3D Metal Printing Enables Hypersonic Propulsion Breakthrough
James Anderton posted on September 11, 2017 | 8834 views

It was going to be fast. Very, very fast:

“We are going forward with research on a new Orient Express that could, by the end of the next decade, take off from Dulles Airport, accelerate up to 25 times the speed of sound; attaining low Earth orbit or flying to Tokyo within two hours.”

On February 4th, 1986, then President Ronald Reagan used the state of the union address to predict a revolution in aviation: hypersonic flight. In 2017, the technical challenges still aren’t solved, but new technology promises to overcome the issues that have dogged hypersonic aircraft for half a century.

Flying at very high speeds has always been a challenge. By the 1950s, the problem was understood. Aerodynamic drag roughly scales with the square of airspeed; double the speed, and the drag goes up four times.

Streamlined shapes have partly overcome this problem, but the solution then, as it is now, is more thrust. Rocket engines can deliver the necessary thrust, but expend their fuel in mere minutes. They must also carry their own oxidizer along with the fuel, adding cost and weight.

Jet engines are the obvious answer, since they take their oxidizer directly from the air. However, compressing that air, adding fuel and igniting it becomes more challenging at higher speeds. Supersonic shockwaves must be managed with complex mechanical systems, and combustion instability must be addressed, also with complex systems.

The biggest problem is simple: heat. From the mid-1950s on, aircraft capable of Mach 2 speeds and faster were frequently limited not by available thrust or drag, but by the heat buildup caused by atmospheric friction. The same is true inside turbojet and ramjet engines: at hypersonic speeds of Mach 5 and higher, existing aerospace alloys simply burn up.

Paradoxically, very high combustion temperatures are key to high thrust, which is essential for high speed flight.

It’s a double-edged heat problem.

Jeff Engel, chief operating officer of Golden, CO-based Reaction Systems, Inc., has a novel solution to the heat transfer issue that may open the door to practical hypersonic aircraft propulsion. According to Engel, “we are developing an endothermic fuel system. In hypersonic flight, as you fly faster and faster, the heat load on the air frame, engine and specifically in the combustor gets so high that materials can’t survive in that environment; you have to continually cool the combustor sections. We’re developing a fuel system to absorb that heat load from the combustor specifically, so that the final speed of the vehicle is faster.”

Why use an endothermic system involving fuel when you could just hang a radiator in the airstream?

At hypersonic speeds, it would burn off the airframe in a fraction of a second, meaning conventional heat transfer processes are out. That’s why Reaction Systems’ endothermic fuel system works: heat isn’t rejected to ambient; instead, the fuel is the working fluid, but not in the conventional sense.

As Engel describes it, “Through the heat exchanger, we would cool the combustor and fuel would be running through it. In historically slower speed flights, the fuel would be used to absorb that heat and it might undergo some chemical reactions to absorb a little bit more heat, but there’s a limit to that.  You can’t get a fuel too hot because you’ll end up coking fuel paths, and if you coke the fuel path with the heat exchanger then you lose fuel and the flight is over. What we’re doing is developing a new fuel and catalyst system that we will put inside the heat exchanger to actually absorb more heat than a traditional fuel.”

Reaction Systems’ heat absorbing fuel is a key enabling technology, but transferring the heat to the working fluid, while providing a maximum surface area for catalysis inside the heat exchanger, is essentially impossible to achieve with conventional heat exchanger fabrication technologies.

Enter additive manufacturing.

Arvada, Colorado-based Faustson Tool Corporation, located near Reaction Systems’ laboratory, isn’t just another machine shop. Faustson has built successful parts for NASA and several major aerospace OEMs, including parts for the F-35 program, and has extensive experience in 5-axis machining and multi-axis EDM.

Mike Mussel and the Concept Laser M2 cusing Multilaser.
Mike Mussel and the Concept Laser M2 cusing Multilaser.

The firm recently added metal additive capability in the form of a M2 cusing Multilaser machine from Concept Laser, a GE Additive company. With a 250 x 250 x 280mm build envelope, dual lasers, 20-80micron layer thickness and, critically, the ability to process aerospace “hot-section” superalloys, the Faustson-Concept Laser relationship was the right process in the right place at the right time.

According to Engel, “I believe for us it is.  We need a certain amount of catalyst inside the heat exchanger channels to undergo the chemical reaction.  And to get the amount of catalyst that we need at typical fuel flow rates that will fuel these engines, we need a lot more surface area than what you can do through a straight serpentine path in a heat exchanger. With additive manufacturing, we’re able to create more surface area, which creates more contact with the fuel.”

