For the colonists, �out� would be our terrestrial equivalent of
�down�
"It is a stupid presumption to condemn as false all that
which may not appear likely to us. There is no greater madness in
the world than to reduce everything to the measure of our capacity
and competence."
-Montaigne
In order to start attacking the problem of designing an orbital space settlement, after deciding the exact location in space (one of the libration points along the Moon�s orbit around the Earth), the next logical step would be to define its outward shape.
This will enable us to start considering the rest of the engineering problems, such as the actual design of the dwelling, distribution of areas, life support system, etc. The definition of the shape is essential in order to continue with the analysis.
Several factors condition the design. Some of these design constraints are human and others are technical, but all of them have to be carefully considered before determining the external shape of the colony.
An important overall consideration must be that the colony will be inhabited by families who should be encouraged to spend several years in space, and, if possible, to live their lives in it. So great pains have to be taken in order to ensure that living conditions are not only adequate but also comfortable, in a safe and pleasant environment.
Population : Obviously the size of the colony will be directly linked to the number of people living in it. This is not an imposed design constraint, but at least an estimate of the stable population must be formulated prior to committing to any design. Again it becomes useful to resort to the objectives expressed in the introduction, which will eventually lead to the design and building of space settlements. If the goal is to colonize space and knowing as we do that the colony will be situated near the Moon, far away from Earth, it would be desirable for the colonists to constitute self sufficient community in terms of services, education, light industry, etc. Even though there are no real standards as to what the minimum population would be to sustain a city in space and self provide for all its needs, we arrived at the figure of 10000 inhabitants approximately, based on the study of local cities. Although this number is by no means exact, it will be used as an orientation for the design. It would be desirable to design a space settlement that could eventually accommodate more people so that if the dynamics of its population make it grow it will be ready to do so.
Agricultural area : The agricultural area is crucial to sustaining life in the settlement. Depending on the life support system to be adopted, the actual requirements for agriculture will vary, but in any case food supplies will not be eternally imported from Earth and so must be grown in space. The presence of plants will also contribute to maintain a breathable atmosphere.
Light industry : Any self-sustaining city must be capable of producing the equipment and supplies it needs for everyday life. There is no reason to think that light industry can�t develop and prosper in space. Again the gravity barrier would make it both technically and economically undesirable to import all supplies from Earth. Like in any city of 10000 inhabitants, not all goods will be produced in it, but at least manufacturing of essential items (medicines, clothing, tools, food processing) must be done in space.
Recreation areas : If people are to live in space for many years on end, the space settlement must include recreation facilities similar to those existent on Earth, like for example parks, sports facilities, etc.
Normal city activities : In the design, all areas that make up our terrestrial cities must be included : services, shops, education, offices, laboratories, etc. A continued human permanence can only be achieved through Earth like conditions.
Transport Hub : The arrival of spacecraft from Earth becomes the only physical link with the mother planet. A transportation terminal allowing fast and efficient access of spaceships must thus be engineered.
Artificial gravity : It is well known (see chapter on Medicine) that human beings rapidly decondition when exposed to microgravity, such as would be found in the vicinity of the libration points. For that reason, artificial gravity will have to be generated in order to again provide Earth like conditions. This means that the structure should rotate, which makes a solid of revolution the most logical choice.
Differentiated gravity areas : Even though microgravity is extremely harmful in the long run, Zero- G facilities will have to be included for investigation, manufacturing and processing of materials that could be better done in the absence of gravity. A microgravity recreation facility would result undoubtedly attractive for the colonists themselves and for potential Earth tourists. Days and nights : Human beings circadian rhythms cannot be indefinitely interrupted by just having periods of total sunlight. The human body needs days and nights, that would have to be artificially created by means of mirrors, etc.
Radiation shelter :Outer space is abundant in harmful radiation. The space settlement will have to shelter and protect its inhabitants from the hostile conditions existent in space. Depending on the actual design adopted, a radiation shelter should be considered in the event of an exceptional radiation event (like a solar flare)
Energy : All man made processes require energy, and it must be remembered that the main economic reason that justifies the endeavor is the collection and distribution of abundant solar energy. Some sort of system must be designed to fully utilize the Sun�s energy and beam it back to Earth.
