The section of the spacecraft that the crew lives and works in is called the Habitat Module (Larry Niven calls it a "Lifesystem"). It is pressurized with a breathable atmosphere, and protects the crew from extremes of temperature and from radiation. Unlike spacecraft in TV and movies, most of a spacecraft is not pressurized. The vast majority of the ship is composed of the propellant tanks, rocket engine, and power plant; all exposed to airless space. The pressurized habitat module is sort of tucked into some convenient corner. Remember rockets are not hotels.
Because every cubic meter of habitat module has to be pressurized and protected from the space environment, interior volume will be at a premium. Due to mass constraints, spacecraft designers will have no choice but to minimize the volume. Which will of course make them very cramped.
A useful document when designing these things is Human Integration Design Handbook. This include info on the minimum volume needed for such tasks as exercise and hygiene, habitability functions, architecture, hardware, crew interfaces, and EVA.
But don't make them too cramped or the crew will start suffering psychotic breaks and go berserk.
Upper end models will have a with Closed Ecological Life Support System, cheaper ones will have life support that requires periodic resupply of food, water, and air.
The various NASA studied for Human Outer Planet Exploration designs use NASA's TransHab habitat module. This is because TransHab came from a NASA study, and the module is easy to add to a spacecraft design. Just plug and play.
It is possible to make a habitat module that collapses like a balloon for storage purposes (Inflatable space habitats). NASA's Transhab was designed to temporarily reduce the diameter of the module, since surface-to-orbit booster rockets have all sorts of problems if the payload has a larger diameter than the rocket. Inflatable structures also tend to have a lower mass than a same volume conventional structure, and Every Gram Counts.
Such inflatable hab mods can also come in handy if one was, for instance, transporting a space station to be established in a remote location. Or little space igloos to sell to mom-and-pop asteroid miners. Inflatable hab mods are also central to the Spacecoach concept.
While it sound incredibly dangerous to trust your life to something that will pop at the touch of a pin, it isn't really. A mere pin isn't going to do anything to these modules. Most such modules are constructed from Kevlar or other bullet-proof material. The walls are probably much stronger than that pathetic aluminium foil the International Space Station uses for its hull.
Bigelow Aerospace smelled a business opportunity. It purchased the rights to NASA's Transhab technology, and are busy prototyping pre-fab instant space stations. These Bigelow modules will be incredibly affordable ($100 million dollars each), have a low mass to reduce boost cost, and will collapse small enough to fit the radius of most boosters.
As a test-bed, Bigelow created the Bigelow Expandable Activity Module (BEAM). It was installed on the International Space Station on April 16 2016 where it will undergo testing for various design objectives. The walls are constructed of a Bigelow-patented Kevlar-like material, whose multiple layers of flexible fabric and closed-cell vinyl polymer foam will act like an anti-meteorite Whipple shield.
The TransHab concept was a NASA project to create an inflatable space station, which is not quite as insane as one would think. The walls include layers of Kevlar, and are probably harder to puncture than the metal walls of the International Space Station. The private company Bigelow Aerospace has purchased the rights to TransHab patents, and is in the process of developing a commercial space station. Bigelow already has launched two prototypes into orbit and they are working just fine.
The standard TransHab module had a mass of 34,050 kilograms (34 metric tons), an inflated volume of 350 cubic meters, an inflated diameter of 8.2 meters, four levels, and could support a crew of six for about eighteen months.
In the Mars Reference Mission, they had a bimodal nuclear thermal rocket on a Mars mission. The rocket could deliver the mission to Mars, come back to approach Earth but with dry propellant tanks. So the rocket would go sailing past Earth into the abyss while the crew bailed out to be rescued. Bye-bye rocket.
However, if you replaced the relatively massive hard-shell habitat module with a lightweight inflatable TransHab module, the increase in delta-V was enough so that the rocket would have enough extra delta-V to be able to brake into Earth orbit and be re-used.
As you look through the various Realistic Design pages, you will be struck with how many designs use a TranHab. Designers figure it is "off-the-shelf" technology. Certainly Bigelow Aerospace is doing its best to make it so.
There is an online calculator for TransHab modules here.
Troy Campbell pointed me at a fascinating NASA report about spacecraft design. The report shows how much easier it is to design a habitat module if it for a one gravity environment instead of free fall (surprise, surprise). It has the spacecraft separate into two parts connected by tethers, spinning for artificial gravity.
