[american institute of aeronautics and astronautics caneus 2004 conference on...

American Institute of Aeronautics and Astronautics1

MNT-based Gossamer Space Frame

Cynthia Y. Cheung* and Steven A. Curtis†,NASA Goddard Space Flight Center, Greenbelt, Maryland, 20771, USA

Michael L. Rilee‡

L3 Communications, GSI, Greenbelt, Maryland, 20771, USA

and

John E. Oberright§

NASA Goddard Space Flight Center, Greenbelt, Maryland, 20771, USA

We have developed a new micro-nano-technology-based architecture called SuperMiniaturized Addressable Reconfigurable Technology (SMART) to guide the developmentand control of a “smart” gossamer space frame. Since the SMART architecture is scalable,designs can be realized in the near-term in the macroscopic scale using electro-mechanicalsystems (EMS) to produce an Addressable Reconfigurable Technology (ART) structure.This will enable us to evolve the design and to examine operational issues such as distributedcontrol, group intelligence and global coordination. As advances in micro- and nano-materials and fabrication processes become available, the design can be “ported” to MEMS-based and NEMS-based systems. ART is applicable to the new NASA Exploration Initiativeby enabling multi-functional systems for Moon and Mars Exploration.

I. Introductionarge rigid monolithic structures constructed using current fabrication technologies run up against packagingand mass limits of launch capabilities and drive up mission costs. The problem is fundamental and concernsall structures to be deployed in space. One solution to this problem is to design segmented gossamer structures

that provide small launch volumes, but large deployed and/or assembled reconfigurable multi-functional forms.We have developed a new micro-nano-technology (MNT)-based architecture called Super Miniaturized

Addressable Reconfigurable Technology (SMART) to guide the development and control of a “smart” gossamerspace frame. The basic building block of the SMART truss is a reconfigurable tetrahedron – a set of Nodes withintegrated MNT controllers and sensors that drive actuators to deploy or retract struts connecting the Nodes (Fig. 1).Multiple tetrahedra can be connected together to form a reconfigurable structure with variable functions without theneed for specialized appendages. The three-dimensional nodes are addressable and can be controlled much as pixelson a two-dimensional liquid crystal display screen. This addressability of each element is key to coordinating themassively parallel system of interconnecting reconfigurable SMART Nodes in a structural network. The control canbe centralized or distributed with local autonomy. This architecture provides a framework for developing globallyreconfigurable, precision-controllable trusses that would find ready application wherever light, flexible, andadaptable structures are required. The MNT design issues include: selection of materials for the structural supportframe; development of advanced materials for functional surfaces; fabrication of interconnects and deploymentmechanisms; and intelligent coordinated control of the gossamer structure during deployment, operation, andreconfiguration.

We developed the MNT-based SMART architecture using NASA vision missions targeted for 2030s. We thendefined a developmental pathway for SMART using technology back propagation. Since the SMART architecture is

* Astrophysicist/Data System Engineer, Planetary Magnetospheres Branch, NASA GSFC/Code 695, AIAA member.† Branch Head, Planetary Magnetospheres Branch, NASA GSFC/Code 695, AIAA member.‡ Scientist, L-3 Communications, GSI, NASA GSFC/Code 695, AIAA Member.§ Systems Engineer, Systems Engineering Services & Advanced Concepts Branch, NASA GSFC/Code 592, AIAAMember.

L

CANEUS 2004--Conference on Micro-Nano-Technologies1 - 5 November 2004, Monterey, California

AIAA 2004-6720

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.


Figure 2. Segmentedsolar sail – Each sectioncan expand or contractto allow the sail tochange attitude oracceleration.

scalable, designs can be realized in the near-term using currently available electro-mechanical systems (EMS) toproduce an Addressable Reconfigurable Technology (ART) structure with little built-in intelligence. Issues ofcontrol, nodal communication, local & global autonomy, deployment, and reconfiguration can be examined usingmacro-scale techniques and components. As advances in micro- and nano-materials and fabrication processesbecome available, the design can be “ported” to Micro-EMS- (MEMS) and Nano-EMS (NEMS)-based systems. Wewill discuss the ART to SMART technology pathway and its near and far-term mission applications. In particular,ART is applicable to the new NASA Exploration Initiative by enabling multi-purpose systems for planetaryexploration. The SMART and ART structures presented in this paper are the subjects of 9 NASA patentapplications12.

II. Vision Mission: ANTS/PAM

A. A Swarm for Distributed TargetsWe were funded in 2003 by the NASA Revolutionary Aerospace Concept

(RASC) to derive innovative mission architecture and operations concepts for solarsystem exploration. The Autonomous Nano Technology Swarm (ANTS1) is theresultant architecture and one of the mission applications is the ProspectingAsteroid Mission (PAM2.3). The ANTS/PAM mission architecture posits a swarmof highly autonomous spacecraft each specialized to a particular mission function(e.g. data collecting, communications, or control) to cooperatively explore theAsteroid Main Belt. The swarm of intelligent pico-class spacecraft (~1 kg) isstructured according to a social insect hierarchy and will self-organized into sub-swarms with functional groupings of instrument capabilities to study manyasteroids simultaneously.

B. Segmented Solar SailEach spacecraft will use individual solar sails to fly directly from the outer

edge of Earth’s gravity well to the targets. The solar sail has a surface area of~100 m2 and will enable the spacecraft to travel to the Asteroid Belt with a transittime of approximately 3.5 years. Since the Asteroid Belt is strewn with planetarydebris, PAM spacecraft will use gossamer segmented solar sails based onSMART that are not susceptible to single-point failures. SMART structures canbe patched and the nodes migrated after suffering damage or failure. The solar

sail achieves orbit and attitude control by changing dynamically its morphology.Each segment of the SMART frame is covered by stretchable sail fabric (Fig. 2 &5) and can be reconfigured to vary its effective area and solar reflectivity, thuschanging the sail’s acceleration and momentum vectors to achieve the requiredorbital maneuvers and orientation. The segmentation also allows the spacecraft tohover around an asteroid with low gravity by “blanking out” large segments ofthe sail.

Figure 1. Vision of an SMART truss consisting of Nodes and their dynamic, variable lengthinterconnections: a) SMART in the stowed, launch configuration; b) extension of the struts duringdeployment; c) SMART in the final mission configuration.

a) b) c)


Figure 3. Simple model ofPAM spacecraft illustratingthe attachment ofinstruments to spaceplatform, tethered to spaceframe.

C. Large Packaging FactorThe science instruments and spacecraft operation subsystems are embedded in

the SMART spacecraft space frame or are attached via multiple tethers to the mainspace frame (Fig. 3), to be deployed by another 3-D instrument tetrahedral truss, sothat both the pre-deployment and post-deployment volume and geometry can beoptimized. The ANTS spacecraft are packaged in SMART pre-deployment shellsfor transportation and launch from the Earth’s Lagrange point. Each spacecraft isstowed into a cube of 10 cm length on a side, and the entire swarm of 1000spacecraft can be launched in a stack of cubes of 1 cubic meter in total volume(Fig. 4). This very compact packaging factor implies that the areal density of thestructural material is of the order of 5 gm/m2. The current areal density achievablefor aerospace structures is of the order of 5 to 15 kg/m2. So advances in the MNTmaterials and their fabrication and processing are necessary before the vision can berealized. For example, research in carbon nanotube composites would likelyproduce materials, actuators, and sensors that are suitable for such aerospaceapplications by 2030s. The required MNT developments include materials with theproper characteristics for the reconfigurable support structure and the solar sail, andmicro-nano- actuators, sensors, electronics, and strut interconnects. The compositematerials and devices need to be examined for their robustness in the extremethermal and radiation environments of space operations and planetary surfaceexploration. In addition, the coordinated control of the gossamer structure, which isa system of system, posts a major challenge to intelligent systems and operationsresearch.