Life Support System : Other life support system tasks must be taken care of, such as the generation and maintenance of a breathable atmosphere, a comfortable temperature and atmospheric pressure and the processing of waste and/or hazardous materials.
Although the following are not strictly design constraints, they would have to be taken into account as guidelines or desirable design conditions :
Separate transport area : The lunch-landing hub is potentially the most dangerous area, due to the obvious risk in handling these operations. Although no air exists in the vacuum of space where all vehicles will park (thus reducing the possibility of an explosion), a spacecraft out of control or a propellant spill could seriously compromise the structure. For that reasons, it would be desirable to design a separate launch facility, well apart from inhabited area and possibly from the main structure.
Industry and waste processing : The industrial and waste processing facility has to be, for obvious reasons, widely separated from the dwelling areas. Although industry will be severely controlled (pollution will become a critical issue in the space settlement), it does generate waste and residues that will have to be treated. It seems logical to separate them from the habitation areas as much as possible.
Good views from habitation areas : The particular geometries of the possible shapes for the space colony allow for fascinating views. In some way, by means of mirrors, views of the Earth and the stars must also be possible, for they will perhaps become one of the most attractive features of living in outer space. A reduced or confined view in an already restricted settlement would entail dangerous psychological consequences for the colonists.
Separate recreation areas : In order to allow for variation in a small world, a separate recreation area, where people can go for an outing , will have to be provided rather than including recreation facilities in the middle of habitation areas.
A non uniform climate : Although the weather will certainly be mild and ranging far from uncomfortable temperature extremes, the colony must be designed in such a way that �seasons� or temperature variations can be implemented.
Artificial Gravity As it can be seen in the Medicine chapter, there is no way humans can live in microgravity for several years without severely deconditioning. Because of this, artificial gravity or pseudogravity must be generated by means of rotation.
As we have already seen, an inertial force called centrifugal force will be generated whenever a body is rotated. The familiar expression for the centrifugal force is :
Fc = m w^2 . R where m is the mass of the object, R the radius of rotation and w the angular speed. The acceleration generated by this force will be equal to the force divided by the mass of the object
: ac = w^2 . R In this way, any acceleration can be radially generated. By giving different values to the two variables that determine the acceleration, the angular velocity and the radius of the circular structure, gravity can be obtained.
Biomedical investigations have been extensively conducted with
respect to microgravity deconditioning. However, no research could
be done with respect to the effects of values of gravity under 1 G,
that can only be found in planetary surfaces or at the Moon. Apollo
astronauts stays at the Moon have been too short for any effects to
be noticeable.
There is little scientific evidence to assist in adopting a value
for the desired pseudogravity. This is not a trivial matter for, as
we shall see, the value of pseudogravity will ultimately determine
the dimensions of the colony.
Adopting a value lower than g would be tempting, for it would be
easier for humans to move about and they would generally feel
lighter. Industrial processes would be carried out more easily and
require less energy.
However, in order to be conservative, we only know so far that
humans can live healthily in a 1 g environment and so that is the
value that will have to be adopted. The rotation of the structure
will be such that 1 g will be generated in the permanent habitation
areas. This acceleration would point radially, so that for a
colonist, �out� would be our terrestrial equivalent of �down�
Although pseudogravity will solve some of the biomedical problems of living in space, it will generate others. Living on a permanently rotating structure can be harmful to human beings. The rotating motion and subsequent Coriolis acceleration can cause equilibrium and inner ear problems. Although research is difficult due to the impossibility to recreate pure g downward rotation on Earth, it appears to show that eventually humans can adapt to rotation rates of several rpm. Most negative effects are hardly noticeable at rotation rates below 2 rpm.
For the colonists, �out� would be our terrestrial equivalent of
�down�
Different combinations of the radius R of the structure and the angular velocity w can generate 1 g (9.81 m/s2) :
g = w^2 . R
If w is expressed in rpm (revolutions per minute), then 1 rpm means that the structure will cover 2 p radians in one minute :
1 rpm = 2.pi / 60 s = 0.105 1/s
Substituting in the above equation
: (0.105 .w)^2.R = g
Expressing R in terms of w :
R (w) = g / (0.105 . w)^2
The following graph depicts the mathematical relationship between R and w :
The table below shows possible radii for desirable rotation rates of between 0.5 and 2 r.p.m.
| w(rpm) | R (m) |
| 0.5 | 3560 |
| 0.75 | 1581 |
| 1 | 890 |
| 1.25 | 570 |
| 1.5 | 396 |
| 1.75 | 290 |
| 2 | 222 |
Due to pseudogravity considerations, solids of revolution are
the obvious choice for shapes. This is so due to the fact that the
lines of equal gravity will be circular, and it would be desirable
to have the same gravity at all points along the surface.