Some of the details of this design cannot be used with, say, a warship. You do not want to used an inflatable habitat module on a ship going into battle. But the lists of required equipment are very useful for your ship designs, as are their masses, volumes, and power requirements.
For its habitat module, the report take a TransHab inflatable habitat module, and modifies it for one g worth of spin gravity. TransHab modules are low mass since the walls are made of woven Kevlar instead of metal. For the report design, interior suspension cables are added to support the decks (since the basic TransHab is designed for free fall), and an anti-radiation storm cellar added to the core. The other main reason for using a TransHab is because the proposed launch vehicles used to boost the module into orbit had severe payload size limits. The TransHab could fit into the limits while collapsed, then inflated to full size when in space. For your design, you probably will not have such payload size limits, so you will not need to use an inflatable habitat.
To cool off the module, a small heat radiator is wrapped around the exterior. This radiator can only collect and reject 15 kilowatts of heat, since it is only for life support. The propulsion system and power system will require a much larger radiator (read the report for more details).
The report gave a sample set of deck plans. The first floor is the lowest, at the 1.03g level. For some odd reason the first floor deck plan is rotated 45 degrees counterclockwise with respect to the other two deck plans, as you can see if you try to match up the ladder and pass throughs on the three plans.
Note how all the crew beds are inside the storm cellar.
The module is designed to house a crew of six for eighteen months. According to the report, the bare minimum internal volume for a crew of six is 101 cubic meters (about 17 m3 per crewperson). This design has more than that. The TransHab has 350 cubic meters of internal volume, and of that 193 cubic meters is habitable (about 32 m3 per crewperson). Please note that this is the total habitable volume, the crew's personal volume is much smaller (basically their bunk and their desk).
The module has an exterior surface area of 233 m2. Just the cylindrical exterior surface has an area of 153 m2.
Again remember that this is for a crew of six and an endurance of eighteen months. The values for mass and volume of all the components will have to be scaled up or down with the size of the crew and the amount of endurance. The air and water are recycled, the main endurance consumable is food. Maybe a bit for space clothing and whatnot.
By simple division, a rough rule of thumb is a TransHab will require 4,606.2 kilograms of structure and equipment per person plus 2.303 kilogram per person-day of food. This is a very rough rule of thumb, but it should get you in the ballpark.
THSM = 4,606.2 * numCrew
THFM = 2.303 * numCrew * numDays
THM = THSM + THFM
THSM: TransHab Structure and Equipment Mass (kg)
THFM: TransHab food mass (kg)
THM: Total TransHab Mass (kg)
numCrew: number of crew members
numDays: number of days in the mission, the TransHab "endurance"
Example: This TransHab is for six people, so the structure/equipment mass is 4,606.2 × 6 = 27,637 kg.
The mission is for 18 months (540 days). 6 persons × 540 days = 3,240 person-days.
2.303 kilograms × 3,240 = 7,462 kg food to feed 6 people for 18 months.
27,637 kg + 7,462 kg = 35,099 kg total which is close enough for government work to 35,097 kg value found below.
|System||Mass (kg)||Stowed Vol. (m3)|
|Power Management and Distribution||625||1.05|
|Displays & Controls||14||0.01|
|ENVIRONMENTAL CONTROL & LIFE SUPPORT||5,030||31.50|
|Temperature and Humidity Control||113||6.32|
|Fire Detection and Suppression||13||0.05|
|Water Recovery and Management||2199||6.02|
|THERMAL CONTROL SYSTEM||576||2.43|
|Internal Thermal Control System||135||0.34|
|External Thermal Control System||167||0.13|
|Galley and Food System||8063|
7,460 is food
|Waste Collection System||327||8.83|
|Recreational Equipment and Personal Stowage||150||3.00|
|Operational Supplies and Restraints||120||0.01|
|Vehicle Support for EVA||291||0.40|
|EVA Translation Aids||123||3.36|
|STRUCTURE AND MECHANISM||12,941||84.51|
|Human Research Facility||289||2.50|
|Crew Health Care Systems||759||3.67|
From the report (which goes into this in much greater detail):
This hair-raising concept is from Earth Return Aerocapture for the TransHab/Ellipsled Vehicle
Aerocapture is a powerful technique for drastically reducing the required delta V and propellant load for your spacecraft, which is so important for those delta-V challenged chemical rockets. Just send your spacecraft on a hot ride through the planet's atmosphere and you can literally burn off enough delta-V for free. Assuming your heat shield lasts long enough.
