A potentially major use for fully reusable launch vehicles and orbital facilities is tourism. Market research performed to date in Japan, Canada, USA, Germany and England suggests that this offers the potential to generate a sufficiently large commercial market to amortize the development and production of both a new generation of reusable launch vehicles and of dedicated passenger accommodation facilities in Earth orbit.

Recently this idea has gained considerably in credibility, with the publication by NASA (1), the AIAA (2) and Japan's Federation of Economic Organisations, Keidanren (3) of reports supporting its feasibility and commercial potential. Indeed, far from being considered science fiction', space tourism is now recognised as the most promising commercial market in space, with the potential to grow to many tens of $billions/year.

In parallel with this growth in acceptance, an international symposium was held in Bremen in 1997 and 1999; sessions on space tourism were held at the 1997, 1998 and 1999 IAF Congresses; the Space Transportation Association in Washington DC established its Space Travel & Tourism Division in 1998, and held its first conference on space tourism in June 1999; and the Los Angeles-based Space Tourism Society held its first "Space Fair" themed on space tourism in 1998. In addition, in 1999, wealthy entrepreneurs Robert Bigelow of Budget Suites of America and Richard Branson of the Virgin Group both established new companies aiming to provide space tourism services to the general public. In particular they both plan to operate hotels in space to enable guests to enjoy a variety of entertainments in Earth orbit and on lunar round-trips, both entrepreneurs having the means to make investments of several hundred million US$.

As the space hotel business matures, orbital facilities will grow in size and sophistication, as hotels do on Earth, competing to attract customers. One feature of advanced space hotels that is expected to be popular with guests is sports centers providing guests the opportunity to move about freely in weightlessness, or "zero- G", and to engage in a range of sports activities. Earlier papers have proposed several different types of orbital sports center - namely a spherical gymnasium 10 m in diameter, a rotating swimming pool 20 m in diameter, and a stadium 100m long and 60m in diameter (4). This paper considers the 3rd of these possibilities, the sports stadium, in more detail. Such a large investment will become feasible at a later stage of orbital tourism, once hotels have moved beyond being just an assemblage of modules, and once the number of guests has reached several hundred thousand/year.

The requirements for an orbital sports stadium will be determined primarily by customers' preferences. These are not known in detail today, but by analogy with terrestrial sports and hotels we can draw up some initial guidelines.

The size needed for an orbital sports stadium will be set by the sports events to be accommodated. Although it is too early to know what will become a "standard" size for orbital sports stadia in future, one likely requirement will be to accommodate flying races by participants using fabric wings. It has been estimated that humans flying in this way will have a turning-circle of some 50 m (5). Consequently a stadium size of 60 m diameter, with hemispherical ends, and an overall length of 100m is selected here.

Consideration of the operation of a sports stadium of this size raises interesting questions concerning its relation to guest accommodation. That is, a facility of this size would tend to dominate attached hotel facilities, becoming a "sports stadium with accommodation" rather than a "hotel with a stadium". This would be particularly true if the stadium was designed to be used for popular sports events, with TV coverage and live spectators, in addition to enabling hotel guests to engage in sports activities. A further consequence is that orbital sports stadia may well be sited in an equatorial orbit, since this would achieve launch cost economies of some 10-20% over inclined orbits, and would also provide many launch and return flight opportunities every day (from near-equatorial launch sites). Use of an equatorial orbit would have the adverse consequence that the facility would offer guests relatively limited views of only a narrow band of land around the equator. However, it is believed that there will be a significant proportion of guests for whom sports will be a more important and continuing motivation for visiting orbit than viewing the whole Earth, and demand from these will drive the development and growth of orbital sports centers. They may well also drive the development of commercial passenger launch-sites near the equator.

It seems likely that there will be demand in future for both rotating and non- rotating sports stadia in orbit, since they offer interestingly different possibilities. Slow rotation would introduce an entertaining difference into sports and games, in particular in the curved trajectory followed by a person or ball through the air. From the point of view of the overall facility design, a major difference is that the direction of the axis of rotation will be fixed in space; thus, as suggested for the case of a rotating swimming pool considered in an earlier paper, it would be convenient to make such a stadium's axis of rotation parallel to its orbital axis (7). A rotating stadium would have the additional constraint that entries and exits would necessarily be restricted to the axis of rotation.

