For this chunk of the website I am building a similar worksheet to the one I developed for Design A Starship for the Sublight Universe. A way to bake all of the math of a massive inter-related megaproject into bite sized forms that update the system-wide calculations, and highlight where the design is being stretched beyond breaking in other areas.
Like on Design a Starship this form allows you to download your work as a JSON file for later reviews and revisions. The goal is to produce something plausible for a hard science-fiction novel, without getting too far into the weeds. The calculations I have derived are from a previous effort to develop an excel spreadsheet for:
The idea of a self-contained colony is that all of the day-to-day needs of a civilization on board are met by agriculture. And not some hand-wavy space magic variety, actual acreage based on actual patterns of actual human civilization.
These worksheets also have a few built in assumptions:
These calculations assume that each space settlement has a design life of <120 years. Humans only started to build massive metal-framed structure in the late 19th century. The oldest steel skyscrapers still standing were built in the 1880s. The first megastructure built entirely of steel, the Benjamin Franklin Bridge was completed in 1926. And while the Empire State Building, completed in 1931, is still standing, keep in mind that it's expected design life was only 60 years.
Now, part of the issue is that we only figured out the technology to allow for massive skyscrapers in the late 19th century.
So far we have been assuming a static structure. The lifespan for ships is even less. The steel vessels with the longest design life are nuclear aircraft carriers. The have an design life in excess of 50 years. And the US Navy actually manages to get those 50 years out of those ships. The USS Enterprise (CVN 65) was in service from 1961 to 2012. With another 5 years of inactive service.
Most other steel ships are only expected to have a useful service life of 20-30 years. 20 being the lowball figured for a commercial vessel that built cheap, rode hard, and put away wet. 30 years for carefully maintained military vessels.
What determines the design life? Partly material, mostly economics. Steel rusts. Stress on structural members causes cracking.
Skyscrapers are subjected to stresses from winds and earthquakes and loads shifting inside of their structure from day-to-day activity. If there is a fire in the building, the heat can warp the beams. When a building gets beyond 4 floors, it needs reliable elevators. And they are immensely complex and expensive devices to maintain. Buildings have plumbing systems and HVAC which also require constant maintenance.
Ships have all of the issues of a building, plus constant exposure to salt water, engines, pumps, and deck equipment. They are battered by storms. Hulls are flexed by wave action, turns, and constantly plowing through the water.
For all of them there gets to be a point where the cost of repair exceeds the cost of replacement. Though for certain historical structures and vessels, there is a willingness on the part of the curators to preserve the ship. But it's not cheap. I cite the ongoing preservation efforts for the Battleship Texas and the Cruiser Olympia.
So while all of the nutritional and biological needs of the settlement are met internally, we can assume regular injections of material from asteroid mines/trading/etc. This influx of material also includes trace minerals like phosphorus that filter through biospheres and are tricky to recycle efficiently. These resources, however, can be stockpiled.
For a settlement at the edge of the Solar System (30+ AU), a fusion powered vessel still needs 6-12 months to arrive. At those distances, planetary alignments might knock a few AU off of the trip. But the bigger the discount on travel time, the less often those planetary alignments come around. There is even a case to be made for placing a massive spaceport in orbit in the L2 point in the Mercury/Sun system. While every craft would have to travel the distance between their launch point and Mercury, Mercury whips around the Sun at such a rate that a favorable alignment to any other point in the Solar System is, max, 88 days away.
So a settlement needs to assume at least a 12 month lead time on supplies. And one should always plan on an accident or worse destroying a shipment in transit every so often. A cautious planner would say that one needs a 24 month stockpile, a paranoid planner would say 36 months. A pragmatic planner would break the shipments up into packets that could be sent periodically enough that the loss of one or two won't matter. But suffice to say, we are just quibbling about how large a stockpile to keep on hand. We aren't trying to operate completely in a sealed bubble. Well, a completely sealed bubble.
For an interstellar vessel, they need to keep enough supplies on board for 120 years. And I have a more in-depth explanation for that. But even in the interstellar scenario, outside supplies are baked into the plan. And while 120 years worth of supplies may sound like a lot, this is only in raw materials that for whatever reason are not economical to recycle. When I ran the numbers for the Clarke class, the spare parts and strategic resources accounted for little less than half of the dry mass of the ship. About 4 million metric tons of a 9.5 million metric ton vessel. So being isolated has its costs.
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