"The X-51 safely separated from the B-52 and the rocket booster fired as planned. However, after 16 seconds, a fault was identified with one of the cruiser control fins. Once the X-51 separated from the rocket booster, approximately 15 seconds later, the cruiser was not able to maintain control due to the faulty control fin and was lost."
Hypersonic X-51 Unsuccessful - Engine Not to Blame, Aviation Week
"No confirmation yet, but sources are telling Aviation Week that [Wednesday's] attempted Mach 6 flight of the US Air Force Research Laboratory's Boeing X-51A Waverider hypersonic demonstrator was not a success - but that the vehicle's Pratt & Whitney Rocketdyne SJX61 dual-mode ramjet/scramjet engine was not to blame."