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The water situation in Arizona is like the one on Mars in that they're both dry places, but otherwise they don't have much in common.

From https://www.arizonawaterfacts.com/water-your-facts:

Here's where Arizona gets its water:

And here's how they use their water:

Some of these points are going to come across as obvious but the implications are significant.  There are major differences between Arizona and Mars here.

Looking at sources, Arizona's water comes from rivers and groundwater.  Reclaimed water is a very small fraction of the whole, which is to say that water is used once, then lost.  Though not truly lost, since it remains on the planet.  Even in dry places like Arizona, the water supply is basically an open system.  On Mars, water is best understood to be a kind of mineral, it's something you mine, maybe like the image below (from Copilot AI).  This makes it more valuable, something worth recycling.  And recycling will be easier, because things will be more closed off.

Looking at Arizona water use, in reverse order, the lion's share goes to agriculture.  The State is a bit circumspect on which crops this goes to, but an investigation into the acreage dedicated to each crop indicates that about 2/3 of this total (half of the total water usage) goes to cattle feed, the rest to food directly eaten by humans.  Given that animal husbandry is an inefficient use of water and land, major gains in food production per gallon are available with a shift from beef to basically any other kind of food (vegan, vegetarian, or poultry).

In any case this is less relevant on Mars, because agriculture necessarily has to happen indoors in a sealed environment.  The water isn't lost.  Most likely it will just condense up against the walls and be reused or be taken out of the air by a dehumidifier in the life support unit.  Unlike Earth, it's always going to make sense on Mars to recycle graywater and brownwater, because it'll still be purer and warmer than having to mine it new.  There will still be some leak rate, but unlike on Earth where that "leak rate" is nearly 100% it'll be maybe a couple percent or less.  (Animal husbandry is still probably going to be prohibitively expensive based on the space and  caloric inefficiency of livestock).

Next is municipal: Homes, businesses, parks, government.  The biggest use here is lawns, personal, commercial (golf courses), and public (parks, schools, etc).  Those will not exist on Mars (whose built environment will be more comparable to Manhattan in density and certainly not suburban in character like the population-weighted average in Arizona).  Human washwater, drinking water, cooking water, etc, is a smaller but obviously very important fraction.  In any case on Mars this water is very likely to be recycled at very high rates, like agricultural water.

The last category is industry.  Water in this category is used for mining and cooling, including cooling of power plants.  Liquid CO2 might be used in some cases.  It's easier to get in some ways since it's just compressed air but the high pressure (and therefore energy) required to produce it might mean it's not worth doing.  On the other hand, the energy needed to freeze it out of the air is lower (followed by pressurization via melting--still with an energy cost), so it might be the preferred industrial solvent.  There's no way around water for some things though.  Mostly that will be recyclable but there are going to be industrial uses that produce non-reusable water. 

I would not describe Arizona as an especially industrial state overall, and for Mars industry is unavoidably necessary.

Thermoelectric power plants are an interesting issue.  On Mars, that means nuclear (or maybe solar utilizing fields of mirrors).  It's a matter of dispute whether nuclear or solar is best overall, but I want to skirt that to talk through the cooling issue.  How do you reject, say, 100MW of heat?  Mostly I've heard it said to use radiators like the hard vacuum, but those are for packaged designs sent from Earth.  That's definitely possible and consumes no water.  We don't do that on Earth though because dumping the heat into a river or (more relevantly) using a big cooling tower that dumps heat via evaporation of water is cheaper and easier.  Boiling water absorbs 2.2 MJ/kg, so 100 MW of heat (maybe 2500 people?) will take 50 kg per second of water (4500 tonnes per Martian day). 