Although it would be physically possible for a single stadium to be used in both rotating and non-rotating modes, this does not seem likely in practice due to the energy that would be used in spinning and de-spinning the structure, the loss of utilisation time during change-over, and the many features that would become sub- optimal for at least half of the time, since the design of most, if not all, subsystems of rotating and non-rotating stadia will be significantly different. (Further in the future, two stadia might possibly be established at a single site, whereby only electric power would be needed to rotate them in opposite directions.)

It will be necessary to be able to accommodate at least several hundred spectators around the central sports zone of a stadium of this size. These will need positioning devices (though not a full seat) as well as catering services, toilets, plumbing and "people movers" to assist their movement into, out of and within the stadium.

It hardly needs to be specified that a stadium will need a high level of safety, including both inherent reliability of the structure and equipment, and effective procedures and equipment for emergency situations, notably fire, leakage of the internal atmosphere, solar storms and other possible dangers.

In selecting a particular approach, several possibilities were considered. While inflatable structures are claimed to be an economical design solution for orbital accommodation, this is only for structures up to a certain size. The present application is so large that it must be launched in many different payloads, making an inflatable structure inappropriate. The approach selected here takes advantage of the stadium's structural simplicity, using large numbers of pre-fabricated aluminium structural segments of a small number of different shapes.

The use of a large number of identical segments will facilitate the automation of the assembly process. The optimum size for the segments will depend on the cargo launch vehicle's dimensions. The proposed "Kamotsu-maru" launch vehicle, a cargo version of the " Kankoh-maru" passenger launch vehicle designed as part of the Japanese Rocket Society's Space Tourism Study Program (8), will have a roughly cylindrical cargo hold some 8 m in diameter by 3 m tall. It is therefore assumed that the maximum dimension of structural segments for the stadium will be some 8 m. In this case approximately 600 individual segments will need to be assembled in orbit.

In an earlier paper the proposed construction method for a 10 m diameter spherical gymnasium involved the use of a robotic arm on a fixed axis (9). This process is limited to structures within the physical range of the equipment. The outer structure of the stadium will be cylindrically symmetrical, making it suitable in principle for a similar approach. However, a robotic arm 30 metres long mounted on an axis 100 metres long would have problems of stiffness and center of mass variation, despite the fact that in micro-gravity stresses will be relatively small. Instead, the use of mobile assembly-robots is selected, which will be able to carry components along the inner and outer surfaces of the already-assembled part of the stadium under construction. Semi-autonomous surface-crawling robots are already used by the construction industry on buildings on Earth for such applications as window-cleaning and condition-monitoring. An extension of this technology will be feasible in the relevant time-scale, 20+ years in the future, and could in principle be applied to stadia of any size. Free-flyer robots as well as surface crawlers may eventually be used for moving segments around.

The assembly of such a large structure will clearly be a larger project than the assembly of a hotel made of prefabricated modules, but because of the extremely repetitious nature of the structure, and the limited internal fittings, it will be simple by comparison with the construction of large modern terrestrial buildings, which are divided internally into many compartments. Once the cost of travel to low Earth orbit falls to some $20,000/person, the cost of employing people to work in orbit will be comparable to that of paying people to work in harsh environments such as Alaska or the North Sea. Thus the work involved in assembling a stadium in orbit will be analogous to construction/assembly work in harsh environments on Earth, although in important respects, such as the absence of unpredictable storms, it will be less dangerous.

Some form of restraints to play the role of seating will be required for spectators in micro-gravity. These may comprise a frame to wrap legs around, a hand-hold, a seat with a velcro pad, or other equipment, lined up in rows. Various concepts have been proposed as people movers in micro-gravity, including handles moving along guide-rails, moving cables, elevators, air-tubes, individual air-jet back-packs, and others. Until more detailed research is done, and/or operating experience is accumulated, it is difficult to judge which means will be preferable.

Spectators will also need more protection from the sports activities which they are watching than is usual in stadia on Earth, since players and their equipment will travel further in the absence of gravity. Equipment such as netting screens may therefore be used, although this will depend on the design of the setting for the sports in question.

