Space Shuttle Challenger

Triggered by ATK Alliant being awarded the Rolls-Royce composite fan case contract. Tom's framing: the State of the Union teleconference with Christa McAuliffe was the political "externality" that overrode the engineer's cold-weather warning. Bracketed slip: `Boss Juliet [Boisjoly]`.

This is potentially a good application for composites. Rolls-Royce just gave the contract to ATK Alliant — Alliant Technology, I can't remember what the K stands for. They're in Magna, Utah. What were they famous for? Anybody know what Alliant built that blew up? Space shuttle Challenger — the seals. That was all Alliant technology. They designed the seals for the Challenger that caused the Challenger to blow up. An engineer, Boss Juliet [Boisjoly] at Alliant, said don't, it's too cold, don't fly it. But they were going to fly the Challenger. Anyone remember the reason why they flew the Challenger?

Referenced as the cautionary example for what happens when joints leak — motivating why the external tank used welded 2219 aluminum.

One alloy in the aluminum-copper series, 2219, has very good weldability. That's what the Space Shuttle main tank was made out of for many years, because you had to weld it. You don't like leaks — they found out with Space Shuttle Challenger what happened with leaks of the O-ring. So they used 2219 aluminum for the tank when we had a Space Shuttle. If you get down to the 6000 series, you get a lot of A and B weldability. 6061 and 6063 — the difference is, this is typically plate or sheet and this is typically extrusions — they have A-grade weldability. The largest tonnage of aluminum alloys is 6061: architectural aluminum, door frames, window frames, you name it. You can get good strength in the base plate, but we're going to see you only get about half the strength in the weld as heat-treated. The 7000 series — some of them have very good weldability. Some of them, like 7075, which was the workhorse aviation alloy — 2024 was the workhorse alloy from 1920 to 1950, and from somewhere around 1960 to 1970, 7075 was the workhorse aircraft alloy for Boeing and McDonnell Douglas. Since then, Alcoa has come up with other alloys that have better combinations of properties.

There's actually a book — this one right here. [Tom holds up Petroski's book.] Henry Petroski. Does anybody know who Henry Petroski is? He's a professor down at Duke University. He wrote this book, To Engineer is Human: The Role of Failure in Successful Design. He was elected to the National Academy of Engineering the same year I was, for having written this book, with the idea that we only learn from our mistakes. The Hyatt Regency collapse is in here, and if you watch some of the other videos I'll talk about that and use his book. The theory is we keep on building things bigger and bigger until we hit the limit, and then we have a big failure, a bunch of people die, and all of a sudden we say, what's going on? Like the space shuttle Challenger, someone brought that up the other day.

The X-33 was supposed to replace the Space Shuttle. Tom uses the Shuttle's claimed-vs-actual reliability (1 in 10,000 claim, ~4% actual demonstrated by the Challenger failure at flight 25) to establish that NASA's reliability calculations are systematically overoptimistic — context for the X-33's similar failure mode.

So if you look this up, Lockheed Martin — I guess it was Lockheed before Lockheed Martin that did the C5-A and beat Boeing out for the aircraft transport back in the 60s — the X-33 Lockheed Martin was supposed to replace the space shuttle. It was supposed to be single-stage-to-orbit. The problem with $10,000 or $20,000 a pound in orbit is that you have to throw away your vehicle on every trip, and that's expensive. That's one of the things SpaceX has done. SpaceX has a returnable vehicle, in theory, as long as it doesn't blow up on the pad, which they do. We can talk about those statistics. If you can bring back your structure and reuse it — that's what the Space Shuttle was supposed to do. If you go back and look at the claims and the justification, the Space Shuttle was going to take $10,000 a pound in orbit and drop it to $1,000 a pound. It turns out they had enough problems that they actually raised the price to about $40,000 a pound. The Space Shuttle was never cost-effective. But it was our main way of getting things in space for thirty years. We had a fleet of five, and when we lost one we built another. Finally it was just too expensive to maintain and they decided to go back to the old disposable rockets.

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Tufte's "unsuccessful" example. Solid rocket motor o-ring blow-by at low temperature. Tom teaches the engineering design of the SRM, the recoverable-vs-expendable architecture, and contrasts solid-fuel and liquid-fuel control. The launch politics (NASA pressure, $1.3B continuation contract, State of the Union, Christa McAuliffe) frame Boisjoly's failed warning. Feynman's o-ring-in-ice-water demonstration closes the case.

The next one is the Space Shuttle Challenger disaster, 1986. Here's the space shuttle. It has two solid rocket motors, one liquid oxygen / liquid hydrogen tank — the center tank, which is expendable. The solid rocket motors are actually recoverable. They have a parachute, they eject them and they fall into the ocean, and they recover and reuse them. The main tank was not recoverable. It went further out into space and got burned up in the atmosphere. But the solid rocket motors were designed to be reusable.

Opening anecdote on why things fail at joints. Tom recounts his on-the-day prediction that *Challenger* failed at a weld; correct on "joint", wrong on "weld".

There are many failures, and this is kind of where we stopped last time. When I heard about the Space Shuttle Challenger in 1986 — my students told me about it that morning — I said, "I bet it failed at a weld." I was wrong, but it did fail at an O-ring joint. It was at a joint. Things typically fail at joints because they're the highest-stress locations in the structure. It's just sort of natural. They also may not be 100% efficient depending on the material.