and Alternatives

Table of Contents
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Operational Environment
Habitat Requirements
Habitat Envelope
Habitat Power Modules
Module Interconnect Methods
Gas Recycling
Water Recycling
Habitat Rotation Engines
Habitat Balance Control
Habitat Perimeter Modules
Drive Requirements
Drive Envelope
Drive Interconnect
Drive Orientation
Drive access to Habitat
Drive Engines
Drive Power System
FOLDINGSS Conceptualization

The FOLDINGSS Research Spacecraft in a nutshell
Mass 250 metric ton
Habitat Configuration 3 spokes, 2 x 330m^3 envelopes ea 
Inertial Gravity 1/6G inner envelope, 1/3G outer
Habitat RPM @ InG 4 RPM
Habitat Volume  2150m^3
Habitat Living Area 585m^2 (18 floors @ 32.5m^2)
Habitat Power 6 x (30KW UJT Solar Array)
Low Impulse Engines 2 x (4N multi-channel ION)
Low Impulse Propellant Argon or Iodine (8 metric ton)
ION Power 2 x (200KW UTJ Solar Array)
High Impulse Engines 2 x 40N chemical
High Impulse Fuel Cryogenic MetheLOX
Drive Press. Volume 210m^3
Docking 2xUniversal and 18 CBMa available


Operational Environment

The program objectives dictate quite a few of the specifics of the FOLDINGSS research spacecraft. The craft must be large enough to independently support a crew for 900 days. This indicates a lot of storage space. The inertial gravity requirement necessitates a rotating habitat with a radius greater than 30 meters. Any radius of habitat rotation less than 30 meters requires rotation rates that produce coriolis effects that are problematic for the crew. A mass in rotation as large as the necessary habitat creates strong gyroscopic effects. This limits the plane of rotation to the plane of movement of the ship. Most of the foreseeable deep space destinations are within the solar system's plane of the ecliptic. Therefore, the habitat must rotate on the same plane as the ecliptic.

One of the major objectives of the program requires continuous inertial gravity. For this gravity to be consistent, the acceleration of the spacecraft as a whole must be significantly less than the centripetal acceleration. This limitation eliminates high impulse maneuvers. Lower efficiency orbital transfers are therefore required. This strongly suggests solar powered ion propulsion.

There is no requirement for landing on planetary bodies or moons. This means the research spacecraft only needs delta-V for orbital maneuvers. The FOLDINGSS spacecraft is strictly a space ship and operates only in space. No atmospheric capability is required. This will mean that the FOLDINGSS craft must be dockable by ascent and landing vehicles.

Given the emerging desire to conduct a crewed mission to Mars in the near future. The FOLDINGSS program needs to start soon and the research spacecraft cannot require a long development cycle. These realities suggest the ship be assembled in LEO using proven construction techniques. The ISS is a good model for this type of construction, using modules build on the ground that are connected together in space. The CBM connection technique can easily be applied to the FOLDINGSS vessel. The maximum size of any single module is then restricted by the largest available booster fairing size. Assuming this fairing is 5.1m x 14m the number of modules can be extrapolated.

The crew compliment is somewhat arbitrary. The crew needs to be large enough to get multiple data points for every experiment. Additionally, some experiments and lines of investigation may benefit from differing test parameters. Given that the rotating habitat must be mass balanced, this implies at least two spokes to the habitat wheel. Three spokes is a more reasonable minimum as it allows for cross bracing between spokes. Thus two modules must be used per spoke to approach the 30m minimum radius for 1/3G spin gravity production.

The Bigelo Aerospace inflatable B330 module is a likely format on which to base the FOLDINGSS habitat modules. Each of these modules is rated for a crew of 6 for short duration. So, for an initial crew compliment, assume a crew of 2 per envelop and 6 envelops for a habitat wheel with three spokes. Thus a crew compliment of 12. This allows 3 variations of parameters with 4 test subjects each. Variations in recycling technology can be compared for air recycling, water recycling, crew dynamics, exercise regimes, pharmaceuticals, food production, etc.

Given basic assumptions about food requirements of 1T per crew, a total of 12T of dehydrated food should last the crew for 900 days.

The inner habitat envelop of each spoke will experience approx 1/6G inertial gravity, the outer habitat envelop of each spoke will experience approx 1/3G inertial gravity.

