakeholder intervention during this preliminary assessment. They invite designers to erase the possibility of having a multi-stage transportation system, while they suggested evaluating different strategies for the launcher.

For the sake of clarity, Table 3 presents the rational process behind the selection of weighting factors in QFD of Fig. 13. In particular, the last three rows of the QFD have been analysed to make the readers aware of the way, in which on-ground operations can affect the spaceplane design.

Table 3 QFD weightings rationale (on-ground operations influence on the aircraft design)
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As far as Fig. 14 is concerned, the authors decided to analyse the first three rows of the QFD, because they reveal the possible effect on vehicle design depending on the type of launcher (Table 4).

Table 4 QFD weightings rationale (launcher option influence on the aircraft design)
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The iterations performed using the QFD tools allow defining and ranking a high number of options for each element of the mission. As it has been explained before, at first, the mission concepts are identified as simple unions of elements (Figs. 15, 16). Subsequently, the mission concept options that are selected for further evaluations should be studied in the detail. In particular, a description of the concept of operations, with the identification of the main phases of the mission, a timeline, and a sketch of the main operative modes of the main system (such as the spacecraft, in this case) should be performed (Fig. 17).

Fig. 15
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Sketch of the link between mission concept elements and mission concept proposal

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Fig. 16
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From mission concepts proposal to the mission concepts selection

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Fig. 17
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Mission trajectory for the reference mission [11]

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Considering Table 2, it is worth to notice that the high-level operational modes here described refer to the system spacecraft. Indeed, it is not possible to define the operational modes without having identified the system to which they refer to.

Moreover, the identified operational modes are:

  • Un-powered: this is the operational mode during which the spacecraft does not use neither the air-breathing engines nor the rocket one and its motion is governed by the inertial forces,

  • Powered: this is the operative mode, in which the spacecraft exploits its air-breathing engines,

  • Rocketed: this is the operative mode, in which the rocket engine is ignited,

  • Safety: it is the typical operational mode that is encountered each time a minor failure or malfunctioning is identified. Depending on the phase, in which it happens and the associated level of risk, the trajectory could be modified and the spacecraft sub-systems could change their operative modes to overcome the problem,

  • Escape: it is the operative mode related to the highest level of risk. In this case, the spacecraft is considered no more able to carry out the nominal mission. The spacecraft is separated into two pieces. The small one, corresponding to the front fuselage, contains the crew and related vital systems and should be designed to allow the crew and passengers’ survival, landing, after a ballistic, un-powered phase.

3.4 Design reference missions

In this sub-section, the authors aim at showing the great variety of mission concept options obtained from this kind of process. As example, six different mission concepts able to comply with the initial stakeholder requirements are described. Each mission concept option is correlated with a brief textual description of the mission and some comments. Please note that these lists are a direct consequence of the scores obtained by the mission concept options in the QFD tool applications (Tables 5, 6, 7, 8, 9, 10, 11, 12).

Table 5 Mission phases for the sub-orbital reference mission
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Table 6 Spacecraft operational modes
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Table 7 Characteristics of the helicopter-lifted capsule scenario
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Table 8 Characteristics of the helicopter-lifted spaceplane scenario
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Table 9 Characteristics of rocket-launched capsule scenario
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Table 10 Characteristics of rocket-launched spaceplane scenario
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Table 11 Characteristics of powered capsule scenario
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Table 12 Characteristics of powered spaceplane scenario
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3.5 Baseline selection

Among the hundreds of alternatives of mission concepts arisen from the application of the conceptual design methodology, following the results of the analyses, the last option proposed in the previous sub-section has been selected as baseline. The selection has been carried out evaluating the final ranking (based on the previous QFD matrices), and it is interesting to notice that the numerical suggestions are in accordance with the qualitative comments presented in the previous table. The stakeholders and the developers usually jointly perform this fundamental selection, and it is in this special moment that new top-level needs or requirements can arise implying a new iteration of the methodology.