Sharp ends should be avoided for structural reasons. The main
stress of this structure will result from internal pressurization,
so membrane oriented shapes are better suited for the job. Possible
shapes are :
SPHERE
CYLINDER
RING
Two radically different concepts are implicit in the suggested shapes.
Spheres or cylinders of huge radii (several hundred m, to allow
for reduced spinning rates) result in a huge space colony with
considerable volumes of trapped atmosphere. The size of this
atmosphere and of the settlement itself leads to the concept of
naturally regenerative environment, in which the Earth like natural
mechanisms could be recreated.
Being able to reproduce natural processes and having a big sized
colony is undoubtedly an important design factor, for it gives
designers the chance to rely on nature�s buffers for security.
Although life support system solutions would still have to be
engineered, it could be said that, if all parameters are properly
calculated, the system can be switched on and it will take care of
itself. The clear drawbacks to this solution have to do with
research (these same natural processes can only be tested in situ)
and the difficulties associated with constructing such a mammoth
structure and terraforming an atmosphere in it.
On the other hand the ring like structure reduces the size of the atmosphere with opposite advantages and disadvantages with respect to the above mentioned options. That is, the structure itself will be easier to construct but a careful watch must be kept over the now smaller atmosphere. Any mismanagement of the atmospheric parameters could result disastrous, for the relatively reduced size of everything does not allow for nature to compensate imbalances.
These two opposed alternatives seem, however, to fulfill their
own unique role in the goal of colonizing space. While the ring
structure could be envisaged as a first step in achieving the goal
due to its manageable construction and also act as a testbed for
the systems that will need to be engineered, only the mammoth
cylinders or spheres will actually provide a stable, self
sufficient quasi natural environment for humans to dwell for years
to come.
For the above reasons, further studies will be conducted for both
options : a relatively small ring shaped colony as a first step and
a huge spherical or cylindrical colony to follow in the evolution
of space settlements.
A good way to decide upon the suitability of each shape would be
to compare the surface areas that could be utilized in comparably
sized spheres and cylinder.
Taking a sphere of radius R and a cylinder of radius R and equal
height (2R) the surface areas would be :
Sphere = 4. PI . R^2
Cylinder = 2.PI.R. 2 R = 4. PI . R^2
That is, in both cases the surface area is the same.
However, a cylinder can be designed with a greater height to radius
ratio that could increase disposable surface, while the sphere
constitutes a �locked� geometry. Anticipating future
considerations, a sphere appears to be of more difficult
construction and assembly, and will offer a considerable challenge
from the point of view of illumination. Although the final closed
shape of the structure will be determined according to other
considerations, it is then decided that the main body of the �big�
colony will be constituted by a cylinder.
In order to be consistent with the objectives that were outlined
above and the structural design reasons that led to the adopting of
both shapes, a big radius and consequently slow rotation rate
should be chosen for the cylinder and a relatively smaller radius
and a thus faster rotation rate should be chosen for the ring
colony.
In this way, the ring would constitute a space colony that will be
more confined and perhaps not so pleasant to live in, but that will
pave the way for a bigger cylinder that could host colonists in
more favourable earth like conditions.
Based on gravity considerations, a rotation rate of 1.25 rpm is adopted for the ring structure, with an external radius at the surface of 570 m
The diameter of the cross section will determine both the horizontal area and the ceiling of the colony. A possible diameter of 250 m will still roughly (considering a total diameter of 1100) allow for 600m of free space in between the edges of the colony.
If we require at least 200 m for the horizontal section, then we
must find the actual position of the floor.
At a distance x :
[c(x)/2]^2+x2^=R^2 x = {R^2-[c(x)/2]^2}^1/2
Replacing R=125m, c(x)=200m,
x= 75 m
Which allows for a �ceiling� of 200 m.
