Sergio Pellegrino

The dynamic properties of vibration of large spacecraft structures play an important role in the design of these structures, and yet computing the most basic property, the natural frequency of vibration, is challenging for the scalable deployable structures for the Caltech Space Solar Power Project. These structures consist of bending-stiff trapezoidal frames forming concentric squares, attached to prestressed tapes supported by diagonal deployable booms. A fully connected version of these structures is introduced, from which upper bounds on the natural frequencies can be computed rather easily. These bounds are useful to determine the geometry and material properties for the structural components, before moving to the more complex and time-consuming analysis of the actual, prestressed structures. Results from both sets of analyses are presented for structures ranging in size from 10 to 60 m. In addition, a simplified analytical beam model is developed to estimate the fundamental vibration mode with reasonable accuracy.

This paper presents design, manufacturing and testing techniques for elastically foldable flat structures. The structures consist of trapezoidal flat strips, arranged in four identical quadrants that form a square, supported by prestressed diagonal tapes along the diagonals of the square. The structures are folded by forming localized elastic folds in the strips and in the diagonals. The deployment kinematics of the structure are controlled by the corners of the structure. Gravity effects are minimized by means of a passive offloading system consisting of helium balloons. The deployment is monitored by recording the 3D trajectories of selected points on the structure with a motion tracking system. The test results provide evidence that the structure deploys sequentially in strip pairs.

This work explores the numerical simulation of the folding of complex structures consisting of pairs of Triangular Rollable and Collapsible (TRAC) longerons linked by transverse battens and parallel hinges. The analysis uses the Carrera Unified Formulation (CUF), which divides the three-dimensional displacement field of the structure into axial and cross-sectional terms, enabling one-dimensional beam finite elements to be used without sacrificing accuracy. Higherorder CUF models capture cross-sectional deformation through nine-point Lagrange expansions, extending through the whole structure. Thus, the numerical model of a complete structure consists of a single beam. An implicit quasi-static scheme, coupling Newton–Raphson iterations with displacement control is used across all analyses. Contact between the longerons is represented by nonlinear spring elements assigned to prescribed pairs of nodes.

This work presents the performance-driven design of a deployment mechanism for a collapsible tubular mast (CTM). The CTM is coiled around a central hub, which rotates to deploy it through a supporting device. A tensioned tape, co-wound with the CTM, is guided by an idler and wound onto a secondary spool. Whereas the key components of the deployment mechanism are defined, their geometric and positional parameters are left as free variables for a refinement process focused on the structural performance of the deployed booms. The influence of these parameters on the boom’s structural performance is investigated using a high-fidelity finite element model of the system, including both the CTM and the deployment mechanism.

Performing slew maneuvers of large flexible structures in space is challenging and even ground testing which is necessary to establish actuator working, control scheme and corresponding flexible structure dynamics. The paper presents a design of a small scale Control Moment Gyroscope (CMG) prototype and a lightweight flexible structure built for performing slew maneuvers. The 1.2 m × 1.2 m flexible structure is highly compliant along the maneuver direction allowing the study of its elastic deformations. Smooth polynomial maneuvers are performed with the CMG-structure system mounted on an air bearing, the central hub orientation and structural dynamics are measured. Finally, input shaping is applied to the original maneuver using Zero Velocity (ZV) shaper and the results are presented.

Deployable space structures that leverage high-strain composite shells have significant potential for applications requiring lighter and larger structures. However, due to the small thickness of coilable composite shells, often these structures are more compliant than desired. Thus, this paper investigates a novel approach using a curved tape-spring that forms a continuous ring perimeter structure. The double curvature of the composite ring increases its radial stiffness and the circular shape provides a high surface area-to-mass efficiency. The paper explains the deployment scheme for this structure and presents a detailed finite element analysis of the folding process. In addition, a prototype is built using an ultra-thin hybrid laminate and a Kapton membrane with an elastic spring attachment to prestress the structure. A folding and deployment test validates the presented scheme, which incorporates an origami fold pattern for the membrane.