3.6 System and sub-system design

This sub-section aimed at giving some additional details about those systems and sub-systems that deeply affect the vehicle architecture. Indeed, aiming at defining the general architecture of the mission and the layout of the transportation system, the authors mainly focused on those systems having a greater impact on the vehicle and spaceport layout. In particular, crew compartment, propulsion system, and escape system have been in-depth studied. Conversely, the other sub-systems as well as additional details about these elements will be surely evaluated and published as soon as the proper detail level of the design will be reached.

Once the baseline scenario has been selected and fixed, the main attention has been devoted at developing those elements of the mission that have been defined critical during the previous analyses. In particular, in this case, the design of a vehicle with such challenging characteristics has been selected as special system to be developed [11–15].

In particular, at the beginning, considering the very high design level, the major attention has been devoted to the development of those sub-systems with the major impact on the general layout of the spacecraft. This goal has been reached following a typical system engineering approach, in which the methodology presented and applied at high level all along the paper, has been recursively exploited to define the systems and sub-systems levels (see Fig. 18).

Fig. 18
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System and sub-systems design methodology

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From the architectural perspective, the spacecraft Fig. 19 can be regarded as a hybrid configuration, standing between a pure wing-body and a pure lifting body. Indeed, the vehicle shape has been derived after several considerations, considering different design peculiarities, such as sub-systems allocation, weight and balance boundaries, crew safety, and trajectory. Indeed, the vehicle should be able to perform a quite long atmospheric flight, produce sufficient amount of lift at altitudes, where the atmosphere does not allow the assumption of continuous flow, but is very rarefied and resists at the high g and heat loads. As it is possible to see in Fig. 20, the spacecraft hosts the crew compartment in its forward part of the fuselage. This location guarantees the crew compartment the maximum distance with respect to the propulsion sub-systems and allows the creation of a detachable escape system. Indeed, as it is detailed in a following sub-section, this part of the fuselage can be separated from the main body of the spacecraft in case of very dangerous critical events.

Fig. 19
figure 19

CAD drawing depicting a typical spacecraft external layout

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Fig. 20
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CAD drawing depicting typical spacecraft sub-systems

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3.6.1 Propulsion sub-system

Considering the mission statement, mission objectives, and the first list of requirements, some of them could deeply impact the design and the sizing of the propulsion system:

  • to reach the target altitude of 100 km,

  • to vertically take-off and landing from/to the same location, and

  • to safely carry four passengers.

To accomplish all these goals, two different propulsion sub-systems have been developed: an air-breathing-based sub-system and a rocket-based sub-system.

The first sub-system consists of two air-breathing engines and related feed sub-system (with tanks, pipelines and valves) able to guarantee the vertical take-off and the first climb segment, up to 18 km of altitude. Several solutions have been evaluated to implement feasible system architecture for overcoming the problem of the vertical take-off of this configuration. A potential solution is sketched in Fig. 21. During the take-off manoeuvre, the two main engines provide hot exhaust gases to steerable nozzles placed on the bottom part of the main body (Nozzles 3–6 in Fig. 21), while the two primary rear-mounted nozzles are closed (Nozzles 1, 2, in Fig. 21). In this condition, if properly sized, the engines shall be able to produce the total amount of thrust required to lift the spaceplane up to a certain altitude, where the transition to the climb segment shall take place. During the transition, the nozzles placed on the bottom part of the vehicle shall be steered to introduce a horizontal component to the resulting thrust vector. Simulations performed to understand this complex phase revealed that it would be necessary to act not only to modify the direction of the thrust vector but also to change its module. After some seconds, when a TBD angle of attack is reached, the nozzles placed on the bottom surface of the spacecraft shall be closed and the hot exhaust gases shall be directly diverted backward in the main rear-mounted nozzles. This propulsive configuration will be exploited up to reaching the ceiling altitude (or a lower and more convenient altitude). At this point, the air-breathing engines shall be shut down and, at the same time, the rocket engine shall be ignited. The rocket engine (Fig. 22) shall power the spaceplane up to a certain altitude defined as the burn out altitude (in this example 80 km). Starting from this moment, the spaceplane continues its mission performing a parabolic flight, reaching the target altitude (100 km) and starting the un-powered re-entry phase. When the spaceplane reaches its ceiling altitude, the air-breathing engines can be re-started, guaranteeing a higher accuracy in approach and landing phases. In particular, from the point of view of the system operative modes, approach and landing are specular with respect to climb and take-off.

