Other papers
Abstract
As an alternative to the expensive and environmentally sensitive
RTG (Radioisotope Thermal Generators), L'Garde is developing the
Power Antenna concept. This new technology utilizes an inflatable
reflector to concurrently concentrate solar energy for space electrical
power generation, while acting as a large aperture high gain antenna.
L'Garde has conducted a detailed study of the issues concerning
the design and performance of the Power Antenna concept. Much
of the effort was conducted in conjunction with JPL under the
Gossamer Spacecraft program. The technical objectives of the Power
Antenna program are to reduce the mass and stowage volumes of
the Power Antenna. We have optimized the key parameters, and developed
an enhanced, state-of-the-art configuration, based on generic
mission requirements. A generic Jovian mission resulted in a 6.7m
aperture Power Antenna subsystem mass of only 21.9kg yielding
75 watts of electrical power. This low mass yields a power density
of 3.42 watts/kg. A state-of-the-art Earth orbiting power subsystem
yields a 100 watt/kg. This same system in a Jovian orbit, assuming
it retains its conversion efficiencies, would yield about 3.7
watts/kg. The power antenna compares well to this performance,
particularly when it is also includes a large aperture high gain
antenna shown to exceed mission requirements.
Power Antenna Concept
The Power Antenna utilizes an inflatable parabolic reflector to concentrate solar energy for space electrical power generation, while acting concurrently or alternatively as a large aperture high gain antenna. First, the parabolic reflector acts as a solar concentrator, focusing solar energy onto an array of photovoltaic (PV) cells for electrical power generation. In deep space ambient sunlight is severely diminished and a concentrator is required to increase the light intensity levels for efficient use with conventional solar cells. Second, a beam splitter or metallic grid is mounted in front of the solar cell array to deflect Radio Frequency (RF) energy onto a feed (Figure 2). In this way the optical and RF energy impinging on the reflector can be separated and utilized for deep space power generation and/or high gain RF communications.
Reflector Design and Construction
A series of low film stress reflectors were designed using FLATE, a L'Garde gore design tool, and analyzed using FAIM (1), a L'Garde FEA code for analyzing inflatable structures.
FLATE is used to solve the inverse problem of determining a flat gore shape, such that when the reflector is constructed and inflated to a specified pressure it forms a paraboloid. The code uses the desired shape and material properties to determine the flat gore shape. A FLATE output gore shape for the 350 psi, 1m, F/D 1.0 reflector is shown in Figure 3. The waviness in the gore outline is inherent in the plotting routine, the actual gore edges are smooth.
Shown in Figure 4 is the FAIM result for the medium film stress (350psi) gore. Please note this is a half gore model. To minimize FAIM computer time all symmetry is utilized. To construct the full reflector the gore is first mirrored around its centerline, and then repeatedly mirrored to represent the full reflector. The color bands represent the delta from a best fit parabola. The most pronounced differences are found near the gore ends and are up to 10.48*10-3 in. or .27 mm from the best fit paraboloid. Fortunately these areas are very small and the calculated RMS accuracy over the full reflector surface is 0.071 mm. This precision is theroetical and does not include material inconsistencies and manufacturing tolerances. A similar analysis was conducted at the other pressures.
The main requirement for the canopy (which, with the reflector completes the lenticular structure) is that of high optical transmissivity. It must of course also provide a low permeability membrane to contain the gas. The surface precision requirements are not as strict as that of the reflector. We decided to use the same gore template to cut the canopy gores. The modulus of Mylar is 540,000 psi, similar to that of Kapton. Were surface accuracy of the canopy a factor, we would re-design the gore shapes to compensate for the different modulus. However, for the Power Antenna canopy we used the gore shapes designed for the reflector.