A novel method of continuously attaching thin films to deployable thin shell structures, allowing for widely different coefficients of thermal expansion, is presented. This paper proposes a double s-spring border that exhibits a local post-buckling behavior and provides a tunable continuous edge attachment method that can maintain constant preload under large thermal strains. The mechanical behavior of the double s-spring under different mechanical loading conditions is studied both numerically and experimentally.

Deployable composite structures such as TRAC (Triangular Rollable and Collapsible) longerons are highly attractive for large-scale space applications due to their compact stowage and passive deployment capabilities. Conventional TRAC longeron manufacturing relies on autoclave-cured thermoset composites, which limits scalability. This study presents an out-of-autoclave manufacturing approach for ultra-thin thermoplastic longerons using thin-ply PEEK-based prepreg systems. A modular 3-step process (comprising laminate consolidation, web welding, and flange shaping) is applied to evaluate the effects of processing parameters and prepreg quality. Laminates are assessed via mechanical testing, computed tomography (CT), differential scanning calorimetry, and thermogravimetric analysis, revealing that fiber distribution homogeneity and thickness control are critical for void suppression, forming behavior, and overall laminate quality. Complete longerons are manufactured and analyzed using high-resolution laser scanning and CT analysis. Shape accuracy is found to strongly correlate with laminate uniformity, with flange radius deviations of up to 48% being observed in materials with poor fiber architecture. The most uniform prepregs yield nearly cylindrical flanges and geometric distortion of less than 10%. These results demonstrate that, with careful process control and prepreg selection, out-of-autoclave manufacturing can achieve structural and geometric quality comparable to autoclave-based production, enabling scalable fabrication of deployable composite structures. • First demonstration of TRAC longeron manufacturing using thermoplastic thin-ply prepregs. • Characterization of novel commercial thermoplastic thin-ply prepregs for structural applications. • Identification of processing parameters for manufacturing of ultra-thin thermoplastic longerons. • Out-of-autoclave manufactured longerons can achieve near-autoclave quality with controlled processing.

The focus of this study is the numerical simulation of the folding of ultrathin shells, focusing on slender deployable longerons. The proposed solution leverages the finite element method, specifically within the framework of the Carrera unified formulation, to develop one-dimensional finite elements with enhanced three-dimensional capabilities. Refined one-dimensional beam finite elements are used to model the longerons, and the three-dimensional displacement field is computed as a general expansion of the nodal displacements along the axis. The folding of these deployable structures is simulated with an implicit approach, and quasi-static simulation is performed. Moreover, a novel node-to-surface contact is introduced, where nonlinear springs act upon predefined node pairs. Simulations of a 400-mm-long longeron are conducted to fine-tune the parameters of the nonlinear spring stiffness. A deployable structure comprising two 400-mm-long longerons connected by transverse straight battens is analyzed. The results are compared with reference results obtained with the Abaqus software, revealing a noteworthy degree of agreement. The proposed method is also employed to simulate the folding of longer strips (1 and 5 m in length), where elements are not strictly constrained by aspect ratios, and it efficiently handles these structures.

Two different approaches to the in-space assembly of large space structures are presented. The first is based on a six-limb robot that picks up deployable truss modules that are launched into orbit in their stowed configuration. It crawls over previously assembled parts of the structure to place additional modules. The second is based on a box-like robotic truss builder that is launched into orbit with all of the components required to form a prestressed mesh reflector antenna. The assembly operations are carried out within the truss builder and the reflector structure emerges from it.

Isogeometric formulations of thin shells provide accurate geometric descriptions and deformation fields with higher-order continuity. They use only translational degrees of freedom and require smaller-sized models than standard finite elements with bilinear shape functions, which include both displacement and rotational degrees of freedom. This paper analyses the folding of a prototypical thin-shell deployable structure, a tape spring, using both NURBS-based and bilinear Reissner-Mindlin finite elements available in the software LS-DYNA. It is found that the analysis with isogeometric elements is three times slower than the analysis with bilinear Lagrange polynomial elements. Use of high aspect ratio meshes in the regions of the tape spring that do not deform significantly during folding leads to significant improvements in speed for both types of elements, but the difference in performance remains.