However, doing that with a TransHab perched on your spacecraft's nose seems like the height of insanity. Sort of like waving an oxyacetylene torch over a birthday balloon. One large POP! and lots of sad astronaut corpses burning up in reentry.
But the handsome propellant savings from aerocapture are so alluring that some NASA researchers did a study. This would be incredibly useful, especially if you want to reuse the spacecraft for several Mars missions or something. They came up with an aeroshell covering the impact side of the TransHab, dubbed the "Ellipsled" due to its shape.
The Ellipsled has a mass of 3929 kg, while the TransHab in the study was assumed to be 14,522 kg. They also assumed the rest of the vehicle had a mass of 7053 kg, presumably most of the propellant had been already burnt. Total of 25,500 kg. I am unsure from reading the report but I get the impression this represents the TransHab detaching from the rest of the spacecraft, carrying along only the ellipsled and a small rocket engine for change-of-plane maneuvers. Meaning the rest of the spacecraft goes sailing off into the wild black yonder as the TransHab aerocaptures into Terran orbit. The report mentions the TransHab being mated to a new spacecraft for a new Mars mission.
The thing has a lift-to-drag ratio of 0.39 at Mach 24 and above, which is better than the Apollo capsule's 0.3. So it is slightly more maneuverable. The deceleration limit is 5.0 g.
They were aiming for something that could approach Terra at the end of the Mars mission and aerocapture into a 420 kilometer orbit by burning off about 4.3 km/s of velocity. If one wanted to get fancy, it is much easier to brake into a high elliptical parking orbit with periapsis of 407 km and an apoapsis of 120,000 km. Yes it makes it more difficult to refurbuish and reequip the ship for a new mission, but it really cuts down on the required trans-Mars injection delta V.
|Mass (kg)||Stowed Vol. (m3)||Quantity|
|Fiber Li-Ion Battery||0.17||335||1|
|Battery Charge/Discharge Unit||0.09||50||3|
|Main Bus Cable||0.84||7.5||3|
|Secondary Power Distribution|
|Wiring Harness Secondary|
|Power Management and Distribution|
|Galaxy Inverter Boxes||0.04||28||3|
|Custom Built 400 Hz, 115 Vac|
|Unitron PS-95-448-1 400 Hz|
to 60 Hz Frequency Converter
|Vikor AC/DC Rectifiers||0.0007||2||9|
The primary power system for the spacecraft is a pair of nuclear reactors on the other end of the boom. Since they are external to the habitat module, their mass and volume are not included here.
The secondary power system is internal to the module. It consists of three main subsystems:
- Secondary Power
- Power Management and Distribution
These three subsystems can be further broken down to the component level as shown in the table to the right.
The assumption was made that the power entering the habitat would be 115 Vac, delivered at 400 Hz. A final assumption that was made was that the habitat would nominally use 15 kW of power. The final subsystem that needed to be sized for this habitat was the secondary power source. Upon analyzing the architecture and the type of primary power sources, a decision was made to supply 24 hours of emergency power to the habitat that will accommodate 50% of the nominal load (180 kW-h).
Includes a communication system; a guidance, navigation and control system; a crew interface system; and an integrated vehicle management system. It has a peak power consumption of 864 watts. It provides for the command, control, communications, and computation required for the carrying out the mission including insertion into transit orbits. This involves provisions for crew displays; data, voice, and video communications home base, other orbital assets, and EVA crewmembers; an integrated health management system for onboard and ground monitoring of all systems; and a full flight system capability for Guidance, Navigation, and Control. The flight system must also integrate requirements for data communication and computational support for remote commanding of the spacecraft during any uncrewed phase as well as ground commanding during crewed phases. The crew interface must be integrated with data communications and computational support for remote commanding of visiting vehicles.
Air Management Subsystem
This system recycles the oxygen, plucking it out of the carbon dioxide molecules and returning it to the atmosphere to be breathed once again.
First the stale air is pumped through a 4-Bed Molecular Sieve (217.7 kg, 0.6 m3, 733.9 W). It initially removes the water from the air (and sends it to be added to the life support water supply), then it removes the carbon dioxide.
The carbon dioxide and some hydrogen (from a source to be explained shortly) are fed into a Sabatier Reactor (26 kg, 0.01 m3, 227.4 W). They react producing methane and water: CO2 + 4 H2 → CH4 + 2 H2O + energy.
