Airbus Defence and Space has tested and validated what it claims is the first 3D printed metal radio frequency (RF) filter for use in a telecommunications satellite. The filter, a culmination of the research funded by the European Space Agency, was created using a direct-metal-printing (DMP) process on a 3D Systems ProX DMP 320. The RF filter is said to be produced in a shorter time, cost less, and weigh 50% less than the previous design.
Metal RF filters and waveguide filters date back to the development of the first space communication systems. They operate in the megahertz to gigahertz ranges and are designed to filter incoming or outgoing communication signals (broadcast radio, television, cellular, Wi-Fi, etc.).
Historically, RF filters incorporate traditional designs based on libraries of standard elements, such as rectangular cavities and waveguide cross sections with perpendicular bends, to filter signals. Typical reductive manufacturing processes such as milling and spark eroding limit shapes and connection points, resulting in the need for RF filters to be machined as two halves that need to be bolted together.
Airbus D&S explored complex geometries with electromagnetic simulation software to redesign the RF filter for 3D printing. This improved functionality by utilizing shapes and surfaces not previously possible through typical manufacturing. The final design was a depressed super-ellipsoidal cavity that helped channel RF currents and deliver the required tradeoffs between Q factor (a measure of a waveguides' efficiency based on energy loss) and rejection of out-of-band signals.
At first, Airbus D&S engineers were concerned about the different surface topology of 3D printed components.
“The microscopic topology is different in the 3D printed part than in a machined part,” said Paul Booth, Airbus D&S RF engineer. “Machined surfaces have sharp peaks and troughs, while the 3D printed surface is spheroids melted together, so there is less sharpness.”
“The spherical shape of the powder particles used in 3D metal printing lead to a certain waviness rather than steep transitions,” said Koen Huybrechts, Project Engineer for 3D Systems. “But the ability to shape a part for more effective signal filtering more than overcomes any concerns with surface topology.”
Booth added that after Airbus D&S did X-ray CT scans, the company was "impressed with the density of the part and the general surface quality."
Three aluminum sample filters printed using different processing paths were tested by Airbus D&S at its Stevenage, U.K. facilities. Tests mimicked conditions the parts would face during launch and orbit, including vibration, shock, and thermal situations such as temperature extremes and vacuum conditions. All three samples met or exceeded requirements, with the best performance coming from a filter that was silver-plated via an electrolytic process.
Additionally, the additive manufacturing processes allowed for the consolidation of components and fasteners, reduced production time, and decreased weight with maintained or improved material strength.
“Mass reduction was reduced without spending any time on optimization, and it can be reduced further with more aggressive mechanical design,” said Booth. “The reduced mass saves costs by requiring less propellant in the rocket and puts fewer demands on support structures, allowing further mass reduction.”
Such "disruptive innovations" are vital for the telecommunication satellite industry where high-capacity satellites, such as the Airbus D&S Eutelstat KA-SAT—which carries nearly 500 RF filters and more than 600 waveguides—costs approximately $20,000 per kilogram to launch. Reduced design and production times and accelerated innovation are also high priorities, as many of the RF filters and waveguides are custom-designed to filter specific frequencies.
“The success of this project opens up the possibility of much greater integration of RF filters with mechanical and thermal components to reduce part count and overall mass. We will also look at integrating more functionality such as test-couplers as part of the filter or directly integrated into waveguide runs. There is a huge potential for reducing mass while cutting production time and costs,” said Booth.