Long Duration Spacecraft and Space Stations

I thought I should contribute something to our new Mars effort, so here are my thoughts on a future long duration spacecraft or space colony. Unfortunately I haven't had time to present an illustration of my concept, but hopefully the text will suffice.

In looking at the existing designs of either long duration spacecraft or space stations, several problems come to light. Here I address these problems and suggest some very simple alternate solutions to them.

First, these stations tend to be huge, requiring a large leap from things like the International Space Station or likely follow owns to these monstrously large ones. The bigger and more expensive a station is the less likely it is to get funded in anything other than the very remote future. It’s extremely unlikely that Congress would fund a station even a 10th as large as the smallest of these designs. What is needed is a much smaller station or craft that could conceivably make it through the budget process. So this is my quick attempt to come up with something to fill the gap, or possibly form the basis of later designs.

Typical colony interior, as currently envisioned

Second, as the above illustration shows, these stations have low usage density. Many other illustrations are here. The space inside is mostly open with people, tree, houses, and crops scattered in what is really a very low density approach. Space costs money, for materials and shielding, and as you put such a design under the budgetary microscope the program will collapse. Considering the expenses involved, it is more likely an early station will bear more resemblance to a submarine than a park. With the expense per cubic foot being infinitely higher than an ocean going ship, none of which has yet been designed and built as a tree-lined subdivision, the odds that a space station will be built along these lines in anything like the near term is essentially nil.

Third, these designs depend on very large windows and mirrors to provide illumination. This looks simple and low tech, but actually is very intensive. Given the structural strength of glass and accepted design rules for windows in submarines and spacecraft, each of the windows must be quite thick, with the thickness increasing dramatically as the span of the window increases. Years ago I ran calculation of the estimated amount of glass required in such designs. Based on the design of quartz windows, for example, where “The thickness of quartz required to support a pressure of 1 atmosphere, with a (pressure) safety factor of 4, is 0.059 times the diameter of the unsupported part of the quartz [1].” This means that a circular window 1 meter in diameter would have to be 2.3 inches thick and weigh 41 lbs. To provide 100 by 100 meters of clear window area would take 12,732 such windows, totalling 522,000 pounds, and 25 miles worth of O-rings to seal. The mass per fully illuminated square meter is 52 lbs, not including the frame to hold the window. Keeping in line with current space usage, the cheapest acceptable material for the window would probably be Pyrex, at a bulk cost from Corning of about $1,800 just for the sheet, without the precision machining and testing, or $23 million just for the glass.

The same math indicates a square kilometer of a space station would have 1.2 million such windows, 2500 miles of vacuum O-ring seals, weigh 52 million pounds, and just the sheet pyrex costs $2.3 billion dollars. That’s more mass than 1000 shuttle missions could deliver to low earth orbit. Some have held that glass will be produced cheaply on the moon, but I still see no way to underprice a cheap earth factory in bulk production with the massively increased overhead and energy costs of a lunar factory.

The way around the mass problem would be to make each window fairly small, but this brings up the seal problem. The length of the seams necessary to seal each window grows unimaginably large. Under current standards for windows that must exist in a pressurized environment, each seam must be carefully installed and tested, and then retested every few years. Given the thousands of miles of seams involved, the station's occupants would just inspect windows and seams all day. When they finished it would just be time to start all over again. Making the windows larger to reduce the seams increases the total window mass, since the wider the window the thicker it has to be, meaning the total mass of glass is roughly a function of the individual window diameter. For square windows, cutting the length of the seals in half by doubling the size of each window also doubles the mass of each window.

The final two flaws with skylight designs are that the windows provide no inherent radiation shielding and more importantly can suffer catastrophic failure from a relatively minor impact. A tool that gets loose might impact and crack or shatter a window, possibly leading to complete failure, causing the impossibly difficult problem of repairing a window that’s sucking up objects with greater than hurricane force winds. The neighboring windows will be suffering impacts from objects caught up in the airflow (big wheels, lawnmowers, charcoal grills), so the single window failure could easily ripple into multiple window failures. The only way to counteract the problem is to equalize the pressure by sealing the area of the craft illuminated by the window and letting the pressure leak out, meaning that there really couldn’t be a fully open space inside the colony.

Solar Concentrators and Fiber Optics

For all these reasons the massive skylight or greenhouse type station designs probably won’t prove feasible. However, there is a much cheaper and simpler way to achieve the same results without either the glass or the seams. The first thought is to use sunflowers and fiber optics, which have the advantage of not transmitting radiation, eliminating the need for all the mirrors and optical geometry common in current space colony designs. The solar concentrator fiber optic concept allows parabolic solar reflectors to be aimed as simply as a satellite dish while allowing the pressure hull to be completely free of large areas of windows.

