One large limitation in the starships design, is the habitation deck. The Space Settlements design study published by NASA (reference 1), pages 22 &;42 states that most people can adapt to rotation rates of up to 3 rpm, but that this adaptation might not be possible if personnel routinely move between the rotating 1 G. sections and the non-rotating zero-G sections of a space settlement or ship. The design study strongly recommended a rotation rate of less than 1 rpm. But that would require a habitat over 1600 meters across. A 3 rpm limit still requires a habitat 200 meters across, but this at least seems plausible for a large exploration platform. A 4 rpm rate would allow a 110 meter across habitat, but for purposes of this design I'll assume a 3 rpm rate. Putting this 200 meter in diameter ring in the ship, immediately means a very large ship.
Since the ship's required speed will force it to accelerate and decelerate at high rates for months, the habitation decks will need to adapt to everything from zero G, to one G forward or backward. To do this, I'm assuming the habitation ring is made of straight tubes (gold tubes in Hab. Deck image), attached between struts radiating outward from a support centrifuge (blue green structure in image), which rides on the inner edge of a support structure (clear in image) that attaches directly to the inside of the ships outer hull. Each of the hab. tubes can be rotated around their long axis to turn the floor toward the direction of the currently active 'down'. (Rooms in the support centrifuge will need to be constructed so the outer, front, and back walls can be walked on.) For example: If the ship is accelerating (or decelerating) at nearly one gee, the centrifuges rotation can be stopped and the segments turned so their floors points toward the rear (or front) of the ship. If the ship is coasting, the floors will be turned to point outward and the habitat rotated at full speed. If the ship is accelerating or decelerating at less than one gee, the floors will be partially rotated, and the hab ring spun at partial speed.
The Hab. Deck centrifuges are effectively independent space stations 660 feet across riding inside the Explorer class starships depresurized outer hull. The only direct connection between the hab deck and the ship are the tracks the centrifuge rides in and the electrical and data links. Each hab cylinder has its own life support and food storage in the centrifuge segment closest to it. Transit of food and material between the centrifuge and the rest of the ship is via pressurized shuttle cars riding on tracks inside the main centrifuge. This separation maximizes redundancy and isolation of the various areas. Each hab cylinder can be sealed off from the rest in case of fire or contamination. Even if all of the habitation centrifuge was to become contaminated, the pressurized areas of the rest of the ship would still have there own life support.
Note that I have not considered spinning the habitat section of the ship on a tether, or spinning multiple ships tethered to one another. I'm not sure how well a tether (or the hab module) could handle years of unprotected exposure to relativistic plasma impacts, and spinning part of one ship (or two linked ships) could preclude magnetic shielding. Also, this method would not be practical while the ships were accelerating or decelerating, and it would make both shielding the hab module and allowing service access to the rest of the ship harder. Finally, this would greatly complicate the rest of the ship, the vast bulk of which should not be spun for gravity (or any other reason).
Also consideration should be given to counter rotating habitat rings, or other counter rotating masses to compensate for torque on the ship. Otherwise the ship will need to use its attitude jets to compensate when it spins and de-spins the centrifuge.
The Space Settlements, a Design Study, pages 26 and 32, list various breakdowns for crew living space requirements. Per person, these are:
|(tram / halls)|
These numbers were originally developed for space colonies and may be excessive for a starship, but given that the crew will spend almost all of their professional lives aboard the ship, it's probably best to err towards a more spacious design.
Assuming each habitation ring is made of 12, 50 meter segments 10 meters in diameter with 3 m ceilings (more in the central halls and open areas) gives about 16,000 square meters of floor space (about 70 m^2 for up to 224 crew). If the hab ring is instead made up of 12 50 meter segments 20 meters in diameter with 3 m ceilings, you get about 63,000 square meters of floor space (about 70 m^2 for a crew of up to 898). These figures assume farm and/or personal and food storage are outside the full gravity section of the habitat. Of course, we might not need to do much (if any) farming, depending on the mass assumptions.
The drawing above gives 792 people more open area then the above calculations would suggest. Which is probably necessary given how long the voyage is. People are housed two to an apartment/office. With a few single person apartments that can be converted to conference and lab areas. This should be a functional arrangement both for the trip, and for the exploration phase. While not as glamorous. Most people involved in space exploration do support and analysis functions. Only a fraction of the crew will ever leave the ship, and most of the science will be done back on the ship pouring over the data recordings. Thus the need for some office space and a lot of networked personal computers. Given the performance increases in computers. These personal computers should have hundreds of times the power and capacity of a human brain. (Makes you wonder what the software will be like? Or the ships mainframe!)
