The U.S. Air Force is making progress in developing a hypersonic weapon based on the success of an experimental scramjet program, engineers said.

The service in 2013 conducted its fourth and longest flight of the so-called X-51 WaveRider. After separating from a rocket launched beneath the wing of a B-52 bomber, the hypersonic vehicle built by Boeing Co. climbed to 60,000 feet, accelerated to Mach 5.1 and flew for about three and a half minutes before running out of fuel and plunging into the Pacific Ocean.

At that speed, which is equivalent to about 3,400 miles per hour, a missile could travel from Washington, D.C., to Atlanta in just several minutes — making it a potentially powerful weapon against enemy air defenses.

“We are the Air Force. What do we want to do with this technology? We want to weaponize it,” Ryan Helbach, an official with the Air Force Research Laboratory, said last week during an exhibition at the Pentagon to showcase various military research projects. “The follow-on program to this is the High Speed Strike Weapon effort. It’s taking a lot of the lessons learned and the technology and moving to a weapons acquisition.”

The hypersonic missile program comes as the U.S. faces increasing competition from China and other countries working to capitalize on the defense technology.

“Certainly, the U.S. is not the only country involved in developing hypersonic weapons,” Mica Endsley, the Air Force’s chief scientist, said in a recent interview with Military.com “They (China) are showing a lot of capability in this area. The advantage of hypersonics is not just that something goes very fast but that it can go great distances at those speeds.”

She added, “For example, currently today to get from NY to LA is a five hour flight in a commercial aircraft. With a hypersonic weapon, you could do that same thing in about 30 minutes. You can go great distances at great speeds.”

The nine-year, $300 million X-51 program was designed to demonstrate that the military could build a scramjet capable of accelerating, ingesting hydrocarbon fuel, and actively cooling in flight, Helbach said. Unlike a traditional engine, a scramjet, or supersonic combusting ramjet, has very few moving parts and relies on an air-breathing propulsion system to travel faster than the speed of sound.

But it needs a kick, like a boost from a rocket, to get there. So the WaveRider was first propelled by a solid rocket booster, a surface-to-surface missile called the MGM-140 Army Tactical Missile System, to about Mach 4.5, then separated and activated its scramjet engine built by Aerojet Rocketdyne. (A weaponized version of the vehicle would use another missile, not a ground system design.)

“There are no moving parts in the flow path, so that means there are no compressor blades to suck in the air, so we need something to get us up to above Mach 4 in order to get the compression into the engine,” Helbach said.

The Air Force program, which had a couple of failed tests, came several years after a similar NASA effort called the X-43, which in 2004 shattered speed records when it flew at nearly Mach 9.7, or about 6,600 miles per hour, for 10 seconds. But the engine couldn’t withstand the temperatures involved.

“The engine basically melted because it got so hot,” Helbach said. “They didn’t actively cool it. So for our program, we actively cooled the engine, which means that along the outside of the engine, we cycled the fuel around it to suck out the heat from the engine, heat up that fuel, and then inject it into the combustor for the scramjet engine.”

The X-51 was designed to start its engine using ethylene and transition to a hydrocarbon fuel called JP-7 — the same type of endothermic fuel employed by the SR-71 Blackbird spy plane.

“It basically means you can dump a lot of heat into that fuel,” Helbach said. “When you crack the fuel, it actually makes it more combustible. It increases the amount of combustion you can create from the fuel.”

For the follow-on weapons program, the Air Force has teamed with the Pentagon’s research arm, the Defense Advanced Research Projects Agency, to shrink the technology into a hypersonic weapon that could fit on most of the bomber fleet, according to Kenneth Davidson, manager of the hypersonic materials development at the Air Force Research Laboratory.

“If you look at the X-51, the size is slightly too big to put it on our current bombers,” he said. “It was made as a demonstrator. There’s no weapon in it. There are no sensors on board for controlling the guidance. So we’re looking at making it more durable, getting the guidance control developed so that it can become a weapon system, developing the ordnance.”

Carrying a small, conventional warhead, a hypersonic weapon could be used as a stand-off missile, so the military could strike targets at a safe distance without putting pilots and aircraft at risk.

“You could then attack defensive targets, those heavily defended or the time-critical targets in a very timely manner — if it’s a moving target, before it can move,” Davidson said. “And then ultimately, these would have a sensor so that they can track a moved target — not necessarily something that is moving, but if the target moves or it gets into the area, they can see the target and hit it very, very accurately.”

The High Speed Strike Weapon is affiliated with other demonstration projects being developed by DARPA, including the Hypersonic Air-breathing Weapon Concept and the Tactical Boost Glide, both of which have test flights scheduled for 2018 or 2019.

“Our goal is to make sure the Air Force has the knowledge in 2020 or over the next five years to be able to make acquisition decisions using this technology,” Davidson said. “Our goal is to provide a capability to stand off, launch these vehicles off the aircraft to hit time-critical dependent targets … And ultimately from a manufacturing standpoint, it’s got to be affordable.”

-Associate Editor Kris Osborn contributed to this report.

— Brendan McGarry can be reached at Brendan.McGarry@military.com.