In order to generate 1 g, we have seen that the radius at the surface would have to be 570 m. According to the above figures, total external radius would be :
Re = 570m + (125-75)m = 620 m
The internal radius would be
Ri= 570 m - (125+75)m = 370 m
Total surface area would be :
A = 2. 570 m . 200 m = 716283 m2
The volume of the structure can be calculated by �stretching out� the ring :
External perimeter = 2.PI. 620 m = 3895 m
Internal perimeter = 2.PI. 370 m = 2324 m
The volume of the cylinder labeled A is :
Va = PI. (125 m)^2 . 2324 m = 114079083 m3
The two cylinders labeled B and C form a cylinder of height (3895-2324) m and radius 125 m :
Vb+Vc = PI. (125 m)^2 . 1501 m = 73679587 m3
to make up a total volume of approximately 18775000 m3
Calculation of surface areas and volumes become simpler in the
case of the cylinder shaped colony.
In accordance to the above expressed criteria, the cylinder will be
dimensioned in a way that ensures maximum comfort and with enough
of an atmosphere to guarantee the smooth flow of natural
processes.
An angular speed of 0.75 rpm is adopted and this consequently results in a radius of approximately 1580 m.
In order to provide an ample space for development, a height of
5 km will be initially considered for the cylinder.
Surface area is then :
A = 2.PI.R.h = 2 p . 1590 m . 5000m = 49951323 m2 .
This is approximately 70 times the surface area of the ring structure.
The cylinder�s volume would be :
V = PI R^2.h = PI . (1580m)^2 . 5000 m = 3.9 x 10^10 m3
which represents 530 times the volume of the ring.
The following graph shows how pseudogravity varies along the cylinder, becoming zero at the center and gradually acquiring more value until it gets to 1 g on the surface.
This leads to an interesting effect : if an object, or a ball, were tossed upwards, if the initial impulse is enough to overcome the pull of gravity it will encounter less of it as it ascends. Once it passes the center of the cylinder, it will be speeded up towards the farther end!
A valid question would be if the curvature is noticeable, that is, if people will be living over an evidently curved floor.
In order to analyze that, we will calculate the elevation in 10 m of horizontal floor for both the cylindrical and the ring like structure.
The angle subtended in 10 m will be :
a = 2 .PI . 10 m / (2 . PI. R) = 10 m/R
The elevation will then be :
x = R - R cos a = R (1-cos a)
In the case of the ring :
a = 10 m / 570 m = 0.01754
x = 570 m (1 - cos 0.01754) = 0.087 m = 8.7 cm
For the cylinder :
a = 10 m / 1590 m = 0.0062
x = 1590 m (1 - cos 0.0062) = 0.03 m = 3 cm
In both cases the elevation would be barely perceptible.
If a person were to make a complete turn around any of the two shapes, he would find himself always going up, which contradicts all our Earthly perceptions. So our space traveler would find himself perpetually climbing, feeling exhausted from his spacewalks !
This paradox is solved by considering that, in fact, down and out are the same in our colonies, for our artificially induced gravity points radially, that is, it will always be perpendicular to the path and so no effort will have to be made in order to 'ascend'.
In both models of colonies, areas will have to be provided for :
Habitation areas will include some paths or roads that connect the houses and small plazas or green spaces, possibly with fruit trees. No major scale parks will be present, for these will be grouped separately in parks and recreation.
Shops will not be scattered around habitation areas, but constitute some sort of a shopping center. Here it must be stated that in order to allow for some variation, these malls will preferably group a large number of small shops rather than a huge single retail outlet.
Services essential to the life of any city, like banking, insurance, etc. will have to be provided. Government offices and other public services must also be administered.
Although a closer analysis should be made as to what type of industries should be set up in the colony, some essential goods will certainly have to manufactured in order to ensure the city's relative self sufficiency.
Recreation will become extremely important in a place which the colonists will not be able to leave. Parks, sports facilities, aerobics paths, and possibly even lakes etc. should be made in order to provide the colonists with a pleasant environment. Good views, possibly of the Earth and space should be had in this area.
Agricultural areas are very important as they will provide the settlers with fresh food and help to regulate the life support system.
A facility must be provided in order to generate energy to supply all the energy intensive processes of the colony and beam the excess back to Earth
Solid and liquid waste will have to be processed and recycled. The waste processing facility would ideally be separated from the habitation areas.