Two different approaches to the in-space assembly of large space structures are presented. The first approach is based on a six-limb robot that picks up deployable truss modules that are launched into orbit in their stowed configuration. The robot crawls over previously assembled parts of the structure to place additional modules. The assembly of a 100 m primary mirror for a telescope is described. The second approach is based on a box-like robotic truss builder that is launched into orbit with all of the components required to form a prestressed mesh reflector antenna. The assembly operations are carried out within the truss builder and the reflector structure emerges from it.

This paper introduces a novel robotic assembly concept for ring-like space structures with an internal prestressed cable net, based on the AstroMesh Reflector architecture. The proposed concept uses a stationary robotic assembly facility known as the truss builder. The modular components of the structure are packaged into the truss builder for launch into space and, once in orbit, the truss builder carries out the assembly process by executing simple, repetitive operations. A two-dimensional finite element ABAQUS simulation predicts the kinematics of the assembly sequence of such ring-like structures, capturing physics-based events of the actual assembly process. For six-sided and twelve-sided structures, it is shown that the assembly process can be successfully completed for a range of design parameters of the system.

A torsionally soft rectangular frame supporting an internal prestressed membrane undergoes stable torsional buckling when the prestress reaches a critical value. Estimates of the critical prestress vary significantly, depending on how the membrane’s load on the structure is modeled. This sensitivity of the critical prestress value is highlighted by reviewing classical results on the buckling of cantilever rods under different prestress loading geometries. With this background, a numerical study of a novel deployable structure supporting an internal membrane was carried out, considering four different ways of modeling the membrane with increasing fidelity. It is shown that the critical prestress increases by a factor of four when the loads on the structure are changed from fixed-direction to follower loads, and only models that include the membrane predict the correct stiffness of the structure in the postbuckling regime.

This paper presents a distributed space solar power system that converts solar insolation into microwave power and beams it to Earth. This system, composed of a power station of close-flying modules residing in geostationary orbit, can form dynamically programmable focal points on Earth to provide dispatchable power on demand. Modules are composed of flexible phased array sheets hosting a self-synchronizing network of integrated circuits and antennas that convert DC power from photovoltaic cells into radiated RF energy. The sheets are coiled into a compact payload, launched, and deployed in orbit. Here, we present a detailed technoeconomic analysis of the proposed system, with investigations into mass, cost to produce and launch, and a levelized cost of energy (LCOE). Our analyses demonstrate that with 10 years of technology development, maturation, and scaling, the proposed 10 GHz system can deliver electricity at 9.4 ¢/kWh—competitive with the cheapest clean energy sources available today.

This paper presents a method to determine the bending mode fracture toughness of thin-ply composite laminates. Single-edge notch samples are tested using a recently developed compression-bending fixture in the geometrically nonlinear regime until failure. The fixtures impose large bending curvatures at the notch tip and the corresponding moment vs. curvature response is obtained. Post-mortem micro-CT images show the details of the quasi-brittle fracture process zone. The experimental results and micro-CT images of the process zone are combined with the numerical virtual crack extension method to measure the critical energy release rate of the laminate structure, based on a detailed representation of the fracture process zone. The results are presented for a specific cross-ply laminate, but the procedure can be extended to other material systems under large curvature loads.

This paper presents the Deployable on-Orbit ultra-Light Composite Experiment (DOLCE) technology demonstration that flew in low earth orbit in 2023 as part of the Caltech Space Solar Power Demonstration 1 (SSPD-1) mission. DOLCE demonstrated the flight implementation of a novel ultralight composite structure for large deployable arrays. The paper outlines the design and flight operations of DOLCE. The deployment of DOLCE is discussed in detail, particularly the in-flight anomalies that were encountered and the way they were addressed. Lessons that were learned from this mission are discussed.

