It´s time for latest news on the HighPEEK project. Since the traditional cost for every kilogram launched to orbit can be up to $30.000, considerable budget savings can be achieved even with small weight losses. So many players on the space launch market are interested in development of the newest polymers – the topic of our project.
The goal of our materials research is to achieve minimum on 50% weight savings compared to metallic parts and reduce product delivery times. The main challenge is that the alternative polymer parts need to fulfill excactly the same requirements as the originals, no slack given. Thus the materials in question are almost solely ultrapolymers.
But this term requires further explanation. Ultrapolymers is a loosely defined group of semi-crystalline thermoplastics that have significantly higher mechanical and thermal properties in comparison to so-called commodity polymers. Their drawbacks include high price and challenging printability with the Fused Deposition Modelling (FDM) method of 3D printing. Some common ultrapolymers are e.g. polyether-etherketone (PEEK), polyetherimide (PEI) and polyphenol sulfonate (PPSU.)
A Mixture of Parts
One of the latest polymer substitute parts are the fixing brackets of general-purpose honeycomb panels (see Figure 1.) These panels are used for various applications, usually as satellite closure panels.The panels have traditionally been mounted to frame and each other using simple bolted-on aluminium plates angled to the required shape. The substitute polymer brackets are more complex in shape, but yield a weight save of up to 59 percent. The latest revisions of the brackets are semi-hollow with a gyroid infill (Figure 2), printed with a novel CF-PEEK material (carbon fibre filled PEEK polymer.)
Figure 1. Honeycomb panel brackets. The original aluminium version in top left, the latest polymer version in bottom right.
Figure 2. Latest 3D-printed bracet, made from PEEK-carbon fibre composite polymer.
In the following two experimental parts, Maker3D has been involved only in the printing work. The first one is an alternative housing for an IRES-C sensor (Figure 3.) IRES-C stands for InfraRed Earth Sensor, Coarse and is used in satellites for attitude adjustment in orbit. Due to the low accuracy of this version, it is mostly used for reference or as a back-up device for more accurate star trackers (ARTES 2020). The original sensor housing was metallic. The alternative 3D-printed housings were made of PLA plastic, as their purpose was to evaluate the overall quality of printed parts and the suitability of FDM printing method.
Figure 3. Housing of IRES-C earth sensor, 3D-printed with PLA plastic.
The second similar evaluation part was an onboard computer (OBC) housing of the Biomass Earth Explorer satellite. The satellite, which is scheduled for launch in 2022 is part of ESA´s Biomass mission that aims to map the growth of forests to determine the amount of biomass and carbon stored in forests (ESA, 2020a). As the printing work commenced, the best printing practice was quickly discovered. The best way was to print the housing in several pieces, as the PEEK plastic used for the real part is challenging to print. The shape and positioning of the pieces was further optimized to improve the strength of printed parts (Figure 4.)
Figure 4. 3D printed parts of the Biomass Earth Explorer satellite OBC housing. The first attempt was printed in two parts, but the latest experimental model (pictured) was made in 6 pieces.
Real ultrapolymers were used in the next experimental part, which was a power relay housing used in the Sentinel-1 earth mapping satellite. This satellite was a part of ESA´s Copernicus mission together with its sister satellite Sentinel-2 (ESA 2020b). Both satellites have been in orbit and operational from 2016 on, and the original housings were made from aluminium (Figure 5.)
The housing has previously been printed with PLA and ABS polymers, and from one ultrapolymer called polyetherketone-ketone (PEKK-A). In addition to various materials, the structure of the part was altered by making the non load-bearing sections of the housing as lighter meshes (Figure 5.)
Figure 5. Power relay housing of Sentinel-1 satellite, rendered image below and an actual part printed with PEKK-A in top right.
But regardless they need to withstand… a lot!
A common and difficult question for all these weight loss parts is, whether they can withstand the harsh conditions during the launch and after that, the space environment. The very beginning of the journey is mechanically quite stressing, as the parts ars subjected to maximum 6g´s of longitudinal and 4 g´s of lateral acceleration. On top of that, there are constant vibrational and sporadic shock loads. Thus the traditional mechanical strength and stiffness of the part is important, and all materials are tested in tensile and bending modes. Fatigue strength is evaluated with shaker tables.
After reaching orbit, the challenges are different. Parts are now stressed by the high vacuum and various spectrums of electromagnetic radiation.As all materials of organic origin, also polymers are sensitive to UV radiation which is abundant in space. Thus all materials intended for space are stress tested with all UV subtypes (A, B & C).
A third important characteristic of space materials is outgassing. This is a phenomenon related to the stability of the material in high vacuum. Vacuum-stable materials should not contain gas bubbles in its polymer structure nor chemically decompose in vacuum, as the released gases can have a detrimental effect on electronics, or they can smudge optical devices such as lenses.
So the development race continues.
Even high mechanical strength alone is not sufficent on some applications. In 3D printed electronical applications, the challenge is to create conductors on inherently dielectrical material. This can be achieved either by conductive fillers or with ALD (atomic layer deposition) coating. Research on the conductive filler materials is done by our project partner Carbodeon Oy. Carbodeon is a finnish startup that specializes in nanodiamond additives and fillers. So far nanodiamonds have been used to raise the tensile failure strain and lowering the Young´s modulus of material (i.e. softening and stiffening), but now the next challenge is to develop a conductive filler for PEKK and PEEK ultrapolymers.
Even PCBs can be printed…
Figure 6. Model of printable circuit board. The conductors will be ALD deposited on the core material.
The functionality of ALD coated electrical wiring has beed tested in this project by printing “circuit boards” from two materials (Figure 6). The wires have been deposited on the channels on the core material by ALD method. The most successful material combinations were achieved with nanodiamond enhanced PLA as core material and PVDF (polyvinylidene fluoride) or PPGF30 (30% glass fiber polypropylene composite) as outer shell. Other materials were also tried, but the adhesion between various core and shell materials was often poor.
Figure 7. Printed circuit boards after ALD copper coating. Nd-PLA/PVDF combination on top and Nd-PLA/PPGF30 on bottom.
The ALD copper coating process resulted in a 10 Ohm resistance over the conductors at best. Some copper was also deposited inside the vias seen on the models but it did not form an electric connection (Figure 7.) The coating process also revealed differences in the porosity of materials, with glass fiber filled polypropylene being least porous. For best vacuum outgassing results, materials should have as little porosity as possible.
Next stages of project
The project will continue in 2021, and Maker3D´s role continues as the producer of printed parts. In addition, we will offer our expertise on design and 3D printing techniques. Quite often, a part needs to be designed to match the intended manufacturing method, this applies also to 3D printing. With our design expertise, we can ensure the success of printed parts. We will also assist in the testing of new nanodiamond enhanced and conductive printing materials.
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