United States Air Force is Collaborating With DARPA for X-51A Follow-on

The Air Force has partnered with the Defense Advanced Research Project Agency, or DARPA, to advance hypersonic flight and launch a new joint developmental effort, service officials said.

The initiative, now being finalized, is designed to build upon the heels of a successful May 1 flight 60,000 feet over the Pacific Ocean wherein the X-51A Waverider demonstrator aircraft accelerated to Mach 5.1.

The new program is designed to advance the research and capabilities of hypersonic flight following the May 1 demonstration or test flight, said Air Force Chief Scientists Dr. Mica Endsley.

link.

huh.  Crazy thought here.  I'll post that another time.   However, imagine very stealthy bombers sneaking up and lashing out with a mix of hypersonic and subsonic stealthy cruise missiles.  It makes the nuclear bombers even more scary and difficult to protect against.  Do you only chase down the missiles going 8 km/s or will you split your forces to hunt for the slower, harder to see missiles?

There are also concerns if you have hypersonic weapons (or other 'prompt global strike' class weapons) whether they are conventional missiles...or not.