The methane is vented into space. The water is fed into an electrolyser to be split into hydrogen and oxygen. Specifically a Solid Polymer Electrolysis (SPE) Oxygen Generation Subsystem (OGS) (501 kg, 2.36 m3, 2004 W).
The hydrogen is sent back to the Sabatier Reactor to take care of the next batch of carbon dioxide. The oxygen is added to the breathing mix and released into the habitat module's atmosphere.
The TransHab starts out with a tank of high pressure oxygen (20.4 kg, 0.78 m3, 6W, 30 MPa) and a tank of high pressure nitrogen (94.4 kg, 3.6 m3, 6W, 30 MPa). The oxygen tank has three days worth of breathing for six crew, enough to give the Sabatier Reactor time to get started. The nitrogen tank has enough to establish the proper ratio for the breathing mix, and some extra to compensate for any atmosphere leaking into space.
Water Management Subsystem
An aluminum potable water storage tank (145.9 kg, 0.54 m3, 5 W) initially contains a three day supply of water for the six crew members. Waste water is sent through a Vapor Phase Catalytic Ammonia Removal (VPCAR) system (1119 kg, 5.5 m3, 6090.7 W). The VPCAR process is a wastewater treatment technology that combines distillation with high-temperature catalytic oxidation of volatile impurities such as ammonia and organic compounds.
The report mentioned that the VPCAR system was selected over a rival system since it had a lower mass, volume, and turnaround time. The VPCAR's drawback was the larger power requirements.
Waste Management Subsystem
The Waste Management Subsystem uses a Warm Air Dryer (527.2 kg, 11.2 m3, 2,043.7 W). This dries out the crew's fecal matter, reclaiming the water. The dried residue is discarded.
|Fluid mass (kg)||Dry mass (kg)||Volume (m3)||Power (kw)|
The TCS system concept makes use of flexible lightweight body mounted radiators, which are attached to the outer surface. The TCS system has been sized to collect and reject 15.0 kW of heat. Mass, power, and volume are listed below. ITCS refers to coldplates, heat exchangers, and plumbing located inside Transhab, while ETCS refers to similar equipment mounted on the outside. Radiators are listed separately.
A propylene glycol/water coolant is circulated inside the module to collect heat from heat exchangers and coldplates and this heat is rejected to space through the body mounted radiators mounted on the outer shell of the module. Radiator size was determined for the warmest case (0.5 A.U. orbit). The results indicate a required area of 78 m2. This represents 51% of the available area of the cylindrical portion of the shell.
Two other sizing exercises were also conducted for the module. The first determined the radiator area needed to reject twice the average load of 15 kW. Assuming the warmest environment temperature at 0.5 A.U., the analysis indicated approximately 157 m2 was required. This is just slightly over the total cylindrical area of the shell of 153 m2, therefore rejecting just under 30 kw on average is the maximum amount of heat rejection possible without adding something like a heat pump to raise the radiator temperature.
Another sizing exercise determined the heat rejection given the following scenario: The module is in Mars orbit and the crew has left the module for the Martian surface leaving the AG module uninhabited. If the heat loads are reduced and the TCS fluid is allowed to approach its freezing temperature of -50°C, the question becomes how much heat can be rejected. The analysis indicated that the radiators could still reject up to 11 kW of heat with the TCS fluid just above its freezing temperature. This is in part due to the much colder environment at the low Mars orbit assumed. At the 0.5 A.U. orbit location heat rejection would be approximately zero because the radiator and sink temperature would be identical for this scenario.
Propylene glycol was selected for the working fluid. The relevant options are water or 60% propylene glycol with 40% water or some other working fluid. While water is non-toxic and has greatest thermal capacity per mass of working fluid, it also freezes at 273.2 K and thus may not allow sufficient radiator availability for some mission phases. 60% propylene glycol with 40% water is also non-toxic but, compared to water, it is a less desirable thermal working fluid. However, 60% propylene glycol with 40% water freezes at roughly 223 K, a significant advantage over water. Thus, tentatively the working fluid for the thermal control fluid loops is 60% propylene glycol with 40% water. As above, complete resolution of this issue also requires in-depth thermal environment modeling focusing on radiant rejection from the habitat.
