At a time when fewer than 300 people have visited space, and only two of these have been private citizens front outside the space industry, the Japanese journalist Akiyama Toyohiro and the British food rescarcher Helen Sharman, it may seem premature to discuss the possibility of large scale commercial space activities such as space travel by ordinary citizens. However, after 35 years, the space industry is in the midst of rapid change, as the end of the cold war has eliminated the main reason for government funding to date. In addition, the very high cost of using the expendable launch vehicles developed from missiles is becoming unsupportable.

Following the development of the partly reusable US space shuttle, the development of fully-reusable launch vehicles has become the subject of considerable research around the world. In particular, in connection with their plans for development of the fully reusable, single-stage-to-orbit, vertical take-off and landing ( SSTO VTOL) "Delta Clipper", McDonnell Douglas Corporation are projecting launch operating costs as low as $1 million for delivering 11 tonnes to low Earth orbit (1, 2), or a total cost of perhaps $200/kg of payload.

At such low costs many more industries could become involved in commercial space activities. Consequently the next 35 years promise to show much greater growth in access to space than the first 35 years of space activities, which were dominated by government utilisation. In this situation it is valuable to consider possible future commercial markets for launch that have the potential to become large - of the order of thousands of flights per year. One possibility that has received increasing attention in recent years is that of space tourism (3, 4). This seems likely to start with short return trips to low Earth orbit, and to lead on to stays in orbit big accommodation, as being considered by the Japanese Rocket Society (5).

One reason for expecting space tourism to become popular, and to grow rapidly once it has started) is because of the wide range of unique and entertaining activities that are possible in "zero gravity or "zero G", as the micro-gravity environment in side orbiting spacecraft is popularly known. This is a very attractive world which many people are keen to experience at least once in their lives. Both relaxing activities such as viewing the Earth, space and heavenly bodies, and energetic activities involving moving around in zero G, have been greatly enjoyed by those who have visited space.

One field of such activities that seems likely to become popular is that of "zero-G sports", perhaps played initially by athletes selected through competition. Such sports would seem to have good potential to earn profits from commercial broadcasting world-wide, as championship professional boxing, F-1 Grand Prix and other sports do today. In the following we discuss some of the niain possibilities. The analysis performed in this paper is only preliminary, but we hope that, being the first paper to be published on this subject, it will stimulate the imagination of the general public, and also encourage commercial companies to study these possibilities further in order to bring them about.

Zero-gravity sports centers

To date, the cost of western space facilities has been extremely high due to the very high cost of launch, and to the "one—off" satellite design process that the low rate of production has entailed. Also, since all such facilities have been built for government purposes, they have comprised mainly scientific research facilities using very advanced equipment, and have been small, roughly the size of a small apartment.

However, at launch costs of the order of $200/kg, it becomes realistic to consider constructing large accommodation and leisure facilities in low Earth orbit, with masses of even thousands of tons. At such low launch costs minimization of niass will be less important than it is today, and in the case of commercial facilities will be replaced by minimization of cost as a more important design objective.

For this and other reasons the design of leisure facilities, such as hotels and sports centers, will be very different from the design of scientific research facilities such as the currently planned US/international space station. In particular the dominating design-drivers for orbital hotels will be safety, comfort, entertainment and low cost. Their design will thus be more like the design of a modern in tell igent building today. In the following we consider three cases of sports centers: a gymnasium for individual and group entertainments; a swimming pool large enough to accommodate both individual and group sports activities such as water polo; and a large stadium suitable for full-scale team-sports which could become a valuable target for broadcasting companies, such as the commercially-sponsored Olympics, or the new "J-League" professional soccer league in Japan.

When space hotels are developed they will offer guests a range of entertainments which is much wider than that possible in side a passenger launch vehicle. If space tourism develops as a normal commercial business, then as demand increases a succession of Earth-orbiting hotels will be constructed which will provide progressively more extensive offerings to guests.

It is likely that from the earliest phases of space tourism guests will wish to practise i moving in zero-gravity, and that this will be catered for. Although it will of course be possible for guests to practise in their own rooms, many will probably enjoy the opportunity to move 3-dimensionally in a larger space, and to learn how to keep their body-balance. The people who lived in Skylab in the 1970s said that the space inside it was large enough to provide theft with a relaxing sense of spaciousness (6).

Consequently at a certain stage a dedicated public space is likely to be constructed for use by guests for physical recreation, as proposed for example in, (4). In the following we consider the design of such a gymnasium.

From a structural point of view, a sphere is the most efficient shape for a large pressurised chamber. Consequently we consider a spherical chamber, assembled from curved pentagonal and hexagonal segments of two sizes. If we use regular pentagons and 20 regular hexagons with equal-length sides, like a soccer ball, the size of each segment will be as shown in Figure 1.

The main stress on such a structure will be the tension in the walls due to the internal pressure of 1 atmosphere. Consequently the strength required in the walls is given by

T = P.r/2

where T is the tension per meter, P is the internal pressure, and r is the radius of the chamber.

Thus we see that for a chamber with radius of 10 m, and pressure of 101,000 Pa, the tensile force in the wall is 101000 10/2 = 505,000 Pa.m

The thickness of the walls will be determined by the tensile strength of the material used. We consider two different aluminum alloys, 2219-T87, used in aerospace applications, and 6061-T6, used in architectural applications, which have yield strengths of 400 MPa and 240 MPa respectively.

Thus the thickness required is given by 505,000 / 400,000,000 = 1.26 mm for 2219-T87 and by 505,000 / 240,000,000 = 2.1 mm for 6061.T6.

Thus if we use a safety factor = 2, the required thicknesses will be about 2.5 mm for 2219-T87 and 4.2 mm for 6061-T6. For a sphere of diameter 20 m, the area is some 1260 sq m, and so the volume of material required will be some 3.15 cubic rn of 2219-T87 or 5.3 cubic m of 6061-T6. These will have masses of some 9 tonnes and 16 tonnes respectively.

Thus, if we allow a further factor of three for structural skeleton, liner, connections between segments, fittings such as power, lighting, HVAC (heating, ventilation and air—conditioning), insulation etc, the total mass will be some 30 to 50 tonnes.

At a launch cost of some $200/ kg, crewed activities in space will be much more common and cost-effective than they arc today. Assembly of such structures as Space hotels will therefore be routine. All the components for the gymnasium described above could be launched in a few payloads and assembled by construction workers operating front aii orbital base. In order to be easily accommodated in cargo launch vehicles, the larger segments might be divided into smaller pieces.

The main construction operations would be positioning the segments for assembly, the - welding, bolting or joining segments together, either by clamping, and attachment of a lining for extra resistance to air leakage. In addition there will be work to install fittitings such as lighting and HVAC, and external connections to the host hotel.

Water sports will also be possible in zero gravity. These seem likely to be popular in orbit as on Earth, in particular because of the entertaining novelty introduced by zero-gravity. Most people know that blobs of water float in the air inside orbiting spacecraft. It is interesting to try to imagine how to swim in a large spherical mass of water in orbit. Hazama Corporation has begun to consider zero G swimming (7), as illustrated in Figure 2.

Even in zero G, body movements will be effective in moving through water or towards the surface, by creating a reaction against the water However, in zero G there will be no buoyancy forces acting to pull a person to the surface of the water, or to guide them towards tlte nearest surface. There may therefore be a need for people to wear compact emergency air-breathing equipment such as that commercially available today as mini-scuba equipment. As evidence of the practicality of this, it is interesting to note that the pressurized air cylinders used in this equipment have already been used in space on board MIR, as part of the "Frogs in Space" experiment performed by Akiyama Toyohiro, and designed at the Institute of Space & Astronautical Science (8).

However, "real" swimming will require partial gravity in order to create "up" and "down". It would be possible to provide this both by using "gravity gradient" forces in a structure that extends in the vertical direction, perhaps using a tethered section of a hotel, and also by rotation of part of a hotel. The optimum level of such artificial gravity is not clear, but the size and strength of the structure required, and so also the cost, will increase as the level of artificial gravity increases.

In the case of a space tether, in order to achieve partial gravity even as small as 0.01 g would be necessary to use a tether several kilometers in length. Although this may be realized at some time in the future, in the following we consider the more compact design of a rotating, cylindrical swimming pool. Figure 3 is an illustration of such a pool by the British space artist David Hardy.

For clarity, in the following we assume that the entire pool is inside a separate one-atmosphere pressure vessel. The structure of the pool does not therefore need to support an air pressure differential. The possibility of using such a pool for a game such as "water polo" raises interesting design questions. Various different layouts could be used for a game similar to water-polo on Earth. Alternatively, novel layouts could be used that would require partial redesign of the game. On Earth water polo is a tough sport requiring players to lift their bodies above the surface of the water against the force of gravity. However, in space very little force will be needed, and so more people will probably enjoy playing.

Using rotation for obtaining artificial gravity, it will not be necessary to enclose the water completely. For example water could be contained within a rotating wheel with a trough-shaped cross-section. Various layouts would be possible, within the overall constraint that the inner surface of the water will always remain cylindrical, ie locally "flat". For example, by selecting an appropriate shape for the "bottom" of the pool, the water depth need not be uniform but could be varied along the length of, or around the pool, as in Fig 4b. Also, the pool might be closed on one side and open on the other (Fig 4c).

For the purpose of this analysis we assume the simplest design case of a cylindrical pool with constant water depth. If the level of pseudo-gravity is chosen to be 10% of that on the Earth’s surface, the dimensions and rotation rate will be given by:

a = r

where a is the acceleration at the "bottom" of the water. Thus, if the radius is 10 m we require a rotation rate of 0.3 radians/second, or 1 revolution per 21 seconds. If the structure is made of acrylic, such as is used for trattsparent tank walls in modern aquaria, only a thin wall will be needed due to the very slight pressure of the water. If the water depth is 2.5 meters, the length of the inner water surface will be 47 meters, which, if the pool width is 10 meters, will be sufficietit for most purposes.

The direction of the axis of rotation of such a pool will remain fixed with respect to the distant stars, and consequently, unless mounted in a spherical bearing, it would generally be convenient for the pool’s axis of rotation to be parallel to the hotel’s orbital axis. In this way t he hotel could point continuously at the Earth.

A major concern of designers will be to prevent leakage of water from the swimming pool to other parts of the hotel, which would occur more readily than in the one-G environment on earth. A high level of hygiene will of course also be necessary, and will require the installation of water purification systems like those used in onsen and swimming pools today.

The largest component that can be delivered will depend on the launch vehicles available at the date in question. The volume of water will be nearly 1400 cubic meters. To launch 1400 tons of water would require 28 dedicated flights of a 50 ton capacity launch vehicle. Alternatively it might be more economical for water to be delivered to orbit incrementally as additional payload whenever a scheduled launch vehicle flight had spare payload capacity. Once the pool is assembled, the water will need to be given angular acceleration by rotation of the water mass.

Progressively more advanced leisure uses of zero-gravity will require progressively larger sports centers, up to and even beyond the size of a stadium on Earth. Two possible uses for a true stadium are team-sports, and zero-gravity flying, neither of which would be possible in the small gymnasium described above. Both of these possibilities would provide business opportunities for the broadcastittg industry and for commercial sponsorship, as popular sports events do today, which would not arise in the case of a smaller gymnasium.

Trying to imagine 3-dimensional American football or 3-dimensional soccer is an interesting test for the imagination. What shape should the goals be? In what directions should they point? What kind of surface should surround the pitch - a padded wall, a hard wall, a sprung net?

One team sport that is likely to become popular is a zero-gravity variant of rugby and/or football. One interesting aspect of such games is the range of new tactics possible in zero G. The fundamental basis of movement in this new world is different from that on Earth; it might be called an "action & reaction world", or a "kinetic energy world". That is, moving does not require continuing effort, as on a sports-field on Earth, but depends on pushing-off from a wall or another person. The following basic moves are some of the new possibilities that will be possible:

1. intercepting a member of the opposing team and trying to push off from them in a chosen direction;
2. members of the same team collaborating in order to move in desired direction
3. members of the same team throwing another member at high speed in a chosen direction; and other tactics.

Like the case of water polo, it is also possible to imagine further developments of 3-dimensional football that exploit zero G more fully, such as the use of several balls in a game, each with a mass of several kilograms, that could be used to transfer momentum between players.

In micro-gravity it will be possible to fly, propelling and steering oneself witlt the use of suitable equipment, and using much less force than is needed on Earth. One possibility that will surely become popular is to fly like birds, using flapping wings (9). This would lead to a range of possible activities including races over different distances both speed and slalom, and "synchronized flying".

