SR-71 Blackbird

One of the first titanium aircraft; titanium skin because aluminum couldn't tolerate the frictional heating at Mach 3–4 / 90,000 feet.

Lanthanum — I don't know any use of lanthanum except to make lanthanum hexaboride filaments for scanning electron microscopes. It's the lanthanide series, so it's famous for that. Titanium — 60,000 tons. Titanium is a fantastic metal in many ways. I have a little pacemaker can from forty years ago. Pacemaker that you put in your heart, to signal your heart with a pump. They've been making them out of titanium for fifty years, because titanium just doesn't corrode in the body. It also doesn't corrode in heat exchangers. It's got good high temperature capability. The Concorde flies on temperatures — the SR-71 Blackbird had a titanium skin. It was one of the first titanium aircraft. Aluminum wouldn't make it to 90,000 feet at the temperatures you get from frictional heating at the speeds those things go, Mach 3 or 4. The B-2 bomber is mostly titanium in terms of its skin.

At 60–70,000 feet at supersonic speeds, skin temperature requires titanium not aluminum. Used as the high-temperature endpoint of the aluminum-titanium tradeoff.

Yeah, air friction on the skin at 40,000 feet. Now you start getting up to 60 and 70,000 feet where the spy planes go, they actually had to use titanium because they couldn't use aluminum. Don't ask me why — I'm not sure I understand why as you get more rarefied you get more friction. Did you know, Sam?

Mach 3 / 100,000 ft skin temperatures exceeded aluminum capability, requiring titanium skin.

We use lots of titanium dioxide — all the white in this room, it's titanium dioxide. But it takes a lot of energy to refine titanium to its metallic state. It also doesn't have very good fracture toughness. It is lightweight, and the SR-71 had titanium skin, because when you're flying at a hundred thousand feet at Mach 3, the skin temperature is above the temperature at which aluminum will maintain its strength. The Concorde supersonic jet didn't fly on speed; it flew on skin temperature. Colder day, fly better across the Atlantic.

The exemplar of when only titanium will do: aluminum over-ages at the skin temperatures, steel is too heavy, beryllium too expensive. Used to close the market-size argument by showing why a tiny-market material gets built at all.

But there are some things steel can't do, like fly light. When you need to build an SR-71 Blackbird, titanium skin — can't do it with aluminum, can't do it with steel, too heavy to fly. Aluminum basically over-ages at that surface temperature, doesn't quite melt but almost, because of the melting temperature of aluminum. Material choice — titanium. You could make it out of beryllium except you can't afford it. The price of beryllium is another factor of 10 to 50 above the price of titanium.