The natural frequencies of vibration of large, flexible spacecraft structures play an important role in the design of these structures, and yet computing them is a far from trivial exercise for the latest generation of structures that are under development. This paper presents a range of results for the scalable deployable structures for the Caltech Space Solar Power Project. These structures consist of bending-stiff trapezoids forming concentric squares, attached to prestressed tapes supported by deployable diagonal booms. The paper introduces a fully-connected version of these structures, from which upper bounds on the natural frequencies can be computed rather easily. These bounds are useful to determine the geometry and material properties for the structural components, before moving to the more complex and time-consuming analysis of the actual, prestressed structures. Results from both sets of analyses are presented in the paper.

This paper presents a new controlled deployment scheme for elastically foldable thin-shell structures. A previously developed kirigami-inspired architecture is used that allows folding and tight coiling of flat sheets divided into concentric squares of trapezoids. The deployment behavior is enhanced by introducing a set of hinges between the trapezoids to couple their elastic deformation. Thus, a quasistatic actuation concept is introduced for future flat deployable solar or antenna arrays. The study develops an explicit finite element model to simulate the folding and deployment of these elastic structures, to ensure robust deployment. The results are verified experimentally, for gravity either assisting or opposing the deployment. In both cases quasistatic and controllable behavior is verified.

This study analytically calculates the critical prestress of torsionally soft square frames supporting an internal thin film. The study highlights the role of the attachment scheme, which has a very significant impact on the critical prestress. The analytical calculation is verified via numerical finite element analyses and an experiment. It is concluded that distributed edge attachments significantly increase the stability of space structures against torsional buckling.

Thermoplastic composites (TPCs) offer many advantages compared to thermosets in terms of mechanical properties, reprocessability, and flexible manufacturing. However, their implementation in aerospace systems has lagged behind due to manufacturing challenges, insufficient availability of ultra-thin prepreg materials, and lack of information on material processing. The current study aims to fill this knowledge gap by demonstrating a design strategy for manufacturing and testing TPC Triangular Rollable and Collapsible (TRAC) longerons. To explore the design space, both analytical and numerical models of the longeron’s coiling were implemented, using experimentally determined mechanical properties of an ultra-thin (55 microns lamina thickness) PEEK prepreg. The results indicated that a [0/90/0] laminate could only be coiled to a 50 mm radius without failure due to the formation of a buckle, which acted as a stress concentrator. To verify these results, a TRAC longeron was then manufactured by exploiting the re-shaping capabilities of the matrix after initial consolidation, while the shape accuracy of the longeron and bond quality within the web region of the TRAC were assessed to identify any imperfections. The TRAC laminate was then coiled under the same conditions of the numerical model and the buckle formation was recorded and post-processed using image correlation software to compare directly with the simulation.

This study explores vibration damping induced by local buckling in ultra-thin composite shell structures under frequency-dependent loading. High periodic forces near a resonant frequency can trigger local instabilities, diminishing the shell's ability to transfer loads and strains reducing it dynamic response. Understanding this behavior is essential for optimizing system design. Both experimental and numerical approaches are employed. Experimentally, two boundary conditions are analyzed. Sine sweep loadings are applied to a 500-mm-long Triangular Rollable and Collapsible (TRAC) longeron near its natural torsional frequency at varying amplitudes, with damping ratios determined via the Half-Power Bandwidth Method. Numerically, the structure is modeled using a refined one-dimensional beam finite element based on the Carrera Unified Formulation, with three-dimensional capabilities. Nonlinear dynamics are simulated with an implicit scheme using the Newmark method of the Hilbert-Hughes-Taylor type. Results highlight damping effects caused by local buckling in the flanges of the longeron, as evidenced by variations in damping ratios across the two setups. Simulations closely match experimental results, providing valuable guidance for structural optimization.