Water requirements and habitat air will depend upon the effectiveness of recycling and losses over time. Heating and cooling requirements will also vary depending upon other factors. For the sake of this preliminary document, we will assume sufficient water, Oxygen, Nitrogen, and CO2 can be accommodated by this habitat configuration.

As the habitat wheel will be in constant motion, a connected but movement disconnected (bearing attached) drive section (s) must be utilized for moving the habitat section between LEO and research distant Earth orbits. This drive section is also required to simplify docking for visiting spacecraft. Docking to a spinning habitat would be very difficult and potentially dangerous. Two obvious methods for attaching a drive section would be either attaching two drives with bearings on the central hub of the wheel forming a sort of stationary axle. The other method would be to create a track around the circumference of the habitat wheel. The stationary drive section would then mount on the track and push against the perimeter of the habitat wheel.

Solar collection is the likely power source for this mission. The drive section which is responsible for docking visiting spacecraft and moving the research ship between orbits, will need sufficient power collection to drive the Ion engine array. The habitat will need sufficient power for life support and scientific research. The life support will need heat, cooling, lighting, also the gas recycling will need power for Sabatier cycles. Likewise the water recycling will need power as well. ISS style water recycling hardware can be used initially. Horticulture will need power for lighting as well. Waste management and recycling will also require power.

Each drive section can keep a relatively large array pointed at the sun except for shadow cast by planets or moons. The solar arrays on the wheel will tend to shadow each other briefly during each revolution. Also, the solar arrays will need to constantly track the sun as the habitat wheel rotates. This will necessitate a custom built habitat solar collection module.


Habitat Requirements

Each spoke of the habitat arm requires The following independent systems:

  1. Solar collection with sun tracking and at least 48 hours reserve batteries or hydrogen for fuel cells
  2. Water recycling system
  3. Air recycling system
  4. Thermal radiation system
  5. Electric heating system
  6. Food Stores for 900 days for at least 4 persons
  7. Water storage for 900 days (including recycling) to supply needs for all water dependent habitat systems. Must be a 45 day excess water storage for days that the water recycling systems are not functional.
  8. Envelope atmospheric gas storage for 900 days (including recycling). There may be exchange between water, heat transfer, and atmospheric gases per Sabatier processes and other recycling/purifying cycles. There must be a 45 day excess storage as a contingency for days that gas recycling systems are not functional.
  9. Two Ion drives at the spoke perimeter for slowing or speeding habitat rotation as well as a sufficient supply of propellant to support the maintenance of consistent rotation of the habitat wheel as well as a reserve for modulating the speed between minimum rotation speed and maximum speed at least 4 times.
  10. An air humidifying/drying system.
  11. A sanitary system for both habitats on the spoke.
  12. A sanitary waste desiccating and sterilizing system.
  13. A fabrication station in the higher G habitat.
  14. A medical facility in the lower G habitat.
  15. A galley for each habitat
  16. A common area for exercise, socialization, meetings on each habitat envelope.
  17. A sleeping / office quarters for each habitat envelope.
  18. A data server.
  19. A central passage way from the central hub all the way out to the perimeter mechanical module. This passage way goes through 4 CBM port pairs and the corresponding 8 pressure bulkheads and hatches. Some of these hatches might be made to close automatically in an emergency. The strategies for this type of operation may be one of the areas of research for the FOLDINGSS program.

Each spoke also shares parts of a common system for the following:

  1. A water based balancing system. Each spoke will have piping and pumping to rapidly shift ballast water from it's ballast tank to the other spokes tanks as commanded by a central control system in the habitat hub.
  2. A means for power sharing, via three switch able busses.
  3. A means for water/gas transfer between spokes.
  4. Several wired data connections provide several data networks between the spokes and the long distance communications equipment contained within the drive sections.
  5. Wireless data connections are also maintained between habitat spokes and anything else within 802.11 range.
  6. Obviously a passage way to other habitat spokes and the drive sections via the central habitat hub.

The central hub has the following additional system, controllable from other data points.

