One thing I didn't address in part 2 was why the vessel provides the entire food supply for passengers rather than using some stored food. After all, ECLSS-style supplies for a trip to Mars are only about 1.3 tons per person while a complete food supply takes around 3 tons of equipment per person and a whole lot more power. The reason is safety. If the vessel arrives at Mars only to find that all the landers have been destroyed then the passengers can wait in orbit until the next return window (or as long as necessary) and get back to Earth safely.
At any rate, here are the concise results:
Habitat Module (2 per vessel)
| Height | Volume, m³ | Crew | Mass, t | (dry) | tons per |
| 6 meters | 8,380 | 97 | 1034 | 592 | 10.66 |
| 8 meters | 11,140 | 131 | 1301 | 794 | 9.93 |
| 12 meters | 16,660 | 196 | 1818 | 1180 | 9.28 |
Vehicle Configurations - Chemical
| Configuration | Crew | Power (kW) | Dry Mass | (Fueled) | Thrust (MN) |
| Short (6m) | 194 | 1,583 | 2,221 | 5,842 | 5.74 |
| Tall (8m) | 262 | 2,138 | 2,796 | 7,354 | 7.22 |
| Grande (12m) | 392 | 3,199 | 3,909 | 10,281 | 10.1 |
Vehicle Configurations - Ion
| Configuration | Crew | Power (MW) | Dry Mass | (Fueled) | Thrust (N) |
| Short (6m) | 194 | 61.11 | 2,306 | 3,114 | 3115 |
| Tall (8m) | 262 | 77.06 | 2,903 | 3,920 | 3921 |
| Grande (12m) | 392 | 107.95 | 4,059 | 5,480 | 5481 |
Details after the jump.
Habitat Section
For reasons of launch volume, an expandable hull is the simplest solution. This will be essentially a TransHab hull with a second restraint layer inside the airtight bladders. This second restraint layer will keep the hull in the shape of a cylinder and will provide support for floors and fixtures without requiring a bunch of complex penetrations. Each end of the module's rigid core will include a very large docking port (up to 6.4 meters diameter) with built-in electric motors to apply torque, an airlock to allow isolating the module and potentially a swivel fixture for transferring fluids. We already know the necessary deployed diameter, so we need to determine an appropriate height (which will be the width of each floor). I used 8 meters in the previous version, but let's look at the consequences of 6 and 12 meter versions.
I'm assuming that the minimum cabin space for a single passenger is about 2x4 meters. We could stack bunks three deep and pack people in like a submarine crew but this is supposed to be for average people. My last attempt used 17 square meters as a family cabin (2 to 4 people), so this measure is slightly more generous. Rather than specify how many cabins of each size I'll just assume that all cabins are 8 square meters and no less than 2 meters wide. Beyond that, I'm assuming (without any basis for the assumption) that the ship needs at least 5m³ of engineering space and 10m³ of storage space per person in addition to 40m³ of hydroponics space. These are pretty simplistic assumptions; engineering should be split between vessel requirements and life support requirements, and storage should be split between stores, spares and cargo. Access between levels is by ladder; there are no structural concerns with openings between levels so these can be wherever they are handy, even roll-out rungs covered by hatches.
As one might expect, larger habitats are slightly more efficient in terms of mass per passenger. This is because the water shield scales with surface area while capacity scales with volume, the classic square-cube law. (More precisely, since the radius is a fixed number we have to pay the cost of one circular wall of shielding regardless of the length of the habitat. The volume to surface area ratio is constant for additional lengths, so a longer habitat averages the sidewall cost over a larger volume.)
