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Abstract
As an alternative to parabolic antennas and Synthetic Aperture Radar (SAR) systems, waveguide arrays offer another method of providing RF transmit/receive communication apertures for spacecraft. The advantage of the membrane waveguide array concept, in addition to its lightweight and low packaged volume, is its inherent shape. Relative to parabolic antennas, the requirement to make an accurate doubly curved surface is removed. L'Garde and Langley Research Center (LaRC), are currently working in this area to develop lightweight waveguide array technologies utilizing thin film membrane structures. Coupled with an ultra-lightweight inflatably deployed rigidizable planar support structure, the system offers a very compelling technology in the fields of space-based radar, communications, and earth resource mapping.
Introduction
After significant development, and several prototypes, a packagable
2.2mx1.1m RF prototype was constructed and sent to the NASA Langley
Electromagnetics Research Branch and the Georgia Technical Research
Institute for RF testing (Figure 1). The membrane waveguides are
visible as the large planar array in the center of the structure.
Also visible are the radiating slots cut into the upper membrane
to transport the RF energy in and out of the waveguide cells.
The structure surrounding the waveguide is constructed of rigid
aluminum tubes and is designed to simulate the geometry of a deployable
rigidizable structure. The innovative RF distribution network
is visible on the far end of the array, and distributes the RF
signal from a single source to each of the eight waveguide feeds.
Work is currently ongoing to develop a rigidizable inflatably
deployed structure to deploy and support the waveguide array thus
completing the antenna system.
Figure 1. Waveguide Array in NASA LaRC RF Test Chamber
This paper discusses the mechanical and structural aspects of the waveguide array and support structure. Excellent RF performance was achieved by the test article, Figure 1, during a comprehensive RF test program at LaRC and GTRI utilizing far and near-field RF testing. The reader is urged to review references 1-3 for a detailed description of the test methods and RF performance results.
System Design
The design of the thin film waveguide is based on a series of waveguide elements or cells which are bonded together to form a large array [4,5]. Each waveguide element is a rectangular-shaped prismatic closed shell fabricated from a 0.01mm thick metalized Kapton membrane. Each waveguide cell features a feed assembly located near one end. The waveguide array, shown in Figure 2, is made up of eight waveguide cells; the cell cross-sections are just visible at the foreground end of the array toward the right. Slots etched into the metalization are clearly visible and direct RF energy incident on the array down the waveguide cell onto the feed assembly just visible toward the end of the array toward the left. Two catenary sheets form the top and bottom membrane surfaces of the waveguide array cells. The catenaries are supported at the corners by cables that transfer the tension loads in the array to a simulated rigidizable inflatable support structure.
Figure 2. Waveguide Array in GTRI Near-Field Test Range
Between the adjacent walls of each cell is a 0.13mm thick Kapton "stiffener." These stiffeners provide the additional rigidity needed in the vertical direction along each of the waveguide cell walls. These stiffened vertical walls of each cell are perpendicular to the top and bottom surfaces of the array when tensioned by the catenaries.
Baseline Mission
Much interest has been shown in this concept as a water sensing
radiometry antenna for Earth resource mapping. A highly enhanced
inflatably deployed rigidizable support structure has been developed
around the baseline 9mx9m radiometer mission. To make up the array
for the 9mx9m unit, 55 waveguide cells, each with radiating slots,
will be bonded together. A structural optimization resulted in
the geometry shown. The tube geometries were selected to withstand
the tension loads in the array with minimum safety factors of
2.0 in local buckling, and 4.0 in Euler buckling, while minimizing
the mass of the overall structure.
Figure 3. Optimized 9m*9m Radiometer Array
Feed Network
To transport the RF signal from the transceiver to the waveguide
cells a packagable integrated feed and microstrip network has
been designed and fabricated. The design, developed by LaRC and
GTRI and fabricated by L'Garde, utilizes metalized membranes supported
by a low dielectric foam material to provide the proper impedance
for efficient RF transport. The network is mounted above the waveguide
array, which concurrently acts as an RF groundplane for the network.
Feeds are integrated into the network, which transport the signal
to and from the waveguide cell. The network is designed to divide/combine
the signal through 8 separate and equal length paths. A picture
of the network mounted to the waveguide array is shown in Figure
4, note the feed elements extend through circular holes in the
cell walls.
Figure 4. 8:1 Combiner Microstrip Network
Novel "hinges" in the form of precisely positioned gaps in the foam were integrated into the network design and fabrication. These "hinges" allow the network to be folded and are positioned to align with the array fold lines for packaging. Precise geometries were attained by using a computer-controlled laser cutting table to form the foam and membrane structure.
Array Stowage
The waveguide cells are stowed by folding the top and bottom
membranes between the sidewall stiffeners as shown in Figure 5.
