Rockets and Guns
OK, people, I probably don't need to explain this to everyone, but for the incredibly thick among you:
I think you'll find that regardless of actual need or the state of the budget, etc. the Navy, Air Force, and their ilk are likely to press for weapons development. Currently, the state of the art is a projectile launched from some sort of tube, which, when it strikes its target, explodes and causes all sorts of damage. The projectile can be launched from a so called "gun" which imparts all of its kinetic energy at once, or a "rocket" which trades chemical for kinetic energy as the projectile moves towards its target.
The "new generation" iterations of these ideas are the rail gun, and this mach 9 rocket sled. This was probably just a proof-of-concept test run, given the most favorable conditions possible. Would have been better had they tested this in space, but then what would the Russians or Chinese think? They'd be scared, or they'd race to develop weapons of this sort, also. So, ground testing it is.
I really does not matter what the warhead is, at these speeds. 100 lbs. (50 kilos) of cement traveling at mach 7-10 has so much kinetic energy that it tends to vaporize whatever it hits. Go ahead and classify the warhead. It's irrelevant. What these extremely high speeds tend to do is make it very difficult to intercept the projectile on its way to destroying the target.
The only thing that has a chance against both the "gun" and "rocket" version of these hypersonic projectiles is that South African kill bot that went crazy and decided to kill a few of its human operators.
Personally, I wouldn't want to put my own safety in the hands of a such a paranoid machine. It might decide *not* to intercept, just to spite the humans.
Oh, and a hydrogen-filled tunnel would burn when exposed to the exhaust gases coming out of the rocket motor, assuming, as most people do, that incomplete combustion occurred.