F̶r̶o̶m̶ ̶p̶a̶r̶t̶ ̶1̶ ̶o̶u̶r̶ ̶1̶2̶-̶m̶e̶t̶e̶r̶ ̶r̶o̶c̶k̶e̶t̶'̶s̶ ̶p̶a̶y̶l̶o̶a̶d̶ ̶w̶o̶u̶l̶d̶ ̶o̶n̶l̶y̶ ̶b̶e̶ ̶e̶n̶o̶u̶g̶h̶ ̶f̶o̶r̶ ̶t̶h̶e̶ ̶6̶-̶m̶e̶t̶e̶r̶ ̶v̶e̶r̶s̶i̶o̶n̶ ̶d̶r̶y̶ ̶w̶i̶t̶h̶ ̶3̶0̶%̶ ̶m̶a̶r̶g̶i̶n̶.̶ ̶T̶h̶e̶ ̶1̶5̶-̶m̶e̶t̶e̶r̶ ̶r̶o̶c̶k̶e̶t̶ ̶c̶o̶u̶l̶d̶ ̶h̶a̶n̶d̶l̶e̶ ̶t̶h̶e̶ ̶s̶a̶m̶e̶ ̶v̶e̶r̶s̶i̶o̶n̶ ̶f̶i̶l̶l̶e̶d̶ ̶w̶i̶t̶h̶ ̶w̶a̶t̶e̶r̶ ̶o̶r̶ ̶e̶i̶t̶h̶e̶r̶ ̶t̶h̶e̶ ̶8̶ ̶o̶r̶ ̶1̶2̶ ̶m̶e̶t̶e̶r̶ ̶v̶e̶r̶s̶i̶o̶n̶s̶ ̶d̶r̶y̶ ̶w̶i̶t̶h̶ ̶3̶0̶%̶ ̶m̶a̶r̶g̶i̶n̶.̶ {{Updated payload values have changed things pretty dramatically. The 12-meter rocket would be able to launch the 6-meter habitat dry but with only about 4% margin, not a sane proposition. The 15-meter rocket could handle the 6 or 8 meter habitats dry with adequate margins. The 12-meter habitat looks to be unattainable unless the hull and furnishings are launched separately and assembled in orbit.}}
Let's take a look at each of these sizes in detail. I'll provide data from my estimation sheet (with apologies for the wide tables) followed by a short write-up.
6 meter width
Allocation of volume:
| hydro: | 75 | kg/m³ | ||
| - volume | 3883.95 | m³ | 40.04 | per |
| - mass | 292 | tons | ||
| quarters: | 60 | kg/m³ | ||
| - volume | 2148.85 | m³ | 22.15 | per |
| - mass | 129 | tons | ||
| storage: | 40 | kg/m³ | ||
| - volume | 1184.7 | m³ | 12.21 | per |
| - mass | 48 | tons | ||
| public: | 60 | kg/m³ | ||
| - volume | 754.36 | m³ | 7.78 | per |
| - mass | 46 | tons | ||
| engineering: | 120 | kg/m³ | ||
| - volume | 499.89 | m³ | 5.15 | per |
| - mass | 60 | tons |
Mass estimates (in tons):
| water | 441.35 |
| hull | 8.44 |
| core | 8.44 |
| hydro | 292 |
| quarters | 129 |
| storage | 48 |
| public | 46 |
| engineering | 60 |
| total: | 1033.23 |
Level geometry:
| level | radius | circum. | cyl area | floor area | volume | gravity |
| 1 | 21.2 | 133.204 | 371.336 | 799.221 | 2228.018 | 37.93% |
| 2 | 18.2 | 114.354 | 314.788 | 686.124 | 1888.726 | 32.56% |
| 3 | 15.2 | 95.504 | 258.239 | 573.027 | 1549.433 | 27.20% |
| 4 | 12.2 | 76.655 | 201.690 | 459.929 | 1210.141 | 21.83% |
| 5 | 9.2 | 57.805 | 145.142 | 346.832 | 870.849 | 16.46% |
| 6 | 6.2 | 38.956 | 88.593 | 233.734 | 531.557 | 11.09% |
| 7 | 3.2 | 20.106 | 32.170 | 120.637 | 102.944 | 5.73% |
Level layout:
(The five categories are assigned a 'width' of each floor; this is not necessarily one long strip, just representative of how much volume is assigned to that purpose. For example, each habitat includes 1m of public space on level 1; this works out to 133m² of space for education, meals, exercise and assembly.)
