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Abstract
With today's high launch costs, and tightening launch opportunities, low mass, cost, and packaged volume can determine the mission feasibility. L'Garde has developed the Inflatable Torus Solar Array Technology (ITSAT) to supply power to the growing fleet of small satellites in the 1kW class making forays into the space market. The ITSAT configuration with low mass and stowage volume and the inherent reliability of inflatably deployed structures is an excellent solution for these low power applications. The ITSAT is able to provide power densities more typical of a much larger system, despite its small scale.
Utilizing L'Garde's next-generation stretched aluminum inflatable
rigidizable tube technology, and Northrop Grumman's new polymer
cover-glass photovoltaic cell technology, power densities as high
as 105 watts/kg can be achieved for a 0.5kW class array. Further,
the packaging efficiency inherent in inflatable structures allows
the complete system to be packaged in 0.04 m3 for a packaging
density of 20kw/m3. L'Garde's proto-flight unit, utilizing older
technology 13.8% cells, is achieving 73W/kg, a production system
with the same cells and upgraded components would achieve a power
density of 93W/kg. However, when this lightweight structure is
integrated with a competitive 28% efficient blanket the power
density would rise to a very competitive 109W/kg.
Introduction
The ITSAT configuration is the culmination of several programs [1-4]. The initial design work was conducted under an SBIR Phase I funded by DARPA, in which a feasibility study was conducted on various point designs for power systems for LEO, GEO, and Molnaya orbits, with array sizes from 100W to several kilowatts. The subsequent Phase II effort refined this design, generated detailed drawings, fabricated the hardware, and tested the components producing a proto-flight system, Figure 1. This phase included a successful deployment at temperatures of 90C in the Naval Research Laboratories (NRL) 9 meter vacuum chamber, and subsequent dynamics testing and thermal cycling in the deployed state.
Figure 1. ITSAT Solar Array
In 1999 an SBIR was awarded to prepare the ITSAT configuration for launch on the shuttle in early 2001. The flight experiment was sponsored by the Air Force who were interested in the ITSAT technology to provide power for their small satellite programs. Under technical direction from JPL, the Phase I effort further defined the flight experiment, and instrumentation while defining the qualifications required for flight. Soon after the award of the Phase II program, the shuttle flight manifest identified for the ITSAT flight was cancelled in favor of a Space Station Freedom payload. Instead, the remaining resources were used to prepare the ITSAT for a generic flight, should one become available, and integrate several new technology upgrades developed in the interim. Specifically, new lower mass and volume, higher strength helically-wound stretched aluminum laminate tubes were developed and retrofit to the system with no loss in structural strength and stiffness. A new more capable lower mass and volume inflation system was designed, fabricated, and retrofit to the system to further enhance it's competitiveness. Successful ambient deployment and launch vibration qualification tests were conducted to validate the new components and upgraded system. Additionally a new canister design has been developed to reduce the weight even further.
The result of these efforts is a flight ready system with a measured power density of 73 W/kg of on-orbit performance for a system generating 275W of B.O.L. power. This performance is achieved with older technology 13.8% efficient crystalline silicon cells. A production unit, incorporating improved components previously identified, will have a power density of 93 W/kg. With new technology polymer cover glass technology from TRW, a production ITSAT system generating 500W of power can reach power densities of up to 109 W/kg, a very competitive system without the complexities or constraints of concentrator hardware or cells.
ITSAT Design and development
Requirements
The ITSAT was designed in the 0.2-1.0kW range for a mission life
of 3 years in LEO to GEO with the associated thermal and space
environments. Structurally, the ITSAT is designed to handle 0.03g's
in any axis, while maintaining relevant safety margins. The deployed
array exhibits a natural frequency in excess of 1.0Hz. In the
stowed configuration, it must be able to withstand the shuttle
launch vibration environment of 12g's in all three axes.
Aluminum Laminate Struts
At the heart of the inflatable solar array concept are the
inflatably deployed/rigidizable aluminum laminate struts [5,6].
These struts provide the force needed to fully extend the array
during deployment, and after rigidization, provide the strength
and stiffness to absorb the static and dynamic loads during the
mission. The strut is a monocoque cylinder fabricated from a three-layer
laminate consisting of an aluminum foil sandwiched between two
layers of Kapton. Before inflation and deployment the laminate
is very malleable and is easily flattened and packaged in a Z-folded
arrangement. Upon inflated the strut returns to its cylindrical
shape. Once deployed, the strut is inflated to its 206.84MPa rigidization
pressure, the stresses are carried by the Kapton but are high
enough to yield the aluminum, removing wrinkles and distortions
introduced during packaging. The aluminum is work hardened during
this process raising its modulus. Once rigidized, the internal
pressure is vented leaving a straight, rigid, geometrically stable
tube capable of supporting the mission loads.