  1. The habitat rotation system. The central hub has accelerometers and gyros that detect the rate of rotation of the habitat wheel. On the basis of this information it automatically fires the ion thrusters on each of the habitat struts as necessary to keep the spin rate within the commanded rate.
  2. The habitat balance system. The central hub uses these same accelerometers and gyros to sense the center of mass of the habitat wheel. On the basis of this information, the control system will command pumping from ballast tanks of the highest moment spokes to the ballast tanks on the lowest moment spokes. Balance is affected primarily by crew movement. This system will also alert the crew and earth based monitors if the center of mass of the habitat moves out or range or moves in an unexpected way.
  3. The drive section orientation system. In conjunction with the habitat rotation system, the central habitat hub keeps the orientation of the drive sections as commanded. This is controlled with electric motors on the two main bearings. This system can be used to reverse the thrust of the main drive engines by allowing the drive sections to rotate 180 degrees.
  4. The two habitat robotic arms. The two robotic arms attached to the two main hub bearings can are controlled by a system on the central habitat hub. Software ensures that the arms never interfere mechanically with the habitat or drive sections. These arms are used during initial construction of the habitat wheel, maintenance of the habitat wheel, and external cargo transfer between the drive section or craft attached to the drive section and the perimeter of the habitat spokes. The arms can be made to rotate at the same rate as the habitat wheel or to rotate in a manner that they are stationary with respect to the drive section.


Habitat Envelope

Each habitat spoke has two habitat envelopes which are likely to be based upon Bigelow Aerospace's B330 space stations. These envelopes are 13.5 meters long counting their central passageway structure and CBMa berthing ports. Each envelope has an internal volume of 330 cubic meters. This space is divided into three floors with configurable decking. Each floor is octagonal in shape and is in rigid connection with the central passage way but not in physical contact with the outer wall of the envelope. This creates passage ways for systems in the voids between the floor walls and the envelope wall. Each floor has an area of approximately 19 ft^2 after the center passage area and wall void area is deducted.

For each habitat a possible configuration would be one 19 ft^2 top (least gravity) floor for food stores and other storage. the Ceiling of the top floor is not full height over the entire area. One 19 ft^2 middle floor common area for exercise, fabrication, horticulture or other uses. One bottom (highest gravity) floor for sleeping, galley, office, and sanitary purposes. The bottom floor may be partitioned into the equivalent of four 9.5 ft x 9.5 ft rooms, though the actual shape is more triangle or pie shaped. Below the bottom floor is an irregular shaped space as high as 6 feet with an area of 19 ft^2 that is used for mechanical systems purposes.

The habitat system in FOLDINGSS is predicated on the assumption, that once spin gravity is established, it will be maintained through the service life of the spacecraft. A minimum rotation rate (something in the range of 0.5 RPM generating 1/24G and 1/48G in the outer and inner habitats respectively) must be maintained with the thruster systems on the habitat wheel. To conserve main bearing wear, the drive sections will be allowed to rotate in tandem with the habitat wheel when the ship is parked and idle.

The habitat build out can then be predicated on the assumption of constant downward acceleration. This allows for more Earth traditional Bathrooms and Galley functions. Clothing will come in contact with crew's skin, Crew's digestive feedback will be traditional, Heating foods will behave in an Earthlike manner. Furniture and Desktop surfaces will also be used in Earthlike fashion. Altogether, the FOLDINGSS crew experience should be much more familiar and tolerable to the crew. This is presumed to be a benefit given the extreme length of missions. With the proposed crew compliment of 4 crew per habitat spoke, each crew will have approximately 14 ft^2 of private area in the form of sleeping, office, and storage space and 43 ft^2 of shared area, though some of that shared space is taken up as storage area. Assuming three habitat spokes, there will be an additional 86 ft^2 shared area that low G restricted crew can visit. High G permitted crew can visit up to 172 ft^2 of additional shared area. The G restriction is imposed by experiment constraints for crew observations. Some missions may have no Crew G restrictions.

In addition to the habitat enclose spaces and area. Crew have access to the central passage ways of the habitat spokes as well as the micro G space of the habitat hub. This amounts to an additional volume of  5600 ft^3 per habitat spoke and a projected 3950 ft^3 central habitat hub volume. These volumes are reduced by the volume of piping, ladders, landing gratings, hatches, and habitat hub mechanical equipment. This still leaves a substantial volume to be utilized by a crew experiencing cabin fever.

In addition to the pressurized space and floor areas of the habitat wheel, the two angular transfer air locks can be used to access the pressurized space within the drive sections (north and south). These air locks have the ability to spin and can match the spin rate of either the habitat wheel or the drive section. Crew and cargo can transition between the pressurized drive space and the habitat wheel space as necessary. The drive sections have reduced radiation shielding and more hazards so the drive section should only be used as a maintenance access, docked ship access, or EVA airlock access only. Loiter time in the Drive sections are to be kept to a minimum.

