3D printing or additive manufacturing is ongoing revolution in manufacturing with its potential to fabricate any complex object and is being utilized from aerospace components to human organs, textiles, metals, buildings and even food. Additive manufacturing is defined by ASTM International as the process of joining materials together, layer by layer, based on three-dimensional model data.
For the last several years, aerospace companies have been examining ways to use additive manufacturing, or 3D printing, to aid the production of rocket engines. These industries are using this technology both for prototyping and production from motors to complete rockets. At their core, rockets consist of four main systems: payloads, guidance, propulsion, and structures. The payload is whatever the rocket is carrying. The guidance consists of sensors that keep the craft on target, and propulsion is made up of the fuel and engine that make it go. The structures are the rest of the frame, cone, and fins of the rocket—parts that are typically fabricated using ultra-precise CNC milling machines and hand welding.
Now Industry is examining 3D printing for emerging hypersonic platforms and weapons are powered by scramjet engines. The scramjet — from ‘supersonic combustion ramjet’ — only becomes operational at around Mach 5, or five times the speed of sound (more than 5000 km/h). Scramjets have required materials that could withstand the extreme heat created at such speed. Like a conventional turbojet engine, a scramjet inhales air through its inlet, compresses and mixes it with fuel, and ignites the mixture to produce thrust out its nozzle.
Unlike a turbojet engine, it does not have any turbine blades to compress air, but instead relies on air being forced through its inlet as it is pushed through the atmosphere at high speeds, often propelled by a rocket booster. The result is a jet engine with no moving parts, which cannot produce thrust on its own from a standstill. There aren’t even any moving parts inside of a scramjet, it’s little more than a carefully shaped tube with fuel injectors and ignitors in it.
The first scramjet, an airbreathing jet engine capable of pushing an aircraft beyond Mach 5, was successfully flown in the early 1990s. But while pretty much any other technology you could imagine has progressed by leaps and bounds in the nearly 30 years that have passed, the state-of-the-art in hypersonic scramjets hasn’t moved much. We still don’t have practical hypersonic aircraft, military or otherwise, and any missiles that travel at those sort of speeds are rocket-powered. Because of this, the operation of a scramjet has often been likened to trying to light a match in a hurricane; the challenge isn’t in the task, but in the environment, you’re trying to perform it in.
The best option for increasing the amount of time the engine has to burn the fuel and air mix, referred to as the “residence time”, is to complicate its internal geometry. Dotting the inside of the combustion chamber with small flameholder cavities gives the gasses somewhere to linger, and research has shown this greatly improves overall engine stability at hypersonic speeds.
“Designing ramjet and scramjet inlets and nozzles are challenging both aerodynamically, in terms of shape, and structurally,” Dr Kevin Bowcutt, Boeing Senior Technical Fellow and Chief Scientist of Hypersonics said . “For a ramjet or scramjet that operates over a speed range greater than about one Mach number, inlets and nozzles must employ variable geometry to efficiently compress the ingested air and expand the engine exhaust respectively. This is particularly challenging because the inlet and nozzle structures, and the seals they employ, get very hot and must be made of special, high-temperature materials.
There are a few competing ideas in regards to the shape of these cavities, but the most common approach uses indentations with a 90° leading edge and sloped back wall. According to the research paper Cavity Flame-Holders for Ignition and Flame Stabilization in Scramjets, when these indents are located aft of the fuel injection ports, the sudden drop at the front of the step creates a void in which gasses will recirculate. The angled back wall helps prevent the shockwaves which would otherwise be generated if the flow struck a flat surface after dipping down into the cavity.
In a traditionally manufactured scramjet, these indentations would be milled into the walls of the combustion chamber. But with additive manufacturing, they can be integrated at the time of manufacture. Not only will this save time and money during the production of the engine, but it also allows for the size and position of the cavities to be experimentally adjusted. Research so far indicates that the more cavities the better, and that a “wavy” surface on the inside of the combustion chamber may be ideal.
Many companies including Aerojet Rocketdyne and Northrop Grumman have turned to 3D printing for their scramjet engines, so that they can not only iterate through design revisions faster, but produce them far cheaper than they’ve been able to in the past. Even more importantly, it enables complex internal engine geometries that would have been more difficult to produce via traditional manufacturing.
In 2016, Orbital ATK successfully tested a 3D printed hypersonic engine combustor at Nasa Langley Research Centre in Virginia. The breakthrough could lead to planes that can travel 3,425mph (5,500km/h) – 4.5 times the speed of sound. The combustor was created through a manufacturing process known as powder bed fusion (PBF). In this, a layer of metal alloy powder is printed and a laser fuses areas of together based on the pattern fed into the machine by a software program.
