The hallway inside scoop that I've heard is the X-43A flew at constant Mach number while under scramjet power. If I was better engineer, I would have figured that out on my own. This is an initial flight test of an unproven technology. The inlet duct would almost certainly be of a fixed geometry (not a variable geometry like the inlets on the XB-70 or the SR-71). A fixed geometry inlet means the vehicle would have to fly within a very narrow range of Mach number. Otherwise the oblique shockwaves bouncing down the inlet would not set the combustion chamber at the correct thermodynamic state. Because the inlet's geometry was fixed, the vehicle's thrust would have been throttled to precisely match drag and cruise at constant Mach number.

Given that constant Mach number was the design, it is interesting the vehicle cruised at Mach 9.8 and not Mach 10. This means their CFD was not dead nuts accurate (I wouldn't expect it to be this early in the game).

During the NASA television broadcast, I noticed they were doing many angle-of-attack and side slip oscillations as the vehicle descended as a glider. This, of course was intelligent. It means NASA now has a comprehensive flight data validated aerodynamic model for this vehicle from Mach 10 all the way down to impact. I was told that all flight will be kept classified. I was also told that NASA is now out of the air breathing hypersonics game and the baton will be passed to DARPA. My old office mate, Prof. Alan Paull at the University of Queensland must now be smiling like the cat who ate the canary. He's running the joint Australian-US scramjet program and is funded by DARPA. Assuming the US government isn't stupid (a big assumption), the classified data from X43a along with the CFD software used to design it will be handed over to Alan for the follow-on work. Alan is now positioned to become mankind's best hope for achieving Cheap Access to Space. The politics will be brutal but the potential rewards are enormous.

Gary