Other papers
Abstract
Solar sails reflect photons streaming from the sun and convert
some of the energy into thrust. This thrust, though small, is
continuous and acts for the life of the mission without the need
for propellant (1). Recent advances in sail materials and ultra-low
mass structures have enabled a host of useful missions utilizing
solar sail propulsion. The team of L'Garde, Jet Propulsion Laboratories,
Ball Aerospace, and Langley Research Center, under the direction
of NASA, has been developing a solar sail configuration to address
NASA's future space propulsion needs. Utilizing inflatably deployed
and Sub Tg rigidized boom components, this 10,000 m2 sailcraft
achieves an areal density of 14.1 g/m2 and a characteristic acceleration
of 0.58 mm/s2. The entire configuration released by the upper
stage has a mass of 232.9 kg and requires just 1.7 m3 of volume
in the booster. After deployment, 92.2 kg of non-flight required
equipment is jettisoned resulting in a sailcraft mass, including
payload and control system, of 140.7 kg.
Copyright 2003 by L'Garde Inc. Published by the American Institute
of Aeronautics and Astronautics, Inc. with permission
This document outlines the accomplishments of a Phase 1 effort
to advance the technology readiness level (TRL) of the concept
from 3 toward a TRL of 6. The Phase 1 effort, the first of three
proposed phases, addressed the design of the solar sail, its application
to several missions currently under review at NASA, and developed
a ground test plan to bring the technology toward a TRL of 6.
Introduction
Early in the program, with the support of NASA, solar sail
missions under consideration were researched and a mission set
of interest was developed. Relevant mission parameters and environs
were identified and defined. These requirements were used to refine
a solar sail design meeting and exceeding these requirements.
This design includes all elements required for power generation,
communications, and navigation. An equipment list was generated,
components selected, and mass properties developed. To enhance
the flight performance of the concept, a carrier concept was developed
to jettison all non-essential deployment-related components before
the mission.
An important aspect of the Phase 1 effort was to generate a test plan to raise the TRL from 3 toward 6. A list of test articles was developed to validate section properties such as boom modulus, torsional stiffness, and deployability. Sail sections and quadrants will also be fabricated for testing and validation. A 10 m subscale system test article will be fabricated for ground testing at L'Garde and will then undergo a vacuum deployment and structural test in Langley Research Center's (LaRC) 16 m vacuum chamber. Finally, a 20 m square test article will be built and tested at NASA's Plum Brook 30 m thermal/vacuum test facility. This test, which will validate the sail system at space thermal and vacuum conditions, will bring the sail system toward TRL 6. Achieving a TRL level of 6 requires testing in a "relevant environment". Our tests will simulate space thermal and vacuum conditions but will still be conducted in 1 g. Many issues related to the 1 g environment will remain after testing of this large and gossamer structure. As a result, achieving a full TRL of 6 on the ground will not be possible, however, we will come as close as possible in a ground testing environment.
Design Overview
The baseline design is shown in figure 1. This 100m square
configuration was designed around the solar sentinel or sub L1
sun observation mission. This solar sail mission utilizes thrust
from the sun to descend below the L1 Lagrange point providing
a stable vantage point closer to the sun yet remaining in the
same orbital period as the Earth. This same configuration can
be used for a host of other missions with no or minimal modification
other than scaling.
Baseline Mission
As future phases of this program will require test articles of
a specific configuration, a baseline mission was selected around
which to optimize the design. The Solar Sentinel or Geostorm (2)
mission was selected as a likely candidate for future missions.
This scenario takes advantage of the constant thrust available
from the solar sail to place a payload in a solar orbit inside
of the L1 point, yet remain in the same period as the Earth's.
This position provides an excellent vantage point for solar observation
and warning of adverse solar activity. Satellites can station
keep at the L1 Lagrange point without the need for additional
propulsion, see figure 2. This point is about 230 Earth radii
(Re) from the Earth toward the sun along the Earth-sun line. Using
the constant thrust from the sail it is possible to descend to
a closer orbit to the sun yet remain in the same period as the
Earth, and remain on the Earth-sun line. An orbital analysis shows
that the baseline sail design can descend an additional 520 Re
closer to the sun. This location can provide information of solar
activity with a lead-time 3 times greater than current solar observing
missions.
