Usually on regular sized Satellites, structural components are machined following custom specifications related to the payload that is needed to put in orbit such as budget and time among other things. CubeSats on the other hand, gain popularity because they try to follow standard parts that reduce costs, but at the same time they usually impose restrictions on the payload design itself. Since the development team is subject to using the existing structural and support parts to build their CubeSat and scientific payload there are many more constraints.
Additive manufacturing presents an incredible opportunity for CubeSat development teams to produce their own structures following the basic requirements for a CubeSat satellite (general dimensions and so on). But all at the same time, allowing teams to customize the interior, and up to some point also the exterior of the structures, supports and other parts of the satellite. For instance, it allows the research and development of different deploy experiments outside the cube, mechanisms for antennas and solar panels expansion, or to even attach several cubes together. The opportunities are almost infinite since AM ,as we mention, is “Geometry Free”. AM gives more liberty to cubesat designs while maintaining the ability to meet the given requirements appropriately.
Today AM technology provides the ability to print in materials that are approved for use in Aerospace applications as we will see during this module. But even without those high end materials or 3D Printers, a simple 3D printer can construct a CubeSat component to easily test ideas and prototypes without the need to wait for machining parts or other delays while under development. Once the part is fully tested it can be sent to final manufacturing; greatly reducing the development cost of the part. The development of "Custom Tools" (clamps, fasteners, base plates, holders, etc.), is another area where development teams can benefit from AM since they can manufacture not only component parts, but also the tools that are require to handle those or other components.
Figure 1: "Foldable CubeSat design", winner of the "Stratasys CubeSat Challange 2016" .
Fuse deposition modeling, stereolithography, and laser sintering are different methods used for rapid prototyping. When considering these technologies, laser sintering appears to be the best due to its ability to produce durable plastics with consistency. Nylon 11 (developed by Boeing) and Nylon 12 (used by Northrop Grumman) are two materials which can be used in production through laser sintering. Windform XT 2.0, another material used in laser sintering, has already been utilized for constructing parts for CubeSats. It is a polyamide-based material reinforced with carbon microfibers. Originally developed for motorsports teams, Windform materials have been used to manufacture end-product parts such as intake manifolds, air inlet ducting, electronic enclosures, cold air ducting, and retaining clips. NASCAR, Formula One, and Lemans GT series all utilize Windform XT 2.0 to build components used on the cars. Parts for the PrintSat and KySat-2, which will be referenced again in this section, were also additively manufactured using this material. On the FDM side, we can find ULTEM 9085, a thermoplastic filament used by Stratasys printers but also by many other low end 3D printers, to manufacture high temperature resistant parts that can be used in aerospace applications.
AM Parts on the KySat-2
The KySat-2  included five unique additively manufactured parts produced with the Windform XT 2.0 material. The satellite included some duplicates of these parts for a sum of ten individual AM parts. The off-the-shelf parts used for the KySat-2 imaging system were not optimized for placement in a CubeSat, so the team designed two AM parts that would fit together and mount the purchased parts. Two "straps" were also additively manufactured to hold the necessary batteries. Four solar panel clips were also produced additively to hold antennas when stored, which allowed the KySat-2 to only need one burn wire instead of two. Finally, two deployment extensions were also produced with the Windform XT 2.0 material. These components would present many difficulties if they had to be manufactured with traditional methods.
Figure 2: Revisions of KySat-2 battery holders (only bottom holder was made with Windform XT 2.0) along with CMOS cover and lens holder (L). Assembled imaging system just before integration into Flight Model (R) .
Whatever method and material a CubeSat team chooses, the satellite must be able to withstand the conditions it will encounter on its mission. This means that the material and its structure must be tested. Tests must be undergone to see the effects of different environments on the satellite. Some things to consider are different pressures, radiation exposure, and varying temperatures. As an example the Windform XT 2.0 material was verified as viable for space applications through a NASA Launch Services Program regimen, which included material strength, Coefficient of Thermal Expansion (CTE), Total Mass Loss (TML), Collected Volatile Condensable Mass (CVCM), and surface roughness testing.
Advantages and Disadvantages
- Opportunity for building functionality of mechanisms directly into the structure.
- Build time is independent of object geometry complexity, but dependent on part volume and build height.
- Lead-time between submission and construction is very short.
- It is possible to reduce mass by creating hollow parts with internal reinforcement.
- 3D printed/SLS parts may be easily machined to maintain critical dimensions.
- The layered construction process is free from geometrical constraints, and is possible to build hollow parts, undercuts and internal ducting.
- It is possible to integrate multi-functional capability within printed structures or mechanisms. For example, it is possible to embed thermodynamic control, mechanical reinforcement, and/or electrical features into the typical build process. These may be used for such applications as embedded antennas, electrical interconnects and transmission lines.
- Additive Manufacturing (AM) is more efficient with material usage. This in turn requires less material to be sacrificed in post processing, such as polishing.
- Because parts are 'grown' as Stewart has often described, designers may approximate the designs in nature that are grown and attain the specific stiffness and strength of nature's design. That is, a single part can be made instead of many, deleting the bureaucracy of mechanical design.
- Minimum feasible structure thickness is 1mm, but recommended at 1.5mm to 2mm.
- Keep features greater than 1mm.
- Excessive structure thickness, greater than 10mm, can cause undesired warping or shrinking.
- Avoid enclosed volumes. Hollow parts will need holes to allow the removal of unsintered powder.
- When designing parts that are to be assembled later, maintain a minimum 0.2mm clearance between the parts.
- Hole features will often need to be machined to achieve a truly round hole.
- Over the 100-millimeter distance of a CubeSat it is probable to measure a 1-millimeter difference from nominal dimension. Consequently engineers in charge of the 3D printing need to manipulate the CAD model to counteract this tendency.
- Dimensions of printed parts continue to change as a function of time, (possibly due to humidity and temperature cycling).
- CTE mismatch between metals and Windform can be problematic.
- In mechanisms, such as the antenna stow and release system, where precision is highly important, the variation of nominal dimensions, necessitates the printing of moving mechanical hardware in the assembled state as a means to nullify the effect of the variation. For example, if you want a door hinge out of Windform, print the two halves at the same time in the assembled state. Though the whole assembly may be off dimensionally, the relative variation will be negligible, allowing the mechanism to work best.
- Windform is neither a good electrical conductor nor insulator.
Here is an interesting video about how today's design tools can help us with many of the parameters needed for a successful production of AM parts that follow tight requirements, including CubeSat structures:
Let's review the Pros of using AM technologies in CubeSat development:
Let's also review the cons of AM in CubeSat development: