The BGM-109 Tomahawk missile has served the Navy well, providing combat ships with the ability to strike targets on land or at sea from ranges of 700 nautical miles (nm) up to about 1,350 nm. The Tomahawk, a subsonic missile, can travel at speeds of up to 550 mph to hit these targets.
Beyond that, however, defense researchers are investigating the feasibility and potential of a missile that could travel at supersonic speeds, reaching up to Mach 6, or more than 4,500 mph.
“What speed brings to the table in particular for strike weapons is the ability to go after time-sensitive or time-critical targets, those for which there is a limited window for which they are vulnerable or present so we need to go after them,” said Gil Graff, program manager for the Hypersonics Flight Demonstration program (HyFly).
Graff, who manages the Applied Research and Weapons Technology Directorate at the Office of Naval Research (ONR), is working jointly with the Defense Advanced Research Projects Agency (DARPA) to produce missiles for the Navy that can travel at such speeds.
“What speed [it does] gives you the capability to go after those targets from a long range,” Graff said. “What that means then is that you can reduce substantially the number of forces that are close by or in jeopardy. You can go after time-sensitive targets with a bomb or a mortar or a rifle if you are close enough. But there are targets that may appear and we may have very short notice, and you would have to have a lot of forces in place to cover the unknowns.”
The HyFly program’s goal is to demonstrate the key technologies that would move a missile at Mach 6 in an environment that would enable the Navy to adapt the technology to a next generation of high-speed cruise missiles. According to DARPA, the HyFly program aims to demonstrate a vehicle range of 600 nm “with a block speed of 4,400 feet per second,” or 3,000 mph.
At Mach 6, a HyFly missile would not break any speed records, Graff noted. It takes a great deal more speed than that to escape the Earth’s gravitational field, and even a spatial re-entry or ballistic missile re-entry through the atmosphere can generate speeds greater than Mach 20.
The challenge is to build something that can travel at Mach 6 and remain as small as a naval missile. When NASA builds a vehicle capable of making it to the moon or further, it uses rockets, which carry both a fuel and an oxidizer. An air-breathing engine, the alternative to rocket propulsion, would be much lighter and more compact for a cruise missile.
“An air breather is getting the oxidizer straight out of the air, so the storage requirements are reduced,” Graff said. “You take a volume penalty because you have to put in a duct system that is going to now suck that air in. But overall, if you are talking about longer ranges, these air breathers are much more efficient in terms of the range that you can get, or what we call the total impulse that you can get, for a regular-sized vehicle.”
The fastest engines are those propelled by rockets. While air breathers won’t reach those speeds for a very long time, Graff said, they have shown the potential for sustaining speeds such as the Mach 6 goal of the HyFly program.
Ramjets and Scramjets
The government already has run several demonstrations with vehicles powered by air-breathing engines. The two main classes of these air breathers are called ramjets and scramjets.
“Your typical ramjets are limited in speed,” Graff said. “Turbines today are probably limited to around Mach 3.”
Ramjets become less and less efficient above Mach 3, and the technology basically tops out before Mach 5, he added. A ramjet compresses air and injects fuel into the air.
“But the air at the point at which you are burning it is moving below subsonic speed,” Graff said. “That is to get efficient burning and to get all of the energy out, and also it’s pretty hard to hold a plane in a supersonic air stream.”
The speed of the airflow in a ramjet is only about 10 percent to 20 percent of a Mach, Graff estimated. The duct system in a ramjet slows the air down as the vehicle gains speed. But as a ramjet travels beyond Mach 3 or 4, the shock to the duct system becomes too powerful and shock heating of the air inside the ducts occurs.
“The whole process becomes less and less efficient at the very time as your drag is going up as you gain speed. So you get a crossover between thrust not going up, unfortunately, but your drag is going up,” Graff said.
Where the ramjet technology tops out, scramjet technology picks up. A scramjet is a supersonic combustion ramjet. At present, scramjet technology is required to truly reach speeds above Mach 4. In a scramjet, the air stream becomes supersonic and combustion occurs within that supersonic air stream.
NASA has conducted the first successful flights of scramjets with its X-43 aircraft, and it has done so only within the past three years. The first flight of the X-43 reached Mach 7, and the second flight was at Mach 10. NASA says its scramjet could get to Mach 15.
But the X-43 technology is not applicable to a cruise missile system, making the tests not directly useful to the military, Graff said. The X-43 uses hydrogen and oxygen, which have very strenuous infrastructure requirements for cruise missile applications.
So the HyFly solution, Graff said, is to combine the best features of a ramjet and a scramjet and to make the demonstration vehicle fly using conventional hydrocarbon fuels.
Test Flights
Boeing conducted a successful demonstration of the boost phase of the HyFly hypersonic strike demonstrator vehicle during a trial at the Naval Air Weapons Center at Point Mugu, Calif., in fall 2005.
Over the Navy sea range at Point Mugu, a Boeing F-15E Strike Eagle launched the HyFly vehicle from the air into an unpowered flight. The vehicle used a solid rocket booster, which successfully ignited and took the HyFly demonstration to a speed of more than Mach 3, according to Boeing.
That demonstration was the second of five planned flight tests for HyFly, all of which are scheduled to occur by 2007.