As each layer is fused, a second is printed until the final product is complete. Any additional powder is removed and the product is polished. The combustor was successfully put through a range of hypersonic flight conditions over the course of 20 days, including one of the longest duration propulsion wind tunnel tests ever recorded. Orbital says it is part of one of the most challenging parts of the propulsion system,the scramjet combustion. This houses and maintains stable combustion within an extremely volatile environment.
Northrop Grumman is 3D printing all of its hypersonic engine using advanced materials, including the critical combustor. “The use of additive manufacturing techniques also enables better performance and faster production,” says Wilcox. “Advancements in this area of manufacturing have made a big difference in printing precision parts that used to be too complex and time consuming to do by other techniques. Our successful tests have confirmed that additively manufactured materials perform as intended in simulated conditions.”
Moreover, a 3D printer could help create scramjet engine geometries not possible before. Changing the contours of the inside of a scramjet could help to better control the flow of air and fuel so as to improve performance, though it has to be durable in high-temperature environments, says Javier Urzay, a senior research engineer in aerospace at the Center for Turbulence Research at Stanford University.
Aerojet Rocketdyne and Northrop Grumman have partnered with Lockheed Martin and Raytheon respectively to develop entries for the Defense Advanced Research Projects Agency’s (DARPA) Hypersonic Air-breathing Weapon Concept (HAWC) program, which aims to develop an affordable air-launched hypersonic missile. The program is the logical progression of the research performed during the development of the Boeing X-51 Waverider, which in 2013 set the record for the world’s longest flight of a hypersonic scramjet.
DARPA and the US Air Force now want to take the knowledge gained during the X-51 program and apply it to a mass-produced missile. With that shift naturally comes the need to build the engines as quickly and as cheaply as possible. There’s also a desire to miniaturize the weapon; the Waverider had to be carried aloft by a B-52 bomber, but a hypersonic cruise missile small enough to be carried by a fighter jet would be faster and less costly to deploy.
To achieve that goal, both teams have announced they are utilizing 3D printed scramjet engines. According to Aviation Week, the scramjet engine developed by the Raytheon and Northrop Grumman team is less than half the mass of the one that was used in the Boeing X-51 Waverider.
In April 2022, it was reported that scramjet engine built by propulsion system manufacturer Aerojet Rocketdyne has been successfully flight tested as part of a US hypersonic missile research project. By 3D printing the drive system, the firm is said to have been able to construct it using 95% fewer parts than were needed to build its previous iteration, which powered the Mach 5-capable United States Air Force (USAF) X-51A Waverider.
Developed via a military R&D program with the Defense Advanced Research Projects Agency (DARPA), Air Force Research Laboratory (AFRL) and Lockheed Martin, the company’s engine is designed to propel an air-launched missile system, that can be built and fired with greater efficacy than the weapons of US rivals.
As long ago as 2017, Aerojet Rocketdyne announced the successful hot-fire testing of a 3D printed thrust chamber for its RL-10 rocket engine, before achieving the same with its 3D printed AR1 propulsion system. That same year, the firm was also contracted by DARPA alongside Boeing to design a new experimental hypersonic spaceplane for launching satellites, though this has since been scrapped.
Since then, Aerojet Rocketdyne has bought 3DMT, in a move that boosted its manufacturing capabilities, with its acquisition believed at one time to have had one of the world’s largest metal 3D printing facilities. Fast forward to this month, and the company has unveiled an optimized 3D printed quad thruster as well, showing that its efforts to harness the technology’s potential remain alive and kicking.
When used in a recent flight test to power a HAWC vehicle prototype, the second to come from the program, the system is said to have allowed it to travel at Mach 5 or higher for over 300 miles. Capable of flying at altitudes of 65,000 feet or higher, the vehicle is also understood to be ‘air-breathing,’ in that it utilizes air captured from the atmosphere to achieve sustained propulsion. As such, air breathing missiles have the potential speed and maneuverability to evade ground defense systems, while the kinetic energy they generate enables them to destroy targets without using high-explosives.
“Aerojet Rocketdyne is well-positioned to support our nation’s hypersonic development and production,” said Eileen P. Drake, CEO and President of Aerojet Rocketdyne. “By applying decades of advanced research and development, together with engineering know-how and innovative manufacturing and materials, our products optimize performance while dramatically reducing costs and development time.”