When completed the canopy was mounted to the canopy rim and attached to the tub using quarter turn latches. The completed canopy mounted to the solar test fixture is shown below in Figure 5. The canopy is shown here during setup (below its operating pressure), at its proper operating pressure it is considerably less wrinkled
Reflector Surface Precision Measurements
Surface accuracy measurements of the reflectors were conducted using the V-Stars (2) video-grammetry system. The system uses a high-resolution digital camera and software to measure the surface accuracy of the reflector. One of a series of pictures used in the measurement is shown below in Figure 6. The optical targets used to locate positions are visible as bright white dots. Visible on the lower right portion of the rim is a geometrically precise bar with targets on the ends used as a reference for the photo-grammetry algorithm. Other references visible are the four targets arranged in a cruciform arrangement toward the lower left. Note, the DCS 460 used by the V-Stars system is black and white only. Color is not required to measure surface accuracy and was sacrificed to provide a higher resolution imager chip.
SOLAR TESTING
A new photographic method has been researched and developed to characterize the Power Antenna reflectors and is described briefly below. This technique uses a high-resolution highly calibrated CCD camera to photograph the Sun's image cast by the concentrator. With these images and software the performance of the reflector can be discerned, documented, and compared. The effect of materials, film stress, reflector configuration, and canopy can be viewed directly and the data used to optimize the Power Antenna configuration.
The photographic method uses a Kodak/Nikon 460 digital camera to photograph the reflection of the sunlight concentrated by the reflectors on the target. A water-cooled target is mounted near the focal plane of the reflector. The target is movable so as to traverse the planes of interest near the focal point. Various filters were placed over the camera so as not to saturate the image at the high intensities expected. The resulting digital images were high resolution with the values of the pixels representing the intensities of reflected light
The camera is mounted near the edge of the reflector and has an oblique view of the target. Software has been developed that electronically rotates the image into a normal plane and adjusts the perspective. Once this normalization is complete the value of each pixel is run though a calibration curve and the image plotted on a contour plot. Software was also developed that allows us to define areas of the image for concentration ratio calculations.
The data reduction flow diagram is shown in Figure 8. The picture at the top left is an actual datapoint from the Power Antenna. There is a heavy filter on the lens so as not to overexpose the frame. The edge of the target is visible as a hazy ellipse in the middle of the picture. The first process in the data reduction is the perspective change algorithm. This routine down-samples the image and electronically rotates it into a normal perspective. The target image is now circular. After the perspective change the image is "gamma corrected", this correction uses the camera calibration and corrects the pixel intensities to be linear with light intensity. The data is then corrected for ambient conditions to produce a concentration ratio. Contour plots of the corrected data are produced for comparison. Another routine was written to produce the familiar intensity plots shown on the bottom left. This routine finds the "centroid" of the concentrated energy and sweeps a radius around this point and averages the intensity within various radii to generate the shown plot. This plot is a 2-dimensional representation of the image. As it is seen in the contour plots the image is not axisymmetric. The intensity plot is a very convenient way to characterize and compare images but it does not fully represent the concentrated image.
Test Technique
A picture of the functioning test stand is shown in Figure 9. Visible in this picture are the hoses used to supply cooling water to the target. One hose is connected to a faucet and the other vents to a drain. There is a thermo-couple mounted behind the target face to measure temperature. Tests were conducted to determine the proper cooling flow and at this flow level it was found the target stays relatively cool. On a cloudless summer day only a trickle of water was required to keep the target under 100F. Equilibrium temperature with the flow off was not much higher. There is enough convection, conduction, and radiation to keep the target cool.
A picture of the concentrated sunlight is shown in Figure 10. The target brightness appeared much brighter in person than it does in the photo. It was not possible to view the spot for any length of time with the naked eye, and welding goggles were required.
Another view of the test stand is shown in Figure 11. Visible
at the lower left of the reflector is the camera used to acquire
the images. The oblique view the camera has of the target is shown.
To keep the view angle the camera has to the target constant the
camera is translated back and forth with the target. The slot
used to relocate the camera is just visible at its base. Also
note the white covering material surrounding the camera used to
keep it cool during testing.
