SPACE.com has learned that the failure of the NASA X-43A hypersonic aircraft in June 2001 was the result of inaccuracies in computer and wind-tunnel tests that were based on insufficient design information about the vehicle itself.
On June 2, 2001, the X-43A "stack" -- a modified Orbital Sciences Corporation’s Pegasus XL booster topped with the Hyper-X research vehicle -- was released from a B-52 carrier aircraft. Booster ignition went as planned, with the aircraft accelerating on its predetermined high-altitude ascent. Seconds later, however, booster fins broke off and the aircraft spun out of control. The vehicle was then destroyed by range control.
NASA convened the X-43A Mishap Investigation Board (MIB) to look into the failure on June 5, 2001 at NASA’s Dryden Flight Research Center at Edwards, CA.
The report has yet to be officially released by NASA, but SPACE.com obtained details of the mishap board’s findings that were approved in May 2003. Though the report clears the way for the return-to-flight of the X-43A later this year, no one contributing factor is singled out as being the prime cause of the vehicle's failure.
High-altitude ascent
NASA initiated the Hyper-X Program in 1996 to advance hypersonic air-breathing propulsion and related technologies from laboratory experiments to the flight environment.
This program was designed to be a high-risk, high-payoff program, drawing upon the cheaper, better, faster philosophy tightly embraced by NASA during the tenure of space agency chief, Daniel Goldin.
The X-43A was to be the first flight vehicle in the flight series, built to obtain scramjet technology data. The Hyper-X Phase 1 is a NASA Aeronautics and Space Technology Enterprise program being conducted jointly by the space agency’s Langley Research Center in Hampton, Virginia and the Dryden Flight Research Center at Edwards, California.
Root cause
According to the MIB documents, the X-43A Hyper X launch vehicle "failed because the vehicle control system design was deficient for the trajectory flown due to inaccurate analytical models (Pegasus heritage and Hyper X launch vehicle specific), which overestimated the system margins."
The investigative group reports that "during the pitch-up maneuver the X-43A stack began to experience a control anomaly" which led to rudder control failure. Shortly after the rudder stalled, the starboard fin was torn from the vehicle, quickly followed by the port fin, then the rudder and wing.
The X-43A fell into Pacific Ocean waters within a test zone off Point Mugu, California. No attempts were made to recover physical evidence from the X-43A. It now rests some 1,200 feet below the surface.
Modeling inaccuracies
In its report, the board states that the failure occurred because the control system could not maintain the vehicle stability during transonic flight. The X-43A mishap board points to a series of "modeling inaccuracies" used in ground tests, as well as "misinterpretation" of wind tunnel data due to insufficient data.
Furthermore, changes were made in thermal protection to the Hyper-X launch vehicle wing, fins and body due to the increased thermal loads predicted for the trajectory to be flown. But this additional thermal protection was not taken into account in preflight wind tunnel test modeling. Computer and wind tunnel tests to help understand what caused the failure showed that the new thermal protection did alter the booster’s aerodynamic characteristics.
Insight and oversight: bad marks
"No single contributing factor or potential contributing factor caused this mishap," the document states. The board found that the flight mishap could only be reproduced when all of the modeling inaccuracies are taken into account in post-failure work dedicated to unraveling the cause of the X-43A mission loss.
The X-43A MIB report underscores the fact that the Hyper-X launch vehicle contract was developed under the faster, better, cheaper philosophy.
There were "strong indications", the report observes, that contractor staffing at Orbital Sciences Corporation in critical technical areas, such as aerodynamics, software, and guidance, navigation and control, needed to be supplemented.
Moreover, the board concludes that increased technical insight and oversight by NASA itself might have resulted in detection of critical errors in readying the X-43A for its maiden flight.
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