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49th International Astronautical Congress Sept 28-Oct ... - Team-Logic

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					   IAF-98-I.1.02

   INFLATABLE SPACE STRUCTURES:
   A NEW PARADIGM FOR SPACE STRUCTURE DESIGN
   D. Cadogan, M. Grahne
   ILC Dover, Inc.

   M. Mikulas
   University of Colorado




      49th International Astronautical Congress
      Sept 28-Oct 2, 1998/Melbourne, Australia
For permission to copy or republish, contact the International Astronautical Federation
3-5 Rue Mario-Nikis, 75015 Paris, France
                       Abstract                                 ♦   Structural Support Elements

The planned increase in satellite launches over the next
decade will emphasize reduction of space hardware                        2.0 Inflatable Deployable Structures
mass and launch costs. Inflatable structures present
many benefits over current state-of-the-art +mechanical         2.1 Inflatable Structures
technologies and are principally attractive because they        Inflatable structures are manufactured from flexible
can be packaged into small volumes, thereby reducing            materials (thin films or coated fabrics) that are made
program costs.       Reduced costs are realized in              structural via internal inflation pressure. They cannot
development and production as well as in enabling               be made completely leak free for extended periods of
smaller launch vehicle size. Inflatable structures will         time in space.        Environmental threats such as
reduce total system mass and complexity, thereby                micrometeoroids and debris (MMD), atomic oxygen,
increasing system reliability.                                  and radiation damage eventually result in leakage of the
                                                                system. In addition, gases utilized for inflation are
This paper describes several types of inflatable                typically inert with small atomic sizes, resulting in gas
structures and potential applications for the technology.       permeation over time. Therefore, this classification of
Rigidization methods and results from preflight test            structures is used only if the system has 1) a short
programs are also presented to support the viability of         operational life (less than a few years); or 2) the
the technology.                                                 potential for unlimited supply of ‘make-up-gas’ exists
                                                                (mass and storage penalty).

                   1.0 Introduction                             2.2 Inflatably Deployed, In Situ Rigidized Structures
                                                                ILC Dover has developed proprietary methods for
ILC Dover has been designing and fabricating space              hardening or ‘rigidizing’ structures in orbit (in situ).
structures, including micro-precision space structures,         These technologies enable a structure to be fabricated
with leading edge space materials for over 30 years.            from a composite structural laminate, densely packaged
Recent technology enhancements have made space                  for launch, deployed via inflation on-orbit, and finally
inflatable structures a viable alternative for many             rigidized or cured in situ – no longer requiring inflation
additional applications including RF antennas, radar            pressure to maintain shape or provide structure. This
arrays, solar arrays, structural members such as booms          technology can be employed to fabricate many shapes
& trusses, impact attenuation devices, habitats, and            such as tubes, toroids, dish structures, etc., that can be
shields to protect spacecraft form thermal, light, or           used in the design and manufacture of numerous types
micrometeoroid/debris impingement. Their size can               of structures. In situ rigidized components can be
range from a few meters to hundreds of meters.                  deployed in various orbits and are designed to remain
Inflatable structures can be deployed in various orbits,        operational for typical satellite lifetimes of seven to 15
rigidized in situ, and remain operational for years             years without concern of damage from MMD impacts,
without the concern of destruction from micrometeoroid          radiation exposure, atomic oxygen exposure, or other
and debris impacts, loss of inflation gas, or damage from       environmental concerns.
other orbital threats such as radiation or atomic oxygen
exposure. Depending on the required system life and             Rigidization is achieved through a number of methods
relative environmental threats, a purely inflatable (non-       including:
rigid) structure could further reduce system mass and
cost. Inflatable structures can be classified into two          ♦   Thermal Heating
groups:                                                         ♦   Passive Cooling
                                                                ♦   UV Exposure
♦   Inflatable, Non-rigidizable Structures                      ♦   Inflation Gas Reaction
♦   Inflatably Deployed, In Situ Rigidized Structures           ♦   Thin Wall Aluminum
                                                                ♦   Foam Inflation
Each inflatable structure is comprised of three major
subsystems:                                                     Many of these technologies have been investigated
                                                                sporadically since the 1960’s but are now yielding
♦   Inflation System                                            reliable structural elements through the use of advanced
♦   Controlled Deployment System                                materials and revolutionary design practices.


