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.”