D. Manufacturing and AssemblyFor the efficient manufacturing and assembly of the thousands of SMART frames for the spacecraft swarm, we

envision the use of a series of two-dimensional templates in an autonomous fabrication facility. Two types oftemplates are used: (1) a surface type composed of a equilateral triangular mesh, and (2) a middle type composed ofa more complicated triangular pattern to build up the three-dimensional tetrahedral frame. There are at least threegossamer space frames for each spacecraft: the solar sail, the spacecraft platform and the instrument platform. Foreach platform, the truss surface template, middle template, and instrument platform templates differ in the number ofconnecting struts and node structures for the gossamer structure. All MNT-based components: electronics,interconnects, strut nodes, sail fabric stowage nodes, and instruments will be embedded or attached via nanotubetethers to the space frame during assembly. Each completed spacecraft will then be attached to the SMART shell,which will fold up into a SMART cube. The SMART cubes for a swarm will be stacked together in a package fortransportation and launch. A movie illustrating the manufacturing sequence and the ANTS/PAM mission scenariocan be downloaded or viewed from the GSFC ANTS website4.

III. SMART Space FrameThe SMART space frame is an interconnecting network of tetrahedral nodes. Key to their dynamic structural

role is that they can reversibly extrude rigid truss members (struts). By changing the length of the interconnectingstruts the shape of the structural mesh can be reconfigured and controlled. Because deployment and retraction of thestruts are completely reversible, the space frame can make the most of its degrees of freedom to dynamicallyreconfigure itself as needed (Fig. 5). For minimal volume, maximal strength launch configurations, the nodes willhave the struts fully retracted. The nodes swivel (Fig. 6c) so that as the interconnection geometries change during

Figure 4. ANTS spacecraft deployment from SMART shell.


deployment and reconfiguration the system is not over-constrained. To maintain the stability and shape of thestructure, closed loop control will provide feedback to the actuators in the nodes that control the deployment. Thisfeedback may make use of radio- or other electromagnetic-wave based metrology for a flight-capable version. Smallchanges of strut length will suffice to maintain the desired configuration after deployment. Since the SMARTarchitecture is scalable, designs can be realized in the near-term using EMS to produce an ART structure. So theSMART architecture implementation does not need to wait for the maturation of micro-nano-technologies. We shall

describe below the current ART implementations and the micro-nano-technologies that are required to realize thegossamer space frame in the vision mission.

A. Tetrahedral Strut StructureThe fundamental element of the ART/SMART architecture is a strut that may be individually addressed and

whose length may be controlled. Several extension/retraction schemes are possible and are currently beingdeveloped. The ends of these reversibly extensible struts are connected via swiveling mechanisms at nodes. Thusnodes become vertices and struts become edges in a geometrical structure. One can go through the entire set oftessellations of one, two, and three-dimensional space. The simplest is a single segment consisting of one strutbetween two nodes. For two-dimensions a triangle with three nodes and three struts is the simplest structure, forthree-dimensions it is the Tetrahedron. For most applications, only the lengths of the struts are controlled, leavingthe swivel-connections at nodes free to accommodate the constraints thereby set. However, the control system muststill be careful not to over-constrain the system and push the components beyond their capabilities.

An ART structure that consists of a single Tetrahedron (Fig. 6a), 1-TET, can display interesting behavior andcan function as an innovative rover that is capable of traversing rugged planetary terrain by toppling over usinggravity (see the 1-TETanimations at the ANTS website4). The packing scale of a 1-TET is set by the physical limitsof the retractability of the struts and the size of the nodes. A payload can be mounted at the center of the frameprovided by a 1-TET by four struts connected to the vertex nodes. Such a configuration is a 4-TET because theoriginal volume of the 1-TET has been divided into 4 tetrahedral areas. A 12-TET consisting of 12 tetrahedral sub-volumes and 26 struts and 9 nodes forms a duo-decahedron (Fig. 6b) with remarkable reconfigurability. (The 12-TET rover animation is available at the ANTS website5.) In fact, nodes and struts may be connected together in atetrahedral mesh to form columns, sheets, or space-filling volumes that are actively reconfigurable. Examples ofthese applications include the multi-functional system, Lunar Amorphous Rover Antenna (LARA4), which could bedeployed to support the NASA’s Exploration Initiative (Section IV).

