DARPA’s HTV-2 Fails to Complete Second Hypersonic Flight Test

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The wedge shaped HTV-2 capsule is designed to fly about 20 minutes, proving the capability to endure atmospheric flight conditions at Mach 20 speed. Photo: DARPA

The second and final flight test of DARPA’s Hypersonic Test Vehicle 2 (HTV-2) failed today as the capsule, launched by the Minotaur IV missile, was lost upon reentering earth atmosphere, about nine after launch. DARPA has assembled an independent Engineering Review Board to review and analyze the data collected through the experiment. The findings will inform policy, acquisition and operational decisions for future Conventional Prompt Global Strike programs — the goal of which, ultimately, is to have the capability to reach anywhere in the world in less than one hour.

The wedge shaped HTV-2 capsule is designed to fly about 20 minutes, proving the capability to endure atmospheric flight conditions at Mach 20 speed. Photo: DARPA

The wedge shaped unmanned hypersonic test vehicle was launched on 07:45 PDT by a Minotaur IV rocket from Vandenberg Air Force Base on the pacific coast, The flight was expected to take about 20 minutes. The Minotaur IV vehicle successfully inserted the HTV-2 into the desired trajectory, confirming separation by a rocket installed camera. At this phase the aircraft transitioned to Mach 20 aerodynamic flight. According to DARPA, this transition represents a critical knowledge and control point in maneuvering atmospheric hypersonic flight. More than nine minutes of data was collected before an anomaly caused loss of signal. Initial indications are that the aircraft impacted the Pacific Ocean along the planned flight path. The second test vehicle failed at the re-entry phase, where extremely high thermal loads develop. It was the same mission phase where the first test failed last year. “We know how to insert the aircraft into atmospheric hypersonic flight [but] we do not yet know how to achieve the desired control during the aerodynamic phase of flight.” said Air Force Maj. Chris Schulz, DARPA HTV-2 program manager.

“Prior to flight, the technical team completed the most sophisticated simulations and extensive wind tunnel tests possible. But these ground tests have not yielded the necessary knowledge. Filling the gaps in our understanding of hypersonic flight in this demanding regime requires that we be willing to fly,” said DARPA Director Regina Dugan. “In the April 2010 test, we obtained four times the amount of data previously available at these speeds. Today more than 20 air, land, sea and space data collection systems were operational. We’ll learn. We’ll try again. That’s what it takes.”

HTV-2 is packaged in a special capsule launched by the Minotaur. After the Minotaur positions the HTV-2 near the planed orbit, capsule will separate, accelerate and fly at a hypersonic glide trajectory within the earth’s atmosphere Mach 20 speeds, approximately 13,000 miles per hour. Photo: DARPA

The goal of this test was to collect data on the vehicle’s handling, stability and maneuverability at Mach 20 hypersonic speed, enabling scientists to validate current assumptions about vehicle behavior and control under such extreme conditions. “Prior to flight, the technical team completed the most sophisticated simulations and extensive wind tunnel tests possible. But these ground tests have not yielded the necessary knowledge. Filling the gaps in our understanding of hypersonic flight in this demanding regime requires that we be willing to fly,” said DARPA Director Regina Dugan.

A technology demonstration and data-gathering platform, the HTV-2 is packaged in a special capsule launched by the Minotaur. After separation and reentry into the atmosphere the capsule would have accelerated to fly at a hypersonic glide trajectory within the earth’s atmosphere Mach 20 speeds, approximately 13,000 miles per hour (20,900 km/h). The capsule itself was not required to transmit telemetry through its flight as its status and behavior would be monitored by more than 20 land, air, sea and space test assets, collecting the test data.

The Minotaur IV rocket stands beside Space Launch Complex-8 at Vandenberg AFB, from where it will launch the HTV-2 hypersonic test capsule today. Photo: U.S. Air Force photo/Staff Sgt. Scottie McCord.

“Wind tunnels capture valuable, relevant hypersonic data and can operate for relatively long durations up to around Mach 15. To replicate speeds above Mach 15 generally requires special wind tunnels, called impulse tunnels, which provide milliseconds or less of data per run,” Schulz said. “To have captured the equivalent aerodynamic data from flight one at only a scale representation on the ground would have required years, tens of millions of dollars, and several hundred impulse tunnel tests.” According to Schulz, impulse tunnel testing is required to create a portion of Mach 20 relevant physics on the ground. “And even then,” said Schulz, “we wouldn’t know exactly what to expect based solely on the snapshots provided in ground testing. Only flight testing reveals the harsh and uncertain reality.”

HTV-2’s inaugural flight collected data that demonstrated advances in high lift-to-drag aerodynamics; high temperature materials; thermal protection systems; autonomous flight safety systems; and advanced guidance, navigation, and control for long-duration hypersonic flight.

Approximately nine minutes into its first test flight in April 2010, telemetry assets experienced a loss of signal from the HTV-2. The vehicle’s onboard system detected a flight anomaly and engaged its onboard safety system—prompting the vehicle to execute a controlled descent into the ocean. “We gained valuable data from the first flight, made some adjustments based on the findings of an engineering review board to improve this second flight, and now we’re ready to put all of that to the test.” For its second test flight, engineers adjusted the vehicle’s centre of gravity, decreased the angle of attack flown, and will use the onboard reaction control system to augment the vehicle flaps to maintain stability during flight operations.

During its second test flight, “DARPA looks forward to conquering more unknowns about long-duration hypersonic missions. The HTV-2 program is what remained from an ambitious ‘Prompt Global Strike’ weapon known as ‘Falcon’, designed to be based in the Continental U.S. and reach any point on earth within 60 minutes. The ambitious program was reduced to the development of two flight tests of two hypersonic capsules, exploring the unknown hypersonic speed regime. We need to increase our technical knowledge to support future hypersonic technology development,” said Dave Neyland, director of DARPA’s Tactical Technology Office.

Flying at a speed of 13,000 miles per hour – a speed x22 faster than commercial jetliner – it would take less than 12 minutes to get from New York to Los Angeles creates aerodynamic and thermodynamic conditions, dealing with extreme pressures that cannot be fully replicated by simulation or wind tunnels. Long-duration flight at such speed generates surface temperatures in excess of 3,500 degrees Fahrenheit on the surface, hotter than a blast furnace that melt steel. To maintain an operable environment inside the vehicle, carbon composite material is used, to create a ‘glove’ that keeps the instruments cool only a few inches away from the inferno outside.

As the wedge shaped vehicle rips the air apart at such high speeds, controlling the capsule at such speed is another challenge, requiring precise sensing and near simultaneous response to flight path disturbances, requiring hybrid controls combining Reaction Control System (RCS) and aerodynamic effects.