Recently, L'Garde has improved the aluminum laminate concept substantially through the addition of a thin helical PBO wrapping around the tube. This helical wrapping supports much of the pressure load in the hoop direction allowing greater yielding in the longitudinal direction resulting a stronger, and more geometrically precise cylinder. Additionally, the spiral wrapping, as it supports much of the inflation pressure, has raised the burst margin of the tubes significantly to about 3.0, specifically, the burst pressure of over >620.53MPa is at least 3.0 times higher then the rigidization pressure.
Figure 2. Aluminum Laminate Helical Winding
This innovation in the inflatably deployed, rigidizable struts has improved the ITSAT concept greatly. The new strut design allowed us to reduce the previous 10.16cm diameter tubes to 6.35cm while maintaining the same overall stiffness, and resistance to buckling. The addition of the chord mass is far outweighed by the reduction in tube diameter, volume, and endcap mass resulting in a substantial decrease in strut mass. Despite an increased rigidization pressure over the previous tube configuration, the reduction in internal volume of the new struts has resulted in a 45% decease in the required inflatant, allowing a similar decrease in the overall tank volume and mass.
As aluminum is a good conductor of heat it distributes heat quickly around it's perimeter, further, carefully selected coatings on the interior allow it to radiate heat within the structure itself very effectively distributing the energy. As thermal gradients are quickly dispersed within the structure MLI is not required for good thermal stability.
Canister
The canister and lid are built of 12.77mm and 3.18mm Nomex
honeycomb with 0.13mm thick graphite-epoxy facesheets bonded on
both sides. Reinforcements are used on all corner joints. The
lid is constructed similarly, with a small flange overhanging
all four sides. The finished canister weighs only 0.86 kg. and
completely enshrouds and contains the stowed array and struts.
The stowed dimensions are 16.25cm x 21.33cm x 113.28cm, with a
stowed volume of 0.04m3 including the inflation system which resides
on the back of the canister.
A new lower mass canister has been designed. The new open configuration saves mass by leaving the walls open. Thin walled composite struts sized to withstand the launch vibration loads carry the packaged loads. "Stems" are incorporated to further constrain the packaged array during launch and ascent. The new design weighs only 0.54kg, saving 0.32kg of mass.
Array
The existing array uses 0.05mm Kapton as the blanket substrate,
see Figure 1. The blanket is 10% populated with cells with the
remainder populated with mass simulators including cover glass.
The functional cells are interconnected to each other, with 31
cells per string. The interconnects are soldered cell-to-cell
and allow for differential thermal expansion. In addition, resistance-temperature
devices (RTDs) are mounted on the back of the working cells to
monitor cell temperature. Cell temperature was measured and kept
constant during electrical current-voltage (I-V) testing. Temperature
measurements will also be required for on-orbit performance measurements.
By-pass diodes are incorporated to allow for damaged cells and
partial shading.
Thermal cycling of the blanket was a concern. To test the strength of the cell-to-substrate bond and the cover glass bonding, a sample of the solar blanket was sent to NASA Lewis Research Center for rapid thermal cycling. No damage or degradation occurred during the test. This coupon was cycled from 100C to +80C for a total of 2000 cycles.
To predict the power output of the fully populated array many
factors were considered. The Silicon Crystalline cell chosen was
rated at 13.8% efficient, but including mismatch, line, and temperature
losses, the final B.O.L. efficiency was 11.38%. Including UV degradation,
thermal cycling, cover glass and adhesive darkening, the E.O.L.
efficiency is 9.16%. For a fully populated version of the existing
blanket and array, the B.O.L. power is 274 watts, dropping to
an E.O.L. power of 220 watts. The total blanket mass including
lines, hinges, and tensioning chords is 1.1kg.
Array Enhancements
Recent work at L'Garde has focused on integrating the lightweight
inflatable structure with new technology high efficiency cells.
L'Garde and Northrop Grumman formed a team to investigate relevant
cell technologies for the ITSAT array. Four cell technologies
were selected for further investigation: high efficiency (26%)
GaAs, high efficiency (16.5%) lightweight (3 mil) silicon, advanced
amorphous silicon (9.5%), and copper indium gallium diselenide
(CIGS, 10.14%). In addition, it would be beneficial to minimize
the weight of shielding and thermal control materials. Thus, eliminating
the cover glass on the solar cells helps reduce mass significantly.
Northrop Grumman has been testing polymeric materials that may
be substituted for the glass. While this is not a new idea (fluoropolymers
were tested in the late 70's for cover material [7] and flown
as adhesive [8]), the new elements for this are advanced plastics
and, as important, protective coatings.
Seven polymers have been selected for additional space testing on the Materials on the International Space Station Experiment (MISSE) [9]. For purposes of simplicity in this presentation, Aclar, an advanced fluoropolymer, has been assumed to be the polymer medium. Each of the polymers is coated with a Northrop Grumman proprietary combination of metal oxides to result in a tenacious UV protective, very low energy proton protective, solar antireflective, and electrically conductive surface that provides ESD protection. (Proton protection is afforded by a thick layer and serves to reduce the quantity of particles, not spectrum of energies). The coating also serves to protect the polymer from atomic oxygen attack.