In the first test in 2005, Boeing launched an unpowered HyFly vehicle from an F-15E Strike Eagle, demonstrating safe separation from the jet in addition to vehicle guidance and control functions.
“That was a separation test flight,” Graf said. “This had a motor behind it, not a live motor but a mass simulator, and it had the real avionics and controls. The idea was to demonstrate that we could survive and understand the carry and eject loads and separate, then have the avionics and controls actually capture the vehicle during the transience you have after you have ejected.”
The next three flight tests, the first of which originally had been scheduled for January 2007, call for HyFly vehicles to use a booster and dual combustion ramjet (DCR) in a powered flight to reach speeds up to its target of Mach 6. The goal of HyFly ultimately is to use this DCR engine to power a scramjet that can operate in a missile configuration that is compatible with Navy ships, submarines and aircraft.
The burning of fuel through the system provides the power, but the combustion must occur fast enough to prevent gas from coming out of the back of any missile, Graf said. In a ramjet, the relatively slow speed of the air stream provides the fuel time to burn.
“If the air stream where the combustion is taking place is now supersonic, that chemistry has to be proportionately faster to do that. That’s been a challenge,” Graf said.
To overcome the challenges involved, DARPA and ONR are building a rocket that will get the HyFly vehicle to a certain speed.
“Typically, what we envision for a tactical system is an insertable [sic] rocket that is basically ejected at the end,” Graf said. “Another approach is what is known as an interval rocket. The reason that works is that the scramjet combustor is essentially a hollow tube, which is not in use while you are boosting it up.
“That’s the kind of configuration you would have for launching it from an airplane,” he said. “An airplane is flying at some high subsonic speed, and they drop this thing off and light it. The motor will then burn and burn out, and then this will get ejected out. Covers would cover up the inlets, and the covers would come off. Flow will come down, and you would have an ejection of the rocket.”
A submarine would require an extra rocket on the missile to launch the same HyFly concept because it is below sea level and must provide more power to boost the missile, Graf said, but DARPA and ONR are refining a single configuration that would launch from air, sea, below the sea and ground.
The rockets in use for HyFly cannot be too big because they must fit within the footprint of a standard missile configuration, Graf said, which is why the Navy is interested in the DCR engine. The DCR engine makes HyFly a “ramjet-piloted scramjet,” he said. It was created at Johns Hopkins Applied Physics Laboratory.
Johns Hopkins provides technical support and testing to the project, and the DCR engine is made available through Aerojet. The Naval Air Warfare Center-Weapons Division at China Lake, Calif., provides the solid rocket booster for HyFly. Boeing integrates the pieces through Phantom Works, its advanced research and development unit.
“The key aspects of this thing are to demonstrate that the engine and the airframe are sufficiently efficient to enable long-range flight at high speeds,” Graf said. “That there is enough thrust and that the drag is low enough so that the combination gives us the range and the performance within the flying constraints of a tactical missile.
“This is really a demonstration of the engine and the airframe,” he said. “Part of that is demonstrating the efficiency and performance of the engine of the air vehicle. The other part is to demonstrate that we have materials that can withstand the high-temperature environments that go along with these things.”
FASTT Craft
The Boeing effort, using a full-scale vehicle that will undergo three powered flights, is only part of the HyFly program, however. The other program under HyFly is called Free Atmospheric Scramjet Testing Technique (FASTT).
The goal of the FASTT project is to achieve data in flight on how the engine concept would operate, Graf said. For these tests, the engine is built to a smaller scale with a contractor company named ATK-GASL.
“They have been supporting the X-43 program with NASA,” Graf said. “They have supported a number of high-speed initiatives. FASTT is designed for short flights just to get data on the engines. The flight time is only 10 to 30 seconds. There are no missile controls or fancy fuel pods. We are not flying it over a large Mach range, but are focusing on a single Mach range for the tests.”
The FASTT vehicle uses a smaller version of the HyFly DCR engine and its Terrier-Orion unguided solid rocket system, which is integrated by DTI Associates, Graff said. The FASTT vehicle flew a successful short test in late 2005.
“We successfully got this thing up to Mach 5.7, with a little bit more altitude than we wanted,” Graf said. “The next big step is to separate the booster on this thing at Mach 5, then separate the inlet covers and get the engine operating.”
In the HyFly vehicles, the duct system inlets are covered while the rocket provides the initial thrust, and then the covers drop away to allow the DCR engine to take air into its ducts, Graf explained.
“The key thing now is to basically validate our engine model,” he said. “Our engine model was designed based on ground testing. And there are differences. You cannot replicate on the ground the air flight environment. Those differences become important as you go faster.”
All demonstration vehicles of the HyFly program use JP-10 hydrocarbon fuel. ONR and DARPA plan to use the same conventional fuel to power the rocket of any final cruise missile that results from HyFly.
“We implement what is called the ‘wooden round’ concept,” Graf said. “You take a missile, like a Tomahawk, you fuel it, put it in a box, put the box in a launch tube, and you then forget about it for a few years.
“So you want the fuel to be stable for a length of time, and throughout different temperatures, but you essentially forget it about it,” he said. “The process of getting a new fuel qualified is really onerous and takes a very long time. To be able to use an existing fuel is a really big deal, and for the Navy, it’s especially because of the constraints of the environment onboard ship and concerns about hazards of fire when they are under attack.”
