Constructing a Cubesat using additive manufacturing
The use of Windform® XT and additive manufacturing (AM)/3D printing (3DP) technology demonstrates the fast adaptability and freedom of design possible in nearly any field. For the developers of the RAMPART Cube Sat, the use of this material and technology enabled them to modify, change, and add experiments without concern for having to develop tooling or modify an existing cube structure.
By Franco Cevolini, CEO, CRP Technology S.r.l; Walter Holemans, Planetary Systems Corporation; Adam Huang, Department of Mechanical Engineering, The University of Arkansas; Stewart Davis, Director of Operations, CRP USA, LLC; and Gilbert Moore, Project Starshine
As electronics and sensor systems become smaller and more complex, there is a push for developing advanced Cube Sats, the smallest and most flexible satellite for practical experiments in space. AM/3DP technology let us adapt the structure to carry new sensors or optics. Using CAD and additive manufacturing, the internal structure of the Cube Sat can be built to adapt to the components, instead of the other way around. This allows the use of the CAD system to adapt to new technology as it is introduced, eliminating the concern for legacy systems. A new optic or sensor can be fitted immediately and the benefit deployed to the field as quickly as possible.
In addition, standard components can be placed into a CAD library that will allow for parametric generation of portions of the satellite. In this example, laser sintering of the RAMPART BUS module shows a clear mixing of standard and custom board modules for solar panels, wire routing, and the addition of the load cell. Another benefit was the reduction in fasteners and ease of assembly. Positioning features and “snap fit” devices can be incorporated into the design to speed assembly. The extensive baffle design in the tank structure leverages this consolidation of components, removing the need for a complicated assembly.
Introduced in 2004, Windform® XT is a laser sintering material specifically for the creation of functional prototypes. It uses a base polyamide and reinforces this with carbon microfibers. Racing teams were the first to use the material and determined that it could be used in “on car” applications. Windform is currently used by both F1 and NASCAR teams to replace components that would typically require injection-molded materials.
The entire structure of RAMPART is plated in a high phosphorus electroless nickel to provide radar reflectivity for tracking purposes. The upper BUS module consists of several experiments as well as test solar cell panels mounted to the exterior of the module. An additional experiment related to the performance measurement of the Windform® XT material was placed at the top section of the BUS. The load cell (designed by Walter Holemans, Planetary Systems Corporation) measures the change in preload of Windform® XT. A 400 lb compression load in the load cell will cause 400 lb tension and about 2,500 PSI stress in the material. This integrated load cell measures creep or fracture as a result of exposure to radiation and thermal cycling over time.
The excitation is 10.0V, response ~0.002 V; thus, the response times the calibration factor will equal the load in Windform® XT. The load will vary with temperature and time; Lower temperature equals a higher load.
RAMPART’s solar panels are spring–erected following the satellite’s deployment from its launch vehicle.
The aft portion of the satellite consists of a Micro Electrical Mechanical (MEMs) propulsion system designed and developed by Dr Adam Huang of University of Arkansas. The RAMPART propulsion system incorporates a miniaturized resistojet thruster core with a microfabricated de Laval nozzle and integrated heater. Upstream of the nozzle/heater assembly is an injector fed by three miniature solenoid valves. Prior to arriving at the valves, the propellant passes through a two-phase separator membrane where only the gas phase of the liquid propellant can pass through its micron-sized pores. The membrane also serves as a filter for preventing the valve seats from becoming contaminated with debris. The propellant used is the refrigerant R-134a, which is considered nontoxic and nonflammable. The compressed fluid nature of R-134a provides relatively high self-pressurization for delivery throughout the thruster system.
Although resistojets do not represent the state-of-the-art in thruster technology and offer relatively low thruster efficiencies (RAMPART’s design specific impulse of 90s), their simplicity and practicality apply well to pico- and nano-satellites. A key factor to the performance of the propulsion system is the lightweight and cell structure of the propellant tank made from Windform® XT. Instead of using state-of-the-art technology, RAMPART uses high propellant mass fraction to provide the required Delta-V (320m/s for RAMPART) for Low-Earth-Orbit in-plane orbital maneuvers.
Since the dimensions and the weight of a CubeSat are its main constraints, Windform® XT allows these parameters to be optimized through the use of multiple and interconnected near-cubic cells. This offers the advantage of maximizing the propellant volume, use of integrated baffles provided by the interconnecting walls, and the possibility of correlating material strength tests of a unit cell to the entire propellant tank design. One can easily compare this design to traditional pressure vessels (hemispherical caps) and realize a two times difference in the propellant storage volume.
After physically testing the small test cubes that represented the chambers up to 600 psi, an FEA model was developed by Whitney Reynolds of the U.S. Air Force Research Laboratory, Space Vehicles Directorate (AFRL/RV), to simulate the reactions of the small chambers and then correlate the results to a simulation of the final large propulsion unit.