However, this design also presents the problems of massive amounts of high quality optical fiber, which again increases cost. It also replaces the relatively simple flat plate window designs with massed arrays of parabolic reflectors and optical interfaces. Some background cost estimates are here, and show that to fully illuminate an area 100 by 100 meters would require a cable cost of around $2.4 million dollars per meter. Considering that for a very direct run the light would still likely have to be piped at least 10 meters to clear the dish and mast, the cost comes to about $24 million for an area 100 by 100 meters. Not that expensive as space craft costs go. Considering some slight efficiency losses and probably higher pricing, plus development, costs could be as high as $100 million for the 100 by 100 meter area, or about $10,000 per square meter of full sunlight illumination inside the station. It’s possible that light pipes might correct this cost problem, but it would take more design work to investigate the feasability of this.

Solar Cells and Sulfur Bulbs

So I’ll suggest a more tried and true method of illuminating a space station. Current satellite solar cells can deliver electricity at around $1.20 per watt and around 360 watts/square meter. On earth the average surface solar radiation received is around 1000 watts per square meter, potentially as high as 1370 watts per square meter. To properly illuminate this area we have to select a light source, and the microwave sulfur lamp seems to offer the best potential and has no toxic metals to create a disposal problem in a closed system. This would offer an off the shelf long life lamp, possibly nearly infinite life lamp if they replace the existing magnetron microwave generator with a solid state model. NASA researchers have also already enhanced it for plant growth. Even at retail prices and market up for light pipes used in aquarium layouts, the cost of the lighting would be about $20 million for a 100 by 100 meter area. Given a roughly 65% efficiency, which would be greatly improved by going to a solid state microwave source, the 100 by 100 meter area would require 15 megawatts of power and a solar array cost of about $18 million dollars. Given that wiring is relatively cheap, the cost comes to about $38 million for our sample area and around $3,800 per square meter, significantly cheaper than the probable costs of the sunflower fiber optic cable combination. As an added bonus no significant new technologies and production systems have to be developed for this type of lighting system to be utilized. Not only is it all equipment we currently build and use, but it’s also all electrical equipment that is easy to design, wire, and maintain.

The Design

So let’s keep this station small and cheap. Let’s allow 100 square meters to feed each inhabitant, about 5 times more than is absolutely required. If we decide on a crew of 100 people, then we’d need about 10,000 square meters of agriculture, which is the same as the 100 by 100 meter area I’ve been discussing. But since we’re using high efficiency grow lamps and hydroponics we can stack the plants in racks, much like a greenhouse. Let’s assume our hydroponic garden modules each 1 meter high, so essentially it has half-decks 1 meter high with walkways in between. If we use a sphere 27 meters (88.5 feet) in diameter it would have 24 half-decks providing roughly 10,300 square meters of floor area, or about 111,000 square feet. Conversely, I could also go with a cylinder 13 meters (42.6 feet) in diameter and 78 meters (256 feet) long and get 10,400 square meters in 13 half-decks.

Dumping the Heat

Now comes the fun part. How hot will it get inside?
We paint the sphere white so its absorbtivity is around 0.2 and emissivity is around 0.8. At the distance of earth from the sun we get 1400 watts per square meter, multiplied by both the cross sectional area of the sphere and the absorbtivity (0.2), and we have 160,000 watts of thermal energy being absorbed by the sphere. To this we add the 15 megawatts we’re dumping in to illuminate all the plants. After all, eventually all that electrical energy feeding the lights becomes heat, even if it has to spend some time as a potato, a dinner, and a lap on the treadmill. So the heat absorbed directly by the ship itself is relatively insignificant compared to all that energy we’re dumping in through the lights.

We have to radiate this energy back into the cold of space. The energy radiated by the sphere is simply the emissivity of the surface (0.8 for the white paint) multiplied by the total surface area of the skin (2290 square meters), the Stephen Boltzman constant 5.67e-8 watts per square meter per degree Kelvin to the forth power. Solving all this says the spherical ship will reach thermal equalibrium once the temperature is 618 degrees Kelvin, or 653 degrees F. We’ve built a ship that runs hotter than a broiler. That’s a definite problem.