The main stresses on the stadium structure will be the tension in the walls due to the internal pressure. For a stadium with a radius of 30 m and an internal pressure of 1 atmosphere, this tensile force is given by

101,000 . 30/2 = 1,515,000 Pa . m

The thickness of the walls will be determined by the tensile strength of the material used. We consider the aluminum alloy, 2219-T87, widely used in aero- space applications, which has a yield strength of 400 MPa. Using this alloy and a safety factor of 2, the thickness of material required for the walls will be 7.5 mm.

Design standards for this application do not exist yet, but aircraft and maritime practice suggest that a double skin might be specified, connected by an internal framework of longitudinal and circumferential ribs and other members. A structure that was assembled from segments no more than 8 metres in dimension would be less mass-efficient than one with longer uninterrupted ribs. There will be a trade-off between structural elegance and low mass versus ease of transportation and simple repetitive assembly of identical modules.

The amount of additional mass that should be allowed for debris shielding will depend on the state of orbital debris at the time of construction and as anticipated through the stadium's lifetime. Due to a stadium's relatively high risk of collision due to its large cross-section, it may be that such large structures will not be developed until orbital debris has been removed (as discussed further below). In any case, the debris environment over the stadium's lifetime will be modeled and used as the basis for the shielding design.

There are several different possibilities for the mechanical relation between the stadium and the accommodation for users. At one extreme the accommodation could be co-orbiting, possibly connected by tethers. At the opposite extreme, the rooms of the associated accommodation could form the external wall of the stadium. A further possibility would be for the accommodation to form a rotating structure around the stadium, as illustrated in Figure 2.

These different possibilities will clearly have a major influence on the overall structural design of the stadium, and could significantly reduce the dedicated structural mass of the stadium itself as estimated below.

Appropriate procedures and counter-measures will clearly be needed for different possible accidents. These will include such design features as locally autonomous control and communication systems, safe havens with autonomous air supplies, guaranteed minimum reaction-times for counter-measures, and eventually the propagation of standards for these. However, just as companies do not perform disaster rehearsals at most public facilities on Earth, simulation will be the main method for estimating appropriate figures of merit.

In the case of a puncture of the structure, perhaps from collision by an approaching vehicle, even a sizable leak will not pose an immediate threat to most users, due to the large volume of the stadium. In the case considered here the contained air has a volume of some 230,000 cubic meters, and hence a mass of some 230 tons: this is sufficient to substantially buffer the loss of pressure in the event of a puncture of the exterior wall. For example, from an initial internal atmospheric pressure equal to that at Earth sea-level, a leak through a hole even 10 centimetres in diameter would take several hours to reduce the internal air pressure substantially.

A puncture even several times larger in diameter would take minutes rather than seconds to reduce the air pressure sufficiently to be dangerous to occupants. Thus emergency counter-measures such as air replenishment systems and equipment and procedures for sealing holes in the outer wall will need to be designed to take effect within a time-scale of minutes rather than seconds. Due to the large area of wall visible from any point inside the stadium, laser doppler air-movement sensors mounted inside the stadium might be used to automatically identify the position of any leak.

Due to the very large scale of the stadium under discussion, and of the 230 ton air-mass which it contains, the design of the heating, ventilation and air-conditioning (HVAC) system raises a range of new issues that will need to be studied in depth before a detailed design can be completed. This is because, to date, all facilities in orbit have been relatively small: Russian and US space stations have comprised cylindrical modules a few metres in diameter, and the control of air quality and air- flow have not posed particular problems. The largest chamber in orbit to date was the main segment of the first US space station "Skylab". Although that was much larger than any room in the international space station currently being developed, it was also of only moderate size, and the contained air was less than 1 ton in mass. By comparison, a stadium 100 metres long and 60 metres in diameter will contain a single 230-ton body of air, necessitating new approaches to a number of design issues, that will amount to the development of a new design philosophy.

In the design of large open volumes of air on Earth, such as in sports stadia, concert halls, hotel atria and exhibition halls, typically the condition of only part of the air mass is tightly controlled, namely the relatively small volume which people use for sitting, standing, walking, running, etc. There are also parts of the air volume which people cannot use, such as areas high above the ground, near the ceiling, and so on.

