Because we cannot realistically predict the ultimate outcome of the pursuit for a solution to the Space Wasting phenomenon associated with microgravity, we must consider the contingency of the artificial gravity habitat and how this too would be implemented with EvoHab technology and its same strategy of evolutionary development. This is a somewhat challenging proposition since, by default, the gravity habitat must incur greater complexity in the support of a rotational component and its integration with non-rotating portions of a habitat. A much larger minimum structure scale often becomes necessary. Also, such habitats are far less efficient in their utilization of volume and structural materials since most functional space is limited to the thin volume along the inner surface of a rotating structure. This has led to rather elaborate and gigantic habitat concepts in the past which, because of their untenable minimum scales and lack of functionality until their very protracted completion, have so far proven unlikely to realize.
The chief engineering and logistical problem for the artificial gravity habitat is the hub; the interface between rotating and non-rotating portions of an overall structure. No known materials or mechanisms can allow for a hub to link rotating and non-rotating pressurized structures continuously and perpetually without wear on the hub itself. If a hub must be replaced periodically, the entire development of the habitat becomes limited by the frequency with which the entire rotating structure must be stopped. For habitats like those envisioned by the great space colony visionaries like Gerard O’Neil, stopping such vast habitats once started is virtually unthinkable. It would result in a cataclysm for the cultivated terrestrial environments within. Equally unthinkable would be the spinning of any large microgravity structures on the other side of the hub, given their tendency for light mass-conserving structures and asymmetrical forms. Even in the realm of nanotechnology such an engineering feat would be extremely speculative, requiring the development of a pseudo-viscous material that behaves, in a select region, like a liquid in one direction and a solid in the other, never breaks-down over time, and is gas impermeable under some pressure. This is the secret ‘science fantasy’ element underlying many otherwise plausible space habitat concepts.
Frictionless magnetic hub systems are, of course, more technically plausible but have no ability to maintain a pressurized seal. (they aren’t truly frictionless, of course, since magnetic drag produces some mechanical stress and heat and requires opposing structures make an expenditure of some energy to maintain the relative delta-V) They become more complicated the smaller they are in relation to the mass of the two structures they link, as they become a concentrator of axial loads between the two structures. In order for this sort of hub to communicate between two pressurized zones, one rotating and the other not, some rotating airlock mechanism becomes necessary to shuttle between them. This can, in fact, be made sophisticated enough to be a simple spacecraft in its own right, which would have the potential to completely eliminate the need for any hub mechanism at all by employing a complete physical separation of rotating and no-rotating structures. However, one is still left with a key logistical problem. Whether separate or linked with a magnetic hub, the only continuous communication possible between two such structures is for power, light, and thermal energy. Each structure must thus be largely independent and self-sufficient in its other forms of infrastructure, particularly life-support systems. Given that the cost of space in a rotating structure is inherently greater than in a non-rotating one, this adds to the expense of the rotating habitat while creating a logistical complication for most of its external interaction. This may ultimately prove an unavoidable and inherent limitation of gravity structures that one can reduce the impact of through clever design but never completely eliminate.
The issue of the need for artificial gravity will likely become crucial at about the point in time when the first Asgard residents start spending more than half their residence time in space. As noted previously, for some time the first space settlers will likely live a ‘binary lifestyle’ where their habitation is divided between Earth and space, the Aquarius colonies significant as places where this binary living will be most practical and culturally accepted. How effectively Space Wasting can be clinically compensated for will determine the ratio of Earth to space residence time and will likely come to a critical point where people start to spend more than half their time in space. If, at this point, no complete clinical answer to this problem is realized it will compel progressive experimentation in gravity structures, altering the path of evolution in EvoHab structure development.