Convection

Now for my interesting solution to this problem. Since our ship is rotating at the end of a very long boom, to maintain some artificial gravity, we can play some games with air convection. To get the ship to equilibrate at 72F it needs to radiate much more heat out into space, so it needs a much larger surface area. We need a surface area of about 44,000 square meters, or a special radiator area of 41,708 meters with an emmisivity of 0.8 (white or black paint), because we’ve already got some surface area for the sphere itself. We’ll use a big giant fin which radiates from two sides, so it will have to be 20,850 meters in area, or 144 meters (473 feet) on a side if it was square. That’s about ten times larger than the surface of our habitation module.

Now, if we keep it oriented parallel with the sun we can provide a simple shade for it with a piece of aluminum or mylar so that it’s not absorbing any additional heat from the sun, despite its area. Even more interesting is using the large solar panels that power the lights to be positioned perpendicular to the radiative coolers, on the sunward side of the ship, so that the solar cells not only mount on one of the major air arteries, which is structurally simple, they also shade the cooling fins. So what we do is provide a large pipe, or chimney, from the top of the module that runs up through the center of this large fin. Warm air is drawn up via convection and rises to the top (hub end) of the fin, where it branches out into many individual pipes and channels, radiating heat back into space, which cools the air. This air continues to cool and descend through the network of channels in the fin until it finally returns to be reintroduced into the bottom of the habitation module. There it is warmed by the gardens and grow lamps, getting warmer and warmer as it rises through all the decks, to once again channel itself into the chimney and back up through the cooling fin. That gives the craft passive cooling and passive but strong air circulation.

For the cylindrical design, which is almost identical, we can play a slightly different trick. We don't make the cylinder exactly parallel to the axial rotation of the ship. We make the cylinder tilt on a slope, with a low end and a high end. The high end has the chimney to draw up heated air, which then goes up and encounters a network of channels that slope downwards toward the lower (cool) end of the craft. These channels come together to feed the main return duct which reintroduces cool air at the low end of the cylinder.

Thus the cylindrical ship has a lower, cooler end, where cool weather plants would thrive, and grows progressively warmer as you walk slightly upslope to the upper end. There is a constant breeze moving from the low cool end to the warm high end. The air flow is more uniform in the cylindrical design, but the external surface area is half again as much as the spherical ship, and if the entire ship is radiation shielded this means it also weighs half again as much.

The network of cooling pipes can be made from thin extruded aluminum, with a network of valves to shut off flow to any pipes that are punctured. By laying the network out like a circulatory system any number of ruptures can be sealed off with little effect on the ships habitability or safety. As an added bonus this network of pipes also serves as the structure that provides either a part of the counterbalance of the ship in a single habitation module configuration (a ball stuck to a long stick and pivoting about the mutual balance poinit), or the connection to the hub in a multi-module configuration. The main vertical air ducts, or arteries, can also double as the access tube from the hub to the habitation modules. And the whole system is passive, so there's no possible power failure in a forced air circulation scheme that would result in frying the crew. Additionally, by closing valves to parts of the air circulation network, or altering the on/off cycle of the lights, the temperature of the craft can be varied over an extremely broad range. And if these valves are placed along the human accessible tubes that lead to the axis or hub of the ship, the valves could also be either manually operated or even simply plugged. This allows for graceful failure of the system and complete recovery by human intervention.

So the design succeeds in dense packing the crew and agriculture, thus minimizing the required mass of radiation shielding, while providing passive cooling for all the heat being dumped into the crew module, while using this passive cooling system both as a structure component and access path to the axis. The system can easily be tested on earth with something as simple as setting up a test bed greenhouse inside an old airliner fuselage, and the airflow patterns and fin design can be built and experimented with out of nothing more than some stamped metal. The design is also extremely scalable, since the ship could be very large or small while retaining the same configuration, whereas many current designs for an artificial gravity ship don’t function well on a small scale. If can also grow incrementally, with new solar panels and cooling channels added to an existing ship, while still maintaining its thermal equilibrium. You could also double the size of the solar panels for full power operation out as far as Mars, yet be able to maintain thermal equiliibrium all the way in to Mercury without any reconfiguration or geometrical manipulations. In essense, that part comes automatically.

What I like about the design is that once built it could stay in service for an extended period, with solar panels and sulfur lamps being replaced every decade or so. In many of the moon and Mars ship proposals the craft are basically one time use affairs, or so overly specialized that they would likely not see any additional use. This type of craft could serve as a Mars ship, then return and act as a research station, the crew quarters for a construction station, and eventually just a farm with a small crew to harvest the crops. It would give the space program some staying power, as it were.