September 11, 2002
9/11 Department
Defense in Depth
While looking up information to write another post, I stumbled across this old Charles Murtaugh posting:
So here's my first blog rant of Spring. The papers have been full of detailed scientific stories about the physical circumstances that led to the collapse of the World Trade Center. I.e., did the physical impact of the hijacked planes make a difference, or was the fire sufficient to cause the collapse, were the fire suppression systems in the towers inadequate, was there something wrong with the fireproofing insulation, blah blah blah. And now apparently a two-year, $16 million federal study is underway to consider changes to building codes and standards, to help prevent this happening again.
Excuse me, but this is just so September 10th. Asking these questions is like asking why Daniel Pearl died, and concluding that he died because of lack of oxygen to the brain. Well, yes, that's because his throat was slit by a fucking terrorist! And the World Trade Center collapsed because planes were hijacked and flown into it! Any response to prevent it happening again that is not based on this first cause, rather than on the inconsequential and contingent secondary causes, is pathetic and misguided. And, unfortunately, depressingly, predictably, very American.
I don't know what programs Murtaugh saw, but I agree that plane-proofing is a very American response, and I think it's a great idea.
First of all, these reports were fascinating to anyone with even a passing interest in engineering. The way the towers collapsed surprised a lot of people, engineers included, and most surprising phenomena are worth further investigation.
Second, while a good offense may be the best defense, the second best defense is defense in depth. In fact, a good offense is part of a deep defense.
A single line of defense is brittle: If it fails, you're doomed. With multiple lines of defense, you have a fallback against failure of one of your defenses.
You also force your enemies to go through a lot more work to win. If your house has a locked door, it forces an intruder to break it down or pick it before he can harm you. If you have an alarm, it forces him to move fast before the police arrive. If you have a dog, it forces him to fight the dog. If you have a gun, it forces him to fight you. However, if you have a locked door, a dog, an alarm, and a gun, it forces him to break down the door while fighting both you and your dog simultaneously, all before the police arrive. It's a lot harder.
Currently, our country is planning extensive deep defenses to defend against another 9/11-style attack: We're increasing airport security, encouraging passengers to fight back, stiffening passenger resistance with sky marshals, strengthening the cockpit doors, arming the pilots, patrolling the skies with F16's, and protecting targets with anti-aircraft missiles. Adding yet another layer of defense by hardening the targets is not a foolish idea.
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