The U.S. government took another step toward the development of air-breathing rockets by awarding a $150 million contract to a Tennessee company that will build and fly the first of three identical hypersonic demonstration vehicles in 2007.
THE SERIES of experimental aircraft, funded by NASA and the U.S. Air Force under a joint project dubbed X-43C, will be powered by a novel engine that sucks in oxygen from the atmosphere and combines it with jet fuel to propel the vehicles to speeds faster than 100 kilometers per minute (3,700 mph).
The X-43C fleet will be built by Allied Aerospace Industries, the Tullahoma, Tenn.-based contractor that built the X-43A experimental vehicles.
By drawing in oxygen from the atmosphere, so-called supersonic ramjet engines like those being developed by NASA and the Air Force have the potential to dramatically lighten the load of future launch vehicles. More than one-fourth of the space shuttle’s weight at liftoff, or about 600,000 kilograms (1.3 million pounds), is the liquid oxygen that is needed to combust the 100,000 kilograms (220,000 pounds) of liquid hydrogen the vehicle’s three main engines burn on the way to orbit. Eliminating the need to carry so much liquid oxygen aloft would go a long way, NASA officials say, toward helping to blur the line between launch vehicles and aircraft.
“We think it is a very important leap-frog technological capability that will help with affordability, safety and so on for future space lift requirements,” said John “Roe” Rogacki, NASA’s director of space transportation technology.
LIGHTENING THE LOAD
While air-breathing engines are as old as the jet airplane, they have yet to find their way into hypersonic applications like rockets, despite more than 30 years of research. By perfecting so-called supersonic ramjet engines — and thus eliminating the need to carry tanks of liquid oxygen aloft — NASA sees the potential to dramatically lighten a launcher’s load at liftoff.
But supersonic ramjet engines, or scramjets, may be only part of the solution. NASA is also working on complementary propulsion systems, like a high-speed turbine engine, that would power future launch vehicles from zero to around Mach 5 before a scramjet takes over and propels the vehicle to speeds as high as Mach 15 or greater needed to reach space.
NASA’s long-term hypersonics technology roadmap, closely coordinated with the U.S. Air Force, lays out at least four distinct flight demonstration efforts on the way to building a large-scale demonstrator capable of taking off from a runway and hitting Mach 15, according to Paul Moses, NASA’s X-43C program manager.
Looking even further down the road, Moses said, NASA could be ready by 2025 or so to field a fully reusable air breathing launcher.
While the Air Force is also interested in low-cost, reusable launchers, it could see benefits from its hypersonics investment somewhat sooner. Ron Sega, director of defense research and engineering at the Pentagon, told an industry audience in July that hypersonics research could yield a swift-moving, air-breathing cruise missile within the next decade.
But there is a lot of technology development to come before air-breathing launchers, or even hypersonic cruise missiles, become reality. Most of what NASA knows about scramjet engines has been learned either in wind tunnels or from computer simulations, Moses said.
In fact, NASA has yet to successfully flight test a single scramjet engine, but it has not been for a lack of trying.
RECOVERING FROM SETBACK
The X-43C program is meant to serve as a follow-on to Hyper-X, a separate flight research project that suffered a major setback in June 2001 when the first of three vehicles spun out of control shortly after launch. NASA plans to resume the Hyper-X project, also known as X-43A, in December with the launch of the first of two remaining vehicles. If successful, NASA will get about 10 seconds of engine data from each of the remaining X-43A vehicles before they exhaust their cryogenic fuel and glide to a controlled splashdown off the California coast.
Like X-43A, the X-43C vehicles will also be boosted to an initial velocity on an air-launched rocket and operate under its own power for only a short time before gliding into the sea. But the slightly larger X-43C will carry more fuel (jet propellant instead of hydrogen) and get about five full minutes of powered flight before its more robust engine is shut down, Moses said.
During that brief but important burst of powered flight, NASA hopes to show that the scramjet can produce enough thrust to boost the vehicle from Mach 5 to Mach 7. To put that into perspective, the SR-71 Blackbird is still the world’s fastest aircraft able to sustain a cruising speed of Mach 3-plus, or more than 3,000 kilometers per hour (1,875 mph).
To survive the intense heat the X-43C’s scramjet will see at such blazing speeds, Moses said, it was necessary to use its fuel to cool the engine. Because the X-43A’s engine has no cooling system, if left to run, it would overheat and begin to melt in less than a minute, he said.
Lowell Keel, Allied Aerospace Industries’ program manager for both efforts, said the X-43C will look like a larger version of the X-43A but with a flatter nose and a deeper throat to gulp down all the additional oxygen it will need for its much longer flight.
Keel expects the X-43C to come in about 1.2 meters (4 feet) longer from nose to tail than the 3.6-meter-long (12-foot-long) X-43A.
The fully autonomous X-43C will also take advantage of many of the same subsystems used for its X-43A predecessor, such as the thermal protection system and avionics, but with minor tweaks, he said.
“This is first and foremost a propulsion experiment, and we want to keep it focused on proving out the engine and doing it with as low of risk as we possibly can,” he said.
Keel said Allied Aerospace and its industry partners, Boeing Phantom works and Pratt & Whitney, are working to deliver the first non-reusable X-43C for flight testing in 2007. If all goes well, he said, the remaining two vehicles would be flown off over the following year and a half.
NASA hopes before then to secure funding for the next two steps on the way to a large-scale demonstrator. First up, Moses said, would be a joint effort with the Air Force to build and fly a reusable vehicle that would have both a high-speed turbine engine and a robust scramjet similar to the one in development for X-43C.
The proposed vehicle, dubbed the Reusable Combined Cycle Flight Demonstrator, Moses said, would over a series of test flights demonstrate the ability to transition from a subsonic air-launch to powered flight up to Mach 7. The last piece of the puzzle, Moses said, would fall in place with the proposed X-43D, a fleet of vehicles, most likely expendable, designed to hit Mach 15.
There is a lot of work to be done between now and then. But Moses and Keel say the payoff could be tremendous.
“We’re trying to use air-breathing engines to do some of the same things that we’ve typically used rockets for,” Keel said. “The whole idea is not having to carry the oxidizer with us.”
Brian Berger is a staff writer for the weekly Space News.
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