                                                            2
Thermal Heating - Rigidization occurs by heating a               Thin Wall Aluminum - In this method of rigidization a
composite system which is composed of a thermoset                laminate is manufactured with a layer of ductile
matrix resin and a fiber reinforcement such as graphite.         aluminum at its center. Kapton film is positioned on
The resin hardens after being heated to a specified              both sides of the aluminum. To rigidize the cross-
temperature and maintains its shape and structural               section, the tube is inflated and the aluminum is slightly
properties after expulsion of the pressurization medium.         yielded to eliminate the wrinkles in the laminate. An
The properties of the composite material are consistent          example of a typical aluminum laminate cross-section
with those of the composite materials typically used in          can be seen in Figure 2.
spacecraft design. This system can be designed to cure
from energy derived from the spacecraft, or utilizing the
sun’s solar energy, or a combination of both. An
example of a typical composite laminate cross-section
can be seen in Figure 1.




                                                                     Figure 2 Aluminum Laminate Cross-Section

                                                                 Foam Inflation - In this approach, the inflatable tube is
                                                                 deployed with an inner feed tube. The liquid chemicals
                                                                 are then pumped through the tube and discharge ports to
                                                                 fill the inflatable beam. The chemical reaction in the
    Figure 1 Composite Laminate Cross-Section
                                                                 liquid foam reactants then expands and fills the tube,
                                                                 yielding a rigid element.
Passive Cooling - In this case the packed structure is
warmed to a temperature where it is soft, then inflated
                                                                 2.3 Analysis & Testing of Rigidizable Structures
and deployed. Over time the structure releases its
                                                                 Structural and thermal analysis of inflatable rigidizable
thermal energy to the environment and becomes rigid.
                                                                 structures is conducted to predict system behavior in the
These laminates are also comprised of composite
                                                                 prescribed environment. Analyses range from simple
materials that exhibit properties similar to the thermally
                                                                 beam calculations on rigidized beams to complex non-
heated composite laminates.
                                                                 linear NASTRAN analysis of systems. Analyses are
                                                                 verified, where practical, with component fabrication
UV Exposure - This approach works similar to a
                                                                 and test.
thermal cured system. Its only difference is that the
resin system hardens by contact with UV light. Use of
                                                                 For example, sections of rigidizable beams have been
this approach requires the use of an UV resistant
                                                                 analyzed and tested to predict their deployed strength as
reinforcement that does not block the UV light from
                                                                 well as their thermal performance during rigidization.
penetrating the entire laminate.       Fiberglass is an
                                                                 Figure 3 shows the results of a thermal analysis
inexpensive option that transmits and diffuses the UV
                                                                 conducted on rigidizable thermoset beams which was
light very well. Some mass penalty results with this
                                                                 then verified during test in a vacuum chamber at -160°C
approach but a low cost, simple solution is realized.
                                                                 and 10-6 torr (Figure 4). In this case the analysis and
                                                                 test data correlated well and proved the viability of
Inflation Gas Reaction - This method utilizes the
                                                                 thermally curing tubes in a worst case cold environment
reactants contained in the inflation gas used to
                                                                 in space.
pressurize the system. The inner wall of the laminate is
permeable and allows migration of the reactants and
thus rigidization occurs.


                                                             3
                           Maximum and Minimum Temperatures of Lower Sphere, Tube & Upper Sphere-Heater Cured
                                                      Cold Case With Heater On                                                                  3.0 Inflatable Structure Subsystems
                  280
                                                                                 5 LAYERS OF MLI, E*=0.050
                                                          OUTER LAYER IS 1.0 mil KAPTON-ITO COATED: ALPHA=0.31 & EMISS=0.63
                                                               INNER LAYER 1.0 mil BLACK KAPTON: ALPHA=0.85 & EMISS=0.81
                                                       TUBE IS 29 mil LAYUP, TUBE OD=6.0", LENGTH=321.0" WITH SPHERICAL END CAPS
                                                                   EQUATORIAL LOW EARTH CIRCULAR ORBIT-R=450.0 NM
                                                                                                                                       3.1 Inflation System
                  275                                                           TIME 0.0 IS SUBSOLAR POINT
                                                                                HEATER POWER=0.040 W/In
                                                                                                             2
                                                                                                                                       Inflation gas sources for space inflatables have included
                  270
                                                                                      NODE 16-LOWER SPHERE
                                                                                      NODE 166-TUBE                                    compressed gas, gas generators, and sublimation of powders
                                                                                      NODE 316-UPPER SPHERE
  F




                                                                                                                                       and liquids into gasses. Each of these methods has
  o
  Temperatures-




                  265
                                                                                                                                       advantages and disadvantages depending on the
                                                                                                                                       application. Gas generators are lightweight systems, but
                  260
                                                                                                                                       generally produce gas at rates that are higher than desired
                                                                                                                                       for controlled deployment of Gossamer inflatable space
                  255
                    0.00          0.20       0.40       0.60         0.80              1.00           1.20       1.40     1.60
                                                                                                                                       structures. Gas generators are also incapable of providing
                                                                    Orbit Time (hr)                                                    make-up gas if the structure is expected to maintain
                                                                                                                                       pressure for a significant amount of time. Although not
                                Figure 3 Thermal Analysis Results                                                                      suited for Gossamer structures, they do provide a
                                                                                                                                       lightweight approach for systems such as the Mars
                                                                                                                                       Pathfinder inflatable landing impact attenuation system.