A study was conducted estimating the mass and performance of the above cells and optical treatments when applied to the ITSAT's 2.41m2 (0.74m*3.25m) array. The results of the study are shown in Table 1. The highest overall system power densities were achieved using 28.1% efficient gallium arsenide Triple Junction cells. This cell efficiency has been demonstrated in limited production runs, and the cells are currently under qualification testing. With system losses the array is generating 503 watts giving the system a power density of 109 W/kg.
Table 1. Cell Study Results
Qualification Tests
Deployment
The ITSAT deployment sequence is depicted in Figure 6.
Upon deployment initiation, twin pyro-cutters sever cables holding
the ITSAT lid in place. Simultaneously, the inflation system slow
rate pyro-valve is fired initiating the inflation sequence. The
mass flow rate of the initial inflatant is chosen to ensure a
positive steady deployment rate void of any extreme dynamics.
As the tubes are slowly inflated, the extension of the tubes pull
with them the folded array. The orthogonal folds in the Z-folded
tubes and array tend to "guide" the system deployment
in a linear fashion. Lanyards tying the array to the struts keep
the components together and deploying in a controlled manner.
Once fully deployed the inflated tubes tension the array to its
proper 11.57N of tension. At this point, 60 seconds into the deployment
sequence, the high rate inflation is initiated bringing the aluminum
laminate tubes to their proper rigidization pressure. After the
tubes have sustained rigidization pressure, a third pyro valve
vents the gas to space through a zero thrust nozzle completing
the deployment and rigidization sequence.
Random Vibration
The packaged array has endured random Vibration tests to simulate
the shuttle launch environment, Figure 7. These tests were conducted
on L'Garde vibration machine and exposed the array to vibrations
in all 3 axis (8.8g RMS, 12.45g peak, 20-2000Hz.). The tests indicated
no visual damage to the structure and no degradation in cell continuity.
Thermal Vacuum Deployment
Thermal vacuum deployment tests were conducted at the NRL
facility. The test was conducted in simulated eclipse conditions
(vacuum with shrouds at 90C) The ITSAT deployment test stand
is shown in Figure 8. To simulate the effect of zero-G the test
stand has a slight downward tilt to it to negate the effect of
sliding friction of the lid. The deployment test performed flawlessly,
and all equipment and sensors performed well. I-V tests performed
before and after the deployment showed no measurable change in
the cell performance.
Thermal Testing
For a 56 hour period, the deployed configuration was thermally
cycled from 85C to +70C in vacuum conditions, see Figure
9. The front of the tubes were exposed to the shroud temperatures
(-80°C), while the back surface was exposed to the cold plates
(-180C). The difference in the tube external surface was only
9C even though they were exposed to a temperature difference of
100C. The radiative and conductive heat transfer within the structure
between the front and back surfaces of the tube is large enough
to overcome the large difference in exposure temperature. Because
of this small temperature gradient, no thermal distortions were
measurable in the tubes under simulated worst-case thermal loading
conditions. This feature underlines a key advantage of the aluminum
laminate concept; no MLI is required to reduce thermal gradients.
Dynamics Testing
While deployed in the thermal vacuum chamber dynamics testing
was performed on the full system under simulated space conditions
to determine the system natural frequency. By exciting the structure
with a vibrator and measuring the tip deflections, a natural frequency
of 1.04 Hz. was determined.
While thermal vacuum testing of the new configuration with the updated helically wound stretched aluminum laminate tubes has yet to be conducted, a system natural frequency of 1.03Hz. has been determined analytically using NASTRAN. No change in the system natural frequency was expected however, since the biggest driver of the lowest natural frequency mode is the array blanket itself which is governed by it's overall mass and it's 11.92N total longitudinal tension.
Summary
L'Garde has developed an inflatably deployed, rigidizable solar array for small satellites. Using new aluminum laminate rigidization and an upgraded and improved inflation system, the current structure and array, utilizing older generation 13.8% efficient cells, can achieve a credible 73W/kg of on-orbit power generation. Utilizing gallium arsenide 28.1% efficient cells, and Northrop Grumman's new polymer cover glass technology the power density can be further enhanced to 109 W/kg for a 4.62kg array capable of generating over 500W of power.
The existing hardware has been fully qualification tested for a shuttle mission. Qualification tests conducted include vacuum/thermal chamber deployment, launch vibration testing, thermal cycling, and dynamic testing.
The Inflatable Torus Solar Array Technology (ITSAT) was designed to supply power to the growing fleet of small satellites in the 1kW class making forays into the space market. The ITSAT configuration with low mass and stowage volume and the inherent reliability of inflatably deployed structures is an excellent solution for these low power applications. Further, it also represents a substantial savings for the satellite designer; the cost per watt is about one half that of competing mechanically deployed systems
Acknowledgments
This work was funded initially by the Advanced Research Projects Agency, and monitored by USAF Phillips Laboratory. Additional work was conducted under an SBIR monitored by JPL.
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