Short Wave - A Quick Dive Into How Submarines Work

Episode Date: March 17, 2021

Submarines can descend thousands of feet below the surface of the ocean, but to do so, they have to deal with an enormous amount of pressure. In this episode, engineer and pilot Bruce Strickrott of the Woods Hole Oceanographic Institution explains some of the fundamental engineering principles that allow submarines to dive so deep without imploding under the pressure.Have any questions you'd like us to try answering? Send us an email, [e-mail]. See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hey, everybody, Emily Kwong here. So we check our listener inbox every day. And may I just say that we love your messages. They are thought-provoking. They get us thinking about story ideas. And every so often, there's a question so interesting, we must investigate. Like this one from five-year-old Corwin.

I want to learn how do scientists make sense. Submarines stay very, very not falling apart when they go to the bottom of the ocean. Bye. Submarines, excellent question, adorably asked, bonus points. Corwin's mom, Britta, also wanted us to know they're both super fans of the show. Well, Corwin, we are super fans of this question. And guess what? We called up an ocean engineer to help answer it.

My name is Bruce Strickrott. I am the manager of the Deep Suburgence Vehicle Alvin program at the Woods Hole Oceanographic Institution. So, Corwin, Alvin has nothing to do with chipmunks, by the way. It's a research submarine. And Bruce is a senior pilot. Alvin is a remarkable deep diving machine that carries three humans inside of an approximately six-foot diameter titanium sphere. that's inside of a 23-foot-long submersible machine. Isn't that so cool?

So Bruce has worked on Alvin for almost 25 years, helping researchers collect data and explore some of the deepest parts of the ocean. And we mean deep. Think about this comparison. Mount Rainier is a volcano out near Seattle. It's a beautiful place. Mount Rainier is a little over 14,000 feet tall. And that was the depth at which,

We could dive almost basically to the depth in seawater as the height of Mount Rainier is from sea level, a little over 14,000 feet. After some upgrades this year, Bruce says Alvin should be able to dive to almost 22,000 feet, or about 6,500 meters. And at those depths, Alvin has to withstand a lot of pressure to keep the humans inside safe. So today on the show, a brief look at how submarines, when they're deep underwater, water, stay in one piece. Or as Corwin puts it, stay very, very not falling apart. This is Shortwave, the Daily Science podcast from NPR. Today we're talking about submarines by focusing on Alvin. Alvin, as we said, is a research submarine that has taken scientists into the deep ocean. And it looks like a tiny underwater ship.

So way smaller than the giant cylinder-like submarines you might see in the Navy. But similar to them, because there's some fundamental engineering principles. at work, keeping Alvin and other submarines safe underwater. So first, how much pressure is Alvin under? Let's say, 22,000 feet below the ocean surface. Every square inch of Alvin's surface has almost 10,000 pounds of force applied to it. Which is a lot of pressure. And here's another way to think about it. Bruce calculated the amount of pressure that Alvin's hatch, so just its door is under. and he came up with around 4 million pounds. And that really was a number that was a little bit astounding to me. So I started figuring out how many 747s are parked on top of that 21-inch disk that is our hatch.

And the pressure he calculated is equal to four 747 planes stacked on top of that hatch. So you may be wondering what's keeping Alvin and the other people inside from getting crushed under all that pressure? Well, there's a lot of engineering at work. But Bruce says it's largely because of a part of the submarine called the pressure hull. That's the capsule everyone sits safely in, keeping the pressure inside equal to the pressure level at the surface. It's also known as Alvin's personnel sphere. We had a chief pilot once that used to say that the personnel sphere keeps the Big O out of the people's face. By Big O, he means all of that ocean.

So the personnel sphere is about six feet in diameter made out of a titanium alloy. And yes, sphere shaped. And Bruce says that shape is key. Think of pressure as giant hand squeezing the capsule that people are in. The water pressure is a big powerful hand that wants to squeeze all that low pressure space inside. The pressure wants to squeeze that tight. Well, a sphere, when you squeeze a sphere with your hands, if you apply that force from your hands equally, it's distributed around that sphere throughout the whole spherical shape.

That's how spheres are. They distribute the pressure evenly over its surface, making it an incredibly strong shape. You might be thinking, though, what about those big Navy submarines shaped like a tube? A tube, a cylinder, can handle pressure, but only to a certain extent. there's this unfortunate outcome called buckling where the pressure would actually squeeze that tube. Imagine the way you might squeeze a tube of toothpaste, only this is a big thick tube of metal, and they are limited in the depths that they can go to. What makes Alvin so special is its shape and size, and it can take people far deeper than those big military submarines.

But here's the thing, Alvin's hull, so it's wall, which is about three-eastern, inches of that titanium does actually deform as the submarine descends. So within Alvin, we know for a fact, because we've measured it when we tested it in this great pressure chamber down in Annapolis, Maryland, that the shape of the hole gets smaller as you get deeper underwater. That's a natural thing. And it actually gets smaller by about only a 16th of an inch on the inside. But when you add up that 16th of an inch, eighth of an inch difference perhaps over the whole surface area, it changes. changes volume by approximately 11 gallons at 6,500 meters. 11 gallons. Go to the supermarket and think about how much milk that is.

But don't worry, Alvin's titanium hull returns to its original shape when it gets back to the surface. Bruce says that predictability is in part why titanium is an excellent material for Alvin. They know how it will respond to intense pressure, to being tugged and pulled. And another thing about titanium, it doesn't react to seawater like aluminum and sand. steel. You get seawater on a piece of steel, it ruts. It's a chemical reaction. It corrodes. So we don't want corrosion. Corrosion isn't something we like. Titanium was almost like it was made for use in this kind of use. Shape, design, and material. These are the things that helped Alvin explore and survive the deep ocean for decades. It was commissioned in 1964, although the submarine

itself has been overhauled and upgraded many, many times. Bruce calls it a constantly evolving machine. It's been around since the year I was born, and I think it's a wonderful program that does amazing things, and we really love when young people pay attention because we think we'd love to have them come out and dive with a scientist. The future of Alvin could last easy another 50 or more years, so folks listening to this could easily be the next person,

we hire in 15 years. Corwin, hear that? That could be you. Thanks to Bruce Strikrott of the Woods Hole Oceanographic Institution. And of course, thanks to Corwin and his mom, Britta, for the great question. We loved answering it. Have any questions you're curious about? Send us an email at shortwave at npr.org. This episode was produced and reported by Rasha Arredi, with production help from Thomas

Lou, fact-checked by Burley McCoy, and edited by Viet Le. The audio engineer for this episode was Neil Rouch. Special thanks to Jeffrey Falzerano and Dave LaVolvo for their help. I'm Emily Kwong. Thanks for listening to Shortwave from NPR.