[z303]

The biggest problem I see with intercepting a SR-71, is not getting another plane up to the same sort of altitude but getting the plane in the right piece of the sky, taking the wikipedia stats as an example, Mach 3.2 (2,200+ mph, 3,530+ km/h,1900 knots+) at 80,000 ft (24,000 m), if your interceptor take 2 minutes to get up to 80,000 ft, even doing some sort of zoom climb your aiming at a point 73 miles in front of the SR-71 position at take off, and I would expect the real world time to be longer from detection to interceptor at altitude. So you could only do it with a large radar network to track the SR-71 plus airbase in front of the current flightpath.

Then even if you did launch a missile it would not have an easy job of hitting the target