This provide crew accommodations systems and layout to make an 18-month mission habitable for six crewmembers. Functions covered include the following: crew support (meal preparation, eating, meal clean-up, full-body cleansing, hand/face cleansing, personal hygiene, human waste disposal, training, sleep, private recreation and leisure, small-group recreation and leisure, dressing/undressing, clothing maintenance), and operations (facilities for meetings and teleconferences, planning and scheduling, general housekeeping). It is also responsible for configuring work and personal stations such that traffic congestion are minimized. Work efficiency, space use, crew comfort, and convenience should be maximized.
The EVA system is designed to be used for three planned, two person EVA days per mission. The airlock will transfer two crewmembers per cycle. If full crew transfer is required in LEO, this system assumes all three EVAs are used to transfer crew out of the habitat. EVA days are sized to be 8 hrs, and are accomplished with a personal life support system (PLSS) that is sized for eight hours. The system includes a single flexible airlock with umbilical support and PLSS recharge system; no gas reclamation is planned due to the minimal number of EVAs (3). Two EVA tools boxes are provided. Translation aids are provided to aid crew transportation about the vehicle. EVA system spares are also provided.
Included in the airlock arrangement is a single flexible airlock that allows two persons to egress the AGH at one time. A staging area by the inside airlock door is included in the concept. This area provides volume to store all space suits as well as space suit spares and expendables. Provisions for donning, suit expendables recharge, and checkout are included as well. An unpressurized area by the outside airlock doors is included in the concept. It provides a place for EVA tool storage and allows handling of large objects.
EVA tools provided consist of two toolboxes containing mechanical, electrical, and storage/tie downs. The tools are stowed in the unpressurized area just outside the airlock. EVA system spares as needed to support the six suits and airlock suit recharge provisions are stowed in the AGH in the EVA staging area and remain stored there until needed.
|Unpressurized End cone||650|
|Pressurized End cone||800|
|Internal fixed structure||2,120|
|Internal deployable structure||1,870|
|Crew Quarters Radiation Insulation||1,500|
The structure and shell are to provide a safe habitat for the crew and the necessary space to store supplies and equipment to sustain them for the duration of the entire mission. The inflatable module design was chosen because it is the best means to effectively increase the habitable volume of a spacecraft while keeping the diameter of the core within acceptable payload size limits set by current launch vehicles. The airlock system is to provide the crew with the capability to perform extravehicular activities. It is to be located atop the habitat module, so as to allow the fully suited EVA astronauts to take advantage of a slightly lower gravitational pull.
The medical operation capabilities onboard the artificial gravity habitat during transit will provide medical contingencies to promote successful mission completion, crew health, safety, and optimal crew performance.
The potential medical contingencies that are to be addressed include those currently required for International Space Station and additional procedures unique to a continuously rotating spacecraft. Following the convention for classification of medical contingencies onboard ISS, the artificial gravy habitat will enable the practice of emergency medicine, environmental medicine, countermeasures or preventive medicine, rehabilitation, and dentistry. Emergency medical procedures will provide for Advanced Cardiac Life Support (ACLS), Basic Cardiac Life Support (BCLS), and trauma. Additionally, emergency medical contingencies may include shock, behavioral, compromised airway or breathing, drug overdose, and smoke inhalation. Environmental medicine will enable treatment for exposure to toxic and hazardous materials. Countermeasures/Preventive Medicine and Rehabilitation will enable countermeasures to prevent neurovestibular dysfunction resulting from the Coriolis effect induced by the rate of rotation of the spacecraft. Coriolis effects induced by rotation of the spacecraft develop within the neurovestibular system and impacts motor performance, behavior, and motion sickness. Exposure to partial gravity, 0.38G, may greatly impact musculoskeletal and cardiopulmonary systems. Dentistry onboard the artificial gravity habitat will enable basic cleaning, crown replacement and treatment of exposed pulp.
These are from NASA's Human Integration Design Handbook (warning: 42 megs!). As an aide to designing the architecture of a habitat module, they identified certain critical tasks and determined the minimum volume necessary for those tasks.
The NASA Space Tug is a modular design. And one of those module is the Crew Module (CM). This can give us a valuable example of what a bare-bones habitat/control module looks like. The information is from a North American Rockwell Corp. document entitled Pre-phase A Study for an Analysis of a Reusable Space Tug. Volume 4 - Spacecraft Concepts and Systems Design Final Report. The document is solid gold, but be warned it is 640 megs in size and over 600 pages long.
