DARPA pushes autonomous Mach 20 drone program underground

The race for hypersonic speed has never been hotter

In August 2011 a superfast unmanned military drone, the Falcon HTV-2 (Force Application and Launch from the Continental US Hypersonic Technology Vehicle 2) travelling at 20 times the speed of sound, or 13,000 miles per hour, managed to demonstrate stable, aerodynamically controlled flight for several minutes before an anomaly prompted the drones autonomous flight safety system to kick in and guide it to a controlled splashdown in the Pacific Ocean.

Falcon HTV-2 demonstration

The HTV-2 was part of an American Lockheed Martin Skunk Works advanced weapons program called Conventional Prompt Global Strike (CPGS) which ten years earlier had set out to develop systems that could reach any enemy target anywhere on Earth in under an hour. The first test flight of the HTV-2 had taken place in April 2010 and, similarly, that flight had also lasted for about seven minutes, only ending when the aircraft detected a yaw anomaly that its autonomous piloting system couldn’t correct. Analysis of this anomaly prompted engineers to adjust the HTV-2's center of gravity and make several other changes before Flight 2 in 2011.

When quizzed at the time about the failure of the second flight Air Force Major Chris Schultz, DARPA’s HTV-2 program manager told the press that their initial assessment indicated that the Flight 2 anomaly was unrelated to the Flight 1 anomaly. In other words, whatever had happened this time was new and the learnings from the previous flight had been successfully applied to the second prototype helping them to avoid a repeat of the incident. Later investigations would show that the failure of Flight 2 was caused by excessive heat that warped the drones high-temperature composite aeroshell.

Fly fast, fail fast

For the Pentagon's advanced research agency, DARPA, and its consortia of partners, blazing a trail in hypersonics had proved problematic and after a decade the HTV project was, to all appearances, cancelled. Yes there had been some hard lessons learned but there had never been any flight successes.

As a consequence many outside observers believed that the HTV program had simply ceased to exist - another victim of the Governments nefarious budget cuts but if you really think that the US would give up research into such a lucrative technology as hypersonics then think again.

So what happened to the program?

We can now reveal that the program didn’t die, it, like so many of its predecessors simply went back underground and morphed, from a being a “project” to an “initiative”. Rather than focusing on a global strike capability the new initiative, still run by DARPA, relies more on software based modelling rather than real world testing and  on shorter, tactical ranges that include hypersonic flight demonstrations of air breathing cruise missiles and other, new classes of unpowered “Boost-Glide” weapons systems.

At Mach 20 you can cross the Atlantic Ocean in under 12 minutes and needless to say that gives the country wielding the technology an unrivalled offensive, or defensive, strategic advantage. For example, new hypersonic weapons and platforms would let the US neutralize a North Korean missile launch against the homeland in a third of the time it takes today before it could get anywhere near anything that mattered.

Since the HTV-2 program the US, according to DARPA program manager Peter Erbland, “has got kind of lean in hypersonics competency and we’ve refocused.”

“Investment has declined from the heady days of the X-30 National Aerospace Plane, and we have to be careful assuming our existing design paradigms are adequate when developing a new class of hypersonic vehicles,” he said.

“The HTV-2 project sprung some surprises on its two failed flights. First with aerodynamics then with hot structures and materials. Working out what had happened required us to mine all the competency in hypersonics that we have and took a team assembled from US Government, the services, NASA, the Missile Defense Agency, industry and academia.”

Erbland says the decision not to fly a third HTV-2 was influenced by “… the substantial knowledge gained from the first two flights in the areas of greatest technical risk – Flight 1 in aerodynamics and flight performance, Flight 2 in the high-temperature load-bearing aeroshell. We've learned a lot so what is the value of other flights?” he asked.

“From the second flight, we learned a lesson on how to design refractory composites, to improve our understanding of how to model hot structures under thermal load,” says Erbland.

“We learned a critical lesson about variability and uncertainty in material properties. That is why we are taking time to fund the remediation of our models to account for material and aero-thermal variability.”

While it looks like the HTV program itself will be consigned forever to the history books the knowledge that was gained was invaluable and the data is being used to build better algorithmic models that help the teams overcome the packaging and launch challenges faced by tomorrows hypersonic air and surface launched Boost-Glide systems.

“The mass and volume constraints between the two launch variants are different. We had a very high fineness (length to diameter) ratio for global strike so we will have to be very innovative to get high lift to drag without a high fineness ratio,” says Erbland.

“On the other hand, trajectory insertion velocities for air based systems are lower, and the booster problem could be easier to control and influence. The problem with global range is that orbital launch systems with the energy needed are not designed to put a vehicle on an ideal start of glide, so we have to make them fly in ways they don't want to and that is why we are tackling tactical ranges first.” he says.

“None of the lessons we have learnt would have been possible without the flight tests, because of the limitations associated with ground testing. Now, going forward we know how to use modelling, simulation and ground testing to give us more confidence that we can design a successful system.”