| level | cabins: | hydro: | (hvol) | quarters: | (qvol) | storage: | (svol) | public: | (pvol) | Eng: | (evol) | (rows) | (width) | (depth) |
| 1 | 0 | 5 | 1856.68 | 0 | 0 | 0 | 0 | 1 | 371.34 | 0 | 0 | 0 | 0.00 | 0.00 |
| 2 | 53 | 1.75 | 550.88 | 3.75 | 1180.45 | 0 | 0 | 0.5 | 157.39 | 0 | 0 | 1 | 2.13 | 3.75 |
| 3 | 44 | 1.75 | 451.92 | 3.75 | 968.4 | 0 | 0 | 0.5 | 129.12 | 0 | 0 | 1 | 2.13 | 3.75 |
| 4 | 0 | 4 | 806.76 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 403.38 | 0 | 0.00 | 0.00 |
| 5 | 0 | 1.5 | 217.71 | 0 | 0 | 4.5 | 653.14 | 0 | 0 | 0 | 0 | 0 | 0.00 | 0.00 |
| 6 | 0 | 0 | 0 | 0 | 0 | 6 | 531.56 | 0 | 0 | 0 | 0 | 0 | 0.00 | 0.00 |
| 7 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 3 | 96.51 | 3 | 96.51 | 0 | 0.00 | 0.00 |
Description:
This 6-meter wide habitat module includes hydroponics and public spaces on level 1. Levels 2 and 3 each have one row of cabins, a walkway and hydroponics. Level 4 has more hydroponics and most of the engineering gear. Level 5 has the last of the hydroponics and about half of the storage. Level 6 has the other half of the storage space. Level 7 (the rigid core) is half engineering equipment and half microgravity recreation / storm shelter.
8 meter width
Allocation of volume:
| hydro: | 75 | kg/m³ | ||
| - volume | 5339.1 | m³ | 40.76 | per |
| - mass | 401 | tons | ||
| quarters: | 60 | kg/m³ | ||
| - volume | 2868.9 | m³ | 21.90 | per |
| - mass | 173 | tons | ||
| storage: | 40 | kg/m³ | ||
| - volume | 1491 | m³ | 11.38 | per |
| - mass | 60 | tons | ||
| public: | 60 | kg/m³ | ||
| - volume | 887.38 | m³ | 6.77 | per |
| - mass | 54 | tons | ||
| engineering: | 120 | kg/m³ | ||
| - volume | 709.25 | m³ | 5.41 | per |
| - mass | 86 | tons |
Mass estimates (in tons):
| water | 507.04 |
| hull | 9.69 |
| core | 9.69 |
| hydro | 401 |
| quarters | 173 |
| storage | 60 |
| public | 54 |
| engineering | 86 |
| total: | 1300.42 |
Level geometry:
| level | radius | circum. | cyl area | floor area | volume | gravity |
| 1 | 21.2 | 133.204 | 371.336 | 1065.628 | 2970.690 | 37.93% |
| 2 | 18.2 | 114.354 | 314.788 | 914.832 | 2518.301 | 32.56% |
| 3 | 15.2 | 95.504 | 258.239 | 764.035 | 2065.911 | 27.20% |
| 4 | 12.2 | 76.655 | 201.690 | 613.239 | 1613.522 | 21.83% |
| 5 | 9.2 | 57.805 | 145.142 | 462.442 | 1161.133 | 16.46% |
| 6 | 6.2 | 38.956 | 88.593 | 311.646 | 708.743 | 11.09% |
| 7 | 3.2 | 20.106 | 32.170 | 160.850 | 102.944 | 5.73% |
Level layout:
| level | cabins: | hydro: | (hvol) | quarters: | (qvol) | storage: | (svol) | public: | (pvol) | Eng: | (evol) | (rows) | (width) | (depth) |
| 1 | 0 | 7 | 2599.35 | 0 | 0 | 0 | 0 | 1 | 371.34 | 0 | 0 | 0 | 0.00 | 0.00 |
| 2 | 57 | 3.5 | 1101.76 | 4 | 1259.15 | 0 | 0 | 0.5 | 157.39 | 0 | 0 | 1 | 2.00 | 4.00 |
| 3 | 41 | 4 | 1032.96 | 3.5 | 903.84 | 0 | 0 | 0.5 | 129.12 | 0 | 0 | 1 | 2.29 | 3.50 |