Gravity Lounges: Early gravity structures are likely to be built within the confines of existing EvoHab settlements, first employing relatively small centrifugal structures on magnetic hubs attached to the core truss of the larger pressurized habitat enclosures. These will be simple affairs based on space frames and the usual light materials employed with EvoHab urban tree dwellings that avoid the need for independent life support systems by being wholly contained by the larger habitat. Designed primarily as a clinical facility which residents use periodically for ‘gravity treatment’, these early gravity structures would feature lounge space with media and communications systems, clinical monitoring systems, and exercise facilities and would be stopped completely when serviced. They would be almost entirely enclosed in order to prevent vertigo from the site of their relative motion.
Gravity Decks: These early gravity structures would culminate in the development of the Gravity Deck structure which relies on the outer structure of the EvoHab hull as a kind of hub. The Gravity Deck would be based, again, on a space frame structure in the form of a large thin ring running along an equatorial meridian inside the enclosing hull. This structure would be reinforced by tension cable wound about the interstitial space of the framing, leaving frame nodes free on all sides of the structure. These exposed nodes would be used to mount decking and partition structure on the inner surface and a set of magnetic bearings and linear motor elements on the outer surface, matched to similar components mounted in the inside of the habitat hull. When complete, the Gravity Deck ring would be suspended within EvoHab hull by magnetic confinement and when spun-up to provide gravity would rotate as a physically disconnected structure within the larger habitat, its loads borne entirely by its ring structure. The gap between the Gravity Deck and hull may be several meters.
The Gravity Deck would be more multi-functional than earlier gravity structures, incorporating dwellings and non-industrial work spaces, but would still have the benefit of relying on the larger habitat for its life support needs. Still a relatively light structure, it would feature three levels, one for utilities, one for habitation, and one for gardening and topped with a light diffuser screen that obscures the view of the core truss to prevent the sense of vertigo. The architecture of gravity deck dwellings would represent a hybrid of style trends on Aquarius colonies and the light materials and structural systems employed on the urban tree habitat. However, the need for full visual enclosure to preclude vertigo and the lack of any conventional windows viewing the larger habitat area or the exterior space environment essentially relegates the Gravity Deck inhabitant to a kind of interior-only living that is only mitigated by designated interiorized open spaces. Thus the general approach to design would be based on series of atriums as the centers of individual homes and other buildings linked by tunnels or avenues. Additional views would rely on virtual window wall installations. This foreshadows the approach to subterranean habitat design likely with lunar and planetary settlements. These atriums and avenues may penetrate the upper deck or be fully enclosed, using their own artificial sky dome/vault light diffusers, in order to allow the uppermost level to maximize its space for gardening. Optical coupling across the gap between Gravity Deck and the outer hull using secondary light collector panels may be employed to allow the light-transmitting hull system to communicate light to a fiber network in the Gravity Deck. Given this low-ceiling interiorized nature, dwellings of the Gravity Deck would be very easy to emulate and mock-up on Earth.
Physical communication between the Gravity Deck and the rest of the settlement would be accomplished using ‘transfer carriage’ systems in one of several forms; hub carriages, edge carriages, counter-rotation rail carriages, transfer spiral rails, and free transfer carriages. Hub carriages would employ a rotational transfer carriage at the hub to shift cargo between moving and non-moving sides of a light magnetically isolated hub structure. This carriage would basically take the form of a ring or smaller carriage module that uses magnetic brakes to link its motion to one or the other side of the hub and synchs up with an elevator port. Suspended on a series of three or four active linear motor tethers like those employed by StrapHanger transport units, the elevators would then transfer cargo between the hub and the surface of the Gravity Deck. These cargo transfers could be completely automated, while a specialized passenger system is used just to move people.
The edge carriage system would work by placing a kind of shuttle carriage at the edge of the Gravity Deck which, like the hub carriage, shifts in relative motion between the deck and the hull by using alternate magnetic breaks, one side supported on a magnetic bearing rail on the hull, the other on the deck. A series of transfer stations mounted on the habitat hull would be used as loading points, linked to the core truss either by FlyBot pallet carriers or a StrapHanger transit line. Goods and people are then loaded into shuttle carriages that then ‘spin-up’ to link to terminals on the edge of the Gravity Deck.
