Six months after its maiden flight ended in the aircraft self-destructing, the US Defense Advanced Research Projects Agency (DARPA) says it knows what went wrong with the Falcon Hypersonic Test Vehicle (HTV-2).

Nine minutes into the flight - which was to take the HTV-2 4,160nm (7,700km) across the Pacific at Mach 20 along the edge of space – eight DARPA telemetry assets lost signals from the dart-shaped glider because of “flight control limitations to operate at the angle of attack the vehicle was programmed to fly for the speed and the altitude of flight.”

The US agency says the independent engineering review board concluded the probable cause of the anomaly was a “higher-than predicted yaw, which coupled into roll, thus exceeding the available control capability at the time of the anomaly.”

A second Lockheed Martin-built prototype will fly in late 2011, but not before some adjustments are made, though there will be no major changes to the vehicle or its software, according to David Neyland, DARPA’s tactical technology office director. “Engineers will adjust the vehicle’s center of gravity, decrease the angle of attack flown and use the onboard reaction control system to augment the vehicle flaps when HTV-2 flies next summer,” he says.

DARPA never intended to retrieve the vehicle itself, which would have been destroyed when it splashed down in the Pacific Ocean north of the Reagan Test Site in Kwajalein Atoll. Instead, flight information was to be gathered by six sea-based and two airborne telemetry assets.

In spite of losing the aircraft so early in its flight, DARPA still counts the HTV-2 test as a success, with “the first ever use of an autonomous flight termination system” and the first launch of the Minotaur IV booster.

HTV-2 was originally planned as the lead-in to the development of an HTV-3X vehicle, known as Blackswift, which would have formed the basis for deployment around 2025 of a reusable Hypersonic Cruise Vehicle, an unmanned aircraft capable of taking off from a conventional runway with a 5,400kg (12,000lb) payload to strike targets 9,000nm away in under 2h.

While the hypersonic cruise vehicle programme continued to receive reduced funding, Blackswift was cut from the fiscal year 2009 defence budget and subsequently cancelled.

Source: Flight International