Once the cells are folded in this manner, the sidewall stiffeners
are brought together in an accordion fashion.
Figure 5. Waveguide Cell Packaging Concept
The thicker Kapton sidewall material resists folding and must be rolled in a radius to avoid permanently deforming the material. The resulting serpentine packaging concept for the stowed waveguide is visible in Figure 6. The serpentine waveguides are looped back resulting in a very efficient packaging arrangement. The feeds and microstrip network can be seen fully stowed toward the foreground.
Figure 6. Packaged Waveguide Array
Sub-Tg Rigidization
Sub-Tg or Cold rigidization takes advantage of the increase
in modulus of certain materials below their glass transition temperature
(Tg). The system is made up of a specialized composite weave utilizing
strong, lightweight fibers impregnated with a L'Garde formulated
sub-Tg elastomer. Though sometimes called cold rigidizables they
don't necessarily require cold temperatures. Elastomers can be
formulated which rigidize at temperatures tailored to specific
mission requirements. Cold rigidized structures can be constructed
for a variety of missions, from LEO to deep space applications
and this technique was selected to form the support structure
for the waveguide array
A lightweight sub-Tg rigidizable truss built by L'Garde is shown in Figure 7 [6,7]. This structure has been mechanically tested at LaRC and withstood 2473.21N in compression, though its own mass does not exceed 4.31kg. This truss uses a Sub-Tg resin formulated with a Tg (glass transition temperature) of +40°C and is rigid at room temperature. Utilizing this system, this very large structure can be tested at room temperature conditions.
Figure 7. Sub-Tg 8m Rigidizable Space Truss
Sub Tg rigidizable structures are simple and reliable. They are completely passive and in general require no heaters or vents. However, since their rigidization depends on temperatures below their Tg, a thorough understanding of the thermal environment is required. Ideally, an elastomer is selected with a Tg below the equilibrium temperature of the spacecraft during deployment. After inflation and deployment, outside of the spacecraft's thermal environment, the structure cools and becomes rigid. If the deployed structure must endure large thermal excursions, it will be tailored to have a higher transition temperature and heaters may be required to "soften" the structure for deployment. MLI is required to mitigate the effects of on-orbit thermal gradients and to retain the initial softening thermal energy during the deployment sequence.
Conical Deployment
Figure 8 shows the conical boom packaging and deployment scheme
developed for deployment control of the inflatable rigidizable
support booms. The technique uses a unique concentric packaging
arrangement about the boom axis and provides a high degree of
deployment control (patent pending). In this technique, a tapered
tube is packaged in a manner similar to a telescoping tube. Because
there is essentially zero excess mass associated with the deployment
control method, it scales very well and provides excellent packaging
efficiency. To deploy the conical boom, inflation gas is introduced
at the base. Pressure is exerted against the walls of the tube,
the tip, and base endcaps. The inflation pressure unfolds and
deploys the leading folds. The resulting deployment is very smooth
and predictable. As the tube is under pressure during deployment,
it is able to withstand some load while deploying.
Figure 8. Conical Inflation Schematic
A deployment sequence of a conical boom is shown in Figure 9. The deployment proceeded smoothly and in a linear, consistent, and predictable manner. This particular tube is fabricated of a specialized Kevlar weave impregnated with L'Garde's L6 20°C sub Tg resin. The tube is 20.32cm in diameter and is over 2.54m long. Similar tubes have been mechanically tested under simulated space thermal conditions. The tubes achieved a material modulus of 26.89GPa after enduring conical packaging and deployment.
Figure 9. Conical Deployment Sequence
Stowed Configuration
A stowage and deployment concept has been developed around
the 9mx9m baseline mission. The concept marries the stowage capabilities
of the waveguide array and the unique highly controlled deployment
of the conically deployed sub-Tg rigidizable struts. The stowed
configuration is shown in Figure 10. The conically packaged struts
are shown stowed around the outside of the serpentine folded array.
Since some clearance is required between the stowed tubes to encompass
the stowed array, a small portion of the front lateral tube seen
in the foreground is partially deployed and flattened to connect
the corner braces. The volume at the base of the canister contains
the inflation tanks and inflation/deployment system.
Figure 10. Stowed Waveguide Configuration
Deployment
A schematic of the deployment sequence of the 9mx9m baseline
design utilizing conically deployed booms is shown in Figure 11.
Upon initiation of the deployment sequence, the top and sides
of the canister are opened to release the array and support structure.
The deployment in the longitudinal direction is initiated first
to extend the stowed waveguide array. Two opposed double conically
deployed struts are used to control the sequence in the longitudinal
direction. Conical struts on either side of the canister (1,2)
are deployed in parallel. Once these longitudinal struts are at
full extension, the waveguide sidewalls are also fully extended.