Transportation Hub will be external to the structure for security reasons. It will have to provide a terminal for passenger shuttles and cargo vehicles.
A microgravity area near the center of the structure will host material processing industries and investigation laboratories. A possible recreation facility in microgravity can host colonists enjoying for a limited time the freedom of movement of microgravity and attract potential tourists from Earth.
The figure shows an estimated distribution of areas for the cylinder.
Areas are approximate and, as it can be seen, several empty surfaces remain. These could be completed with further agricultural areas or allow for future expansion.
The distribution of areas tries to follow the above criteria. The view from the city as the cylinder unwinds up reveals recreation and habitation areas.
Sunlight will enter the colony through its caps, which means that areas adjacent to the ends of the cylinder (agricultural) will receive maximum exposure.
In the case of the ring like structure, determination of areas results critical due to its reduced dimensions. A detailed study should be made so as to determine exact dimensions based n the type of housing, industries, etc. However, intensive processes and constructions that maximize usable space can help host 10000 inhabitants with no problems.
The ring will receive sunlight through its ceiling, and will obviously not present transverse views of the opposite part of the structure as in the cylinder where the sky is simply the other side of the world and there is no horizon. In accordance to this, the distribution criteria has taken into account that city areas should be adjacent to parks and agriculture and away from industry and waste processing facilities.
Due to radiation hazards, the colony will have to be completely shielded by means of a composite material. (this will be discussed in both Medicine and Construction chapters) This will apparently be incompatible with the requirement of sunlight for the settlement and views of space.
In order to comply with both requirements, mirrors will have to be used. They will fulfill the function of redirecting the sunlight and absorbing part of the harmful radiation (acting as a sort of atmospheric screen) and also to supply the colonists with the long sought after views.
These mirrors will have to be moved in order to simulate days and nights.
In the case of the ring sunlight will enter naturally through the ceiling. This seems to be the most natural arrangement, and it helps from the point of view of radiation shielding. The ring will be oriented in such a way that its axis of rotation is always parallel to the orbital path. In this way mirrors are uniquely placed for both sections of the ring. Because light will enter the ring in an opposite direction from the Sun's rays, it will help in adequate shielding.
In order to achieve the day - night effect, 3 plane mirrors and 1 concave mirror will be used. The sunlight path is shown in the diagram. As the 'day' progresses, shutters will be drawn (not shown in the diagram) across the concave mirrors that will first dim the sunshine and then lead to semi darkness.
The sunlight
path for the ring
As it can be seen, in this way the structure is totally shielded. A transparent glass separates the structure from the external mirrors.
Glass for the
mirrors can be obtained from silicon which is readily available in
the nearby Moon.
A similar mechanism can be engineered for the cylinder shaped space settlement. A novel idea is considered in the design : two mirrors, one on each side or cap of the cylinder, that will present two suns instead of one. Because the intensity of the sun's rays will be dimmed by the mirrors, having two sources og light instead of one will result in amore uniform illumination.
The sunlight
path for the cylinder
The figure depicts the dual sunshine path for the cylinder. The two ends or caps of the cylinder are coated with reflective layers that serve as mirrors. Three external mirrors redirect sunlight and absorb dangerous radiation in the same way as in the ring type colony.
To generate days and nights the external mirrors will be movable and tend to close into the structure. When this happens, the sunshine will first be atenuated and finally semidarkness will prevail.
In both cases, areas must be provided for investigation and recreation in the reduced gravity zone, that is, towards the center of the ring and the cylinder.
In the case of the ring, a long pole like central corridor can be constructed along the rotation axis. Sphere like structures can be implanted into the pole and host all zero g facilities.
The cylinder will have several structures suspended in microgravity along the rotation axis and supported again by poles or vertical corridors that will transverse the cylinder from side to side.
Although final design of transportation hub will ultimately depend on the transportation system to e utilized, passenger transport services will dock in a zero g facility separated from the main structure but connected to it via a pressurized corridor.
Although no landing will take place, but rather a gentle docking and drifting in the case of arrival or departure respectively, the transport area is still a dangerous part of the colony. Fully loaded craft will leave the colony and although vacuum reduces risks of explosions due to nonexistent pressure and oxidizers, there are still some risks associated with the handling of propellants.