As a thought experiment, consider using a nuclear reactor to generate electricity to produce H2/LOX propellant.  Obviously the water used for propellant is lost.  But let's say it takes 20 MJ/kg of electricity to generate that (some inefficiency losses) and 20 MJ/kg takes 60-80 MJ of heat: For every kg of propellant, you lose about 30-40 kg of water to your cooling tower.  That water is inherently nonrecoverable.  Maybe that's fine.  Maybe it's not--we wouldn't waste other mined products that way. 

For reference, assuming perfect emissivity, at 0C you'll reject ~300 W/m2: 33,333 m^2/182 x 182m for 100MWt.  At 50C, 600 W/m2: 16,667 m^2/129 x 129m.  At 100C, 1100 W/m2: 9090 m^2/95 x 95m.  Less area than you'd need in solar panels and easier to build (ignoring the nuclear reactor the radiators are attached to which is just a different kind of thing).  You can also imagine a system where something like an evaporative cooling tower is serving as a simpler version of a radiator, moving heat from a point source to the metal walls of a pressure vessel (this might sacrifice efficiency for cost). 

Rockets, even those using methane, will also be big water-wasting machines.  Vehicles too where they use meth-lox or similar.  Unavoidable.

Otherwise, there's a concept of "embodied water": It'll take some number of kilograms per person to build out structure and infrastructure, that water being found both as the liquid circulating within and in other ways: Water contained in concrete, water lost in the making of the things the habitat is made from, in addition to the flow into and through the habitat.  When the population is growing, as you'd hope it would, it's likely that much of the water will go towards these "embodied water" needs.

In summary, water on Mars is going to be more expensive than Arizona, driving demand for much higher recycling rates, a difference in quantity becoming a difference in kind.  But this recycling will for the most part be made easier by the way we respond to other challenges on Mars, namely by the need for self-contained pressurized structures.

As far as Arizona goes, yes the state is dry.  Direct human needs are so high-value and low-quantity that desalination is economically viable.  This may also be the case for some forms of agriculture (Might not be though! Depends on the alternatives, since food production *has* to be viable in aggregate), but other forms--namely beef--produce such little value per gallon of water that they are not viable if forced to pay the true market value of the water they consume. They may be able to buy water rights in the Colorado from California by paying California to build out their desalination infrastructure (Water is fungible!) and further conservation efforts.

Hey Tahanson,

I appreciate your kind words.

Maybe you can answer a question I have—what is the purpose of this thread as compared to replying directly to my post in the topic where I posted it?

The way the forum is organized these days seems quite strange to me

ymmv but the engineers I know mostly regard spinlaunch as a joke

SSTO is a funny problem.  On a very slightly smaller planet (for example, if Earth were as big as Venus), it would likely be the optimal solution for launch.  As things stand, our planet is just slightly too big: Available chemical exhaust velocity is too low to enable lower mass ratios, and available materials are too weak and heavy to withstand the high mass ratios needed.

The problem is such that a relatively small boost of 500 m/s (~1100mph), if it came with no other downsides, would be enough.  This is why you see a reasonably high number of 1.5 stage vehicles, like the space shuttle.  It's why SpaceX's partially reusable concept (F9) still employs a pretty low staging velocity, and it's why airlaunch is a perpetually attractive option (even though nobody has yet built a SSTO H2/LOX airlaunch, taking advantage both of the plane's speed around 320 m/s and the fact that a separation in the stratosphere allows you to use a regular ~vacuum-optimized engine.

The benefits to a stationary device that gives you even this much are undeniable.  Unfortunately, the drawbacks--harsh tradeoffs between very high up-front capital costs, separation speeds, and high gee-loading--mean that it's hard to imagine an actual device that does what we'd want it to do.  The gee-loading from chemical launch is already a significant constraint on crewed launch and satellite design alike.

One way to avoid this is to have a trackless track: We're back at airlaunch, or a first stage, basically. Maybe a long enough track pays off once payload volumes are really high.

For what it's worth (nothing), extrapolating the time trend from the first four flights gives you July 26th for the fifth.  Looks like that has decidedly slipped.