| 4 | 33 | 3 | 605.07 | 3.5 | 705.92 | 1 | 201.69 | 0.5 | 100.85 | 0 | 0 | 1 | 2.29 | 3.50 |
| 5 | 0 | 0 | 0 | 0 | 0 | 4 | 580.57 | 0 | 0 | 4 | 580.57 | 0 | 0.00 | 0.00 |
| 6 | 0 | 0 | 0 | 0 | 0 | 8 | 708.74 | 0 | 0 | 0 | 0 | 0 | 0.00 | 0.00 |
| 7 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 4 | 128.68 | 4 | 128.68 | 0 | 0.00 | 0.00 |
Description:
This 8-meter wide habitat is similar to the 6-meter version but with cabins on three floors. Level 1 is hydroponics and public space. Levels 2, 3 and 4 each have a single row of cabins, an aisle and hydroponics on the other side. Levels 5 and 6 provide storage and engineering space, while level 7 (rigid core) is split between engineering and microgravity recreation / storm shelter.
12 meter width
Allocation of volume:
| hydro: | 75 | kg/m³ | ||
| - volume | 7905.5 | m³ | 40.33 | per |
| - mass | 593 | tons | ||
| quarters: | 60 | kg/m³ | ||
| - volume | 4297.7 | m³ | 21.93 | per |
| - mass | 258 | tons | ||
| storage: | 40 | kg/m³ | ||
| - volume | 2482.5 | m³ | 12.67 | per |
| - mass | 100 | tons | ||
| public: | 60 | kg/m³ | ||
| - volume | 1137.4 | m³ | 5.80 | per |
| - mass | 69 | tons | ||
| engineering: | 120 | kg/m³ | ||
| - volume | 1120.4 | m³ | 5.72 | per |
| - mass | 135 | tons |
Mass estimates (in tons):
| water | 638.41 |
| hull | 12.2 |
| core | 12.2 |
| hydro | 593 |
| quarters | 258 |
| storage | 100 |
| public | 69 |
| engineering | 135 |
| total: | 1817.8 |
Level geometry:
| level | radius | circum. | cyl area | floor area | volume | gravity |
| 1 | 21.2 | 133.204 | 371.336 | 1598.442 | 4456.035 | 37.93% |
| 2 | 18.2 | 114.354 | 314.788 | 1372.248 | 3777.451 | 32.56% |
| 3 | 15.2 | 95.504 | 258.239 | 1146.053 | 3098.867 | 27.20% |
| 4 | 12.2 | 76.655 | 201.690 | 919.858 | 2420.283 | 21.83% |
| 5 | 9.2 | 57.805 | 145.142 | 693.664 | 1741.699 | 16.46% |
| 6 | 6.2 | 38.956 | 88.593 | 467.469 | 1063.115 | 11.09% |
| 7 | 3.2 | 20.106 | 32.170 | 241.274 | 102.944 | 5.73% |
Level layout:
| level | cabins: | hydro: | (hvol) | quarters: | (qvol) | storage: | (svol) | public: | (pvol) | Eng: | (evol) | (rows) | (width) | (depth) | (avail) |
| 1 | 0 | 11 | 4084.7 | 0 | 0 | 0 | 0 | 1 | 371.34 | 0 | 0 | 0 | 0.00 | 0.00 | 0 |
| 2 | 107 | 3.5 | 1101.76 | 7.5 | 2360.91 | 0 | 0 | 1 | 314.79 | 0 | 0 | 2 | 2.13 | 3.75 | 0 |
| 3 | 89 | 3.5 | 903.84 | 7.5 | 1936.79 | 0 | 0 | 1 | 258.24 | 0 | 0 | 2 | 2.13 | 3.75 | 0 |
| 4 | 0 | 9 | 1815.21 | 0 | 0 | 2 | 403.38 | 0 | 0 | 1 | 201.69 | 0 | 0.00 | 0.00 | 0 |
| 5 | 0 | 0 | 0 | 0 | 0 | 7 | 1015.99 | 0 | 0 | 5 | 725.71 | 0 | 0.00 | 0.00 | 0 |
| 6 | 0 | 0 | 0 | 0 | 0 | 12 | 1063.11 | 0 | 0 | 0 | 0 | 0 | 0.00 | 0.00 | 0 |
| 7 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 6 | 193.02 | 6 | 193.02 | 0 | 0.00 | 0.00 | 0 |
Description:
This 12-meter wide habitat is wide enough to fit multiple rows of cabins into a single level. I did a bit of manual exploration through the design space but by no means found a definitive 'best' solution, so these numbers are an 'acceptable' layout that could probably be improved. As with the other versions, level 1 is hydroponics and public space. Levels 2 and 3 each have two rows of cabins, an aisle and hydroponics space (though in this case there is actually a 1m hallway between the rows of cabins rather than a shared aisle with the neighboring hydroponics section). Level 4 is mostly hydroponics but begins the transition to storage and engineering that is continued on levels 5 and 6. As with the other examples, level 7 is split between engineering and microgravity recreation / storm shelter.
Engineering Section
The habitat modules have to spin in order to produce pseudogravity. Each ship will be built with two modules spinning in opposite directions, each capable of spinning up the entire habitat section. These modules will be connected using one of their large docking ports. The remainder of the vessel's systems including power, cooling and propulsion will be attached to the habitat section via large docking port. There will not be a central structural member running through the habitat modules; the docking port must be rigid enough to withstand all expected loads while keeping the module aligned.
I'll present data for two versions, a phase 1 LOX-methane propulsion system and a phase 2 SEP argon system. Since the docking port would remain standard, these parts would be interchangeable. A larger propulsion bus could transport more than one pair of habitat modules if desired, or could carry exchangeable cargo payloads using the same interface. Other roads to expansion are available, but I will focus on the baseline design for now.
Phase 1
Most of the engineering section's structure is provided by the walls of its propellant tanks. A bit of additional support is added to this strong foundation providing anchor points to which solar panel arrays and radiator arrays are attached. During normal flight the vehicle's engines will be pointed at the Sun, allowing the structure and propellant mass to provide substantial shielding against solar radiation for the habitat cores (level 7). Multi-layer insulation plus powered cryogenic cooling will allow zero boiloff indefinitely. Sizing of the engine and tanks will depend on the final loaded mass of the vessel, with a design goal of 0.1g acceleration at full mass and delta-V of 3.5km/s. That would allow for the trip from LEO to EML2 uncrewed, and would give about 15% margin from EML2 to Mars orbit. Power requirements run from 1.6 to 3.2 MW depending on which habitat module is selected (assuming 6.8 kW per person which is 20% margin over hydroponics demand).
