The U.S. Navy took the first step to reintroducing to the fleet an old-but-much-needed technology when it successfully tested a solid-fuel ramjet engine at Naval Air Warfare Center Weapons Division (NAWCWD) in China Lake, California.
As threats emerge that require an ability to strike targets from farther out to sea than ever before, the Navy is revisiting the solid-fuel ramjet—an air-breathing engine that can propel a missile up to three times the distance and at higher speeds than a standard solid rocket motor.
“The combination of range and speed is an absolutely enabling technology for the warfighter that they need now,” said Matt Walker, head of the Airbreathing Propulsion Section at NAWCWD.
In a traditional rocket motor, oxidizer can make up roughly 90 percent of the rocket’s propellant, Walker said. Meanwhile, a ramjet engine—a technology first conceived in Europe in the 1910s—uses its vehicle’s forward motion to draw in oxygen from the surrounding atmosphere, allowing more room for fuel and making it four-to-five times more fuel efficient than a solid rocket motor, he said.
Larger fuel stores also allow a ramjet to sustain high speed during flight, making it harder to shoot down than a solid rocket motor, which exhausts its fuel shortly after launch and then rapidly slows down, he said.
Though most ramjets are liquid fuel, which generally burns more efficiently, solid fuel can be packed more densely, meaning a solid-fuel rocket can typically fly farther, Walker said.
“We are consistently seeing that solid fuel ramjets will fly about three times the distance as a solid rocket motor of the same size,” Walker said.
The Navy began experimenting with surface-launched ramjets toward the end of World War II and air-launched ramjets during the mid-1950s. Development on air-launched ramjets continued throughout the Cold War, but the fall of the Soviet Union shifted the nation’s military focus to conflicts in the Middle East, where adversaries with inferior air defenses rendered the ramjet’s range superfluous.
But now with Russia re-emerging as a threat, and joined by a resurgent China, the Navy is once again in need of the ramjet’s unique capabilities.
Unique Acquisition Strategy
To that end, NAWCWD Commander Rear Adm. Brian Corey and Weapons and Energetics Department Director Dan Carreño challenged Walker and his team at the end of April to develop, build and fly a solid-fuel ramjet in six months, a timeframe made possible by a unique acquisition strategy.
Walker said his team determined the best course was to keep the project small and use parts that could be purchased off-the-shelf for far less money and in far less time than it would take to design and develop them independently.
“It doesn’t have to take 10 years to get something done,” said Nick Quigley, an aerospace engineer on Walker’s ramjet team. “We took stuff that would come off-the-shelf, we were able to order from somebody, and then we would make that integrate into a solid-fuel ramjet. So, we made a solid-fuel ramjet out of things we could get readily available.”
The team sought out manufacturers popular with model rocket hobbyists. It found a rocket booster that could propel the vehicle to Mach 2-at which point the ramjet would take over-for $900, a pittance compared to the tens of thousands it would have cost to develop one in-house, Walker said.
To his knowledge, this acquisition strategy was unprecedented. Fortunately, many of the parts Walker’s team needed could be purchased with a credit card.
“We had to keep things inexpensive, so we couldn’t rely on the contract world. To buy things this quickly is very difficult to do with the standard contract process we have,” he added.
Walker also stressed the importance of keeping his development team small and agile.
“If you have a small team, you can just get together and draw on a white board on the fly and not have to worry about getting the large team involved and buy-in from everyone,” he said.
Walker believes the strategy could be applied broadly throughout the Navy, but acknowledged that it might not fit larger programs and systems as well as it did his project.
Walker said he sees the greatest impediment to developing such an acquisition culture is an institutional unwillingness to assume risk, both technical and professional.
“The risk-averse approach is to do all kinds of analyses and testing so you understand every aspect of what you’re looking at, but that can take a long time to complete,” Walker said. “Too much as a DoD, we aren’t willing to fail and have a ‘black mark,’ so to speak. We learn a lot from our failures, and as a culture, we need to be willing to fail, to take some chances.”
Walker and his team flew the ramjet about three months into the project, before it had every aspect of its design nailed down. The first flight failed when the booster separated at the wrong time, preventing the engine from igniting.
“But we learned things in that flight that we would not have even known to look for, so ‘fly early and often’ was our mantra,” he said.
As a result of the flight, the team discovered it had a timing issue with the ramjet’s ignition. A quick fix, and the second test flight saw the engine fire at the right time, but the fuel took too long to ignite, causing the ramjet to decelerate to the point it missed its target.
The team changed the ignitor to provide more direct flame to the ramjet’s fuel, and on the three subsequent test flights, “it ignited perfectly,” Walker said.
Now the team is working on making its solid-fuel ramjet “more tactically relevant,” integrating the rocket booster with the propellant inside the combustion chamber, rather than having a separate booster that has to detach from the rocket mid-flight, Walker said.
Another goal is to install a high-performance inlet that will allow the ramjet to more fully realize its speed potential. The ramjet tested last year did not accelerate much once the booster got it up to Mach 2, but the next rocket “will accelerate like crazy,” Walker promised.
Walker believes his team could have a ramjet-equipped missile to the fleet within three-to-four years, “which is a very fast schedule,” he said. “People doubt it, but I’m very confident we can do it if we can change the acquisition culture. We’ve tested the technology and know that it works.”