In the case of an orbital sports stadium there are no places which people cannot use, at least in principle, as there are in terrestrial cases, and so the decision on which zones will need to be controlled and to what specifications requires consideration "de novo". For example, it might be decided that the outer 5 metres of the stadium volume, in which spectators will sit and move around, and the central 50 metre diameter sports area in which players perform might be operated to somewhat different standards, with different requirements for air flow, temperature and other parameters. However, sports activities will themselves cause significant air movements, which will also need to be allowed for. Furthermore, since the air movements themselves would be significant for some sports activities, such as flying, it may be that clear plastic screens rather than nets might be used to separate the sports zone from spectators.

As terrestrial sports stadia are used to accommodate a variety of events, so an orbital stadium is likely to be used in different ways at different times for which different requirements might be appropriate. For example, use of the whole stadium for a sports event with spectators might have different HVAC requirements from its use for general guest activities, such as free flying without spectators (similar to the use of an ice-rink by the general public). Alternatively, its use divided into separate sections to accommodate several separate activities simultaneously, such as gymnastic activities by smaller groups of guests, might have another set of requirements.

In order to accommodate these different operations in a commercial context, the use of the stadium could be expected to change every few hours, for which the capability would also need to be designed. In order to achieve this, an appropriate suite of sensors, control-systems and effectors including fans, air heaters and coolers, filters, air-quality conditioning systems and others will be required.

Parameters that will need to be monitored will, as on Earth, include air temperature, humidity, size and concentration of suspended solids, gas composition - particularly oxygen and CO2 content - noxious gases, mixing by air movement, smells, noise and others. Supplies of "fresh" air will, unlike the case on Earth, have to be generated from air tanks, rather than drawn from outdoors.

Due to the extreme variation in the external radiant heating environment of the stadium, extensive insulation will be needed to control the stadium's overall thermal balance. It will also be needed in order to limit differential heating and cooling in order to prevent large dimensional changes, as well as structural stresses and fatigue from thermal cycling. Radiators also will be required for active thermal control, as well as inter-connection with the host hotel's thermal control system for mutual back-up.

Another major difference from comparable terrestrial structures is the fact that there will be no convection causing hotter air to rise or move differentially. It will therefore be necessary to prevent the build up of "hot-spots" and "cold-spots" within the stadium by ensuring effective mixing of the internal atmosphere. Somewhat like the management of closed stadia on Earth it will also be necessary to plan the rate of change and replenishment of the atmosphere, allowing for its large "inertia" in regard to each controlled parameter. Designers will of course draw on experience of terrestrial buildings, but much will clearly need to be new: for example designing air-flow paths that are comfortable for users. Fans used for air moving and mixing will consume power and will also generate heat and noise, and possibly other contamination. The control of noise levels will involve identifying possible sources of noise, including the air conditioning system itself, and suppressing them appropriately.

In summary, the novel aspects of the HVAC design process can be said to add up to the need to develop a new philosophy for designing satisfactory HVAC systems for orbiting sports stadia. Though this may seem a large new subject, competitive pressure to exploit the known potential of sports facilities in orbit to attract customers will create strong incentives for companies to start to develop the necessary know-how as soon as possible. At each phase of evolution the first company to offer guests a significant new experience - particularly the first company to provide a volume large enough to permit guests to experience genuine bird-like flight - are sure to gain market share and further increase the total pool of customer revenues.

A range of other services, including plumbing, catering, lighting, electronic sign- boards, communications, and shelter from solar storms and/or major damage will also all need to be designed.

The above preliminary review of the issues involved in designing an orbiting stadium show that, though there is nothing requiring fundamentally new technology, there are nevertheless many different topics for which the most cost-effective solutions remain to be identified and developed. Resolving all these issues will involve the combined experience of architectural firms specialising in sports facilities, marine architects and designers of large aircraft, as well as the range of business skills.