Faustsons’s M2 cusing Multilaser can build with a variety of high-performance alloys, including cobalt-chromium grades, Ti6Al4V, pure titanium and the material for Reaction Systems’ heat exchanger, Inconel 718.

The overall plan was simple in concept, according to Engel. “We had a Phase Two project with the Air Force that was coming to an end. We approached the Air Force and said that we wanted to take this technology to the next level with a demonstration test in a more relevant environment. And so, we suggested that we manufacture a cooling panel to replace one of the side walls of their ground-based scramjet engine at Wright Patterson Air Force base. They liked that idea, but they wanted the panel to be additively manufactured. Faustson printed it, and then we were able to get CT scans and X-rays of a mock-up panel section showing the internal geometry that we had designed. It was a really key part of what gave the Air Force the confidence that we could do what we said we were going to do with these panels,” said Engel.

Building a part with the Concept Laser M2 cusing Multilaser, however, is an entirely different process from machining or EDM, the basis of Faustson’s aerospace business. Making the move to additive was customer-driven, said Faustson vice-president Heidi Hostetter. “We listened to the voices of customers like Woodward, Ball Aerospace, Lockheed Martin and a whole menagerie of others, small, medium and large. Hands down, everyone said they would like somebody in this area to really learn the nuances of additive manufacturing.”

Hostetter chose manufacturing manager Mike Mussel to spearhead the Concept Laser project. Mussel explained how the thin metal powder layers are fused into parts. “Our Concept Laser M2 cusing Multilaser is a dual laser powder bed machine.  It uses a powder chamber and a build chamber, which goes down into the powder chamber. It then retracts, and a wiper comes across and disperses the powder. Then the laser fuses it in the build chamber.  The machine that we have, the M2 cusing Multilaser, is able to do reactive materials so we can do titaniums, aluminums and other alloys. It’s possible to change out powders in an inert environment if that’s needed.”

One way in which DMLM part making differs from machining is work holding. There’s no Kurt vise or mechanical gripper in an additive process.

Mussel agreed. “Yes, the thing about additive is that it’s both a heat problem and a geometry problem, so creating support structures that are going to work with the part are very important.  If you mess up your support structure, it can cause the part to not build, or to overbuild. Then you have to stop and start over.  But as far as set up goes, it’s simple because you’re just putting it on a build plate and printing it. I’d say more of the challenges are in figuring out how to orient and support the part, rather than the actual setup of the part.”

For Reaction Systems’ heat exchanger, the M2 cusing Multilaser worked well, he added. “With their part, it was the right part size; it’s something you can hold in your hand.  So, for the ‘ten by ten’ build chamber that we have on the Concept Laser dual laser machine, it fit very nicely in the chamber.  Beyond that we had support structures on the bottom of the part. We can put it in one of our CNC machines to machine the part off the support itself.”

Mussel notes that dual lasers are the key to productivity in what is traditionally a slow process. “On this particular part, if we did just a single laser the run time would be about five and a half hours.  With a dual laser, we get that down to three hours and forty minutes.  So, you can have both lasers working on the same part at the same time, increasing speed, which for productivity is very nice.”

When asked whether other shops should hesitate in moving to additive processes, Mussel notes that the experience with the M2 cusing Multilaser caused few issues. “Just go for it,” he said simply. “You have to get your feet wet at some point, so I think the more people we get involved with it, the better off we’ll be.”

Half a century after rocket-propelled X-15 test flights brought us into manned hypersonics, it may soon be possible to create air-breathing scramjet engines that will allow us to travel from America to Asia in two hours or less.

That would be as radical a change in aviation as the introduction of turbojet airliners in the late 1950’s; however, from a manufacturing standpoint the significance may be even greater.

Additive technology, as proven by Faustson’s Concept Laser 3D metal printer, now allows small and medium-sized shops to team with small research teams to create new aerospace technologies previously limited to big air framers such as Lockheed Martin, Boeing, Northrop Grumman and powerplant heavyweights like General Electric and Pratt and Whitney.

The potential for small entrepreneurs to innovate and speed technological progress in aviation is obvious, perhaps bringing the industry full circle to two brothers in a Dayton bicycle shop, with a high-tech twist. Reaction Systems, Faustson Tool and Concept Laser allow a small Golden, CO research firm to create game-changing technology. As additive technology spreads, the future of aviation may come from small garages instead of huge factories.

Find out more about how Metal Additive Manufacturing Keeps Legend Flying

To learn more, visit the Concept Laser website.

Concept Laser has sponsored this post.  All opinions are mine.  --James Anderton

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