Three conically deployed struts (two single and one double) are
used to control the lateral deployment; struts (3,4) are extended
and coordinated with the far lateral strut (5). Once all the struts
are fully extended and rigidized, the waveguide array is fully
deployed.
Once the structure cools to space steady state temperatures rigidization is complete and the inflatant can be vented, as it is no longer required for structural stiffness. The coordination of the conical deployment and inflation will be computer controlled, and techniques for monitoring the deployment extension of the booms are in review.
Mass/Volume Estimates
Preliminary mass and volume estimates have been generated
for the updated 9mx9m design. These estimates are based on analytic
predictions of prototype waveguide units, measurements of component
hardware, and L'Garde experience in other space inflatable programs.
The total subsystem mass for the inflatable waveguide is 56.7kg,
yielding an aperture areal density of 0.7kg./m2. The stowed volume
of the canister is 0.57 m3.
A comparison of the predicted 9mx9m waveguide subsystem mass with a similarly sized rigid deployable antenna is shown in Figure 12. The rigid system masses are for Astro Aerospace's AstroMesh parabolic deployable antennas [8]. The inflatable waveguide system mass compares favorably to the rigid parabolic reflector masses. Note, however, other factors are introduced when comparing parabolic and planar antennas. Parabolic antennas can function well over a large frequency range while the waveguide is optimized for a small frequency range. The waveguide, however, can be electronically beam steered, by phasing the waveguide feeds, decreasing pointing requirements, and increasing mission applicability. This comparison is meant only to show competitive system masses with other technologies.
Figure 12. Waveguide Mass Comparisons
Conclusions
In this series of programs, L'Garde, LaRC, and GTRI have continued development of the membrane Waveguide Array concept and the deployable/rigidizable tensioned membrane support structure. Several prototypes have been fabricated and a fully functional and packagable membrane waveguide array has been fabricated and demonstrated. The array features a novel packagable feed and signal distribution network. RF testing at LaRC and GTRI has shown the feasibility and performance of the design. A support structure featuring sub-Tg rigidizable conically deployed booms has been designed and test articles of these booms have been fabricated, deployed, and mechanically tested under cold conditions to validate the deployment and mechanical properties. A NASA SBIR Phase II effort is currently underway which will further develop the waveguide array support system, culminating in thermal vacuum deployment and rigidization of the waveguide array and support structure under simulated space conditions. The commercial applications of the membrane planar waveguide concept are numerous. The lightweight, low stowed volume, and simplicity of the concept, coupled with the geometric precision of the planar array, will find many applications in the fields of space-based radar, communications, and earth resource mapping.
Acknowledgments
Much appreciation is extended to Tom Campbell and M.C. Bailey, formally of NASA LaRC, for their vision, advocacy, and technical expertise in development of the membrane waveguide array. The authors would also like to thank Wes Lawrence of NASA LaRC Electromagnetics Research Branch for his continued support and expertise. L'Garde would like to thank Dr. Gyula Greschik for his assistance in the waveguide array and deployable support structure design and analysis.
References
1) D. Fralick, R.L. Cravey, G. Hopkins, D. Lichodziejewski, M. C. Bailey, "Performance Evaluation of a Membrane Waveguide Array Antenna", IEEE-2003-1148 IEEE Aerospace Conference, March 8 2003, WY
2) G.D. Hopkins, "Near-Field Measurement and Evaluation of Tensioned Membrane Array Antenna", NASA LaRC NAS1-02056, Task Order 6003, August 10, 2002
3) R.L. Cravey, H.-J.C. Blume, C.P. Hearn, M.J. Scherner, L.C. Schroeder, and R.F. Harrington, "Electromagnetic Losses in Metallized Thin Films for Inflatable Radiometer Applications," Proceedings of the AIAA 1995 Space Programs and Technologies Conference, September 26-28, 1995, Huntsville, AL, AIAA Paper 95-3741.
4) D. Brandt, "Thin Film Waveguide Conceptual Design Study", L'Garde Technical Report, LTR-98-DFB-003
5) M.C Bailey, T. G. Campbell, "Inflatable Tensioned Membrane Waveguide Antenna Array for Space Applications", IUTAM-IASS Symposium on Deployable Structures, Cambridge, UK, September 6-9, 1998.
6) K. Guidanean, D. Lichodziejewski, "An Inflatable Rigidizable Truss Structure Based on New Sub- Tg Polyurethane Composites", AIAA-2002-1593, 43rd AIAA Aerospace Sciences Meeting and Exhibit, Apr. 25, 2002, Denver, CO
7) K. Guidanean, G. Williams, "An Inflatable Rigidizable Truss Structure with Complex Joints", AIAA-98-2105
8) M. Thompson, "The AstroMesh Deployable Reflector", Proceeding of fifth International Satellite Conference, June 1997