The capability to build very large space structures will enable new kinds of space missions, however traditional deployable structures are restricted by the size of the launch fairing. This study proposes an in-space assembly architecture for large mesh reflectors. It uses an in-space truss builder that assembles the reflector with repetitive operations at one specific location. Important design considerations are identified and addressed for a successful assembly process. The proposed assembly operation is investigated by carrying out a manual assembly test of a modular reflector prototype. A proof-of-concept demonstrator of the proposed assembly facility is manufactured and an autonomous assembly operation is demonstrated.

Characterizing the geometric imperfections of ultra-thin composite structures is important since imperfections create weak points where local buckling is likely to occur. This work develops a thorough method for measuring the geometric imperfections in thin-shell composite longerons. The mean geometrical parameters, the axial twist, and the lengths, amplitudes and locations of both local and periodic radial imperfections in experimental composite longerons are measured using this method. The results of this study identify the dominant imperfections commonly introduced during the manufacturing of these structures.

This paper begins with the formulation of a general, rapid design method for deployable mesh reflector antennas based on the AstroMesh architecture. This method is then used to obtain estimates of the total mass, stowed envelope size, and fundamental natural frequency of vibration for antennas with a range of aperture diameters and focal lengths, assuming an operational radio frequency of 10 GHz. A study of the scaling trends of this reflector design shows that the distribution of prestress in the inner tension structure has a major impact on the mass of the outer perimeter truss. Based on this result, a prestress optimization problem to design reflectors of minimum mass is formulated, and analytical scaling laws are obtained for the mass, stowed envelope, and natural frequency of optimally prestressed reflectors with aperture diameters up to 200 m. It is then shown that aperture diameters of 70–100 m are at the limit of launchers that are currently available or under development. A semianalytical homogenization model that accurately estimates the fundamental natural frequencies for batten-supported and free-free boundary conditions is also presented.

Large apertures in space are critical for high-power and high-bandwidth applications spanning wireless power transfer (WPT) and communication, however progress on this front is stunted by the geometric limitations of rocket flight. We present a light and flexible 10 GHz array, which is composed of dipole antennas co-cured to a glass-fiber composite. The arrays can dynamically conform to new shapes and are flexible enough to fold completely flat, coil into a rocket payload, and pop back up upon deployment in orbit. The array is amenable to scalable, automated manufacturing—a requirement for the massive production necessary for large apertures. Moreover, the arrays passed the standard gamut of space-qualification testing: the antennas can survive mechanical stress, extreme temperatures, high-frequency temperature cycling, and prolonged stowage in the flattened configuration. The elements exhibit excellent electromagnetic performance: a return ratio better than −10 dB over ≈ 1.5 GHz, a single-lobe half-power beamwidth of greater than 110° suitable for broad beamforming, >92% efficiency, and excellent manufacturing consistency. Moreover, its mechanical durability vis-a-vis extreme temperatures and protracted stowage lends itself to demanding space applications. This lightweight and scalable array is equipped to serve a host of new space-based radio frequency technologies and applications which leverage large, stowable, and durable array apertures.

Thin-laminate composites with thicknesses below 200 μm hold significant promise for future, larger, and lighter deployable structures. This paper presents a study of the time-dependent failure behavior of thin carbon-fiber laminates under bending, focusing on establishing a fundamental material-level understanding of this type of failure. A novel test method was developed, enabling in-situ micro-CT imaging during long-term bending. Time-to-rupture experiments revealed the stochastic nature of failure, prompting a statistical approach to account for initial imperfections. The total probability of failure was calculated using separate Weibull functions for instantaneous and delayed time-dependent failures. The resulting function, dependent on curvature and aging time, is a design guideline for the design of future deployable space structures. Time-lapse micro-CT imaging identified kink bands and fiber–matrix debonding as primary failure mechanisms, providing essential insights for the design optimization of composite laminates.