Solar sails provide thrust normal to the sail, by changing the angle of the sail normal with respect to the sun-sail line (b), thrust vectors can be developed with components normal to the sun sail line. With a thrust component normal to the plane of the ecliptic, the sailcraft can orbit the sun above the plane of the ecliptic in a non-Keplarian orbit as shown. The line shown represents a family of solar orbits depending on the b angle of the sail. The baseline design can maintain a position relative to the earth-sun line at a point 700 Re above the plane of the ecliptic, and 800 Re closer to the sun. Additionally, the sailcraft can maintain this distance from the Earth-sun line while orbiting in a halo orbit. Thus several sailcraft can provide multiple simultaneous observation points of the solar wind. By combining the data gathered by these instruments, a 3-D map of incoming solar anomalies can be developed.
Control System
Control vanes resembling one scaled quadrant of the solar sail
have been integrated into the structure to provide full 3-axis
control. By modulating a small amount of reflective area near
the boom tips, forces are generated large enough to control the
sail orientation. Actuators mounted at the tips of the boom provide
the torque required to rotate the vane.
Support Structure
During earlier solar design work at L'Garde, it was discovered
that the sail suspension technique can have a large impact on
system mass and scalability. Several attachment techniques were
reviewed (figure 3) and the stripe architecture was selected as
the most efficient (5).
Stripe architecture is highly scalable. For larger sails/booms additional attachment point along the booms can be added to keep the load length L to a minimum. The other methods require large increases in L, which require significant increases to the strength and mass of the booms to defeat the Euler buckling.
Spreader System
The booms are not sized to withstand the bending generated by
the solar flux alone. A tensioned truss or spreader system is
used to increase the moment of inertia of the boom to absorb the
bending, see figure 4. The spreader system consists of lightweight
rigid spreader bars mounted to rigid rings integrated into the
boom
Iso-Grid Boom Design
The booms are designed in an iso-grid configuration. High modulus
fibers are oriented as shown in a boom built for Team Encounter
(figures 5,6). The fibers are impregnated with a Sub Tg resin
to rigidize the structure after deployment (this is described
in the Sub Tg section). Longitudinal uni-directional fibers are
oriented to absorb the compressive loads in the booms, while the
lateral fibers absorb the inflation loads and stabilize the longitudinal
fibers and the cross section. These lateral fibers provide the
burst margin required for deployment contingencies.
Conical Deployment
Figure 7 shows the conical boom packaging and deployment scheme
developed for deployment control of the inflatable rigidizable
support booms (3). The technique uses a unique concentric packaging
arrangement about the boom axis and provides a high degree of
deployment control (patent pending). To deploy the conical boom,
inflation gas is introduced at the base. The resulting deployment
is smooth and predictable. As the tube is under pressure during
deployment, it is able to withstand loads during deployment.
A deployment sequence of a Team Encounter sail conical boom is shown in figure 8. This deployment took place while the boom was floating in a water trough to simulate a 0 g deployment condition in one plane. The deployment proceeded smoothly and in a linear, consistent, and predictable manner.
Sub Tg Rigidization
Sub Tg or cold rigidization takes advantage of the increase in
modulus of certain materials below their glass transition temperature
(Tg) (3,4). Sub Tg structures can be constructed for a variety
of missions, from low Earth orbit (LEO) to deep space applications
and this technique was selected to form the support structure
for the sail.
A solar sail boom undergoing cold rigidization testing for Team Encounter is shown in figure 9. The boom is housed in a foam test chamber. While not visible through the chamber walls, the position is indicted as shown. The arrows depict the positions and loading orientation of cables used to apply compressive loads to the boom. The cables simulate the static loading of the striped sail architecture after deployment and exposure to the solar flux. This strength is achieved by using the Sub Tg resin at the expected space equilibrium temperatures
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 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. Multi-layer insulation (MLI) is required to mitigate the effects of on-orbit thermal gradients and to retain the initial softening thermal energy during the deployment sequence.
Sail Material
Mylar (6) has been selected for utilization as the sail membranes.
