Due to COSMIC-1’s success, U.S. agencies and Taiwan have been working on a follow-up project called FORMOSAT-7/COSMIC-2 that will launch six satellites into orbit in late 2016 and another six in 2018. NASA’s Jet Propulsion Laboratory (JPL) has developed satellite technology to capture a revolutionary amount of radio occultation data from GPS and GLONASS that will benefit weather prediction models and research for years to come.
COSMIC-2 design and development began in 2011 at JPL. Critical components of the COSMIC-2 design are the actively steered, multi-beam, high gain phased antenna arrays capable of receiving the radio occultation soundings from space. The amount of science the COSMIC-2 can deliver is dependent on the custom antenna arrays. Traditionally, only large projects could afford custom antennas. COSMIC-2 was a medium size project that required 30 antennas so minimizing manufacturing costs and assembly time was essential.
A standard antenna array support design is traditionally machined out of astroquartz, an advanced composite material certified for outer space. The team knew building custom antenna arrays out of astroquartz would be time consuming and expensive because of overall manufacturing process costs (vacuum forming over a custom mold) and lack of adjustability (copper sheets are permanently glued between layers of astroquartz). The custom antenna design also contained complex geometries that would be difficult to machine and require multiple manufacturing, assembly and secondary operations, causing launch delays. JPL decided to turn to additive manufacturing technology to prototype and produce the antenna arrays.
The manufacturing chosen to build accurate, lightweight parts while maintaining the strength and load requirements for launch conditions was Fused Deposition Modeling (FDM). FDM could produce this complete structure as a single, ready-for-assembly piece. This would enable quick production of several prototypes for functional testing and the flight models for final spacecraft integration all at a low cost. FDM can also build in ULTEM™ 9085 resin, a high strength engineering-grade thermoplastic, which has excellent radio frequency and structural properties, high temperature and chemical resistance and could be qualified for spaceflight.
Instead of purchasing an FDM printer to produce the parts internally, JPL turned to Stratasys Direct Manufacturing for its large FDM capacity and project engineers who have experience with the aerospace industry and its requirements.
The antenna array support structures were optimized and patented for the FDM process. All shapes were designed with an “overhead angle” of 45 degrees at most to avoid using break-away ULTEM™ resin support material during the build. “Designing the antennas with self-supporting angles helped with two things,” said Trevor Stolhanske, aerospace and defense project engineer at Stratasys Direct Manufacturing, “it reduced machine run time so that parts printed faster, and reduced the risk of breaking any parts during manual support removal.” JPL was also able to combine multiple components into one part, which minimized technician assembly and dimensions verification time and costs.
Although FDM ULTEM™ 9085 resin has been tested for in-flight components, it had never been used on the exterior of an aircraft, let alone in space. Therefore, in addition to standard functional testing (i.e. antenna beam pattern, efficiency, and impedance match), FDM ULTEM™ 9085 resin and the parts had to go through further testing in order to meet NASA class B/B1 flight hardware requirements. Some of these tests included:
ULTEM™ 9085 resin’s properties met all required qualification tests, proving the antennas are space-worthy. However, the highly reactive oxygen atoms present at the operating height of the satellite could degrade the plastic. To protect against oxygen atoms and ultraviolet radiation, ULTEM™ resin was tested for compatibility and adhesion with some of NASA’s protective, astronautical paints. In this case, S13G high emissivity protective paint was chosen to form a glass-like layer on the plastic structure and reflect a high percentage of solar radiation, optimizing thermal control of the antenna operating conditions.
Stratasys Direct produced 30 antenna array structures for form, fit and function testing. Throughout each design revision, Stratasys Direct's project engineering team worked closely with JPL to process their STL files to ensure the parts met exact tolerances and to minimize secondary operations. Stratasys Direct's finishing department deburred the parts where needed, stamped each with an identification number and included a material test coupon. They also reamed holes for fasteners that attach to the aluminum honeycomb panel and the small channels throughout the cones to the precise conducting wire diameter.
“Not only did NASA JPL save time and money by producing these antenna arrays with FDM, they validated the technology and material for the exterior of a spacecraft, paving the way for future flight projects” said Joel Smith, strategic account manager for aerospace and defense at Stratasys Direct. “This is a great example of an innovative organization pushing 3D printing to the next level and changing the way things are designed.”
As of 2014, the COSMIC-2 radio occultation antennas and FDM ULTEM™ 9085 resin are at NASA Technology Readiness Level 6 (TRL-6). Stratasys Direct Manufacturing was able to successfully enter the JPL Approved Supplier List and delivered 30 complete antennas for final testing and integration. The launch of the initial six satellites is scheduled for 2016. Another constellation will launch in 2018. The FORMOSAT-7/COSMIC-2 mission will operate exterior, functional 3D printed parts in space for the first time in history.
References
* Turbiner, D. “Phased Array Antenna For GNSS Signals”, CIT-6243, USPTO US 20130342397 A1 https://www.google.com/patents/US20130342397dq=Turbiner&hl=en&sa=X&ei=3pc9VJDKAYWHyATy1IDgBQ&ved=0CB0Q6AEwAA
