(not available online) about the scramjet test. The article is "Last Mile" and is on pages 57 & 58. It discusses using scramjet technology for a booster first stage. Here are some bits:

A reusable air-breathing space-launch first stage growing out of the successful X-43A Hyper-X test flight in March could be ready to send 20,000 pound payloads to low Earth orbit (LEO) by about 2015, say the engineers who developed the Mach 7 testbed.

Holding an operational first stage to that speed instead of trying for stage separation in the Mach 12-15 range would make development challenges manageable and perhaps affordable, the team says in a paper delivered to the International Astronautical Congress (IAC) in Vancouve. The Mar. 27 X-43A flight test showed that existing wind tunnels could provide the "relevant environment" for testing the remaining technology needed, eliminating the requirement for an expensive large-scale flight demonstrator or new ground facilities."

...

An application like an air-breathing first stage that could be available just as the exploration initiative begins to need a lot of rides to LEO could revive the hypersonics effort. While McClinton concedes "we're never going to get heavy lift; we're not a threat to the rocket guys," a vehicle able to take 20,000 lb. to LEO would cover 80% of the launch market identified by NASA's old Next Generation Launch Technology program.

Although operational costs would be somewhat lower than for expendable rockets - $1,700 per lb. to LEO verses $2,500 for a baseline expendable - the main benefit would be in safety and reliability. Citing a study published by the American Institute of Aeronautics and Astronautics in July 2003, the X-43 team claims a Mach 7 hypersonic first stage could cut the risk of vehicle/payload loss to one in 4,000 from one in 50, chiefly through its abort capabilities.

The Mach vehicle proposed by the authors of the IAC paper, including Hyper-X Program Manager Vincent L. Rausch, would use a Turbine-Based Combined Cycle (TBCC) engine burning hydrocarbon fuel for the turbojet and liquid hydrogen for the ramjet/scramjet phase of the flight. The turbojets - a variant of the F135 or F110 engine - would push the first stage, an expendable upper stage and the payload from a horizontal takeoff to about Mach 2.5. At that speed the transition to ramjet/scramjet operations is less challenging than at the higher Mach 3.5-4 transition speeds needed for a Mach 12-15 first stage.