Growing enough food for ten million

If we grow food underneath those valley land-strip thingies, does
that give us enough room? Well, let's see.

How much food-producing area does it take to feed a person? I see
numbers of a quarter acre being enough, and I also see numbers of a
tenth of an acre being enough. I'm going to assume that it takes a
whole acre per person, on the grounds that by the time we are doing
Island Three stuff, the people who live in space are going to insist
on good food, good variety of food, and lots of food. We don't want
space-based civilization to be forever a deprived existance. Even
red meat, if the people want it, will be available, as will many
kinds of fresh vegetable, fruits, and seafood. Also, it gives us a
huge margin, if we assume anywhere from four to ten times what is
absolutely necesary. So a full acre per person.

Now, one of these valley thing-a-ma-bobs is 20 miles long by two
miles wide, for an area of 40 square miles. There are 640 acres in a
square mile. Thus a farming deck under the open-to-the-sky valley
would provide 640*40=25,600 acres. Not enough to feed even one
million, even if we did the same with the other two valleys. That
would make the total 76,800 acres, which just isn't enough.

But wait! How tall is your food crop? How high is that farming
deck? If you're growing potatoes, raspberries, and carrots, then
even one metre is a taller farming deck than you need. If you're
growing avocados, then ten metres might not be enough. I'm going to
go with an "average" height of ten metres for the farming deck, on
the grounds that there are a lot more food crops under ten metres
tall than over ten metres tall. Also, most of what people consume
falls into that less-than-ten-metres-tall catagory (you need more
wheat and carrots than papayas and avocados).

Now, why do I care (or expect you to care) how tall a food crop or
farming deck is? Because there's no reason on Eart... I mean, no
reason in space why you can't stack several farming decks on top of
each other. So, what we actually have to work with isn't 25,600
acres, or even 76,800 acres, but rather 76,800*however many decks we
feel like stacking on top of each other. So let's see...

If we stack farming decks ten high, we get 768,000 acres. Not
enough. How about stacking them twenty-five high? That gives us
1,920,000. Almost two million. BTW, if it really only takes a tenth
of an acre to feed a person, then we are there. But if not... OK,
how about fifty decks high? 3,840,000. Almost four million. And if
it takes a quarter acre to feed one person, then we have enough and
then some. But I'm assuming it takes a whole acre to meed the
demands of an affluent, demanding population. Well, how about one
hundred decks? 7,680,000. YEESH! We're still not there!

OK, this is getting serious. One hundred fity stinking decks!
That'll show 'em!

11,520,000 acres. There you go. Just create farming decks the same
area (40 square miles) as the valleys, make them average ten metres,
stack them one hundred fifty decks high (under each valley), and put
you picturesque villages and parklands and such on top of that.
Either artificial light or fiber optics can provide the light needed
for crop growing, and most of the work can be automated. See? No
problem.

Except that you now have your three valleys towering 1,500 metres
over your windows. Is that a problem? Well, maybe if you made it
seventy-five decks high, and did that in both lylinders? That would
be good for the "five million people per cylinder, ten million for
the pair" design.

I have reasons to think things are actually much better than this,
but I wanted to see what could be done with ridiculously conservative
assumptions (takes a whole acre per person, decks average ten metres
high). My next post will have the less bothersome (and I belive more
likely) numbers.