Another possibility will be flight using battery powered, ducted-fan engines. Aerobatics and even "dog-fighting" using such equipment also seem likely to become popular, particularly among younger people.

For the purposes of such sports we consider a stadium roughly equivalent to a commercial sports stadium on Earth 100 meters in length and 60 meters wide. Such large structures will be much easier to construct than on Earth, since there is no problem arising from the weight of members. On Earth the weight of the structural members becomes increasingly important for larger structures, so zero-gee is proportionately more beneficial for building larger structures.

For simplicity of construction we assume that the stadium structure is cylindrical with hemispherical ends. Thus a small number of different types of structural unit should be sufficient. As on Earth, we could take two basically different approaches to construction, a membrane structure, or a frame-and-panel structure.

As for the gymnasium, the main stresses on a stadium structure will be the tension in the walls due to the internal pressure. Thus for a stadium with a radius of 30 m, the tensile force is given by

101000 . 30/2 = 1515000 Pa . m.

Using the same alloys and safety factor as discussed above, the thickness of material required for the walls will be 7.5 mm in the case of 2219-T87 and 12.6 mm in the case of 6061-T6. Alternatively a thinner wall of stronger but denser material such as steel could be used.

A slowly rotating stadium will also be possible. Slow rotation would introduce an entertaining difference into games, in particular in the trajectory followed by a person or ball through the air. Front the point of view of overall design, a major difference is that entry and exit would be restricted to being along the axis of rotation. Such a design has the disadvantage of requiring the axis of rotation to be fixed in space. As for the swimming pool considered above, it would be convenient to make the axis of. rotation parallel to the orbital axis.

Seating will be required for spectators in stadiums, and such novel concepts as moving cables for controlled ntoyernent of the public itt zero G will also be needed. Spectators will also need to be protected from the game. If the game is played inside a large net- or acrylic-walled container, exit and entry of the players might be via an over-lapped opening in the wall.

The stadium will be designed for ease of assembly, like buildings using modern construction methods. Thus it is likely to be assembled in orbit from a small number of identical segments. The optimum size for such segments will depend on the cargo launch vehicle dimensions, which are not known today. Provisionally we assume maximum dimensions of some 10 m. Thus approximately 200 individual segments will need to be assembled in orbit. This will clearly be a much larger undertaking than the assembly of the gymnasium considered above, but still small compared to the complexity of construction of large buildings on the Earth’s surface.

The feasibility of the projects discussed above depends not only on their technological feasibility, but also on whether it would be possible to earn a profit by investing in their development, construction and operation. Thus it is interesting to consider their probable costs, and the possibility of recovering these on a commercial basis.

At a launch cost of $200,000 / tonne, the gymnasium launch cost will be $ 6 to $10 million. Such a simple structure will not be difficult to design and manufacture, so it is probably conservative to assume that the total cost, including launch, will be $20 million.

In order to be commercially justified tltis would need to be repaid at about 10% per year, ie $2 million / year, or $40,000 / week. Allowing for operating and maintainance costs amounting to 100% of this, the required income would be $80,000 per week If 500 people visited the hotel each week, the cost of the gymnasium would therefore add some $160 per person.

Although many of the cost assumptions made here are very approximate, it seents likely that a cost of this size will be considered negligible by comparison with the price of $10,000 or more for a visit to orbit at these launch costs. Consequently such features as a gymnasium seem likely to become popular in space hotels at an early stage.

Compared to the mass of the water the swimming pool structure will be quite small, and so in the following the structural mass is ignored. At a launch cost of some $200/kg the launch of 1400 tons of water would cost some $ 280 million. This would bc a substantial investment, but if repaid at 10% per year interest over 20 years, would require payment of some $28 m / year, or about $ 560,000 per week. Allowing for operation costs of 50% of this, the cost would be some $840,000 per week, or $840 / guest, on the assumption that a hotel at this later stage might accommodate 1000 guests per week Relative to a ticket price of more than $10,000, an extra cost of $840 would seem likely to be acceptable to at least some customers in exhange for the greater entertainment possibilities.

