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News

Additive manufacturing adds flexibility to design process for CUBESAT construction

CRP Technology : 22 March, 2013  (Application Story)
Students at Morehead State University are building an amateur radio CubeSat with a propulsion system that will raise the apogee of its orbit from 500 km to 1200 km. RAMPART, which stands for RApidprototyped Mems Propulsion And Radiation Test CUBEflow SATellite, plans to launch on a Minotaur from Vandenburgh in June 2013. The innovative use of additive manufacturing with Windform XT (a laser sintering material introduced in 2004) enabled the desgners to modify, change, and add experiments without concern for having to develop tooling or modify an existing cube structure.
Additive manufacturing adds flexibility to design process for CUBESAT construction

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.

This 2U CubeSat will use a self-contained, warm gas, propulsion system to adjust the satellite’s initial circular orbit of 500km to an eliptical orbit with an apogee of 1200km and perigee of 500km at a 45 degree inclination. The device will measure the flux of energetic particles in lower Van Allen Belt and test radiation-hardened electronic components and high performance solar cells in a high radiation environment over a period of five years.

The students are proposing a UHF downlink of 9k6 GMSK AX25 packet and a downlink frequency of 437.325 MHzhas been requested.

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. A new optic or sensor can be fitted immediately and the benefit deployed to the field as quickly as possible.

The laser sintered RAMPART Cube Sat module can incorporate a mix of standard and custom board modules for solar panels, wire routing, and the addition of the load cell. A further benefit of using 3D printing technology was the reduction in fasteners and ease of assembling RAMPART. Positioning features and “snap fit” devices can be incorporated into the design to speed assembly, while the extensive baffle design in the tank structure consolidates components, removing complicated assembly steps.

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 mixed standard and custom board modules for solar panels, wire routing, and the addition of the load cell.

Windform XT uses a base polyamide reinforcved with carbon microfibres. 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-moulded materials.

The entire structure of RAMPART is plated in 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 2500psi 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 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 non-toxic and non-flammable. 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.

Since the dimensions and the weight of a CubeSat are its main constraints, Windform XT allows these parameters to be optimised through the use of multiple and interconnected near-cubic cells. This maximises the propellant volume.

After physically testing the small test cubes that represented the chambers up to 600psi, an FEA model was developed by Whitney Reynolds of the US 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.

Rapid prototyping in aerospace

The use of rapid prototyping plastics in aerospace applications has been 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 each have 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.

For the design engineer, laser sintering, as well as other 3DP and AM processes, offers a number of benefits:

  • Not restrained by geometry. It is possible to build undercuts, hollow parts, and internal ducts.
  • Build directly functional “assembled mechanisms” (minimum clearance between parts 0.5 mm).
  • Possible to build many different parts together in the same building volume.
  • Building time 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.

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 that the carbon microfibres 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 moulded 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.

BUS Construction

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 maximise the use of additive manufacturing. Increased complication was not a hindrance to the process : 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.

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.

  • Construction of a CUBESAT using Additive Manufacturing
    Franco Cevolini (CRP Technology S.r.l.), Walter Holemans (Planetary Systems Corporation), Adam Huang (Department of Mechanical Engineering, The University of Arkansas), Stewart Davis (CRP USA), Gilbert Moore (Project Starshine)
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