This material, used in the electronics industry, is low cost and
readily available. An example of a sail fabricated with Mylar
is shown in figure 10. This sail was fabricated and tested for
the Team Encounter program. The sail was deployed in the orientation
to gravity shown demonstrating the feasibility of successful deployment
of these thins films. The tension load in the sail due to gravity
is roughly 600 times greater than the tension load generated by
the solar flux, deployment in gravity is highly conservative and
gives good confidence for deployment in 0 g.
Figure 10. Mylar Sail During Deployment Test
Test and analysis have been conducted to ensure Mylar is compatible with the space environment for the intended mission duration. Special coatings are utilized to maximize heat rejection to space, keeping the Mylar below its melting point in orbits as close as 0.25 AU from the sun. These coatings are concurrently optimized to shield the Mylar from the degrading effects of ultra-violet (UV) radiation. Tests and analysis have been conducted showing that even after exposure to the maximum expected particle radiation doses, the mechanical properties are ample to withstand the expected sail loading conditions. These specialized coatings, coupled with the low stress concentrations afforded by the striped sail architecture, and the low cost and high availability, make Mylar an excellent choice for use as a solar sail membrane.
Space Segment
The Space Segment consists of all items released from the upper
stage. This includes the sailcraft, shown on the top of figure
11, and the carrier shown toward the bottom. After deployment
of the sail, the carrier is jettisoned to free the sailcraft from
all non-flight required components and mass. The 50.0 kg payload
envelope is visible toward the center of the sailcraft portion,
and all of the spacecraft specific elements are shown toward the
top of the configuration. The stowed solar arrays and communication
antennas are visible toward the top.
The space segment fits well inside of the Delta payload fairing as shown in figure 12. With a sufficient payload interface fixture, it may be possible to fit two space segments on a single launch.
Deployment
Once the Space Segment has successfully separated from the upper
stage deployment can be initiated. Vane deployment is initiated
by rotating the vane booms from their stowed position into proper
position for deployment. The vane booms are deployed which pull
the vane membranes into their deployed configuration, figure 13,
(a). Next, the spreader system, which has been pulled together
for stowage, is released in preparation for deployment. The main
boom deployment is initiated by introducing inflation pressure
into the stowed booms. The booms simultaneously deploy the sails
and the spreader system drawing the tension cables into position
by deploying the rigid rings in a sequence, figure 13, (b). An
inflation control system carefully monitors the deployment length
of each boom and modulates the amount of inflation gas introduced
to each boom to ensure the deployment progresses symmetrically,
figure 13, (c). Once equilibrium temperature is achieved and the
structure is fully rigidized, the carrier is released. (d). The
sailcraft is now in its final configuration and providing thrust.
Scalability
Many missions require large sails in order to carry more payload
or to achieve higher specific accelerations. A scaling analysis
was undertaken using L'Garde analysis tools and the results shown
in figure 14. The X-axis depicts the size of the sails in square
meters, while the Y-axis shows the areal density of the sailcraft.
All configurations shown on the chart assume a 50.0 kg payload,
and 43.3 kg of spacecraft elements for power generation, communications,
and guidance and control. In reality these requirements will likely
change with the given mission scenarios, however, in the interests
of this scaling analysis, these parameters were fixed.
The striped sail architecture and excellent mechanical synergism with the conically stowed boom allows scalability without high mass penalties. As shown in figure 14, the baseline design, with minor modification and scaling, is capable of all of the NASA "high-pull" missions shown.
Phase 2 and 3 Test Plan
To raise the TRL level to 6 a solar sail system must be tested
in a "relevant environment". To this end a series of
test articles is planned that will raise the TRL to ~6. We intend
to simulate the vacuum and thermal environs of space during our
tests but we are limited to testing terrestrially at 1 g. With
a structure as large and gossamer as a solar sail, this 1 g limitation
will always be a factor. Suspension techniques will be used to
mitigate the effects of 1 g but issues will remain. An important
aspect of the effort will be to carefully utilize the test results
at 1 g to validate a series of analytic finite element analysis
(FEA) models. With these techniques, validated predictions of
the structural performance of the solar sail configurations at
0 g will be generated. In this way we will raise the TRL as close
to 6 as is possible on the ground, but we will not achieve all
requirements for TRL 6, hence the TRL ~6 designation.