To provide for a smooth parking of the spacecraft, the transportation hub would ideally be disconnected from the rotating motion of the rest of the structure.
In the ring type structure the transport terminal will be attached to the central pole that will, of course, act as a pressurized communications corridor.
For the same reasons, the cylinder will feature a separate transport facility connected to it via a pressurized passageway.
Full utilization of solar energy will only be accomplished if the solar panels are directly exposed to the sun's rays. Solar panels will obviously not rotate with the rest of the structure so as to be always facing the Sun. Power lines can be connected to them into the energy processing and generation plant which could be constructed in a zero G area inside or outside the main structure.
Communication antennas would all be pointing towards Earth constantly, which again makes them apt to be situated along the axis disconnected from rotation.
Radiotelescopes would be ideally moveable so that they could be directed towards the focus of their study.
Other facilities such as waste processing or materials processing and laboratories that for some reason or other would benefit from direct exposure to space will be placed accordingly once they are studies.
A clear distinction is made between habitable and utility areas. Both solutions offer an expandable geometry along their axis.
The architectural design of houses, laboratories and other facilities are not really determinant at this phase. The actual internal layout of houses should not differ much from what we are used to having on Earth.
Buildings will most likely be made of regolith or other rock based composite material, which would make them similar to terrestrial concrete buildings.
Architectural design considerations have to do with the smooth integration of the colonists' dwellings with their settlement. Many energy consuming appliances will be used, for the sun is a cheap and practically unlimited energy source. designers should not worry about consuming too much energy, as we should do here on Earth.
Other considerations have to do with the colony's climate. Despite the fact that seasons and temperature variations will be engineered to avoid monotony, they will always fall within pleasant limits. This means that thermal insulation will not become a critical issue in houses and that large windows should be included.
Awesome views of the colony itself and of the Earth and outer space will perhaps be one of the main attractions to settlers. This reinforces the fact that apartments should be projected for maximum sunlight and allowing large views.
The ring provides us with a very limited area for erecting buildings. The practically only choice open to an architect is to build an apartment type building, to maximize horizontal space. Under these conditions two designs are offered. One of them consists in one apartment per floor that can be occupied by three people and the other option is composed of two apartments per floor that can be occupied by a family of four. Following the same overall structure changes could be made to create more rooms.
In the case of the cylinder, much more space for housing is available. We considered for this structure individual self contained houses that occupy a larger piece of land.
In both designs, large windows are contemplated because thermal insulation is not a critical issue in the mild colony climate. These windows will also enable settlers to enjoy the projected spectacular views. Houses will take advantage of available lectrical energy and host all kinds of modern appliances that will make the colonists' lives easier.
The construction of both types of structure will basically use the same methods and materials. The ring and the cylnbder will be made up of a light and strongly resistant metal like titanium or aluminium, both of which can be obtained from the Moon.
In particular, the decomposition of ilmenite (Fe i O6) will yield iro, titanium and oxygen. The University of Wisconsin has akready designed an ilmenite processing plant.
These metals would constitute ribs that would serve as a framework for the regolith based composite material blocks (NASA has studied the factibility of creting lunar bricks) that could be glued together by special resins. Sealing techniques would need to be especially careful to prevent the gaseous atmosphere from leaking out.
The fact that the structure will be both built and assembled in space complicates matters from the point of view of workers, that could be replaced by robots. But moving things around in microgravity and in the vacuum of space can result helpful.
New techniques currently being developed could be used, like solid freeform fabrication (SFF), vapor molecular deposition, etc.
Although this should of course be studied further, both the ribs and the regolith blocks could shaped by using sliding molds, taking the lunar ore and concentrating solar energy until it is transformed into a hot liquid, then framing it against the mold while it remains hot. Once the desired shape is obtained, we could use the cold of space to solidify it. Puttng it into place requires minimum energy due to microgravity.
The structure would need to absorb various stresses.Both the ring and the cylinder are membrane type shapes, which means that they can absorb efforts by using their shapes, that is, they need not work as flexed beams.
The weight of all buildings, etc, will have to be considered due to artificial gravity and the internal pressurization need not be forgotten, for outside the structure there will be vacuum.
Comments and suggestions : [email protected]
NAS contact: Al Globus
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