The air pressure within the pressurized section will be kept as low as possible to reduce gas leakage rates. The O2 concentration is adjusted to maintain optimal O2 partial pressure for healthy respiration. One objective of FOLDINGSS is to determine the idea atmosphere for long duration missions.


Habitat Power Modules

Each habitat spoke has a power module which contains battery storage and possibly fuel cell support for hydrogen storage of power. The power module has solar array wings, but the area and shape of these wings are limited. They cannot extend too far to the north or south as they will contact the solar collection arrays of the drive sections. The more the shape extends along the spoke, the more that the arrays will shade the arrays of the other spokes as they rotate. Each spoke will likely have between 20KW and 40KW of power. This places the total power capacity for the habitat wheel between 60 and 120KW This level of power is also just on the border of sufficient.

Another set of solar arrays could potentially be added in the perimeter power modules, however these will contribute to the shading problem also. The state of the art with respect to solar collectors may increase which will help to mitigate some of these issues. Also, a tradeoff between can be made between propulsive power use and habitat power use.

As the FOLDINGSS research spacecraft progresses further from the sun, the available power drops as well. Thus, for destination much further than Mars, alternative power sources will likely be necessary. Nuclear is the obvious choice. two small reactors could potentially replace the drive section arrays and provide ample power for both the propulsive and habitat functions. 


Habitat Interconnect Methods

The CBM (common berthing mechanism) is used extensively. The general rule is that modules with the highest level of crew occupation contain the CBMa half of the berthing pair, while modules which are only passage ways or utility modules typically have the CBMp half of the berthing pair. According to this rule, each habitat envelope has a CBMa at each end. The habitat hub has CBMp connections for each spoke and at the distant end of each angular transfer structure.

The habitat power modules have two CBMp ports and the habitat perimeter modules have a single CBMp port. The drive section have all CBMa port halves. The universal docking adapter has a CBMp. There are a few situations in which CBMp modules need to connect temporarily. For this purpose there will be CBM connecting rings that have two CBMa port halves on either side of the ring. These devices have a built in power source and provide control access for the ring externally and internally. Thus the connection can be established or released externally by EVA or internally through one of the hatches.

In addition to the CBM interconnect. There is provision for rigid cross bracing between habitat spokes. Attachment points are provided on habitat power modules and habitat perimeter modules. The exact specification for the interconnect is not decided at this point, but it must be transportable in a 14m fairing, be able to be deployed by simple EVAs and be rigid. Very slight tension pressure is used on the bracing, such that minimum torque would be applied to CBM joints in the event of a brace failure.   


Gas Recycling

Each habitat envelope has its own gas recycling system. At this point, it is assumed that these systems will use Sabatier based processes involving heat, pressure, and other gases reduce CO2 to CH3 for storage as fuel. Water can act as an O2 and H2 source. The CH3 can be burned to increase CO2 as necessary for horticulture. The system is designed to be automatic and not require crew intervention unless their is a problem.

Some basic ventilation functions are provided within a habitat envelop. Supplemental ventilation can also occur within an entire spoke pressurized area, but the habitat hub has its own gas control system.

Gas recycling within the habitat hub and drive sections is controlled with typical lithium based filtering and compressed component gases. This system can be refreshed by exchanging air and filters with habitat hubs which use more sophisticated methods to recycle breathable gases.


Water Recycling

Water recycling is closely connected with gas and waster recycling. Sabatier based processes are used to extract gases from waste and waste water. The remaining solid waste is transported to the perimeter modules where it may be further processed or stored.

Each habitat envelop has a water recycling system and waste recycling system.

Humidity is also controlled within the habitat areas.


Habitat Rotation (RCS) Engines

These are small limited thrust ION engines. These speed or slow the rotation of the habitat wheel. The engines are physically the most distant from the hub of the habitat structures. These thrusters can be accessed be exiting airlocks in the perimeter modules. There are gratings for EVA personnel to stand upon and presumable these persons will be influenced by spin gravity and held against these gratings. There are access panels in the grating that allow EVA personnel to reach the thrusters. The thrusters pivot individually for service so that most activities can be performed while EVA personnel are on the gratings.


