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P Collins, S Kuwahara, T Nishimura, T Fukuoka & S Kuwahara, 1998, "Artificial-Gravity Swimming-Pool", Proceedings of Space 98, ASCE, pp 744-751.
Also downloadable from gravity swimming pool.shtml

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Artificial-Gravity Swimming-Pool
Patrick Collins*, Sunao Kuwahara**, Tsuyoshi Nishimura** & Takashi Fukuoka**

One of the uses that has been proposed for fully reusable launch vehicles is "space tourism", the making of short visits to low Earth orbit by fare-paying passengers. This seems to offer the potential to generate a large commercial market, of the order of tens of launches per day, which could amortize the development cost of a new generation of reusable launch vehicles. Recently there has been a considerable increase in research concerning this possibility, with an international symposium being held in Bremen, Germany in March 1997, a session being held on the subject for the first time at the 1997 IAF Congress, the Space Transportation Association in Washington DC publishing a report on it in collaboration with NASA, and the AIAA holding a workshop on the subject in January 1998, among other activities.

Once businesses start to offer travel services to low Earth orbit, it is expected that orbiting "hotels" will be developed to enable guests to enjoy a variety of entertainments in Earth orbit. One feature of such hotels will be sports centers providing guests the opportunity to enjoy moving about freely in weightlessness, or "zero gravity". This paper considers one possible facility in such a sports center, namely a rotating, artificial-gravity swimming pool.

Figure 1: NASA illustration of lunar surface sports center

Space sports were supported as a promising direction for commercial space development in a report by the IEEE Aerospace Committee (1). NASA has also shown interest in sports centers on the lunar surface: for example Figure 1 shows an artist's impression of a lunar surface gymnasium published by NASA in 1996 (2). Study of this illustration enables a number of conclusions to be drawn about the condition of the lunar economy at that time: there are several thousand spectators in the stadium, and another stadium nearby, which suggests a lunar population of at least 10,000, although a large proportion of these may be tourists. If we assume transport costs of, say, $100,000 per person, and accommodation costs of perhaps $50,000 /person/week, then if 6000 of the residents are tourists, the staff:guest ratio is 1:2 and there are 1000 other residents, then the annual number of visitors will be some (6,000 x 52), giving a lunar turnover of some $15.6 billion/year. This is probably a lower bound for the scale of the lunar economy at the time of Figure 1.

From a business view-point, since travel to the lunar surface is likely to cost some 10 times the cost of travel to low Earth orbit ( LEO), tourism in LEO will probably grow to a substantial scale before lunar surface tourism becomes very significant, and will continue to be a much larger-scale activity than lunar surface tourism - as the number of tourists taking lower-cost holidays outnumber those taking the most expensive holidays several times over.

Thus, before there is a lunar population as large as 10,000 people as shown in Figure 1, the average population in LEO seems likely to reach 50,000 or more. If 60% of these are guests staying in orbital accommodation for 3 days on average, this represents a market of some 3.6 million guests/year, paying perhaps $50 billion/year, and employing more than 10,000 orbital hotel staff. NASA's picture therefore represents an advanced phase of space tourism business activity, well beyond the ongoing study by the Japanese Rocket Society (3), for example, which considers a level of activity up to several hundred thousand guests/year in low Earth orbit.

It therefore seems likely that advanced sports centers in LEO will become profitable businesses long before sports events like those shown in the NASA illustration in Figure 1 are held on the lunar surface. For this reason, this paper, being the third in a series, continues to focus on the design of sports centers in low Earth orbit rather than a lunar surface sports center.

Swimming in weightlessness

Like other sports, water sports also will be possible in weightlessness, and they seem likely to be popular because of the entertaining novelty that will be introduced by the new environment of "zero G". Most people know that blobs of water float in the air inside orbiting spacecraft (many astronauts have demonstrated this phenomenon) and it is interesting to imagine how to swim in a large spherical mass of water in orbit. Hazama Corporation began to consider zero G swimming in 1992 (4). Despite the lack of buoyancy in water in weightlessness, body movements are effective in moving through water by creating a reaction against the water, and so it will be possible to move in any direction, and to hold swimming races in a large body of water.

A notable feature of swimming in weightlessness will be lack of natural buoyancy, so that people will not spontaneously float to the surface of a mass of water in which they are immersed. The situation will be similar to breathing out in order to dive to the bottom of a pool on Earth: it is necessary to struggle to get back to the surface. Since there will be no buoyancy forces pulling a person to the surface of the water, it may be desirable for people to wear compact emergency air-breathing equipment, such as the mini-scuba equipment commercially available today. Such pressurized air cylinders 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 (5).

