I've been banging away at the model, and workshopping different arrangements for crew and physical plant. And along the way I've run across a few problems that needed addressing.

More Crew

Back in March I estimated the target population for the Iliad to be ~385. I was aiming for a minimally plausible crew to cut down on the amount of writing I had to do. I was suspecting I had a problem when I was generating the crew randomly and coming up short on some key personality varieties. Now that I'm trying to fill a real occupation database, the problem is getting to be more acute.

The problem isn't so much the number and composition of people that leave the Earth. That's easy to control. I had to revise the population upward because once you start going 20 or 30 years out, regardless of the composition of the crew at the outset of the journey, a significant portion of your starting crew will be retired, and their children will be the ones in the workforce. But because I'm not running some sort of Dystopia ala Logan's Run most of those old people are going to be around. In addition, the crew's children will have children of their own by then. So if the ship needs 385 people to keep the lights on, the vessel needs to support roughly twice that many people.

In on my March spreadsheet, the workforce was dipping down to ~220 by the time the vessel arrived in the remote star system. The problem is that 220 people was enough to take care of the ship, but leaves no room for training. And in fact you can see that my formulas allocated -1 people to as a trainee. What I wasn't paying attention to was what was going on at 50 years. The workforce is down to 135 people taking care of a ship and crew of 380 people. At that point 25% of the population are children, and 13% of the population are elderly. Both populations that require a lot of human services.

Some jobs shrink or grow with a population. For instance, teachers. You can estimate you need one per, say, 12 students. If there are more students, you need more teachers. Fewer students, fewer teachers. For some jobs, though, there is a minium number needed to have all of the skills required, or to cover a 24 hour schedule. I need at last 8 people, for instance, to minimally staff a medical center. 2 physician, 4 nurses, and and two orderlies to cover a 24 hour rotation. For a nuclear reactor I need 6 people, two for each 8 hour shift to cover 24 hours. With 135 people in the workforce, I keep having to cannibalize other essential services to keep basic life systems running.

It is easy enough to check my work. Even in the roaring capitalism of the United States, where kids go into the workforce as soon as they can add 2+2, and people keep working until they die at their desks, the labor participation rate is only 63%. My model's steady state labor participation rate: 62%. Though I should point out that there are some differences between how my model calculates it and the US Department of Labor calculates it. (They don't count people under the age of 16, and my model considers maternity leave an college education an "occupation".) In the United States retirement is officially 70 years old, but if you look at actual labor participation (using 2016 figures):


There is a sharp drop-off after 55. Even in a culture where we make the elderly eat cat food, we only have a labor participation rate of 19.3% for those over 65.

On the Iliad, there will be older people working. But basically when they want to work, if they want to work. In an engineering formula you leave that as an something nice that you keep in the back of your head and out of your calculations. Which is exactly what I am doing.

Different Population Composition

I am also changing the composition of the initial crew.

In March I used what I call "the Island model". It was based on a theory about how Polynesians set about colonizing new islands. The colony was comprised of unattached people in their 20s. In my model, there were also some older people to supervise, but not many. The Island model was nice because I could use expected population growth to reduce the initial headcount of the crew. (And as you can imagine, keeping the headcount down was really important when transporting your population by canoe.)

That Island population model works fine if your housing is grass huts. Huts are cheap and easy to rebuild or expand. However that model is chaos in a planned economy where I have to build housing to modern standards.

On board I need two different kinds of housing for two different populations of people: Single Bedroom housing and Multiple Bedroom Housing.

Single Bedroom housing is for individuals or couples. They need their own home. Each unit has a single bedroom, a bathroom, and an various configurations of other rooms (kitchen/living room/dinette). People who live there are single or share the residence with a significant other. The residents work, and need to be able to enter at all hours, and leave at all hours. They also need to be able to entertain friends and potential love interests. And life is too short, and morale is too important, to make people deal with roommates. Been there, done that, it sucks.

However, when children enter the picture people need an entirely different kind of housing. They need least 3 bedrooms, 2 bathrooms, a living room, a kitchen, the room you send the kids to play in so you can actually hear yourself think in the living room, and an office/workshop for mom and/or dad (and/or whoever else is in the picture in these enlightened times). Oh yes: and a backyard big enough for the kids and/or family pet to play in.

A Multiple Bedroom Residence is too dang big for a single person or childless couple. Converting 3 single bedroom units into a Multiple Bedroom Unit means you've built a tiny useless kitchen into three rooms, instead of one decent kitchen large enough to service a family of 4+/-1. And the layout is awful. Designing a structure that can be gutted and reconfigured means that on top of everything else going on, you need a dedicated team of housing construction folks. Plus: you have to move everyone out of the unit being gutted.

When I started actually allocating space for housing, there was a problem in my simulation using the Island population model. I had a crush of people at the beginning who needed Single Bedroom Housing. And within a decade, I had an equally large crush of people who needed Multiple Bedroom Housing. Eventually things settled down and stabilized after the initial baby boom. But that surge meant I either needed to build far more of both Single and Multiple Bedroom Housing than I would ever need in steady state. The alternative would be to build virtually all residences as some sort of swiss-army house that could convert to an apartment building.

I switched my population model to a variation I call "The Balanced Model". In the Balanced model, the ship is filled with people from all walks of life in a rough proportion to their distribution in the general population under 55 here on Earth. Families would be moving on board with kids in tow, and possibly an elderly relative or two. There would be plenty of unattached people in their 20s and 30s, but nothing that was going to set off a catastrophic housing crisis decades into the mission. There would also be older and more experienced crew who were middle aged and probably never have kids.

Here is the difference in output from the two simulations, assuming a target population for around 1000 people:

ModelTimeSingle Bedroom UnitsMultiple Bedroom Housing Units
IslandYear 1061315
IslandYear 30344202
IslandYear 100379175
BalancedYear 10267201
BalancedYear 30379178
BalancedYear 100389176

In the long term, in either model, the population eventually reaches a steady state. In steady state, I need ~390 single bedroom units and ~176 mulitple bedroom units. Both models have a baby boom at year 10. But the boom in the balanced model is much less severe. I only have to add 50 new housing units in 10 years, instead of 227. That is something that can be constructed at the shipyard, and simply left unfurnished until needed. And for single bedroom housing, I only need to add 84 units from my launch timeframe to account for population growth. But there I have 30 years to fill it. Again, the best answer it to just build extra and keep some furniture in storage.

Having housing of the right type, in the right quantity is essential for morale.

New Deck Plans

So if you are following along, I'm posting new spreadsheets and deck layouts. I have also added a new data product, the zoning file. This file dump also contains a fresh run of the sector-izer tool. I tweaked the algorithm to err on the side of fewer plots that are larger. This ensures all plots are at least the unit size of 314 m^2. I new algorithm also removed the requirement to have a corridor between the outermost track and the skin of the vessel.

  1. My circle layout already provide at least 2 path at all times.
  2. on many of the agricultural decks I was loosing too many useful chunks of floor to that outer hallway
  3. there are ways of providing an emergency exit that require a lot less in the way of square footage. Simple things like a door through a bulkhead. Or an escape trunk to the deck above and below. And I only need those exits for manned spaces. An agricultural field doesn't have any bulkheads. (To escape, just walk into the next field.)

On my to-do list is to redesign the sector-izer to output proper polygons instead of circles and lines. That will allow me to start generating full-color diagrams with zones in different colors. Though I'm tempted to start a 10 USD level on Patreon to get the pretty, pretty graphics. I'm also fighting the urge to make a coffee table book.

Here are the new yummy, yummy data models: