| There are three mitigating points for that issue: First, you would get choked flow at the inlet, and if you do the math it actually takes a massive opening to really make a dangerous shockwave. Eg, if a joint in the tube disconnects and the two halves pull apart, the inlet area is the tube circumference * gap between halves. The two halves would need to separate by the full diameter of the tube (~15 ft) to get full atmospheric pressure going in both sides. A realistic gap of a couple inches to a couple feet is far less scary. Second, these incidents will be exceptionally rare. Gaps that big are equivalent to a freeway overpass collapsing. It's a total failure scenario, so you see a single digit number of them a year in the US. Lastly, the speed of sound isn't all that quick on the scale of this thing. It's about 13 miles/min, a fifth of a mile a second. If you place emergency fill valves (one time use things, dirt cheap, just explosive bolts in the simplest case) you can flood a whole section of the tube with atmosphere in a controlled manner over say 15 seconds, and your shockwave will be largely stopped after 3 miles. An emergency fill like that would also stop all pods in the region as they hit very high drag, so you don't even need to carry heavy emergency brakes on every vehicle. |