Habitat Balance Control

This system uses accelerometers located in the central habitat hub to command valves and pumps located in the habitat perimeter modules to transfer ballast water as required. This system attempts to keep the habitat wheel rotating in a balanced manner with the center of gravity dead center of the habitat hub.

This system is aware of the acceleration produced by the drive section engines. Thus, even under thrust, the habitat will rotate as smoothly as possible.

The system also provides information to the habitat RCS system and my cause the firing of habitat ION engines to alter rotation rate or the rotation plane.


Habitat Perimeter Modules

The habitat perimeter modules provide many functions. A short list is given below:

  1. It has additional solar energy collection with sun following drive and control.
  2. It has additional energy storage as batteries or fuel cells.
  3. It has ION drives for speeding or slowing the habitat wheel and lateral ION drives for changing the habitat's plane of rotation. This RCS system is controlled from the habitat hub accelerometer sensors and balance control system.
  4. If has ION propellant storage
  5. It has a good sized airlock for access to the ION RCS drives. The airlocks are also used during initial assembly and provide emergency egress in case passage through the spoke to the hub becomes impossible.
  6. It has a bilge and control.
  7. It supports all other habitat functions as necessary for the spoke. This includes water and gases storage. Additional recycle processing and additional waste products processing.
  8. It provides attachment point for any cross bracing within the habitat wheel.


Drive Requirements

The so called Drive segments of the FOLDINGSS perform seven main functions to assist the research space craft in its mission.

  1. Provide a docking point for visiting spacecraft carrying crew and supplies. The Drive section maintains a very slowly changing orientation in space. Therefore, visiting craft can easily approach and dock to facilities on the drive segments, unlike the habitat section who's rotation causes rapidly changing orientation.
  2. Provide a micro gravity airlock for easy ingress and egress of crew during EVA. Each drive section is equipped with two airlocks. Both of these airlock support either conventional balloon pressure EVA suits or mechanical pressure EVA suits.
  3. Provide mechanical robot arms for transferring capsules, assemblies, or crew to and from various places on the exterior of the ship. Two arms on each drive section can reach any point on the drive section exterior and potentially transfer objects to the robotic arms on the habitat section. These arms can also be used for capture of spacecraft and parts for berthing to the many CBMa ports on the exterior of the drive units.
  4. Provide a fixed orientation for thrusters. These thrusters move the FOLDINGSS research craft between orbits and potentially to lunar or Martian orbits. The thrusters are low thrust, thus they must be active for long periods of time, especially the ion thrusters. The drive section facilitates accurate thrusting by providing a stable but point able orientation in space.
  5. The ION engines require a lot of electrical power. The solar collectors on the drive section provide this power and can maintain their orientation toward the sun at all times. This power is used almost exclusively by the engines of the drive section, but may supplement power to the habitat section via slip rings on the main habitat bearings.
  6. The drive section provides a consistent and stable orientation for directional communications equipment and other space science instruments that need stable pointing.
  7. Provides pressurized maintenance access to docked engines, spacecraft, and instruments through the CBM hatches. The CBM ports provide the main mechanical and structural connection between the docked apparatus. In addition to this mechanical connection, the CBM ports can provide a pressurized connection to the docked apparatus if the apparatus has been constructed to provide a sealed access. This can allow for propellant and power connections inside the pressure envelope.

Consequently the requirements are a pressurized module that will fit inside a standard fairing. The module requires a movable attachment for both PV array and robotic arms. The module needs an airlock, independent power system, independent gas system, independent cooling and heating, propellant routing, power routing, data interfaces.


Drive Envelope

The enclosure must be rigid to support thrust forces and docking forces. An aluminum enclosure of an extruded 4m square with a cylindrical outer skin of structural thickness. The module must have a CBMa to attach to the habitat's angular transfer air lock structure. The length of the module cannot exceed 14m which suggests 2 or 3 CBMa ports along each of the four sides. These ports can be used for supplies, propellant tanks, cryogenic fuel/oxidizer tanks, landing craft, chemical thrusters (relatively high thrust), ION thruster arrays, or other apparatus of appropriate size equipped with CBMp ports.

In this way, the envelope is very similar to many ISS modules. Relatively small engineering efforts are required to design and fabricate the drive envelopes.