Figure 5. A reconfigurable space frame concept – Folded MNT membranes are stretchedout by node struts, which actively maintains its shape.

Figure 6. SMART strut design: a) Single tetrahedron concept; b) Multi-tetrahedra concept; c) Node joint showing rotational degrees of freedom


Figure 7. Node Design: a) ART design for 1-TET; b) MNT node size relative to zeros ona US penny coin; c) Enlarged view of MNT connecting nodes, showing 18 struts forinterior nodes and 9 struts for corner nodes in a space-filling SMART space frame.

Direction of travelGearing

Universal joint(for connectinglegsinto nodes)

Trussleg

Mechanismhousing

Spool

Motor

B. Node DesignFigure 7a shows the current node design for the ART 1-Tet demonstration prototype being built at GSFC: three

ball & socket joints connected together in a machined ball with the struts protruding from them. A similar designwill be used for the 4-TET to allow the struts to swivel so that the system is not over-constrained duringreconfiguration. The planned 12-TET ART prototype will have 6 struts in each node. In the future MNTimplementation, the interior nodes will have 18 struts and the corner nodes 9 struts for a space-filling architecture(Fig. 7 b & 7c).

C. Deployment MechanismThe variable length strut can be developed currently using existing deployable boom technology. At the heart of

this technology are low mass, hollow tubes made from shape-memory material that can be flattened and rolled ontospools. By reeling the spool in or out, the tube can be extended or retracted. The end result is a strut housing thatcan extend or retract a shape-memory tube by using existing motor technology to drive the spool (Fig. 8). Anotheralternative design is to employ telescoping tube segments using a string-pulley mechanism or a pneumatic system.Sensors will determine the length of the deployed strut, and feed this data back to control software. Low mass andtight packaging will be employed to the extent possible. Control software and hardware will be built to allow usersto fully control the shape, including fully stowed and fully deployed positions, and to position and orient the 1-TETprototype. Work is underway to build a Web-based 3-D graphical user interface for control of the 1-TET.

A “smart” material that can be used for the MEMS implementation is TiNi alloy that exhibits a shape memoryeffect6. Small diameter TiNi alloy rods or tubes could be used in place of carbon nanotubes. A long small diametershape memory alloy (SMA) rod or tube can be wound on a miniature spool of a strut deployer and this SMA rod ortube can regain its original undeformed shape when heated a high temperature. The miniature strut deployer couldincorporate a microheater at its nozzle to provide the heating required for SMA rod or tube during deployment.Microheaters made with silicon MEMS technology have been reported in literature.

Figure 8. Variable length strut deployment mechanism

a) b) c)


Figure 9. StrutDeployer based onCarbon nano-tubulesforming a nanozipper

Piezoelectric ceramic material such as Pb(Zr, Ti)O3 called as PZT is a very usefulmaterial for designing actuators needed for MEMS SMART nodes. A piezoelectric actuatorhas been achieved using a three-dimensional bridge-type hinge mechanism7, 8. Piezoelectricactuators are also used in commercially available micro-nano-positioning systems9 andMEMS nodes could be designed and fabricated with PZT material.

The NEMS strut deployer is envisioned to be a miniaturized version of a double-spool“tape measure” of MNT nanotubules. Two oppositely wound tapes will be “zippered”together at deployment to provide the necessary strength and rigidity for the strut (Fig. 9).Other possible mechanisms include nanotube bearings and springs14 that can be fabricatedby telescoping multi-wall carbon nanotubes. This is but a short list of advanced materialsthat show promise for the development of small and microscopic devices that will formthe basis of MART and eventually SMART structures. Many promising carbon nanotubedevices such as switches13 and sensors are coming out from research laboratories.

