One thing everyone is overlooking in an escape rappel from a World Trade Center scenario is that rappelling down is only half the battle - if you're 50-60 floors up, you have to get back INSIDE. Modern office window glass isn't bullet proof, but it's the next best thing. What's the point of rappelling down 4 or 5 floors if you're then left dangling on the rope as it slowly melts from the intense heat?