As we saw with the first flight attempt, many things have to work properly before they can even attempt to light the engine. It's a high risk flight profile. The Pegasus has to separate cleanly from the BUFF, ignite, pitch up and climb, level off and accelerate to a very high speed, then shutdown and separate cleanly from the X-43A. The X-43A then has to open the engine inlet and successfully ignite the propellant. Tricky stuff.

Let's all hope for a success.

Positive net thrust at Mach 10 would be a wonderful achievement. I still think that AW&ST article I referenced last week has some good points about developing a Mach 7 scramjet powered first stage. For one thing, 20K pounds to LEO is a pretty good payload, especially with the combination of significantly lower risk (should result in much lower insurance premiums) and about a 1/3rd reduction in per-pound cost to orbit. Combined, those savings could result in a substancial reduction in flight costs, and developing a Mach 7 first stage should cost a lot less than going for Mach 10+. Amortizing the higher R&D costs could more than offset any operational cost savings achieved by making a faster first stage. All too often, I see projects spend an incredible amount of money trying to squeeze the last few percentage points of performance out of a system, only to make the system too expensive to operate.