Component Tests
Initially material and component test will be conducted. The sail
and Sub Tg laminate will be subjected to UV and particle radiation
to validate and expand on tests already conducted. Component tests
of sail and boom sections will be used to validate the mechanical
characteristics. This data will be used to validate the structural
models.
Subsystem Tests
A 10m sail quadrant will be deployment tested in ambient conditions
as will a full-scale vane quadrant. Additionally a boom with spreader
system will be deployed both in ambient and vacuum condition in
LaRC's 16 m vacuum chamber. Finally a sail quadrant with two full
booms and spreaders will be deployed and tested in the LaRC chamber,
as will the full-scale vane and representative actuator. Structural
data will be obtained, as will photogrammetry and laser vibrometry.
All data will be used to validate structural assumption and the
structural FEA models.
10m Test Article
The component and subsystem tests will lead to a 10m sector test
of a full solar sail configuration. As scaling of the various
materials of the concept is not feasible, a 10 m on a side sector
of the full-scale configuration will be fabricated and tested.
While the LaRC chamber is not equipped with cryogenic capabilities,
cold plates will be used locally to rigidize the boom components
and allow structural testing. Again, photogrammetry and laser
vibrometry will be conducted and all data will be used to validate
structural assumption and FEA models.
20m Test Article (Phase 3)
In a planned follow-on contract, a larger 20 m sector of the solar
sail configuration will be thermal vacuum tested in NASA's Plum
Brook 30 m chamber. This ambitious test will bring all of this
work and analysis together. A successful conclusion will see the
solar sail system TRL level raised as close to 6 as is possible
under ground test conditions paving the way for a flight experiment.
Summary
The team of L'Garde, Ball Aerospace, JPL, and LaRC has developed
a highly scaleable solar sail configuration to meet and exceed
the requirements of many of NASA's future missions. This configuration
was enabled by utilizing inflatably deployed and sub Tg rigidized
booms. Striped sail architecture, coupled with L'Garde's conical
boom deployment technique allows scalability without high mass
penalties. A comprehensive test plan was developed to raise the
TRL level of this technology toward 6 by 2005. This focused program
will pave the way for a flight experiment of this highly efficient
space propulsion technology.
Acknowledgments
Much appreciation is extended to Greg Garbe and Sandy Montgomery
of NASA Marshal Space Flight Center, for their vision, advocacy,
and technical expertise in development of the solar sail. Many
thanks to Team Encounter for access to their design and hardware,
their work forms the foundation for much of the solar sail design
shown. The team would like to acknowledge the support of Jeff
Bladt of Ball Aerospace, and Dr. Guru Singh for their control
system expertise. L'Garde would also like to thank Dr. Gyula Greschik
for his assistance in the solar sail design and structural analysis.
Thanks also to TDM for the deployment animations. Part of this
research was carried out at the Jet Propulsion Laboratories, California
Institute of Technology, under a contract with NASA.
References
1) McInnes, C.R., Solar Sailing Technology, Dynamics and Mission Applications, Springer-Praxis, London, UK, 1st Ed.
2) West, John, and Derbès, Billy, "Solar Sail Vehicle System Design for the Geostorm Warning Mission". AIAA-2000-5326, September 21, 2000.
3) D. Lichodziejewski, R. Cravey, G. Hopkins, "Inflatably Deployed Membrane Waveguide Array Antenna for Space" AIAA 2003-1649, 4th Gossamer Spacecraft Forum, April 7-10, 2003, Norfolk, VA
4) K. Guidanean, D. Lichodziejewski, "An Inflatable Rigidizable Truss Structure Based on New Sub- Tg Polyurethane Composites", AIAA-2002-1593, 3rd Gossamer Spacecraft Forum, Apr. 25, 2002, Denver, CO
5) Greschik, G. and Mikulas, M.M., "Design Study of a Square Solar Sail Architecture", Center for Aerospace Structures, University of Colorado, Boulder, CO.
6) DuPont, Inc., "Mylar Product Information," no.
222367D, December 1997, available at www.dupontteijinfilms.com