I also have a limited knowledge but generally speaking every process has a labor input, and labor is in general the largest input cost for any product (at least when you include the labor embodied in eg the machines and buildings and such). Given that there's not currently any people in space, transportation up is expensive, and living there will tend to merit hardship wages, we can expect labor to be more expensive than Earth by a lot for a long while (whether you pay that cost directly by paying for people's ticket up and lodging or indirectly by paying them wages that make it worth their while).

Hey Tahanson,

While I have no specific disagreement with the ChatGPT text you posted, I want to advise against using ChatGPT and similar applications in this way.

The way they work is something like pattern matching, and they're optimized for sounding good, but they're not actually thinking and it's not actually analyzing the viability of the idea—just pattern matching against the kind of responses similar queries have gotten.

One example is that I asked ChatGPT to do a little multiplication: 37383 x 284482

ChatGPT says 10,641,589,506

The actual answer? 10,634,790,606

It got it pretty close, but not correct, because it's giving you an answer that looks right based on the answers in its training set but not actually thinking or doing the math.

So what you're getting here is something that sounds like analysis but isn't really.

It's definitely true that sunny desert locations are better for solar than cloudy northern locations, both based on higher average energy production and on more predictable energy generation.

As far as EROI specifically, I'm not too worried in that I basically think it comes out in the wash when you're looking at $ cost. The US has both solar tariffs and subsidies. I think on net the subsidies are a little bigger (US domestic solar manufacturing is only about 12% of installs ATM) but not enough to dramatically change the equilibrium either way. Different countries have different subsidies but the growth of solar is pretty consistent globally, especially in the developing world where ten hours of electricity can be a big improvement from zero. Likewise energy markets are global enough that it's also not arbitrage of energy prices between the US and China.

And solar prices just keep falling. So maybe West Texas works now and East Texas is more marginal, and maybe in a couple years that won't be so true (or they'll build out more transmission). My high-level theory of solar vs nuclear is that it's so much faster and cheaper to implement improvements in solar that (with the semiconductor boom happening at the same time) we shouldn't be surprised that it eventually won out. Rates vs levels, exponential improvements, etc.

I haven't been following this thread closely and maybe I'm missing something but I notice kbd512 cities 2,100 GWh per 1000T of silicon, which reduces down to 2.1MWh per kg. 1 MWh is one million watts for 1 hour (3600 seconds)—3.6 GJ. So the number being cited is that producing Silicon for PV cells consumes a bit more than 7.5 GJ/kg of silicon. As a gut check, the Gibbs free energy of SiO2 is -856.4 kJ/mol, which is about 30.6 MJ/kg of Silicon. The claim is therefore that the total process efficiency for SiO2 -> Si is about 0.4%. I find this extremely dubious. Is it possible an error was made?

In general the idea that IQ scores are a useful generalized measure of intelligence as a personal characteristic strikes me as silly and obviously wrong—it's maybe as useful as the SAT, which is itself not really that useful.

I don't think there's a single thing that you could call "general intelligence"—I'm pretty confident that ChatGPT would ace an IQ test, for example, just like it aced the bar exam.

Intelligence could be described as a synonym for aptitude, but you wouldn't say someone has a generalized aptitude, that doesn't really make sense. And regardless, hard work is usually more important than aptitude anyway.

Engineers and hard scientists are generally pretty skeptical of the measurements and studies coming out of the social sciences. Good social scientists will also express a healthy skepticism and tell you that their studies are not bulletproof but that they're doing their best to study something complicated but important. In the case of IQ, even social scientists have generally moved on.

As far as outer space is concerned, I'm about as interested in someone's IQ as I am in their grade in pre-calc. If they're a good at what they do and a good member of a team, that will come across from their work and in how they comport themself. Standing next to this, IQ has nothing to add and should be neither a gate nor a recommendation.

Lunar cement idea: 

https://gammafactor.wordpress.com/2024/ … -concrete/