The use of rapid prototyping plastics in aerospace applications has been somewhat limited to prototype work with the exception of Nylon 11 developed by Boeing (ODM) and Nylon 12 used by Northrop Grumman. These two materials are used in production environments in laser-sintering (LS) machines. Selective laser sintering resolution for plastics is commonly run at 0.004 in. per layer using a CO2 laser that can be adjusted to melt the plastics into a fully dense material.
Fused Deposition Modeling (FDM) by Stratasys, Multi Jet Modeling (MJM) by Objet, and Stereolithography (SLA) from 3D Systems were some of the rapid prototyping technologies examined. Each technology and material has benefits for prototyping. Key factors helped steer the material choice, including heat deflection temperature, UV exposure, and the requirement of plating to make the satellite radar reflective.
Based on observations of other materials, samples of Windform® XT were subject to tensile test and the cross sections of the brakes were examined under an electron microscope. The micrographs showed the carbon microfibers were encapsulated by the Nylon base material. In addition, little to no porosity was visible in the internal structure.
Studies have shown that Windform® XT performs in a predictable manner and has been compared to injection molded production materials to determine how it would react in exposure to extended temperature cycling.
The following key factors make Windform® XT a good candidate for the CubeSat application.
• It passes Outgas Screening, ASTM E-595.
• It can be produced in a manner that makes it dense.
• It can be easily machined using conventional methods.
• Heat deflection temperature is above 170C.
• The base polyamide material has been proven to meet performance needs for other aerospace applications.
• Material batches are quality controlled, each coming with a Certificate of Conformance (COC).
• Build volume of the LS system fits well with CubeSat applications: 381 x 330 x 457 mm (14.5 x 12.5 x 17.5 in).
• It can be plated without the need for sealing agents.
Construction of BUS and propulsion module:
Laser Sintering is a powder-based method. A part is developed in layers, with each layer of powder sintered separately by the laser.
The LS machine used in this application consisted of three powder beds and a laser. Two of the powder beds held the feed powder and the third bed held the part. The part bed was in the middle of the beds with the laser acting directly perpendicular to it. A roller was used to push the layers of powder over the part bed and all three beds had their own heater source. The process itself is simple and repeatable.
A roller is positioned beside one of the feed beds. This feed bed then raises a set amount (usually < 0.1 mm) and the roller pushes the raised powder across, covering the part bed with a powder layer.
Then, the laser starts to trace out the desired shape of the part in the powder, melting the powder as it contacts the surface.
The part bed then drops down the set amount and the process continues from the opposite side, with the other feed bed raising and the roller distributing another layer of powder over the part bed, and the laser again traces the shape.
The part is built up in slices, with each layer of powder representing a single slice of the part. As the laser melts the powder, each layer fuses together to grow a solid part. Because the part is made up in layers, very complex shapes and designs can be manufactured that would otherwise be impossible by conventional means.
Windform® XT uses micro fibers as a reinforcing system and is similar to carbon lay-up techniques in that it is non-Isotropic. Similar design rules are applied when building parts to take advantage of the increased strength in one direction over the other resulting from the build process. In the case of the RAMPART BUS, the decision was made to produce the part with the Z build axis vertical to the satellite’s longest axis after assembly. This allows the X and Y axes to be positioned to provide the most strength in the perpendicular axis holding the BUS together.
The BUS underwent several revisions within the period of a few months. Walter Holemans was able to add experiments, create different deployment options, and develop wire routing and harness control devices using design techniques to maximize the use of additive manufacturing. Increased complication was not a hindrance to the process and allowed for greater design freedom. The solar cell experiment was easily tucked into the side of the BUS by implementing a simple feature cut in the CAD system.
The Bus and propulsion modules were created in separate sections to allow the different teams to test and assemble them at different locations. This step allowed the production of prototype modules to try ideas based on the current challenges and then adjust as needed. At the completion of the final design, the parts were grown at 0.004 in. (0.1 mm) layers and then sanded to a smooth finish to aid in the plating process.
The benefits of laser sintering
For the design engineer, laser sintering, as well as other 3DP and AM processes, offers a number of benefits:
You are not restrained by geometry. It is possible to build undercuts, hollow parts, and internal ducts.
You can build directly functional “assembled mechanisms” (minimum clearance between parts 0.5 mm).
It is possible to build many different parts together in the same building volume.
Building time is not dependant on object geometry, but only on part volume and on build “height” (Z dimension).
Building time is short (max 1-2 working days).
Lead time is short ( max 2-3 working days).
To reduce mass and weight it is possible to create hollow parts with internal reinforcing structures.
Using the experimental and simulation data, the propulsion module (pressure vessel) was oriented such that the Z build orientation was again parallel to the longest axis of the CubeSat. Due to the internal structures of the baffled cubes, the module was subjected to an ultrasonic water bath to ensure no material was trapped in the chambers. MPF