Artificial-gravity swimming pool

"Real" swimming in the weightless environment will require artificial gravity in order to create "up" and "down", and a large water-surface. It would be possible to provide this either by using "gravity gradient" forces in a structure that extends in the vertical direction, perhaps using a tethered section of a hotel, or 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 it 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.


In order to provide a surface area large enough to give guests the "feel" of a swimming pool, an inner surface at least 10 meters in diameter, a depth of at least 2 meters, and a width of 8 meters are considered necessary. In this case the inner water surface area will be some 30m by 8m, which should be sufficient for playing team-games such as "water polo" (although the different layout will require some revision of the rules). In Earth's gravity water polo is physically tiring because it requires players to lift their bodies above the surface of the water, but in orbit this will require little force, and so many people may enjoy playing. In addition, a thrown ball will follow a spiral trajectory through the air, presenting new challenges to both thrower and catcher.

Additional requirements are that the level of artificial gravity at the deepest point should be 10% of terrestrial gravity; the water in the pool must not escape from the pool enclosure into adjoining rooms of the hotel; movement of the water in the pool must not transmit mechanical disturbances to the rest of the hotel; and the water must be kept clean and hygienic.

Design Approach

Including a rotating section in an orbital structure has the major design implication that the axis of rotation of that part will constantly point in the same direction relative to the distant stars. This constraint must be considered in integrating it with the rest of the hotel. There are 3 main ways in which a rotating swimming-pool could be housed in an orbiting hotel: a) the entire hotel could rotate about the same axis as the pool, in which case there would be no need for a rotating joint between the pool and the hotel; b) the pool could be the only rotating part of the hotel, in which case there would be a rotating joint between the pool and the hotel; and c) the pool could be housed in a part of the hotel rotating about the same axis as the pool. There are many different configurations for each of these three possibilities; for simplicity, in the following we consider case c) and assume that the pool is housed inside a rotating pressurized room which is attached to a non-rotating part of the hotel by a rotating joint.

Using rotation to create artificial gravity in a cylindrical pool, it will not be necessary to enclose the water completely. For example a wheel with a trough-shaped cross-section of varying depth could be used, although the inner surface of the water will always remain cylindrical, ie locally "flat", as discussed in (6).

Figure 2: Artist's impression of rotating swimming pool
Functional Description

Being housed within a large rotating room which is part of the hotel, a pool would receive "house-keeping" functions - electric power, HVAC (heating, ventilation and air-conditioning), communications, debris-protection, safety systems from the hotel system. This is similar to the case of a zero-gravity gymnasium considered in (7), and so in the following only those factors that would be significantly different are discussed.

Water Management System

From the engineering point of view, the major feature of the proposed pool is the water, which has a mass of some 600 tons. When this is rotating at 3.5 rpm (see below) it will have rotational energy of some 750 kJ. Guests' swimming will make the water turbulent, and it will be necessary to prevent this causing excessive oscillation of the pool's structure. From a technical point of view, several possible "anti-sloshing" systems are feasible. For example, when needed a) plates could be inserted radially from the outer rim to reduce circumferential motion; b) a cylindrical air-bag could be inflated in the center to suppress waves; c) air-jets could be used to suppress waves; d) water pumps could be used to move water in and out of vents placed around the pool. Of these four possibilities, only d) need not inconvenience swimmers and so would be the most attractive from the point of view of hotel operations.

Water Retention

Leakage of water from the swimming pool to other parts of the hotel could occur more easily than in the case of a pool in the one-G environment on earth. Prevention will require an air-lock between the pool and adjoining rooms, drainage vents beside doors to return water to the pool system, and flexible suction-hoses for manually collecting any airborne blobs of water. Water-proofed video-cameras will be used to monitor the pool and adjacent rooms.


Maintaining a high level of hygiene will require the use of water purification systems and expertise used today in swimming-pools and "onsen" (natural hot springs used for public baths). From experience to date on board space stations Skylab, Salyut and Mir, there is no evidence that the behaviour of moulds and microbes is significantly different in orbit than on Earth.

Rotating Joint

Between the room containing the pool and the non-rotating part of the hotel, a rotating joint will be required. This will need to be large enough for guests to move through comfortably, and so will need to be at least 2 meters in diameter. It will also need to house connections for electric power, communications, HVAC and plumbing between the hotel and the pool. There will also be a need for sensors monitoring the motion of the joint. Electric motors will also be required to adjust the relative angular velocity of the two sections of the hotel.