Increasing the stability of thin-shell composite structures is important as they become larger and lighter for future applications. This work focuses on varying the cross sectional geometry of thin-shell coilable longerons in an effort to increase both their critical buckling load and stability approaching the buckling load. Stability metrics to quantify the stability of structures with different geometries are defined. 3D-printed and composite longerons with different cross sectional geometries are tested experimentally, and geometrically perfect composite longerons are analyzed numerically. Based on the dominant imperfection that has been observed, the effects of twist on the stability of these structures is investigated.

Deployable space structures experience thermally induced deformations on-orbit due to solar radiant heating. Deflection and temperature profiles under radiant heating are simulated for a section of a prototype deployable strip structure and verified experimentally. The simulation is extended to model structures with extended lengths (1, 2, and 5 m) under sunlight conditions. Results are compared with those of a small-scale experiment. The resulting deflection and temperature fields yield insight into the factors dominating the behavior of thin composite strip structures, specifically in-plane conductivity and axial twisting.

Deployable solar arrays for the Caltech Space Solar Power Project consist of ladder-like strips of thin-shell composites and use a dynamic, unconstrained deployment scheme. We study the effect of pre-tensioned springs within these structures on the successful completion of their deployment (latching), focusing on a single folded rectangular frame. We tune a finite element model in order to predict deployment timing and latching, and develop an analytical model to identify intermediate equilibrium states for the structure that may prevent full deployment. Our findings suggest that for equivalent pre-loads, the constant-force springs utilized in the current strip architecture generate significantly larger resisting moments and act earlier in the deployment compared to linear spring alternatives, thereby increasing the likelihood of unsuccessful deployment.

We investigate how different root boundary conditions for Omega booms affect the two primary requirements: small stowed volume along with high bending stiffness in the deployed configuration. For each boundary condition two finite element models are developed using the Abaqus software. The first model simulates the coiling process and is intended to get insights into the mechanics of coiling and to retrieve the shape of the transition region between the boom coiled and extended configuration which is used in the second model for computation of the bending stiffness. A specific boom design has been chosen and a physical prototype has been built.

Conventional deployable design approaches have size limitations due to launch fairing size. This study presents a design study of in-space assembled reflectors. Their geometry and structural design focus on mass efficiency. Scaling study of the mass and stowed volume of the reflectors establishes the feasibility of launching reflectors with aperture diameters up to 200m using commercial launch vehicles. A reflector assembly concept is proposed, that enables the assembly of large reflectors with a centralized robotic assembly factory. The proposed scheme is then demonstrated with a lab-scale prototype, proving the feasibility of the assembly concept.

This study presents a general design methodology for deployable mesh reflectors of any aperture diameter, focal length, and operational radio frequency. The reflectors consist of two triangulated cable nets attached to a deployable perimeter truss and prestressed against each other. Scaling laws are derived for the reflector mass, stowed volume, and fundamental natural frequency of vibration. A comprehensive design study is conducted considering the geometry, structural design of the reflector components, and optimization of the prestress distribution for mass efficiency. The launch envelope is identified as the limiting factor in the design of deployable reflectors, for building large reflectors in space. The natural frequency of the designed reflector is evaluated for a range of aperture sizes using a high-fidelity finite element model, and the natural frequency scaling law is established. This study proposes a semi-analytical model for the mesh reflector based on the homogenization of the structural components, which performs with good accuracy while ensuring significantly less computational effort.

This paper studies the performance of a friction damping scheme that uses a pre-tensioned, wound coil that allows interlayer slip during vibration in the context of launch vibrations for spacecraft with practical geometries and properties. A finite-element simulation is used to study the vibration response of a coil wound around a mandrel, supported in a base-fixed configuration. The simulation focuses on material properties and geometries relevant to coiled structures used in space applications. The viability of this concept in realistic contexts is demonstrated using damping ratios and resonant frequencies extracted from the simulated data for each configuration.