3D printed fuel injector demonstrates low-cost missile propulsion potential

Researchers at the Indian Defence Metallurgical Research Laboratory (DMRL) have used 3D printing to create a revamped fuel injector that could allow for the lower-cost propulsion of ground-to-air missiles.

Through the adoption of PBF 3D printing, and the integration of a triangular cross-section into their injector design, the team have been able to consolidate two parts that usually need assembling, into a single, flow-optimized device. In doing so, the engineers say they have not only managed to avoid the use of pricey electron beam welding (EBW), but to build-in unique, latticed weight-reducing elements.

A 3D model of the DMRL team’s topologically-optimized fuel injector. Image via the Indian National Academy of Engineering.

India’s state-backed missile R&D

Ever since 2009, the DMRL’s Novel Manufacturing Technologies Group has been using an Optomec LENS-750 system to print prototype missile parts from steel, titanium and various super alloys. In the process, the group’s engineers have identified the technology’s benefits compared to conventional aerospace production processes, particularly around design freedom and lead times.

However, following the success of a similar thruster project at India’s Vikram Sarabhai Space Centre, the researchers have decided to switch away from DED technology, and design a fuel injector of their own. By turning to PBF, the engineers now say that they’ve been able to take an existing missile part, and redesign it in a way that removes the need for supports without hindering its structural integrity.

“Due to limitations in conventional manufacturing, designers do not have much flexibility to create parts with a better and efficient design that are lighter and stronger, but instead [they] are forced to design components exclusively for manufacture,” said the team in their paper. “3D printing comes as a solution to manufacture the components as conceptualized, designed and modeled by the designer.”

Optical microscopy images of the researchers' 3D printed missile fuel injector.
Optical microscopy images of the researchers’ 3D printed missile fuel injector. Image via the Indian National Academy of Engineering.

A revamped ticket to the skies

For their PBF experiment, the DMRL team chose to redesign a fuel injector part that’s usually used within the reaction control systems of missiles or rockets to provide them with altitude control. Composed of ‘injector’ and ‘ring’ elements, as well as three big holes for fuel and oxidiser output, these components are conventionally produced via CNC machining and EDM, before being fused together with EBW.

According to the engineers, producing the device’s elements separately in this way, makes them “overweight” and “compromises their performance and efficiency,” while necessitating the inclusion of supports for its complex internal geometry.

By switching to PBF and taking a DfAM approach, on the other hand, the DMRL researchers were able to produce their injector as one print, with a new 66.4° cross-section that enabled it to stand support-free. In their revamp, the team also managed to upgrade the part’s flow paths, as well as removing material from its low-stressed regions and introducing ultra-lightweight lattices into its base.

Once the engineers had finished overhauling their fuel injector, they 3D printed a prototype using an EOS-M400 DMLS machine from the IN718 nickel alloy over the course of 30 hours, then subjected it to SEM and mechanical testing. The resulting part was found to feature “well-formed internal cavity regions,” and proved to be “densely built, without any major pores or cracks” to weaken its structural rigidity.

Additionally, during testing, the device exhibited a compressive stress resistance of 500 to 600 MPa, as well as impressive hardness and tensile strength properties, that the team said were “superior to those of conventionally melted and cast IN718.”

As a result, the researchers concluded that they had successfully proven the viability of their 3D printing-based approach and the end-use potential of their fuel injector. However, they also said that more rig testing is needed to assess their part’s “functional efficiency,” while further analysis could lend help identify further device optimization opportunities.

The US has previously tested hypersonic missile technology, but has not yet produced a fully functional weapon. Photo via US DoD.
U.S. defense agencies have long-been experimenting with missile 3D printing technologies. Photo via the U.S. Department of Defense.

3D printing’s launch applications

As the capabilities of large-format metal 3D printers continue to scale, so do their aerospace applications, and the technology has been tested with extensively by defense agencies around the world. Just last year, research organization ASTRO America proposed building a hypersonic missile production facility following a DARPA-commissioned study, which could potentially be fitted with 3D printers.

As part of its own mission to 3D print qualified missile parts, the US Army Research Laboratory has enlisted the help of Senvol and its machine learning software. Using its proprietary AI algorithms, the firm has been contracted to develop a flexible ‘qualification plan,’ that can be applied to any component or additive manufacturing system.

Elsewhere, 3D printing has been deployed to manufacture thrusters in the wider aerospace field too, with Agile Space now joining long-term adopters like Launcher and Rocket Lab in using the tech to develop upgraded propulsion systems.

The researchers’ findings are detailed in their paper titled “3D Printing of Fuel Injector in IN718 Alloy for Missile Applications,” which was co-authored by Saride Ramesh Kumar, V. Srinivas, G. Jagan Reddy, M. Raghavender Rao and T. Raghu.

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Featured image shows a 3D model of the DMRL team’s topologically-optimized fuel injector. Image via the Indian National Academy of Engineering.