One field in which more research could be valuable is market research. However, even more than market research, actual experience of commercial operation of orbital accommodation facilities will generate a wealth of "hard" information about customers' preferences, likes and dislikes. This will be of great value to companies planning sports centers in orbit, which can be expected to grow in size and sophistication to eventually reach the scale of the orbiting stadium discussed here.

By analogy with other industries, notably cruising, airlines and hotels, the development of safety standards and codes will greatly increase business efficiency, by facilitating the task of insurers and investors in assessing individual projects (11). There are features of orbital accommodation facilities that will be closer to ships than either buildings or aircraft, as discussed in (12). Consequently the safety regulations that will be developed may draw on those in all three of the shipping, construction and aviation industries.

Today, organisations such as Det Norske Veritas (DNV) provide certification services to guarantee that a ship meets the appropriate national and international Rules for Classification of Ships (13). Without such certification, insurance of a vessel is impossible, and without insurance finance cannot be raised. This system has developed because it is efficient, allowing technical risks to be minimised, and efficiently handled by different specialised organisations at very low cost. Such certification relating to marine activities is available today for such varied matters as the International Maritime Organisation's (IMO) regulations for different types of ships; materials, equipment and components; safety management following the International Safety Management code (ISM); and the international convention on Standards of Training, Certification and Watchkeeping for seafarers (STCW-95)

The design, assembly and operation of large orbiting structures such as an orbital stadium will also benefit from a system of certification and classification, covering a wide range of matters centered on safety - perhaps offered by an analogous organisation, "Space Veritas" (SV). The development of such standards will arise from accumulated experience, and it will clearly take time to accumulate the experience needed to offer the same range of services for construction in space. However, in view of the efficiency of accumulating, analysing and disseminating information today, we can expect progress to be considerably faster than in the past cases of shipping and aviation.

One of the significant risks for permanently orbiting structures is collision with orbiting debris. Clearly the removal of orbital debris is highly desirable, and recent work has shown that this need not require a major investment of resources (13). Government space agencies currently cooperate in studying the space debris problem: this activity should be extended, and debris removal should be made a major priority of international space cooperation in the near future.

Despite a great deal of analysis in the past, no rotating accommodation facility has been built in space to date. In the case of a rotating sports stadium the design and construction are therefore fields requiring further research. This would need to include not only a range of technical matters in addition to those discussed above, but also ergonomic issues relating to guests' comfort. Research has been done in the past to identify a "comfort zone" for the Coriolis forces experienced, depending on the angular rotation speed and radius of rotation. However, more needs to be known for the design of a large and complex structure such as a rotating sports stadium which would contain many different zones and passages. Research is also needed concerning the assembly process, in order to determine the optimum combination of non-rotating assembly, spin-up and fitting-out in artificial gravity.

The feasibility of an orbiting stadium depends not only on its technological feasibility (which is certain) but more importantly on whether it will be possible to earn a profit by investing in its development, construction and operation. Thus it is essential to consider its probable cost, and the probability of recovering this on a commercial basis from profits generated by customer revenues.

The stadium considered here will have a surface area of some 19,000 sq m and a volume of some 230,000 cubic meters. Thus a double wall comprising two thicknesses of 7.5 mm of aluminium, will have a mass of the order of 800 tons. If we allow an additional 20% for the ribs and members between the two walls, this will add 160 tons, while the mass of air contained within the stadium at 1 terrestrial atmosphere pressure will be some 230 tons. If a further 500 tons is allowed for the interior fittings the total mass of the stadium will be roughly 1,700 tons. At Kamotsu-maru's target launch cost of \20,000/kg ($180/kg) this will have a launch cost of some $300,000,000. In the absence of better estimates, we follow the simple rule of thumb that the manufacturing and assembly cost will be roughly equal to the launch cost, which gives a total cost of $600 million. This is similar to the cost of advanced office buildings today, which are far more complex than a hollow aluminium container assembled from a large number of similar structural panels.

Using Eilingsfeld's estimate of the cost of capital for space tourism projects as 18.6% (14) we can say that the stadium must therefore earn profits of some $112 million/year in order to be commercially viable. If we assume annual maintenance costs of 3% of the initial manufacturing cost of $300 million, this will add $9 million/year. The cost of 40 dedicated staff at $100,000/year would be a further $4 million/year, and their orbital accommodation and transportation costs of perhaps $1 million/year would add an additional $40 million/year. The required annual revenue would then be some $165 million, or some $450,000/day.