The stadium will have a surface area of some 19,000 sq m and a volume of some 230,000 cubic meters. Thus with a wall thickness of some 7.5 - 12.5 mm of aluminium the mass will be of the order of 400 - 700 tons, and the mass of air contained some 250 tons. Thus, if we assume an additional 1000 tons of fittittgs the total mass of the stadium will be sonte 1650 - 1950 tons. This will have a launch cost of some $330- 390,000,000. If we follow the "rule of thumb" that the manufacturing and assembly cost will be roughly equal to the launch cost, the resulting cost of some $400 million is equivalent to the cost of the most advanced office buildings today, which are much more complex than a hollow container built from a large number of similar units. The total cost of the stadium is therefore taken as some $800 million.

In round figures, in order to be commercially viable such a structure must earn profits of some $80 million per year. If we assume additional operating and maintenance costs of $80 million per year, then the required weekly income is some $3 million. If 1500 people used the stadium each week, the cost would be some $2000 per person This would be a significant additional cost compared to a flight cost of some $10,000, but would probably be acceptable to some in view of the wide range of entertaining possibilities in a stadium, both as spectators and as participants in zero G sports.

These calculations suggest that accomodation in orbit is likely to cost less per guest than the flight cost. Trying to predict the actual ratio between flight cost and stay cost is an interesting challenge for business economics.

In order to develop a successful commercial space travel industry, a number of other problems will need to be solved.

In order to ensure the safety of hotels in Earth orbit, it will be necessary to limit, and preferably to eliminate, orbital debris. Various methods of debris removal are being studied, including rendez-vous and capture in the case of larger objects, and deceleration to re-entry in the case of small objects, by means of high-power lasers. Such activities are not economically feasible today, but will become so as launch costs fall to some $200/kg. Indeed, it is possible that salvage of larger objects could become a profitable activity. Even at such relatively low launch costs, objects in orbit would have a value of some $200,000 / ton simply as raw materials, and many inactive satellites contain valuable components. Alternatively it would be necessary to build debris "bumpers" to protect the outside of orbital hotels, which would add significantly to their mass, and so to their cost.

The problem of short-term space travel sickness must also be solved. Existing medication, such as scopolamine-dexedrine is considered unsatis- factory today since it may impair pilot’s capabilities. However this is not necessarily a problem in the case of holiday-makers. Nevertheless, as the phenomenon comes to be better understood, it can be assumed that treatment, including both preparatory measures and medication, will improve.

There are also a number of legal matters that will need to be satisfactory resolved. For example, in order for space traffic to develop to the extent of sea or air traffic, it will be necessary to introduce a number of legal innovations compared to the present situation, such as liability for damage caused to other spacecraft by space debris. Commercial space travel will require suitable commercial insurance. The process of developing the necessary insurance services will clarify the legal changes that are required. In order to be able to provide insurance, companies must be able to assess the risks accurately, and judge them to be tolerable. As reusable launch vehicles fly repeatedly atid generate useful operatitig statistics, this will become essentially similar to aviation insurance.

In order for the possibilities described above to be realized, the single most important condition is for launch costs to fall substantially below their present level. Once launch costs fall to the level predicted for SSTO VTOL rockets, the construction of zero gravity sports centers will become feasible. From the short discussion above it is clear that the variety of space sports is unlimited, and includes sports derived front existing sports on Earth, and sports that will be invented creatively in the new environment of zero G.

Due to the wide range of unique entertainments that will be possible in zero G sports centers, they seem likely to become very popular. Thus, when space tourism has developed to the extent that space hotels are being constructed in orbit, the extra cost of large space sports centers seems likely to be commercially justified. The date when this occurs will depend on the development of low-cost reusable launch vehicles, the earliest of which seems likely to be a SSTO VTOL passenger vehicle such as the Phoenix (10).

The design of orbital sports centers clearly provides many fascinating opportunities as well as design challenges We are very pleased to have been able to write the first published paper on this subject, which seems certain to become popular in future with the general public, and also with engineering designers and manufacturers. The main purpose of this paper is to act as a catalyst, to generate enthusiasm so that other engineers and business people take the subject rapidly forward. HAving been the only paper on this subject at SPACE 94, we would hope to see many other papers from around the world presented at SPACE 96 on the range of topics that we have opened up.

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