They did an analysis of crew modules which were 3.6, 4.5, and 6.7 meters in diameter (because those are 12, 15, and 22 feet respectively. 15 feet is compatible with the Space Shuttle cargo bay. 22 feet is compatible with the Saturn booster.). 3.6 m was far too cramped, unless they made it two decks tall. 6.7 m was too big to be economic, unless they stuck the contents of other modules into the crew module (which kind of defeats the entire "modularization" idea).
4.5 m was just right.
For purposes of analysis, they created designs for three different missions:
|Mission Type||Number of Crew||Mission Duration||Supply|
|Space||6||7 days||42 crew-days|
|Lunar Stay||4||28 days||112 crew-days|
|Rescue||12||1 day||12 crew-days|
For the 4.5m diameter 2.4m tall crew module, they determined the following mass breakdown:
|2.0||Body Structure||1,150 kg||1,150 kg|
|3.0||Induced Envir Prot||154 kg||154 kg|
|4.0||Lnch Recov & Dkg||218 kg||218 kg|
|8.0||Power Conv & Distr||23 kg||23 kg|
|9.0||Guidance & Navigation||86 kg||95 kg|
|11.0||Communication||136 kg||136 kg|
|12.0||Environmental Control||86 kg||89 kg|
|13.0||Growth Allowance||265 kg||274 kg|
|14.0||Personnel Provisions||743 kg||832 kg|
|15.0||Crew Sta Contrl & Pan||70 kg||70 kg|
|SUBTOTALS (DRY WEIGHT)||2,933 kg||3,038 kg|
|17.0||Personnel (90.7 kg each)||544 kg|
|18.0||Cargo, food, etc.||220 kg||735 kg|
|19.0||Ordnance N2 and TK||9 kg||9 kg|
|20.0||Ballast EVA||-||163 kg|
|SUBTOTALS (INERT WEIGHT)||3,706 kg||4,308 kg|
|EPS O2||234 kg||1,182 kg|
|EPS H2||20 kg||123 kg|
|TOTALS (GROSS WEIGHT)||3,960 kg||5,613 kg|
For a Space mission the Space Tug would probably have the crew module mounted on top of the spacecraft stack. There would be a docking port on the top, along with roof windows to assist the pilot with the rendezvous.
For a Lunar-Stay mission, the tug would probably have the crew module mounted on the the bottom of the spacecraft stack. The bottom position would give the pilot a much better view of the landing as opposed to being perched on top of tall spacecraft with no view of what the landing gear (L.G.) was landing on. The landing gear would also be attached to the strong crew module, instead of the aluminum foil thin walls of the propulsion module. The entire spacecraft would have a lower center of gravity, always a plus when trying to land. Once landed, the airlock door will exit only a meter or so above the surface, instead of tens of meters.
The drawback of course is the crew will have a ring-side seat if the space tug crashes. A short view, only until the mass of the rest of the spacecraft (on top of the crew module) accordions it flat like a beer can in a trash compactor. The exhaust nozzles of the propulsion module would be on swing-out engines aimed to fire off the the side instead of hosing the crew module with flaming death. This makes the engine more complex (more points of failure) with a bigger mass penalty.
The crew module is an aluminum honeycomb pressure vessel with a centrally located air lock. There is also an emergency egress hatch in the side wall of 0.91 meters in diameter. The module has a pressurized volume of 31.85 cubic meters.
The control station is a stand-up station similar to the Apollo lunar module. For docking there will be windows for the pilot locate on the roof. For landing there will be angled windows located on the sidewall, again much like the lunar module.
Workstations are chairs with tables. Above the workstation benches are storage cabinets for food preperation, environmental control equipment, and scientific equipment.
Fold-up bunks are provided on the side walls. When folded up, the crew module can be used in rescue mode, with space for twelve persons. Using fold up bunks is far superior to having a totally different design for a rescue crew module.
The airlock is 1.5 meters in diameter with a pressurized volume of 3.78 cubic meters.
Below are the blueprint for the Space Mission (six crew) and Lunar-Stay Mission (four crew) version. There really is not much difference between the two. Basically the Lunar-Stay Mission version has the extra two bunks removed and replaced by additional workstations.
The blueprints show a passive docking ring on the aft end of the crew module. This is the configuration when the crew module is located at the bottom of the spacecraft stack. When the crew module is located at the top, the docking ring will move to the roof, and may be replaced by an active neuter docking system.
Most of the diagrams here are from Integrated Manned Interplanetary Spacecraft Concept Definition. Volume 1 - Summary Final Report, with further data from Volume 4 - System Definition Final Report.