The design point of 3.5 km/s dV plus methane-oxygen engines means we need 1.63 kg of propellant for each kg of vessel mass. Tanks will mass about 4% of their propellant. (I ran some checks and found mass fractions of 2.3% to 2.7%; using 4% will compensate for insulation and cryocooler masses plus add some margin.) Engines need to provide no less than 0.1 g acceleration at full mass in order to gain reasonable Oberth boost, which works out to about 0.26 g peak. I assume an expansion ratio of 120:1. Power requires 2.1 kg/kW for rated output at Mars.
Since I can't be bothered to come up with Exciting! Names! for these configurations I'll simply refer to them as short, tall and grande.
Results: (masses in metric tons)
| Configuration | Crew | Power (kW) | Dry Mass | (Fueled) | Thrust (MN) |
| Short (6m) | 194 | 1,583 | 2,221 | 5,842 | 5.74 |
| Tall (8m) | 262 | 2,138 | 2,796 | 7,354 | 7.22 |
| Grande (12m) | 392 | 3,199 | 3,909 | 10,281 | 10.1 |
Mass breakdown:
| Configuration | Hab Mass | PV Mass | Engine Mass | Tank Mass | Propellant |
| Short (6m) | 2066.46 | 3.32 | 6.7 | 144.9 | 3621 |
| Tall (8m) | 2600.84 | 4.49 | 8.4 | 182.3 | 4558 |
| Grande (12m) | 3635.62 | 6.72 | 11.7 | 254.9 | 6372 |
Phase 2
Once again the propellant tank is the primary structural member, though it is a fraction of the size of the chemical system's tanks. The solar power system is enormous; in addition to the 1.6 to 3.2 MW required for life support and misc. systems, prodigious amounts of electricity are required to run the engines. The exact amount depends on which habitat modules are selected and on how long the engines will fire during the trip (which is not the same as how long the trip will take).
Design goals are 9 km/s for low-thrust transfer and burn time of 90 days (which indirectly sets the required thrust). The 'gear ratio' is 0.35 kg propellant per kg dry mass. I assume a thrust to weight ratio of 0.004 for the engines (magnetoplasma), argon propellant, efficiency of 75% and exhaust velocity of 30km/s (Isp ~3000). Other assumptions are the same as the chemical version.
Results:
| Configuration | Crew | Power (MW) | Dry Mass | (Fueled) | Thrust (N) |
| Short (6m) | 194 | 61.11 | 2,306 | 3,114 | 3115 |
| Tall (8m) | 262 | 77.06 | 2,903 | 3,920 | 3921 |
| Grande (12m) | 392 | 107.95 | 4,059 | 5,480 | 5481 |
Mass breakdown:
| Configuration | Hab Mass | PV Mass | Engine Mass | Tank Mass | Propellant |
| Short (6m) | 2066.46 | 128.32 | 79.41 | 32.3 | 807.3 |
| Tall (8m) | 2600.84 | 161.83 | 99.96 | 40.7 | 1016.2 |
| Grande (12m) | 3635.62 | 226.69 | 139.73 | 56.9 | 1420.7 |
Ideally this architecture would use a capture tether at Deimos with tether transfer to Phobos for docking; that would save about a third of the trip dV, using a 'gear ratio' of 0.22 kg propellant per kg dry mass. Here's what that might look like:
Results:
| Configuration | Crew | Power (MW) | Dry Mass | (Fueled) | Thrust (N) |
| Short (6m) | 194 | 37.48 | 2,213 | 2,699 | 1879 |
| Tall (8m) | 262 | 47.32 | 2,785 | 3,398 | 2365 |
| Grande (12m) | 392 | 66.36 | 3,894 | 4,750 | 3305 |
Mass breakdown:
| Configuration | Hab Mass | PV Mass | Engine Mass | Tank Mass | Propellant |
| Short (6m) | 2066.46 | 78.70 | 47.90 | 19.5 | 486.8 |
| Tall (8m) | 2600.84 | 99.37 | 60.29 | 24.5 | 612.8 |
| Grande (12m) | 3635.62 | 139.35 | 84.25 | 34.3 | 856.6 |
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