Instead of a fixed joint, it will also be desirable to have some degree of compliance in order to permit some relative motion other than about the axis of rotation. (The extreme case of this would be a spherical joint, allowing the pool section to rotate independently of the hotel; however, this would be difficult to reconcile with the need for services between the two sections.) While the design of such a rotating joint is a considerable engineering challenge, it is not a unique requirement of swimming pools, but will be a feature of all hotels which contain both rotating and non-rotating sections. The design of rotating habitats providing artificial gravity is the subject of a growing literature (8).

Design Details

For this analysis we assume the simplest design case of a cylindrical pool with constant water depth. If the acceleration at the outer edge of the pool, a, is taken to be 10% of that on the Earth's surface, the required rotation rate will be given by:

a = v . r

Thus, since the external radius is 7 m, the required rotation rate is 0.37 radians/sec, or some 3.5 rpm.

The excess pressure in the water at the deepest point will be only 0.1 atmosphere, and so if the structure is made of clear acrylic, as is used for the walls of modern aquaria, the thickness needed to retain the water will be only a few mm - less than the thickness needed to resist the shocks of collisions from swimmers.

The direction of the axis of rotation of the hotel section containing the pool will remain fixed with respect to the distant stars. Consequently, unless mounted in a spherical bearing, this will determine the orientation of the hotel. If the pool's axis is the same as or parallel to the hotel's orbital axis, it would be simple, for instance, for a non-orbiting section of the hotel to point continuously towards the Earth.

Research Needed
Orbital assembly

The largest component that can be delivered to orbit in a single piece will depend on the launch vehicles available at the date in question. However, there is no large complicated machinery needed for the pool, and so the rotating joint will be the largest component. The most massive component will be the water which will have a volume of some 600 cubic meters, and a mass of 600 tons. This would require about 100 dedicated flights of a launch vehicle such as "Kamotsu-maru", the cargo version of Kankoh-maru (3). However, being flexible and divisible, water can be delivered to orbit incrementally as additional payload whenever a scheduled launch vehicle flight has spare payload capacity.

Once the pool is assembled, the water will need to be given angular acceleration by rotation of the water mass together with the pool structure and the hotel section which contains it. This will require an appropriate propulsion system and the use of propellant to provide external thrust, even if indirectly. (That is, although the pool section could be rotated relative to the hotel by using electric power, external thrust would have to be used to prevent the hotel rotating in the opposite direction.)

Orbit Boosting

Like other orbital craft, hotels will require periodic boosting in their orbits - typically a velocity increment of approximately 1m/sec each week (9). Consequently, it will be necessary to be able to apply thrust to the rotating pool without causing excessive instability in the water. This will require appropriate procedures, and the specification and placement of the thrusters will have implications for the design of the host hotel. Although thrusting along the axis of a rotating pool would be the most simple, this would not be convenient, since the absolute direction of orbit-raising thrust changes continually during its duration whereas the direction of the pool's axis does not change. In the case in which the pool axis is parallel to the orbital axis, applying orbit-raising thrust at right angles to the pool axis could cause waves, and so use of the pool may have to cease for the duration of orbit-raising and until the water is calm. Developing appropriate procedures requires an interesting combination of spacecraft engineering and hotel/hospitality service management.

Rotating Joint

In the case of a hotel with a non-rotating section that is "de-spun" relative to the pool, the design of the rotating joint between the two parts will be an interesting challenge. Although there is already several decades of design and operating experience of continuously rotating joints on satellites, a large joint carrying several services such as that needed for this application introduces many new design issues. As mentioned above, such joints will be needed between rotating and non-rotating sections of all orbital hotels that are not monolithic, and so they represent an engineering specialization requiring development for which there will be a significant commercial demand in future.


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

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 \20,000/kg ($160/kg) the launch of 600 tons of water would cost some \12 billion ($100 million). This would be a substantial investment, but if repaid at 5% or 10% per year over a nominal lifetime of 20 years, would require payment of some \0.6 - 1.2 billion / year, or about \12-24 million per week. Allowing for operating costs of 100% of this, the cost to the hotel would be some \24-48 million per week. On the assumption that a hotel of sufficient size to contain a rotating swimming pool might accommodate 1000 guests/week, this would represent an additional cost of \24,000-48,000 /guest ($200-400 /guest). Relative to a ticket price of perhaps \2 million /guest ($16,000 /guest), an extra cost of \24,000-48,000 would seem likely to be acceptable to at least some customers in exchange for the unique entertainment of swimming in a rotating pool. Thus, even if the above cost-estimate was in error by a factor of two, such a facility would seem likely to be economically attractive to at least some hotel operators.

