21 January 2020
One of the 3D printers used to create AM lightweight mirrors at the University of Sheffield.
From metals and ceramics to gels, foams and even living tissue – is there anything that can’t be made using a 3D printer? Teams within the UK’s Science and Technology Facilities Council (STFC) have been working together, and with a community of universities and national laboratories to explore just how this technique could be used to create lightweight mirrors for space – when weight and volume limits are critical!
The correct technical term for 3D printing is ‘additive manufacture’ (AM). In AM, material is added in layers to create a 3D object. This is the polar opposite to traditional manufacturing –the mill, drill and lathe approach, which essentially removes material to create the required shape.
Carolyn Atkins, AM technology development lead at STFC’s UK Astronomy Technology Centre (UK ATC) explains “AM really is design driving production, rather than the limitations of production driving the design. For space mirrors, this means we can potentially make a lightweight mirror, more tailored to its function. For example, one of the most exciting benefits AM has for me is the ability to create designs that can be manufactured in one entire piece, because it negates the need for an interface, creating stronger more intricate parts.
“We could therefore,” continues Carolyn, “potentially integrate mirror mountings and a cooling mechanism as part of the same unit. There’s an ‘end of life’ story too. 3D printed mirrors can be made of materials that will burn up on re-entry, mitigating space junk, whereas, often, traditional ceramic mirrors cannot.”
The idea is in its infancy, with near term applications potentially for small mirrors within nanosatellites (which is any satellite weighing less than about 10kg).
CubeSats are one specific class of nanosatellite – which are built from 10cm x 10cm x 10cm units. Three of these units can be fixed together to create a 3U CubeSat, which is about the size of a shoe box. With a multi-disciplinary team, across institutions Carolyn has been leading a research project to evaluate the metals and the machining to create an optimal process for AM space mirrors that could be manufactured and used in nanosatellites in the future. As an example, the project has specifically focused on a 3U CubeSat.
In this article, we hear from some of UK ATC’s engineers on STFC’s graduate programme about the pioneering work they are contributing to, to create the capability for 3D printed mirrors for space research.
Maria Milanova, Mechanical Engineer at UK ATC “I worked on the mechanical design of the AM mirrors. In particular I looked at how to make the mirrors lighter, because weight and volume limits are critical on nanosatellites.
“To do this I considered specifically the different styles of lattice structures. This was exciting because it is very difficult to make this structure with conventional methods of manufacturing. It really is a structure that is unique to AM.
“I also needed to consider how to actually print the lattice structure, so I looked at the orientation that the lattice should be printed in – whether in a horizontal or vertical plane.
“Next I explored different types of metals to use; and I looked at different AM printing techniques such as laser sintering and electron beam melting.
“The team chose the BCC lattice structure; printed in a vertical orientation. Using the 3D print software Inventor® to print it, we found that this particular structure provided us with both a rigid structural design and light weighting at the same time. Printing the part in a vertical orientation ensured the minimum amount of support material was used.
“I then designed a mount to enable the mirrors to be positioned on the chassis of the CubeSat.
“AM for space mirrors is at an early stage, and so it has been great to be involved in such ground-breaking work to optimise the design and function of CubeSat tech. For example, we needed to do some trouble-shooting around the radius of the curvature of the mirror. As a result of that process, we’ve written quality assurance (QA) guidelines to enable other engineers to validate their design for AM.
Naomi Dobson, Systems Engineer at UK ATC “My role in the project was to work on the topology optimisation of the AM mirror design. This means that I looked at where material could be removed or redistributed to make the mirror lighter. But because a mirror still needs to be stiff, I also needed to make sure that, in doing so, the rigidity of the mirror was retained.
“I looked, therefore, at where the volume was best placed in the mirror structure to retain stiffness. To do this I used COMSOL® – a finite element analysis (FEA) tool. It was interesting to take the mechanical designs for the mirror from Maria, and import them into COMSOL®. I then performed topology optimisation on the internal structure of the mirror design to reduce weight and minimise stress; and also to predict what the surface quality of the AM mirror might be like after polishing.
“The results were used as a baseline to show where the designs, created using topology optimisation, had a significant weight reduction and a comparable surface quality. Below shows an image of one of the topology-optimised structures, that has a weight reduction of 37.1%.
“The optimised structures were then used by other members of the team to form a final optimised design of a density-graded lattice, which theoretically is both lightweight and has good surface quality. This process is described in the paper Light weighting design optimisation for additively manufactured mirrors.
“It will be interesting to see if the final optimised design can be manufactured as it really exploits the benefits of AM.
“This was a great project to be involved in because I learnt a lot about AM and how this new tech can be applied in the space sector. It was a great experience too, to learn and work with new software tools. Many of the projects I’m working on are systems engineering challenges for instruments for big telescopes that will be operational in the mid-2020s! So I really enjoyed working on this project, because there was a specific short-term deliverable, and I was involved at all stages – from concept, to troubleshooting challenges, to an achievable outcome.
“I’ve really enjoyed learning about the differences between AM and conventional manufacturing, and seeing the output of my work so quickly. From design to physical product, AM as a process, has a very quick turnaround. And it has been great to use my experience to help set-a-standard through our QA documentation, so that other engineers consider AM as a manufacturing option.”
William Brzozowski, Optical Engineer at UK ATC “I worked on the optical metrology of the mirrors. What this means is that I tested the mirrors – to measure the quality of each mirror surface.
“This was a particularly exciting aspect of the project for me, because the optical quality of the mirror is a critical metric. The mirrors we print through AM techniques need to be as good as a conventionally-manufactured mirror.
“In AM mirror fabrication, the main concern is the porosity within the reflecting surface. If there are too many holes, the quality of the mirror is compromised. I used interferometry as the technique for the optical tests on the mirrors. I looked at two specific measurements – the peak to valley (P-V) ratio and root mean squared (RMS) – which are two best practice methods for analysing the quality of an optical surface.
“On one of the mirrors we tested, we got a particularly poor result, which we attributed to the polishing process on that mirror having reached the porous layer.
The AM mirror mounted in the CubeSat chassis; an AM mirror; the BCC lattice structure of the mirror part.
“For me, this has been a really interesting project. Not only is the technology to create this capability so innovative; if we can create lighter mirrors as well as the current standard, then it creates the opportunity for more space missions and more science. Lighter mirrors means cheaper launch costs making space research a more affordable option for universities, high-tech space start-ups, and more…To be at the forefront of the space optics enabling these possibilities is very exciting.”
“We really didn’t know if the idea of AM mirrors would work,” says Carolyn. “But results so far have been very encouraging. We now know, for example, that the direction that the layers in an AM-manufactured mirror, are built, affects the porosity in the part. And if you polish the porous layer, you get a rubbish mirror! It’s really exciting to exploit the design freedom of AM to create a new methodology for lightweight mirror fabrication.”
This project is a collaboration that has brought together STFC’s UK ATC, Durham University and University College London. Results of the ‘Additive manufacturing of CubeSat mirrors’ project was recently presented at the SPIE Optics + Photonics conference in San Diego. This work was funded by the UK Space Agency (UKSA) as a Pathfinder project, under the National Space Technology Programme (NSTP). In addition, the project benefitted significantly from the kind contribution of expertise and assistance from the University of Sheffield, and the National Physical Laboratory (NPL), as well as STFC’s RAL Space Precision Development Facility and the Diamond Light Source (DLS).
Last updated: 25 February 2020