                                                                                                                                       Sublimation of materials to create inflation gas has been
                                                                                                                                       used in inflatable structures such as the ECHO 30m
                                                                                                                                       diameter balloons created by NASA Langley Research
                                                                                                                                       Center in the 1960s. This technique for inflation is only
                                                                                                                                       viable for large volume structures where very low inflation
                                                                                                                                       pressures are required to generate the required skin stress.
                                                                                                                                       This method of inflation would probably not be considered
                                                                                                                                       practical today because of the uncontrolled nature of the
                                                                                                                                       deployment event and the mass competitive nature of
                                                                                                                                       compressed gas systems.

                                                                                                                                       Compressed gas systems have become very low mass and
                  Figure 4 Vacuum Chamber Rigidization Test
                                                                                                                                       highly reliable in recent years making them the most
A great deal of testing has also been conducted on                                                                                     practical option for inflating most space structures. Most
beams manufactured from various rigidizable materials                                                                                  systems consist of a filament wound or titanium gas
in several diameters and thicknesses. The results of this                                                                              cylinder, regulator, valving, and a control system. These
testing were used to build and verify models of beams of                                                                               systems will control the gas flow rate precisely to achieve
varying constructions. Testing of the beam sections,                                                                                   the desired rate of deployment. It should be noted that the
typically 2.5 meters in length, included compression,                                                                                  relatively small parasitic mass of the inflation system
bending, and torsional strength. Figure 5 shows a                                                                                      becomes relatively insignificant when considering very
typical test set-up for a beam bending test at ILC.                                                                                    large structures.

                                                                                                                                       In the case of a structure that has been rigidized, a
                                                                                                                                       regulated purge of the inflation gas after rigidization
                                                                                                                                       prevents the escaping gas from imparting inertial forces
                                                                                                                                       on the spacecraft over time. This is accomplished in a
                                                                                                                                       non-propulsive manner via expulsion through a “T”
                                                                                                                                       fitting.

                                                                                                                                       3.2 Controlled Deployment System
                                                                                                                                       ILC has developed a number of techniques to ensure the
                                                                                                                                       controlled deployment of inflatable structures. Their
                                                                                                                                       purpose is to 1) keep the deploying structure within a
                                                                                                                                       known envelope, avoiding defined exclusion zones, 2)
                                                                                                                                       improve reliability of structure deployment by avoiding
                                         Figure 5 Beam Bending Test                                                                    entanglement with itself or nearby components, and 3)


                                                                                                                                   4
minimize impulse to the spacecraft structure from which
deployment occurs.

Columnation Devices - Columnation devices, Figure 6,
provide a method of axially extending an inflated beam in a
straight telescopic motion with some degree of beam
stiffness during the deployment. The inflatable tube is
axially collapsed on a short mandrel in its packed state.
The top of the mandrel has a tube engaging feature
(typically a deformable component that expands against the
inside of the tube) that applies some resistance to axial
motion of the tube. Inflation gas is introduced through the
mandrel into the forward end of the tube. As plug load
builds in the inflatable, it overcomes the frictional resistance
                                                                            Figure 7 Roll-up Device (with Membranes)
and advances the tube. This method places axial load in the
tube wall during deployment, allowing it to behave as an
                                                                       2) Reverse roll-up device, Figure 8 - The tube is rolled up
inflated beam and exhibit structural stiffness. Several
                                                                       on a rotating shaft or drum, mounted on the structure,
variations of this device, including collapsible mandrels that
                                                                       starting at the base. Inflation gas is introduced into the far
allow the packed beam to fit into very small volumes, have
                                                                       end. Plug load builds in the tube, pulling the tube out and
been developed and tested with good results.
                                                                       rotating the drum. The drum rotation can be controlled by a
                                                                       friction clutch, or even powered out for complete
                                                                       deployment rate control, while providing interim beam
                                                                       stiffness during deployment. A fixed tube guide controls
                                                                       the direction of the deploying tube. This approach provides
                                                                       the same advantageous translating motion as a
                                                                       Columnating Device and the same benign handling of
                                                                       uncured laminates as the Roll-up device. Its disadvantages
                                                                       are that it requires the rotating drum and tube guide
              Figure 6 Columnation Device                              hardware, and must pay out an inflation hose to provide gas
                                                                       to the end of the tube while deploying.
Roll-up Devices - Inflatable tubes can be rolled up and
flattened such that their deployment is controlled by the way
in which they are unrolled and inflated. There are two basic
approaches:

1) Roll-up device, Figure 7. The tube is rolled up starting at
the end farthest from the base (attachment to the structure).
Embedded mechanisms (springs/Velcro, etc.) cause the tube
to roll up in the prescribed plane, and provide resistance to
unrolling so that interim beam stiffness during deployment
is achieved. Inflation gas is introduced in the tube at the
base. This approach is simple, reliable, compact, and the
roll-up is a benign packing procedure for rigizable                                 Figure 8 Reverse Roll-up Device
laminates. Its primary disadvantage is that interface with
membranes, tubes, or other attachments can be complicated              Internal Compartmentalization, Figure 9 - Inflatable
by the unrolling motion (as opposed to a straight                      structures can be fabricated with internal bulkheads or
telescoping action, where translation is the only motion to            partitions such that there are isolated pneumatic
consider for such interfaces).                                         compartments that inflate and pressurize sequentially. The
                                                                       inflation sequence can be through the compartments
                                                                       themselves, or can be via an external manifold. Inflation
                                                                       through the structure requires that the dividing membranes
                                                                       have burst patches or relief valves that open at a prescribed
                                                                       pressure. This enables one section to inflate and assume

                                                                   5
structural stiffness before the next section begins inflation.       sequentially lets all the other loops go, propagating
Burst patches allow each compartment to equalize at the              outward from the release point.
same pressure when inflation is complete; relief valves
result in consecutively lower pressures in each adjacent
compartment.                                                                             4.0 Applications

                                                                     Several recent applications of systems designed using
                                                                     inflatable space structures are presented below. All of
                                                                     the systems described are currently in development with
                                                                     some moving toward potential flight experiments in the
                                                                     near future.

                                                                     4.1 Synthetic Aperture Radar Array (SAR)
                                                                     The SAR program was part of NASA JPL's Ultra
                                                                     Lightweight Spaced Based Radar Program.             The
                                                                     objective of this program was to create a synthetic
                                                                     aperture radar system that weighed less than 2 kg/m² of
                                                                     radiating area. The prototype system included flexible
                                                                     radiating membranes stretched between an inflatable
                                                                     frame. The radar system operated in two frequencies, X
                                                                     and C band, which was accomplished using three
                                                                     membranes. Spacing between the layers was 1.3 cm and
                                                                     0.5 cm with tolerances of 0.075 cm and 0.05 cm
                                                                     respectively. This spacing was critical to the operation
        Figure 9 Internal Compartmentalization                       of the radar array.

Break Cords/Peel Flaps - Uninflated material, as well as             A unique feature of this radar system was that it rolled
single wall membranes that are supported by inflated                 up for storage, Figure 10, and was deployed via inflation
structures, can be controlled during deployment through the          gas. ILC developed a controlled deployment sequence
use of mechanical softgoods mechanisms such as Velcro                through the use of embedded, negator springs in the
peel flaps, adhesive peel flaps, and break cords. These              rigidizable frame. The springs provided resistance
devices are used to hold the loose material in a defined
position and envelope, and release it in a controlled way
when an opening force overcomes them. The opening force
can be the load of a deploying inflatable, or other externally
applied force. The material is folded in a specified manner,
and the folds are secured with the Velcro or adhesive flaps
or with cords that are sized to fail at a prescribed load.
When the opening force is applied, rather than all the
material being released in an uncontrolled way, it is
sequentially dispensed in increments such that the amount
of untensioned material at one time is very limited. These
devices can also be used for the initial release of the entire
packed inflatable from its stowage container or cover.

Becket Loops - Becket loops, also known as daisy
chains, are a reliable method of releasing a softgoods
assembly from a soft package. The Becket loops secure                                 Figure 10 SAR Array
flaps of the cover to each other, holding the assembly
underneath.      Immediately before deployment, a                    against the inflation gas resulting in a uniform unrolling
mechanical or pyrotechnic device releases the last loop              of the frame. The prototype was tested at JPL's radar
of the daisy chain. Once this loop is released, it                   range resulting in predicted radar signatures with high
                                                                     efficiencies. The weight of the flight unit is projected to


                                                                 6
be only 1.6 kg/m²of radiating area (including the frame,
membranes, inflation system, and packing container)
based upon data generated from the tested prototype.

