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Comment by londons_explore | Hacker News Reader

Why the desalination plant?

Is there not piped water? Is it just another eco friendly sounding thing to put in the plan? Is it to hedge against newspaper headlines saying how much water spacex is wasting?

Desalination has to be more expensive than piped water (both financially and environmentally).

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Boca Chica doesn't have piped water.

"There are no shops or restaurants or amenities of any kind around, including municipal water pipes; Cameron County regularly trucks in gallons of water, which is stored in outdoor tanks."

https://www.theatlantic.com/science/archive/2020/02/space-x-...

The whole piece is worth a read btw, great article.

Yep, a good read!

Take a gander at Google earth, Boca Chica is far enough away from Brownsville that I doubt it has municipal water or would ever be serviced. Salt water and steel don’t mix well so I’d assume they want to stop trucking it in.

They have plans for building launch sites offshore. They're presumably exercising their muscles on as many of the end-game problems as they can.

The bubble wall tech will probably be a big enough problem to sort out once they try to build them.

Can you expand on the bubble wall?

When launching rockets on land you need flame diverter trenches to deal with exhaust and water deluge systems to deal with the sound (the water droplets absorb sound). When launching on water, dealing with the exhaust is more straightforward, but sound travels much better in water than in air, so it's a much bigger problem (mostly for marine life). One idea is to create a curtain made out of air bubbles that effectively blocks sound. The bubbles force sound waves to transition from water to air and back into water, and as you know from putting your head underwater in a pool, sound waves don't like doing that in either direction.

Wait, the water they spray on a land launch is only for sound suppression?

Yes. Without it large rockets like the space shuttle might create more than 200db at lift off, enough to kill someone or to damage the rocket

https://en.wikipedia.org/wiki/Sound_suppression_system

If rocket damage is a concern, why don’t such rockets destroy themselves right after leaving the launchpad? At that point the water suppression is no longer relevant but the sound waves are just as powerful. It can’t just be that the ground reflects sound waves, since that would only be a ≈3dB (2x) increase in sound energy. Rockets with >2x more thrust seem to use the same sound suppression, so rocket damage can’t be the main reason for installing such systems.

My guess is that water systems are needed to reduce damage to the launch pad.

You are proposing...a self destroying launch vehicle?

Would you ride it?

Larger rockets use water as part of the overall suppression system yes, but that doesn't make the whole system the same. The size, depth and shape of the flame pit, the rate of water delivery and the design of the spay system, etc will all make a difference.

The issue is that the sound waves bounce off the pad back towards the rocket. Once the rocket has left the pad, the sound waves proceed behind it and there's no risk of them hitting the body.

Mostly, yes. Some of it is used for protecting the structure (ablative armor, essentially). But that requires much less water than is actually used. This is why Sea Launch's launch platform also had a water deluge system. They're far enough out that the only people that can really hear it are the control ship, but they have the deluge to help keep the exhaust from scorching the launcher.

Similarly, the Saturn V MLP had "rainbirds" to wet it down, even though the vast majority of the water used was down the exhaust trench for sound suppression.

It may be that they need too much for the local water company.

During launches, they need to spray water- a lot of it. Using salt water would probably be problematic. Lots of residue left over.

Salt water is also corrosive.

Much of Texas has brackish water when pumped from wells. It’s the reason a lot of desal tech originated there, FL and CA.

Additionally, Texas also has very few natural lakes and reservoirs.

Specifacally, only one lake is natural. The rest of them are manmade.

Which lake is that?

Lake Caddo way out east basically Louisiana border. Louisiana has a LOT of lakes.

Kind of a fun bit of trivia I picked up working at a geoscience company: Those brackish wells can be very rich in lithium, especially in South Texas.

The site has extensive solar power, I'm not aware of environmental concerns for solar-powered desalination.

I would guess that it is driven by a real need for water and a hedge against environmental assessment difficulties.

What are they going to do with all the brine. It's quite toxic and I don't think it's a good idea to dump that back in the sea.

Is it? I'd guess putting the brine back far enough in the water, away from the seafloor, would provide plenty of dilution. I'm doubtful that they'd desalinate at such a rate that would drastically change local conditions where they return the brine.

I'm just guessing here, but that's my half informed idea.

Brine has set waste disposal costs and methods - near the ocean and with active oil wells nearby there are many options.

Yup. Salinity would be 36 ppt, so for every 1000 tonnes of water they need, they'd produce 2 truck loads of salt (so say maybe 20 truckloads of brine). It's a lot easier to truck out and dispose of 20 loads of brine, than to source and truck in 250 loads of fresh water

Maybe they can use that salt for the Tesla lithium extraction?

Extracting lithium from water is indeed a promising area to keep an eye on: https://www.bbc.com/news/uk-england-cornwall-54188071

The sea can handle it, it's just a matter of how it's introduced back into it.

Dilute it? There’s a whole ocean right there.

If you dump industrial amounts of salt in the ocean, it kills sealife in the immediate area.

The discharge from the Sydney desalination plant has been unexpectedly good for sea life:

https://www.abc.net.au/news/2019-12-19/sydney-desalination-p...

Maybe they can make concrete out of it

https://www.fastcompany.com/90338907/a-new-breakthrough-lowe...

Or they can disperse it atmosphere to fight climate change

https://www.livescience.com/62140-can-salt-stop-climate-chan...

Finally they can serve it with shitload of tequila :)

If you desalinate enough water, getting the salt back into the ocean without killing local marine life can be a bit tricky.

> Is there not piped water?

Not, there is not. All water is currently delivered, and the nearest municipal water is over 15 miles away.

Arranging for a water pipeline would not be cheap, but moreover would take a lot of time. Desalinization plant is plug and play; pipe is going to require a lot of bureaucracy besides a pretty fair capital investment.

Hard to believe 15 miles of roman technology cost more than super advanced solar + membranes plant.

Desalination uses a lot of energy, so if energy is cheap, then so is desalination.

Both solar and wind power have gotten incredibly cheap (~ $15/MW), and more importantly will continue to get much cheaper over the next decade. Boca Chica has lots of wind & sun.

Desalination can also handle the variability of wind & solar quite well: just use water from a big water tank when both wind & solar aren't available. Expensive batteries not required. And to a first approximation it's always windy in Boca Chica, so not much storage is required anyways.

OTOH, piping water from Brownsville to Boca Chica is probably a 9 figure project.

Boca Chica is a relatively isolated location wedged between the Gulf of Mexico and Mexico itself. Texas is a big place, and if you want fresh water you might just have to make it yourself.

Aquifer in the area is brackish, but huge. San Antonio even has their own Desalination plant to process their water that they get from their aquifer.

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