Concorde skin temperature limitation

Heat-treatable aluminum alloy over-aging set the speed limit on the Concorde. Used to illustrate why aluminum's high-strength temper is temperature-limited.

The problem with the heat treatable is they can start to degrade at fairly low temperatures. The supersonic transport Concorde was not limited on speed; it was limited on the skin temperature of the heat-treatable aluminum alloys. They would start to over-age and you'd lose your precipitation hardening. They actually had sensors on the surface that would measure the temperature, and the colder it was up there, at whatever altitude you were, the faster you could go. But you didn't want to destroy your aircraft by overheating the aluminum. The same type of thing — they've just started coming out with aluminum in automotive brakes. The first real problem of trying to use it for calipers — not the drum but the caliper that holds the brake pads — was they would creep. You hit your brakes hard, you get too much temperature, and all of a sudden the caliper holding the brake pads starts to splay out, and you no longer have any pressure on your brakes. It's called brake fade, and it's not a good thing. They fixed some of that.

Skin-temperature-limited operation, not speed-limited. Personal anecdote about flying on a poorly-maintained Concorde.

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.

Stated as a fact ("flew not on speed but on temperature") — Concorde carried skin sensors that gated cruise speed by ambient cold. Functions as the technical hook for the personal anecdote.

Did I tell you the story that the SST, the Concorde that the French and British built, flew not on speed but on temperature? The skin temperature was limiting their speed. They had sensors to measure the temperature of the outside skin, so if it was really cold out, it could go faster. I flew the Concorde once. This was back in the 90s, and I'd never flown a supersonic transport — most people hadn't. I had to go to Europe for something, and MIT had to pay my business-class airfare, and I said, okay, I will spend the $2,400 for the upgrade coming back out of my own pocket just to see what it's like to fly on an SST.

Used to illustrate that aluminum aging temperatures (165 C and lower) are not "that hot," so aircraft skin frictional heating becomes a design constraint. The Concorde was speed-limited by skin temperature, not engine thrust — colder atmospheric temperatures permitted faster transatlantic crossings.

The Concorde, the SST that the French and the British flew, was not limited in speed, it was limited by skin temperature. They had heat treated aluminum alloys, and they had thermocouples — temperature sensors — to measure the frictional heating in the atmosphere. On a really cold day at sixty thousand feet or whatever altitude they were going to fly at, minus 60 degrees Centigrade, they could go across the Atlantic faster. On a warm day up there, minus 30 Centigrade, less cooling. The frictional heat on the skin — I think I knew once — they couldn't exceed something like 200 degrees F on the skin before they were going to get into over-tempering and the life of the aircraft was going to be shortened significantly. You needed a life of the aircraft that's going to go out to ten thousand hours or something, okay.