Like a stadium on Earth, operating an orbital stadium would be a specialised business activity in itself, involving several different sources of revenue, dedicated marketing activities, and so on. If we assume that 1/2 of the required revenue was earned from hosting sports and media events, then the remaining cost of such a stadium attached to a hotel accommodating 400 guests would add about $600 per guest per day. For a stay lasting a few days this would be about 10% of an overall trip cost of some $25,000. Due to the wide range of entertaining possibilities for using orbital stadia, both as spectators and as participants in zero G sports, this will probably be acceptable for a substantial proportion of guests visiting space hotels in the time frame of interest, particularly for a hotel/stadium in equatorial orbit, for which the cost of a return-flight will be 10-20% less than for hotels in a high inclination orbit.

Like the competition between hotels and entertainment facilities on Earth, that between companies operating tourist facilities in orbit is likely to lead to progressive development of larger and more advanced projects, due to the uniqueness and popularity of the new services that they will offer. Technically, the design, construction and operation of a full-size sports stadium in low Earth orbit is clearly feasible, although the most cost-effective design approaches are not known today. This paper suggests that the construction and operation of such a facility could also be commercially feasible once passenger space travel costs fall to the level at which tourism in low Earth orbit reaches a rate of a few hundred thousand passengers/year.

That is, it appears feasible that the extra cost of building and operating a sports stadium could be covered by the additional revenues which it can be expected to generate by providing additional attractions for which some proportion of potential customers will be prepared to pay a premium. Hence large sports stadia can be expected to form a significant part of the orbital tourism industry in 2030, as proposed in (6).

The development of such a large habitable structure will draw heavily on existing experience of architectural design and sports facility operations planning, and will also require innovations in such fields as structural design and orbital assembly procedures. In addition, the novel environment of a large open volume containing an air mass of more than 200 tons in micro-gravity also raises major new issues for the design of the HVAC subsystems. The new approach required can be expected to gradually evolve into an entire new philosophy for planning and management of large air masses in public spaces in orbit, leading to the development of standardised design procedures, codes of practice and certification. Overall the design of such large sports stadia will provide many fascinating opportunities and challenges for their designers - and unique entertainment value for their users.

1. D O'Neil et al, 1998, "General Public Space Travel and Tourism - Volume 1 Executive Summary", NASA/STA, NP-1998-03-11-MSFC; also downloadable from www.spacefuture.com
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3. Anon, 1998, ' Space in Japan', Keidanren.
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5. P Collins and M Graham, 1994, " Flapping Wing Flight in Reduced Gravity Environments", Journal of the Royal Aeronautical Society, Paper No. 1768, Vol 98, pp 177-184.
6. P Collins, 1999, "Space Activities, Space Tourism and Economic Growth", Proceedings of 2nd ISST; also downloadable from www.spacefuture.com
7. P Collins et al, 1998, "Artificial-Gravity Swimming-Pool", Proceedings of Space 98, ASCE, pp 744-751; also downloadable from www.spacefuture.com
8. K Isozaki et al, 1994, " Considerations on Vehicle Design Criteria for Space Tourism", Proc. 45th IAF Congress, paper no IAF-94-V.3.535.
9. P Collins et al, 1996, "Design and construction of zero-gravity gymnasium", Journal of Aerospace Engineering, ASCE, Vol 10, No 2, pp 94-98; also downloadable from www.spacefuture.com
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13. www.dnv.com
14. I Bekey, May 1997, "Orion's Laser: Hunting Space Debris", Aerospace America, May, pp 38-44.
15. F Eilingsfeld, 1999, "The Cost of Capital for Space Tourism", Proceedings of 2nd ISST, Daimler-Chrysler GmbH.

Key words for "Orbital Sports Stadium" by Patrick Collins, Takashi Fukuoka and Tsuyoshi Nishimura:

space tourism, space travel, Kankoh-maru, Kamotsu-maru, orbital sports center, zero gravity, zero G, space sports, passenger space vehicle, space future