It should be noted that DARPA is late to this game. Most of the heaby lifting was done by the DSTO before the US DoD got involved.

The US has a number of FAR MOER EXPENSIVE projects in this area that have not gotten to this point.

DARPA scramjet nudges Mach 10

By Lester Haines
Published Monday 18th June 2007 09:26 GMT

Australia’s Woomera Test Facility last Friday hosted the successful launch and firing of a scramjet engine which reached speeds of “up to Mach 10”, the country’s Defence Science and Technology Organisation (DSTO) has announced.

The HyCAUSE vehicle – a joint project between DSTO, the US’s Defense Advanced Research Projects Agency (DARPA), and the Australian Hypersonics Initiative (AHI)- took off atop a TALOS rocket and reached a heady 530km before firing its way to around 11,000km/h (6,800mph), according to Oz’s parliamentary secretary to the Minister for Defence, Peter Lindsay.

…