A recent United States Air Force scramjet test has hinted at a future where hypersonic vehicles streak through the sky at many times the speed of sound around the world, and perhaps even open up access to space.
The experimental X-51A Waverider used a rocket booster and an air-breathing scramjet to reach a speed of Mach 5 and achieve the longest hypersonic flight ever powered by such an engine on May 26. That technology might not only deliver cargo quickly to different parts of the globe, but could also transform the space industry and spawn true space planes that take off and land from the same runway.
The wealth of possibilities offered by aerospace vehicles that can ride their own shockwaves likely explains why the project has drawn support from the Pentagon's Defense Advanced Research Project Agency (DARPA), NASA, and the U.S. Navy.
"We could have in the future such things as hypersonic weapons that fly 600 nautical miles in 10 minutes," said Charlie Brink, X-51A program manager with the Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio, during a June 1 teleconference. [Most destructive space weapon concepts.]
Scramjet engines can also eliminate the need for much of the huge oxidizer tanks carried by rockets, because the engines capture oxygen from the air to mix with the fuel while moving at hypersonic speeds. That would permit future space-lift systems or space planes to carry greater payloads and operate more efficiently, Brink said.
That concept naturally excites NASA, as the American space agency has continued to consult with the Air Force on the X-51A project, as well as the HIFiRE Flight 2 scramjet experiment.
"NASA would ultimately like to enable large vehicles for access to space using air-breathing propulsion," said James Pittman, principal investigator for NASA's hypersonics project at the NASA Langley Research Center in Virginia.
But first, military and civilian researchers will have to refine the scramjet engines that marks a significant break from past jet engine technologies.
A history of hypersonics
Lockheed's supersonic SR-71 Blackbird could reach speeds of Mach 3.2 during its operation from the 1960s until the late 1990s. The stealthy recon aircraft relied upon turbojet-assisted ramjet engines that compressed the flow of incoming air at high speeds, but slows down the airstream so that engine combustion takes place at subsonic speeds.
By contrast, scramjet designs allow for the airflow throughout the engine to continue traveling at supersonic speed. That raises the speed limit to hypersonic speeds of around Mach 5 or more.
The first flight tests of a hypersonic scramjet vehicle came from NASA's X-43A project, also known as Hyper-X. Its test flight reached Mach 6.8 in March 2004, before a following test flight hit Mach 9.6 in November 2004.
That's about 7,000 mph (11,265 kph), or three times faster than the SR-71.
The Air Force's X-51A Waverider has since drawn on many of the lessons from the X-43A, even if the newer experimental vehicle packs a more sophisticated and complex scramjet engine. The X-51A has claimed the longest scramjet burn during its 200-second flight test, but has yet to break the X-43A's speed record.
"Hyper-X provided a jumping off point for the X-51 program in many areas," said Kenneth Rock, head of the hypersonic air-breathing propulsion branch at NASA Langley.
Rock and his colleagues had already done many wind tunnel tests and computer simulations during the X-43A program, and found that the model data fit pretty well with the actual flight tests. That gave the X-51A program a confidence boost from knowing that there were fairly few unknowns not covered by modeling, Rock told SPACE.com.
The NASA researchers not only shared their experiences with the X-51A program, but also helped participated in an independent review of the ongoing Air Force project. They added some tips about how to streamline flight operations come testing time for X-51A, which flew a route not unlike what the X-43A had taken from the U.S. West Coast over the Pacific.
Rise of the space planes
If scramjet technology advances far enough, it could become part of a system that helps propel unmanned or manned vehicles into space. Space planes might even emerge that can fly into space at just about any time, without launch window constraints.
A scramjet-powered vehicle would need to rely upon a regular rocket or jet engine to reach Mach 4, so that the scramjet could take over for hypersonic speeds during the first stage to Earth orbit.
The X-51A scramjet engine would not be enough by itself to allow a vehicle to reach orbit, said Joseph Vogel, hypersonics director and X-51 program manager at Boeing Phantom Works/Defense, during the teleconference. Both Boeing and Pratt & Whitney Rocketdyne formed part of the private consortium that helped design and build the X-51A.
Any future space-lift system would also need a more energetic hydrogen-based fuel, rather than the JP-7 jet fuel used in supersonic aircraft, Vogel explained.
"I would say that within the next 15 to 30 years — I'll give you the broad side — but probably 15 to 20 years, you could start to see this technology being expanded to the point where you could get aircraft into outer space," Vogel said.
Improved space access represents the driving goal for partners such as NASA, according to Rock at NASA Langley. He added that flexible, on-demand access to space might also allow for even more space exploration opportunities.
"There are certainly operational constraints that have to be overcome, but we believe that this technology can enable missions that aren't possible today," Rock said.
Speeding into the future
Unsurprisingly, the move toward a hypersonic future has not gone perfectly at all times.
DARPA attempted to launch its own HTV-2 hypersonic glider prototype in late April, but lost contact with the vehicle early on in the flight.
Similarly, the Air Force's X-51A Waverider fell short of its intended goal of reaching Mach 6 and burning its scramjet engine for 300 seconds. Unusual readings had appeared throughout the flight, up until the X-51 mysteriously began losing speed and started tipping over toward the ocean.
The Air Force then terminated the vehicle three seconds after losing effective control, but emphasized that the scramjet engine itself had worked perfectly and was likely not at fault. It already has three more X-51 tests scheduled for later this year.
A growing number of hypersonic tests in the future could benefit from having several flexible corridors that could be cleared over the Western U.S., according to Lt. Col. Danny Millman, project pilot for the B-52 bomber that launched the X-51.
The Air Force has also been doing hypersonic tests over the Woomera Test Range in Australia as part of an international project that also involves NASA and Australian researchers.
At least the X-51A has shown enough early success so that the U.S. government should be encouraged to continue investing in scramjet technology, Vogel noted.
"But again, we'll need to fly the rest of these missions to show that the technology is truly viable, because people won't take for granted that you did it once and call it true technology proving," Vogel said.