The counter-rotation rail carriage would be a system supported entirely by the gravity deck which hosts it like a looped ‘train’ line. Using linear motors, the carriage is ‘despun’ to a zero-velocity relative to the rest of the habitat, whereupon it links up with an elevator shaft from the habitat core or has its top surface used as a whole transfer station for free-moving vehicles and people. It then uses magnetic breaks to ‘spin-up’ to a zero-velocity with the Gravity Deck and links up to station terminals there.
The transfer spiral rail concept uses a pair of spiral magnetic rail lines that line-up with a pair of similar but looped lines on the Gravity Deck. Transfer carriages with dual magnetic drive systems on opposing surfaces would travel out from the hub on a ‘spin’ spiral and return on the other ‘despin’ spiral. The spin spiral would accelerate the carriage as it leaves the core truss to match the velocity of the Gravity Deck, the carriage transferring from the spiral to the deck magnetic rail lines where their velocities match. The despin spiral would do the opposite, the carriage synching-up with the end of the despin spiral line, being accelerated by it, and then transferred on it to the core truss. This would be a necessarily large structure that would demand a much larger Gravity Deck to justify it.
The free transfer carriage would combine characteristics of a counter-rotation rail system with a FlyBot pallet carrier or RocShaw vehicle. These free-moving microgravity vehicles would be equipped with a magnetic rail capture interface. They would propel themselves through free-space in microgravity and align with a magnetic rail channel on the Gravity Deck that would draw them to it and magnetically pull them up to speed, allowing them to then dock with a terminal. When the vehicle must return to microgravity, the vehicle rides the rail as a linear motor to ‘despin’ and then disengages when it reaches zero-velocity with the rest of the habitat and propels itself back to the core truss. This is the most structurally simple approach but also the most technologically advanced approach, calling for sophisticated magnetic drive technology. It’s probably unlikely with the earliest of Gravity Deck projects.
Each of these approaches have advantages and disadvantages that will have to be considered in the overall habitat development.
Gravity Deck Habitat: First generations of Gravity Deck structures would likely be quite narrow and light, hosting buildings and dwellings in a single linear series. They would be intended to allow frequent stopping for servicing and their dwellings and gardens would need to be engineered to accommodate that, precluding the use of any open bodies of water that cannot be quickly drained to storage. But, based on the same type of components employed throughout the EvoHab, these structures would be indefinitely expandable. However, while an EvoHab can progressively assume a cylindrical shape suited to progressively wider gravity decks, there is an essential design conflict between the general EvoHab concept and the Gravity Deck. The EvoHab relies on its expansive hull enclosure as a light source, simulated sky, or virtual window to space. A Gravity Deck obscures that and, in turn, requires illumination from the opposite vector. Thus at some point the use of Gravity Deck structures will require a very different habitat design more dedicated to their needs and topology. These dedicated Gravity Deck Habitats may initially be created as large hull chamber extensions of an existing microgravity habitat or, ultimately, as a primary habitat structure. This would result in a radically different habitat configuration from the initial EvoHab design.
Employing the same basic structural systems as the EvoHab, the Gravity Deck Habitat would be based on the use of very wide independently rotating Gravity Deck structures within a hull enclosure dedicated to their use. These would be used in counter-rotating in-line pairs to reduce precessional wobbling given the greater mass of the rotating structures. No longer usable as a light/view transmitting hull system, the basic EvoHab enclosure hull would now be able to fully integrate into the MUOF/MOF superstructure of its industrial facilities, the overall structure assuming the form of an octagon, dodecagon, or cylindrical prism aligned parallel to the Earth’s polar axis and surrounded by radiator wings with a mass of docking ports and industrial structures clustered at the middle. The main body of the habitat would be comprised of a pair of Gravity Deck Habitat pods surrounded by thinner microgravity habitat enclosures used primarily for microgravity agriculture and CELSS.
