Questions are being raised about X-33 performance. Intended to fly to Mach 15, its ability to reach that speed is in doubt. T. Cleon Lacefield, LockMart X-33 program manager, states that it should reach Mach 13, at a minimum. That is still sufficient to test the vehicle's thermal protection system. However, some engineers are reporting that X-33's top speed will really be Mach 10, and that this has been known within the project for some time.
Weight is the culprit.
The weight at main engine cutoff has risen to 83,900 lb.