D. Functional Surface MaterialThe SMART truss can be covered with different surface materials to provide a variety of functions. For

example, reflective membranes can be stretched out by the struts to form an adaptive optical surface, with its shapeactively maintained by the SMART truss (Fig. 5). For the solar sails envisioned for the ANTS/PAM mission, thefabric will use multilayer dendritic polymers consisting of layers of 2-D slinky like chains that can provide 10 to 100times linear stretch, yet maintain sufficient reflectivity when fully extended to provide the radiation thrust necessaryfor navigation (Fig. 10).

E. SMART controlWe describe in this paper the mechanical aspects of the SMART space frame. Of equal importance is the

development of a synthetic neural system for the intelligent control of the SMART structure. The ANTS syntheticneural system is the subject of a companion paper10. We recognize the sheer computational power required by theneural system for ANTS and have begun efforts to achieve in-space high-performance computing capabilities. Weare studying the use of multiple COTS processors for space-based Beowulf cluster in an effort funded by the NASAST-8 program. In addition, we investigate the use of Reconfigurable Data Path Processors11 in a non-Von Neumannarchitecture to increase the onboard data analysis capability and to enable in-situ knowledge discovery.

F. Technology PathwayWe list in Table 1 the projected technology transition timeline from ART to MART to SMART, and the

packaging factor achievable for the gossamer structures. ART is an addressable reconfigurable architecture that canbe realized within the next 5-10 years with available technologies. MART extends ART to include MEMS andcarbon nanotube composites. The full SMART implementation with MNT devices and components is expected in20-30 years.

a) b) c)

Figure 10. Solar Sail Fabric: a) Dendritic polymer, b) 2-D slinky structure,

3) Fabric for one sail component


Table 1. ART to SMART Transition: A breakthrough technology

ART to SMARTImplementation Technology Packaging Factor

Launch vs. Deployed

Time Frame ofMission Infusion

Macro 1:10 Early mid-term ~ 2008Micro (MEMS) 1:104 Mid term ~ 2010s

Nano-technology 1:107 Far term ~ 2020s

IV. Applications to Exploration InitiativeThe near- and mid-term implementations of this architecture provides a new pathway for enabling extensive

reconfigurable robotic planetary exploration in support of the NASA Exploration Initiative, with sustained humanpresence on the Moon and Mars. ART and MART bring reconfigurability down to the level of the basic structuralbuilding block, the tetrahedral element. Systems implemented with this architecture can be optimized for a verylarge range of physical configurations and environmental conditions. For example, the Lunar and Martian landscaperange from relatively smooth flat regions to rugged highlands that are sculpted by crater impacts and past volcanicactivities. We created the concept of a multi-purpose structure, the Lunar Lander Amorphous Rover Antenna(LARA) (Fig. 11). LARA is an ART space frame of more than 100 tetrahedral elements with embedded propulsionmechanisms, sensors, and instruments designed for a lunar exploratory mission. LARA can land on almost any areaof the Moon since its shape could conform to the terrain of the landing site. LARA then morphs into a form mostsuitable for mobility according to the terrain it traverses: (1) a cylindrical rover that rolls or moves by pseudo-podiamotion on relatively flat surface; (2) snake form that slithers up or down a slope. It can bypass or extricate itselffrom obstacles by its flexibility and extreme reconfigurability. When LARA arrives at its destination, the embeddedinstruments can acquire and analyze scientific data in-situ. The struts of the structural frame could also open up toingest rocks or soil samples for return to base station. After it has completed its scientific tasks, LARA reconfiguresinto a high-gain antenna and contacts the Crew Exploration Vehicle for its pickup location for the return flight. Forthe human presence on the Moon, astronauts can assemble gossamer ART structures for crew habitats or as multi-purpose tools.

V. ConclusionWe presented a breakthrough technology for reconfigurable gossamer space structures, SMART, and its near-

term realization of ART and MART using current electromechanical system. ART is applicable to the new NASAExploration Initiative by enabling multi-purpose systems for planetary exploration.

AcknowledgmentsWe acknowledge support by NASA’s Revolutionary Aerospace Systems Concepts program and GSFC Internal

Research & Technology Development Program.

Figure 11. LARA: a) Lander; b) Amorphous Rover, with sample ingest; c) Antenna;d) Astronaut habitat

a) b) c) d)


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