The spaceward axial ends of the habitat would feature a vast tubular heliostat structures built of tensegrity truss structure with a skin of holographic film panels. These ‘solar tunnels’ would concentrate light on second holographic collimating tubes directing it into the polar ends of the habitat and a dense array of optical ports within the volume of a core truss. Inside the habitat, or its individual habitat pods, the traditional but simpler core truss structure would now assume the role of a gigantic light emitter or ‘luminaire’ that illuminates the exposed surface of the rotating gravity decks using holographic film diffusers as though it were a gigantic fluorescent tube lamp. With the habitat located in GEO with an orientation parallel to the Earth’s axis, this solar tunnel heliostat system would be able to collect light continuously and would control the amount of collected light as a function of length and width, filtering unnecessary non-visible portions of light and directing them to concentrated photovoltaic arrays or thermal generators. It would thus become the primary energy source for the entire habitat.
Inside the Gravity Deck Habitat, the use of space on the decks would be very similar to that of early gravity deck structures, though with larger height for the three basic levels of utility, habitation, and garden, the elimination of an overhead diffuser screen (because the core truss would be obscured by its light emitter enclosure), and the use of a more freely varied multi-level terraced combination of atriums and avenues. There would, however, still only be actual window (or more likely, screen door) views into atrium and avenue spaces for most dwellings and no view of space. Transfer carriage systems between the gravity deck and the rest of the habitat would mostly employ the edge carriage systems at far ends of a habitat space.
The core truss would still have its variety of uses as a microgravity habitat but be much reduced in volume since it would be enclosed by its luminaire lighting structures. It would host many utility systems and work facilities, mostly on either side on a single non-brachiating truss structure.
Because the primary structure of the habit would be non-rotating, it become easy to expand the size and length of the gravity decks by adjacent or concentric expansion and increasing the length of the solar tunnel heliostats in proportion. However, it would be necessary to stop the gravity decks periodically to accommodate this construction –adjacent or length-wise expansion being more convenient in this respect. This would ultimately become a limitation on the maximum practical scale of the gravity deck, since the larger it becomes the more energy is needed to stop and start it and the more orbital correction the overall structure may require as a result. This may always limit this habitat to very light dwelling structures and top-level landscaping.
Wound Hull Habitats: At a certain scale of structure the benefits of decoupling a pressurized hull enclosure from an internal gravity structure become outweighed by the need for free structural expansion without the complications of stopping and re-starting the rotation of a very large mass. Even the mass of shielding materials start to become nominal relative to the gross mass of a very large habitat. Thus a simpler more monolithic structural system becomes more practical, even though this comes with a key compromise; greater physical isolation between systems in rotating and non-rotating portions of structure. In fact, incorporating any combination of rotating and non-rotating structure in the same structure starts to become counter-productive. It thus becomes necessary to collectivize more support systems of the rotating structure within its gravity environment.
The basic structural system of the Gravity Deck, composed of a space frame reinforced by tension cables, is a straightforward variation on the EvoHab hull system concept. By combining the elements of the EvoHab hull system with those of the Gravity Deck one arrives at a simple perpetually expandable structural system for a wholly rotating habitat. This would be accomplished by using a modular space frame as ‘winding form’ for tension cables and host for modular shield and pressure-hull substrate panels and an internal decking system. This is called a ‘wound hull’ system and would lend itself to the creation of extremely large rotating habitats based on nanofiber cable that could actually be started from quite small structures expandable length-wise and concentrically while the structure remains in motion. This is accomplished by virtue of the integrated space frame serving as a perpetual scaffold and rigid anchoring system for all later maintenance and construction work. (this work performed by the same types of Inchworm telerobots employed from the MUOL on, though adapted to function in gravity conditions)
The basic configuration of the Wound Hull Habitat would be based on the simple spherical-end cylindrical capsule shape of the classic O’Neil habitat. However, it would employ the Solar Tunnel heliostat light collectors on either end as used by the earlier Gravity Deck Habitat and a series of radial radiator fins on the body of the hull, making it reminiscent in appearance to the earlier Gravity Deck Habitat. A tensegrity truss structure through the core of the habitat would replace the traditional core truss, its functions reduced to only the support of a holographic membrane luminaire and some light structures supported on an axial tether array.