One very useful capability of the drive envelopes would be the ability to purge atmosphere. Since it is very likely that fuel lines or oxidizer lines will be connected or disconnected within the pressurized space of the module, filling with an N2 only atmosphere might be a nice safety precaution. Of course, any deviation from standard breathing gasses will require crew breathing support, either self contained or tethered.

It is very likely the north and south drive sections can be constructed identically. One end of the cylindrical shape has a mount for the solar array and the CBMa intended to mate with the habitat hub. The other end of the cylindrical shape has a mount for both the solar array and two robotic arms. this end of the modules has a CBMa that typically docks with a universal docking adapter. Visiting crew capsules will dock with this docking adapter under most cicumstances.


Drive Interconnect

As mentioned above, the CBM port standard is used extensively for the drive sections. The most crucial of these is used to connect the drive section to the habitat hub. This connects the habitat's angular transfer air lock structure CBMp to the drive sections CBMa. This mating needs to handle all the torque created by any engines pushing the entire craft. The power of these engines is intended to be low while their sustained burn time is assumed to be high. Even the relatively high impulse chemical burns will result in a ship acceleration of only only 0.1 m/s/s. However, given the mass of the habitat wheel, this is a large sheering force applied to this CBM. The mass of the wheel times the max acceleration divided by two as there are two drive sections yields the force exerted on each of these particular CBM interfaces.

The CBM (Common Berthing Mechanism) is a standard developed in the 1970s for interconnecting and sealing pressurized modules. The main mechanical connection is formed by a large (appox 2m) metal ring on either module that is to connect. One side is called the active side (CBMa) and the other is called the passive side (CBMp). The passive side has holes for 16 bolts with captured mating nuts on the back side ( interior ) of the metal interface ring. The active side has a corresponding 16 threaded bolts with motorized drives and torque sensors. There are also 4 "wings" designed to help guide the two parts together during the berthing process.

The center of the large interface rings has a pressure tight plate with a rounded corner square passage with mating hatch. Both of the mating modules typically have this hatch construction. On the outside of the passage there may be additional connections for electrical, gas, or fluid pass through the interface. The specifics of these other connections is determined by the actual devices involved.

There are no permanently installed engines or thrusters on the drive section. These attach via CBM pairs on the "sides" of the module. This allows for greater flexibility in terms of configuration, service, and tanking. RCS control is likewise accomplished through apparatus attached by CBM pairs. RCS can be either hydrazine based or  H2O2 based depending upon the level of bio hazard to be accepted. No bio hazardous fuels are to have access to the interior of the drive sections as decontamination of a spill would be outside the capabilities of the mission.

There may be an instrument platform of unspecified design that can affix to the solar panel truss of the drive section. This would be in addition to steer able high gain RF antennae. By mounting these items to the solar array truss, crew can be reasonably be assured of an unobstructed line of site to most destinations (Earth) at all times.


Drive Orientation

The Drive sections have the ability to pivot around the central axis of habitat wheel. There are only a few orientations of the drive sections that make sense. The habitat wheel controls the angular velocity of the habitat through the use of ion thrusters. The orientation of the drive section is accomplished by controlling electric motors pushing against the habitat section. The most common orientations for the Drive sections are thrusting in the direction of travel of the FOLDINGSS spacecraft or thrusting against the direction of travel of the spacecraft. Either one of these orientations is maintained by running the electric motors for drive orientation to achieve exactly the same RPM of the habitat wheel. This essentially holds the drive section at a consistent orientation relative to space. To save on main bearing wear, the drive section can be allowed to rotate at the same rate as the habitat wheel. Obviously this condition only has value when the drive thrusters are inactive. Holding the drive orientation motors still at zero RPM causes the drive section to rotate in tandem with the habitat wheel.

The control of the drive motors requires careful speed adjustment. A position feedback system is integrated into the main bearings of the habitat section. This sensing system must be robust and redundant as its trustworthy operation is essential for orbital translations.


Drive access to Habitat

A construction called the, "angular transfer air lock" is located on either side (north and south) of the habitat hub. This structure has at its center, a cylindrical enclosure with sealing hatches and docking hardware. This enclosure which we call the, "transfer barrel", is mounted such that it can roll on its axis. This lets the transfer barrel match the spin rate of either the drive section or the habitat section. The barrel can also move in line with its axis to contact and dock with the drive section, or move the other direction to contact and dock with the habitat section.