Fig. 21
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Potential air-breathing propulsion sub-system scheme

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Fig. 22
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Potential rocket propulsion sub-system scheme

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Fig. 23
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CAD drawing depicting the internal view of a possible crew compartment layout

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The propulsion sub-systems can be hosted within the main body of the aircraft and consist of two air-breathing engines and a rocket motor placed between the two (Figs. 21, 22). Considering the air-breathing engines, they are fed by the fuel coming from the two major tanks located within the wing volume. Proper feed system has been designed to allow cross-feed, guaranteeing both the engines to be able to work also in the case, one of the fuel pumps is not properly working a part from the classical engine outlets; additional steerable nozzles are placed on the lower surface of the spacecraft to allow vertical take-off and landing. As far as the rocket engine is concerned, as sketched in dotted lines in Fig. 22, the motor is installed between the two air-breathing engines, while its related propellant is stored in proper tanks placed closer to the centre of gravity.

3.6.2 Cabin crew and escape sub-system

The crew compartment (Fig. 23) has been designed to accomplish some of the primary objectives of the mission:

  • to carry two passengers and two crew members up to 100 km,

  • to make passengers experience microgravity, and

  • to guarantee an amazing view of the earth.

To perform the sizing of the crew compartment, aeronautical regulations [16] have been considered to sketch the minimum structural envelope and to size the room required for the crew. Then, looking at existing manned space projects, the sizing has been further detailed.

Considering the peculiarity of the mission and the advanced technologies that will be implied, a cabin escape system has been envisaged. In case of emergency, the front part of the fuselage can be detached exploiting some cartridges located in the intersection of the two bodies. Then, following a ballistic trajectory, the capsule should be able to decelerate using a set of parachutes and then, to safely land on water or terrain exploiting inflatable bags to reduce the effect of the impact.

The escape system has been conceived to overcome serious failures with the impossibility of safely landing with residual propellant on-board (Fig. 24). Solutions consisting in seat ejections have been considered, but the development of a solution for four seats appears to be too complex (also from the equipment and redundancy point of view) to be further developed.

Fig. 24
figure 24

Detachable crew compartment

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Other sub-systems, such as landing gear, ECLSS, and power sub-systems, in this specific case, will not affect the external layout of the spacecraft, and for this reason, only general evaluations have been performed, while detailed analysis will be performed in the following development stages.

4 Conclusions

This paper deals with an innovative conceptual design methodology for complex aerospace designs proposing a practical approach for the development and the management of a high number of mission concept options. In particular, the methodology is applied to the definition of a transportation system able to perform parabolic sub-orbital flights.

The methodology revealed to be very useful and interesting, because it allows to manage a very high number of possible alternatives, at different design levels, suggesting the best solution among the several options, on the bases of the stakeholders needs expressed at the beginning of the project. Then, the suggested solution shall be in-depth analysed to verify the feasibility of the concept and to go on with following design and development phases.

After a theoretical overview of the methodology, a peculiar application to a sub-orbital transportation system was reported. Special attention has been paid at the accomplishment of the most demanding and challenging requirements and constrained fixed by the stakeholders, such as the vertical take-off and landing capability. The suggested transportation system is a single-stage spaceplane able to automatically take-off and landing, with a double propulsion system (air-breathing and rocket) and a detachable crew cabin to enhance the level of safety of the transportation system.

Future studies will deal with enhancing the degree of automation of this kind of methodology, finding out different algorithms to be implemented for the weights assignment process during the trade-offs. Moreover, in-depth studies will be carried out to continue the design of the envisaged single-stage spaceplane.