Alternatively, the Wound Hull Habitat could employ a variation of the light transmitting hull technique employed with the basic EvoHab hull system, its hull surface covered with an array of heliostat membrane panels. However, instead of passing light directly through the hull, a network of advanced fiber optic cables would channel and concentrate the collected light to the opposing ends of the habitat where it would be directed to the core luminaire. This has an advantage over the Solar Tube heliostat in that it would leave the polar ends of the habitat open for use of a pair of slowly rotating end-to-end docking ports for large payload or passenger vehicles –a feature suited to smaller habitats in an earlier stage of development. It would also produce a more compact form for the habitat. But this would come at the cost of higher mass for the heliostat system and lower light collecting capacity owing to accumulated optical coupling losses, though in general solar insolation on Earth orbits is so very high that the habitat may be exposed to far more light than its interior would need anyway, making such losses nominal.
The habitat would feature no microgravity structures and communication with the exterior environment would be facilitated by a series of external transfer carriage systems running on magnetic rails at either end of the cylinder. These pressurized carriage units would accelerate in counter-rotation to achieve a zero relative velocity, allowing spacecraft to temporarily dock and transfer cargo and passenger containers. They would then use magnetic breaking to spin-up to habitat speed and dock with peripheral airlocks. Microgravity industrial facilities, located in separate but nearby orbital locations, would be based on the standard MUOF design.
Using a thick structure with many cable reinforced levels, the Wound Hull Habitat would expand on the strategy of maximizing upper-most surface area for a naturalistic landscape by using lower level space for habitation, intensive farming, industry, and utilities. Its lowest level would be employed as a vast water reserve, improving its radiation shielding. However, with so many levels it would be able to employ numerous terraces to create a highly variegated landscape that would give its otherwise ‘underground’ dwellings large window frontages like that on the terraced colonies of Aquarius. Its end-cap regions would be the most ‘urban’, employing progressively steep structures rising to the habitat core and incorporating various light industry and recreational facilities based on the reduced gravity.
With the much increased load capacity possible using nanofiber materials, Wound Hull Habitats would be able to far exceed the scale projected for the classic space habitats while being much more efficient in their use of space –though still quite inefficient compared to the microgravity habitat. NASA studies have suggested that nanofiber based habitats could actually exceed 1000km radius, though carbon is generally considered to be a much rarer material in the solar system than alloys favoring smaller more numerous habitats long-term. And unlike the classic habitat concepts, the Wound Hull Habitat could achieve such sizes through incremental growth, habitats being started with the minimum in practical radius. This would be more important for later habitats, particularly in solar orbits, as most GEO habitats are likely to start with the conventional EvoHab structures whose evolution would end with the earlier Gravity Deck Habitat. Simple tether structures or plasma sails would also be sufficient to combat precessional wobble with these structures, though they could also be linked in counter-rotating pairs by tethers as suggested for the classic space habitats.
Gravity Habitats and the Bifrost Space Elevator: Gravity Deck Habitats would be as readily integrated with space elevator systems as the EvoHab structures they are likely to evolve from. However, to overcome their limitations in scale with the use of Wound Hull Habitats they would need to employ a somewhat different structural form based on a thick torus rather than the simpler capsule shape. This torus would be constructed to straddle the SE structure on an array of magnetic bearings, allowing it to compensate for precession while also allowing for low speed transfer carriage systems to link directly to transit terminals along the SE structure.