4.2 Solar Array Deployment Mast
ILC's inflatable solar array deployment mast is similar
in function to the state-of-the-art mechanical
deployment masts.        The purpose of the mast is to
provide a deployment mechanism and support structure
for a folded array. The simplicity of the system allows it
to be easily modified and adapted to a variety of stowage
areas and performance specifications. The mast consists
of a flexible laminate inflatable tube which is stowed by
folding or rolling the tube. The mast is located on the
back side of the array, however, in some cases the array              Figure 11 Solar Array Deployment System
could be bisected by the mast. When dictated by the
mission, the tube would be deployed via inflation gas            4.3 Parabolic RF Antenna
and hardened (rigidized) by one of the methods                   ILC Dover demonstrated rigidization of complex shapes
discussed previously. The inflation gas would then               of high accuracy by developing a 1.5-meter parabolic
undergo a controlled purge so as not to impart any               antenna system, subsequently deploying and rigidizing it
inertial forces on the satellite. What remains is a              in a simulated space environment. The antenna, in its
laminate tube that exhibits very high strength and               pre-deployed state, was a flexible assembly that was
stiffness to weight ratios.                                      packed into a small volume. It was then deployed and
                                                                 rigidized in a vacuum chamber, to its design geometry,
ILC designed a mast deployment system to support an              many times the size of its packed volume.
approximately 3-meter by 10-meter solar array, Figure
11. Due to the scale and limited packing volume (3 x             The objective of the prototype was to demonstrate and
0.3 x 0.15 m) of the array, this design included a               verify that an object could be rigidized in a simulated
deployment tube, whose only function is to deploy the            space environment and maintain its shape. Although
array, and two composite rigidizable booms that provide          not a goal, the first rigidized antenna achieved an
structural support after deployment. This design was             overall RMS accuracy of 1.5 mm. RMS accuracies in
also influenced by a requirement for limiting frontal            the 0.2 mm range are projected with tooling
area in any orientation to reduce drag during orbit              improvements.
raising, thus the multiple booms of smaller diameters.
The array operates in a similar fashion as the one               The antenna consisted of an on-axis parabolic lenticular
described above (single mast) except that the central            structure (bi-concave opposed surfaces) that was
mast deploys the array, then the composite masts inflate         supported by an inflatable torus as shown in Figure 12.
and are hardened (rigidized) prior to expelling the              The rigidizable surface of the lenticular was fabricated
inflation gas from all tubes. This design resulted in a          from a structural composite laminate consisting of three
very robust structure that reduced system cost. The              layers of material.      The opposing surface of the
inflation system, deployment mast, and composite mast            lenticular was an identically shaped canopy made from a
accounted for only 10 kg mass and, more importantly,             single layer of thin film. The perimeter of the lenticular
were stowed in a very small volume.                              was supported by a catenary for load transfer from the
                                                                 lenticular to the supporting torus.




                                                             7
           Figure 12 Rigidized RF Antenna

4.4 NGST Sunshield
ILC Dover, in conjunction with L’Garde Inc. and the                          Figure 13 NGST Half Sunshield
University of Colorado, developed the sunshield for the
Next Generation Space Telescope (NGST) currently
under study by NASA. The sunshield is a multi-layered                                 5.0 Summary
shield that passively cools the telescope to 30OK to better
facilitate astronomy. The sunshield is diamond shaped             The technology discussed above is being used in a
and measures approximately 15 x 35 x 1 meters when                variety of space applications. The flexibility of this
deployed. The system is comprised of 4 membrane                   approach allows freedom of application while
layers that are deployed and supported in operation by 4          maintaining the ability to optimize the design of the
inflatable beams. The membrane layers are “z” folded              structure to obtain excellent mass ratios and, packing
in a small volume around the spacecraft. The booms are            efficiencies while minimizing program costs. Mass
rolled and attached to the tips of the membranes and              reductions using this technology can be significant
contain deployment control mechanisms.             During         approaching reductions of 50% over conventional
deployment the booms unroll and dispense the                      structures.    Packing efficiencies of the inflatable
membrane from the packing container in a controlled               structures can approach 80% (material volume/container
fashion. At full deployment of the membranes the                  volume), which translates to decreases in launch payload
booms, aluminum laminates in this case, are inflated to           volume of up to 90% over conventional mechanical
a prescribed pressure to “rigidize” them and then                 structures.
vented. The fully deployed, unpressurized system is
shown in figure 13. The system shown is a half scale
model deployed at ILC with a gravity negation system
attached              to             the            shield.




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