In addition to its use in a swimming pool, water in orbit has several other uses. It will function as a reserve supply of water which, once purified, can be used for drinking, washing, sprinklers and other functions. In addition, water can be electrolyzed using solar-generated electricity to generate both oxygen and hydrogen - oxygen for replenishing air-supplies, LH2 and LOX for propellants for orbit-boosting, and even for sale to rocket transports. Water is also a good shield from radiation, and so the open center of the pool could be used as a radiation shelter during solar-storms. These possibilities could significantly improve the overall economics of constructing and operating a rotating swimming-pool as one facility in an orbital hotel.

Other Considerations

In order for orbital hotels to become as safe as hotels on Earth, it will be necessary eventually to eliminate the danger of collision with orbital debris. Various methods of debris removal have been studied, including rendez-vous and capture in the case of larger objects, and deceleration to re-entry by means of high-power lasers in the case of small objects. The "Orion project" has recently been proposed in which ground-based lasers could be used to remove all small debris in LEO for a total cost of some $200 million (10). This would be a very attractive investment relative to the cost of space activities today.


There are no known health problems for short-term space travel lasting up to a few weeks, other than radiation from solar flares, for which a storm shelter is required, as discussed above. Motion sickness in space an be prevented by conventional "travel-sickness" medications (11).


It has been recognised in recent years that space law needs to be revised in order to facilitate business activities in space (12). In the USA responsibility for commercial launch systems has been moved to the FAA which is currently finalizing a system for licensing commercial, reusable launch vehicles. The Commercial Space Act of 1997 which is shortly to be enacted will also facilitate this in various ways.

Popular space travel will also require commercial insurance. The process of developing suitable insurance services will help to clarify the legal changes that are required, such as liability for damage caused by debris. 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 they will generate operating statistics, which will make this 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. Fortunately vigorous commercial efforts are now under way to achieve this. Once launch costs fall to some \20,000/kg to LEO, orbital tourism is expected to grow to a substantial scale, and the construction of zero gravity sports centers such as zero G swimming pools will become attractive. From the short discussion above it is clear that the design of an artificial gravity swimming pool offers some interesting design challenges, as well as some interesting business opportunities.

Due to the unique entertainments that will be possible in an artificial gravity swimming pool, they seem likely to become popular. Thus, when space tourism has developed to the extent that space hotels are being constructed in orbit, the extra cost of swimming pools seems likely to be commercially justified. The date when this occurs will depend on the development of low-cost reusable launch vehicles.

  1. T Rogers, ed, 1995, Report of IEEE Aerospace Committee.
  2. P Rawlings, 1996, " The Lunar Games", NASA.
  3. P Collins and K Isozaki, 1997, " Recent Progress in Japanese Space Tourism Research", Proceedings of IAF Congress, Paper no. IAA-97-IAA.1.2.02.
  4. Hazama Corporation, 1992, " Space Traventure", ISTS 92 Exhibition pamphlet.
  5. A Kurotani, 1990, " Space Frog Experiment Onboard Mir", Journal of Space Technology and Science, Vol 6, No 2, pp 1-7.
  6. P Collins, T Fukuoka and T Nishimura, 1994, "Zero-Gravity Sports Centers", Engineering Construction & Operations in Space 4, ASCE, Vol 1, pp 504-13.
  7. P Collins, S Kuwahara, T Nishimura and T Fukuoka, 1997, "Design and Construction of Zero-Gravity Gymnasium", Journal of Aerospace Engineering, Vol 10, No 2, pp 94-98.
  8. T Hall, 1995, "The Architecture of Artificial Gravity: Theory, Form, and Function in the High Frontier", Doctoral Thesis, College of Architecture and Urban Planning, University of Michigan.
  9. T Williams and P Collins, 1997, "Orbital Considerations in Kankoh-Maru Rendezvous Operations", Proceedings of 7th ISCOPS, AAS in press.
  10. I Bekey, 1997, "Orion's Laser: Hunting Space Debris", Aerospace America, Vol 35, No 5, pp 38-44.
  11. H Osawa, 1996, " The History of Japanese Rockets: from Fire Arrows to Space Travel", Mita Press (in Japanese).
  12. P Collins, 1996, "The Regulatory Reform Agenda for the Era of Passenger Space Transportation", Proceedings of 20th ISTS, Paper No 96-f-13.

Note: Papers 3), 7), 8), 9), 12) are downloadable from

P Collins, S Kuwahara, T Nishimura, T Fukuoka & S Kuwahara, 1998, "Artificial-Gravity Swimming-Pool", Proceedings of Space 98, ASCE, pp 744-751.
Also downloadable from gravity swimming pool.shtml

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