Normal radiation shielding is defined as mass per surface area of the area being shielded. The Space Settlements, a Design Study, page 125, lists that for protection of an inhabited volume outside the earths magnetic fields, the surface of the habitat must be covered by about .44 kilos per cm^2 -- 4.4 tons per square meter. This is roughly equivalent to an eight foot thick concrete wall.
For 12, 50 meter long, 10 and 20 meter diameter segments wrapped in shielding mass, you get:
(Surface area of cylinder = Circumference of cylinder * Length
of cylinder. Circumference of circle = pi * diameter)
|Surface area||Shield mass|
|PI * 10m * 600m = 18,849m^2||4.4 tons/m^2 = 82,939 tons|
|PI * 20m * 600m = 37,698m^2||4.4 tons/m^2 = 165,876 tons|
If instead you try to wrap the hab ring rotation area with fixed shielding, you get a lot less torque on the ship. Unfortunately, you also have a greater area to cover.
Say a shielding a 10m length of the outside of the hull cylinder with shield 'bulkhead' walls extending 25 meters in from the surface of the hull. (The 50 x 10 meter tubes would come in 14 meters from the hull edge in the middle of their span.) (100m -[cos(15deg)*100m] = 3.4m of gap between the floor and the outer hull at the center of the hab. tubes. Plus the diameter of the tube.).
(Surface area of cylinder = Circumference of cylinder * Length of
Circumference of circle = pi * diameter, 10 meter length)
= 628.3 meters
Multiply by 10 meters length and you get a surface area of
6283 square meters.
Multiply by 4.4 tons per M^2 of surface,
= 27,646 tons of shielding mass.
For each of the washer shaped shield bulkheads.
(Area of circle = pi*r^2)
= pi*(outer_radius^2 - inner radius^2)
= pi*(100m^2 - 86m^2) =pi*2604=8.180m^2 of surface,
= 35,995 tons of shielding mass.
|Forward bulkhead||35,995 tons|
|Rear bulkhead||35,995 tons|
|Outer circumference||27,646 tons|
|Total||72,017 tons of shielding mass.|
72,017 tons would sink an aircraft carrier, but it is a little less than just shielding the 10 meter hab-segments. On the other hand if you assume 20 meter diameter segments the shielding only goes up to 176,242 tons. Only slightly heavier than just wrapping the 20 meter tubes. Also note that if you use a u-shaped fixed shield (as shown in the graphics image) you could save quite a bit of mass. (20,000-30,000 tons?) Multiple centrifuges sharing the same u shaped shield could save over a third of the shielding mass of independent shields.
Hoping I got something wrong, I checked these numbers against the Stanford torus space colony design. (10,000 person, 69,000,000m^3 of volume [8,000,000m^3 of personal space]) That torus was had a ring 135 meter in diameter, and full torus diameter of 1800 meters.
210,000. tons of structure,
530,000. tons of internal mass
(242,000. tons moist soil,
260,000. tons buildings, trams, cloths etc,
20,000. tons water).
9,900,000. tons of shielding mass total.
Scale the monster down to our hab ring and my numbers seem to be in the right mass range. For a space colony, shielding mass is just a launch cost. For a ship of course, its extra weight that has to be moved. Also note that I only fully shielded the habitation zone. The rest of the ship, the areas the crew may need to work in, is not. Also note that this shielding is only enough to keep the crew safe from the radiation loads a flaring star would put on them. If you want to shield against radiation from particals the ship slams into (a very good idea at high speeds) you probably should put the crewed areas of the ship behind the fuel tanks.
On the good news side. We were considering a Ram Augmented drive ship. The magnetic fields needed to do that can also shield the ship. Also for the vast bulk of the mission time we can wrap the hab ring with reaction mass or fuel tanks. If we save these tanks for last, or as emergency reserve tanks, the crew can be shielded by mass we need to take along anyway. Water for example weighs about 62 lbs per cubic foot and makes a good reaction mass for the auxiliary thrusters, or to spike the main drive. We would probably want to carry along a lot of water and ammonia (H2O &;NH3), so we could break them down to provide oxygen and nitrogen to replenish lost atmosphere. Lets face it, this thing will leak. (Everything does eventually.)
A book published by NASA in the mid '70's on space colonization. Basically, NASA's take on the old L-5 society's ideas. A light technical book written for the general public that covers all the basics from colony construction, space based industrial processes, launching lunar materials, mass flows within the life support / agricultural system, and psychological effects of different colony shapes. Lots of illustrations, graph, the works.