The external structure that houses the transfer barrel is a robust structural element that bears the thrust of the drive section and transfers force to the habitat wheel. This structure directly connects to the main Habitat Hub bearing and on the other side connects with the CBMp that connects to the drive section. Assuming a maximum thrust per drive section of  40N or 9lbf, this structure must be built to withstand the sheer forces and torques that result. Unexpected impacts will also apply resultant forces on this structure. Given the mass of the habitat wheel, projections of the types of forces impacts may produce need to be made to determine what level of risk is acceptable for the strength of this structure.

The transfer barrel is a self contained air lock system. Using extensive safe guards, crew can transfer between the drive section and habitat section without the need for EVA suits. The barrel may also be constructed such that it can dock with both the habitat and drive sections when they are spinning at identical rates. This ability is not absolutely necessary but might be a huge time saver and simplify the process of moving provisions from a supply capsule to the habitat envelopes.

The barrel will have its own battery based power system, its own gas supplies for pressurization, and emergency breathing supplies. It will also have a control interface for control of the barrel and the docking processes.


Drive Engines

The Drive sections have no built in engines. It is assumed that different engine configurations can be attached via CBM ports to drive-side of the section. These engines can be chemical thrusters ( Impulse Power less than 300 N ) used for higher impulse maneuvers to create elliptical orbits. These engines will almost certainly be solar electric ION thrusters as well. ION drives may be used like the University of Michigan's X3, for example. If two were used on each drive unit and solar power collection of 200 KW was achieved on both drive sections. It would be possible to sustain a thrust of 3.8 N for a long period of time. Assuming this level of power could be maintained for 6 months, and assuming the FOLDINGSS craft has a mass near 500,000 lbs, delta-v in the range of 5000 m/s will be possible. This would be sufficient to propel the FOLDINGSS from LEO to a Mars transfer orbit just using ION drives alone. More refinement of the actual masses involved and delta-V requirements needs to be performed, however, this quick analysis suggests the technology already exists or nearly exists to enable FOLDINGSS to reach and return from Mars. Given it's initial target is just a 1,000,000 mile Earth orbit, the technology for propulsion for FOLDINGSS is already available.

The likely propellant for a these types of journeys would be Argon or Iodine. Xenon is too rare, too hard to obtain, and too expensive for this quantity of propellant. A six month burn of an X3 style ION thruster would require on the order of 4400 lbs of propellant (assumes 3000s specific impulse and a thrust of 3.8N sustained for 180 days) So, four thrusters requires 17600 lbs of propellant.


Drive Power System

The primary consumer of power in the drive section is the electric propulsion ION thrusters. Using ISS collection panels as a reference, a 20KW panel requires 1260 ft^2. Scaling this to the 200KW range, a 12,600 ft^2 array is required. This may not be reasonable for the spacecraft. Advancement in solar collection may shrink this area. Multi-junction solar cells, new materials and new coatings may reduce the space required. Given that the size of a drive section is limited to 13 meters ( 42 feet ), solar array wings would need to extend 157 feet in either direction from the center of the drive section. This dimension is greater than the radius of the habitat wheel.


FOLDINGSS Conceptualizations (preliminary to final)

The FOLDINGSS concept has evolved some. Here is a small collection of drawings and writings on the subject.

Early "napkin" drawings

FOLDINGSS research spacecraft concept as viewed from the ecliptic. North is on the left.
CREDIT: Glenn Clark

FOLDINGSS research spacecraft as viewed from the north. The drive section is not pictured. The habitat robotic arms are also omitted. The angular transfer device is only hinted at, by the smaller circle shown on the central habitat hubs. In these renderings, the habitat power unit is only shown as an attachment point for cross bracing. Likewise the perimeter unit is only shown as an attachment point for bracing.
CREDIT: Glenn Clark

This sketch was the first concept for partitioning the Bigelow B330 inflatable habitat envelopes into floors. The radius of the lower floor was also significant in this sketch, as it determines the rotation rate of the entire habitat wheel. Even though the pressurized space inside the FOLDINGSS craft is expansive by current spacecraft standards, it only affords crew a shared space equivalent to a 19' x 19'  room and a private bedroom equivalent in area to a 9' by 9' room. A small office space is afforded each crew member and 2 crew share a galley which is also equivalent to a 9'x9' room. This is the primary space allotted to each crew member for a 900 day mission.
CREDIT: Glenn Clark


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