VEGA LAUNCH VEHICLE: PAYLOAD FAIRING SEPARATION ANALYSIS

VEGA LAUNCH VEHICLE: PAYLOAD FAIRING SEPARATION ANALYSIS P. F. Dezio, E. Mastrella, D. Fotino, G. Leto, M. Linari, F. Battie, D. Barbagallo The Payload Fairing (PLF) is the upper structural part of a launcher. Its function is to protect the payload against the natural and induced environment during the atmospheric flight phase. Once the launcher reaches a certain altitude, the PLF is no more needed and is separated from the launcher. One of the most critical flight phase for a multi-stages launcher is the fairing separation. The present work reports a methodology for dimensioning the sandwich structure of a PLF in order to guarantee its structural integrity at separation instant. The work is based on a PLF structure similar to the one of VEGA Launch Vehicle that, after the successful qualification flight on February 2012, represents one of the European launchers for the space access. 1. Introduction The loop of analysis has been conceived adapting the MSC NASTRAN SOL 400 to simulate the separation phase and MARC nonlinear local approach to verify the structural integrity of critical part, identified in the Cable Connector Door (CCD). Figure 1 and Figure 2 report respectively the detailed transient model of the PLF (NASTRAN sol400) and the nonlinear local model (MARC) of Cable Connector Door (CCD). The FEM structural verification of CCD frame has been achieved putting in place the following activities: Transient implicit analysis of the separation phase imposing its induced loads (global PLF model); Non-linear FE analysis of the CCD using as input the loads derived by means of the transient separation analysis (Local CCD model) 2.Transient Analysis of Separation Phase The PLF considered in the present paper consists of an Figure 1: Fairing Separation Model (transient analysis). A&C - Analisi e calcolo. Num. 65, novembre/dicembre 2014, pagg. 24-33 - Consedit sas 1

ogive structure followed by a cylinder ending in a transition radius to the conic boat tail. The boat tail, finally, interfaces with the launch vehicle. The two half sandwich shells of the fairing are linked together by means of shear rivets that are broken during the separation phase by the Vertical Separation System (VSS). For the present application, the structural part is considered as sandwich panels with laminated carbon fibre face sheets and an aluminium honeycomb core. For the transient analysis, the structure has been built by linear thin shell type CQUAD4 and CTRIA3 using Nastran PCOMP cards. 2.1. Modal analysis The modal analysis has been conducted separately for the two halves. In this paper, however, it will be shown only the modal shape of T-side, being those of TN-side, for symmetry reason, identical. The first T halve elastic mode is plotted in Figure 3. This mode shape is such that the upper part of the halves moves in phase opposition with respect to the lower part. Then it represents an unsymmetrical mode. The second elastic mode (Figure 4) is the so called breathing mode. Figure 4 shows the maximum opening and closing phase that characterizes this modal shape. As we will see, the separation load is able to excite the breathing mode inducing large deformations of the PLF halves, therefore the separation of the Fairing is strongly influenced by this dynamic behaviour (geometric non-linearity). All the remaining modal shapes represent modes of higher order with respect to the first two elastic modes obtained, where the number of nodes (point that remain fixed in the space, thus having no motion) and waves will be increasing as the natural frequencies increase. Figure 2: CCD non-linear local model. Figure 3: PLF T-half first mode shape. 2.2. Separation System Modelling At the end of atmospheric flight phase, the Fairing is separated by pyrotechnic strand from the rest of launcher. The energy delivered at this phase is such that the oscillation amplitudes of the Fairing halves are significant if compared to the Fairing dimensions. Then, the structure elastic behaviour is geometrically non-linear and shall be considered as such. The energy input of the vertical separation system has been simulated with non-linear forces (see Figure 5). 1. The initial load at zero displacement simulates the instantaneous increment of the pressure due to the pyrotechnical charge ignition up to the rupture of the Figure 4: PLF T-half breathing mode. A&C - Analisi e calcolo. Num. 65, novembre/dicembre 2014, pagg. 24-33 - Consedit sas 2

shear rivets; 2. The increasing trend simulates the increase of pressure due to the ignition of the residual pyrotechnic charge (which is completely burnt at the point where the curve is maximum); 3. The decreasing trend simulates the part where the two halves are moving apart and the force on the joints begins to decay until it becomes null. The Horizontal Separation System (HSS) allows to release the mechanical interface between Fairing and Launcher. The HSS load is modelled as a force lying in the separation plane (at the base of the fairing). 2.3. SOL 400 Procedure Since the Horizontal Separation Load does not induce Figure 5: Separation force model. Figure 6: STEP 1 definition for SOL400. Figure 7: BUSH element definition. convergence problems, the present paragraph reports the methodology used for implementing the Vertical Separation Load. The nonlinear force of the VSS, has a non-null force at zero displacement (see Figure 5). This kind of simulation can lead to convergence problem in MSC Nastran, then is required a procedure in order to introduce smoothly this preload condition. MSC Nastran helps in solving these problems by taking advantage of the possibility to use linear combination; in fact, the result of a preliminary static analysis can be Figure 8: STEP 2. A&C - Analisi e calcolo. Num. 65, novembre/dicembre 2014, pagg. 24-33 - Consedit sas 3

Figure 9: NOLIN1 cards. Figure 10: SLOAD cards: time-history displacement definition. added to the linear response to time varying loads in the same job. The SOL 400 [1] works in a nonlinear environment so there is the possibility to execute a static nonlinear analysis followed by a transient nonlinear analysis. It means that the following two steps can be considered: 1. A load equal to the initial load to be considered in transient analysis is applied to the structure in a preliminary nonlinear static step; 2. 2.In the transient nonlinear step a time variable or constant load is applied to the structure. In this analysis chaining approach the program considers as initial condition for the current step the final condition of the previous step. Any further load is applied as an increment of the previous load condition. These considerations can be used to build a modelling solution such that the transient load has the shape depicted in Figure 5. The selected procedure can be summarized as follow : 1. Define a static nonlinear STEP in which the preload is applied. In order to create the preload condition, the PLF shall not be separated. In MSC Nastran this is possible by using CBUSH elements (see Figure 7) with very high stiffness in all their components and the possibility to break when the applied load is slightly higher than the preload value. In fact in the PBUSHT entry the option KN allows to define a table for each of the six stiffness components. In particular a linear stiffness has been defined only in the separation direction with the possibility to break when the internal element force is equal to an appropriate value. 2. Define a transient nonlinear STEP (see Figure 8) whose content will be described in the coming points. a. The first dynamic load consists of the variable loads that starting from the corresponding static preload are reduced to zero in one time solution increment. b. The same nonlinear transient STEP defines the separation forces by using NOLINi cards (Figure 9). In this approach a force in a node is defined as a function of the displacement or the velocity of the same node or of another node. The last field in NOLIN1 entry identifies the TABLED1 ID where the VSS load /displacement values are defined. The force at null displacement is exactly the same than the module of the applied static forces (see Figure 6). c. In order to provide the required force, a scalar point (SPOINT) has been defined. The SPOINT is used in NOLIN1 card as a variable through which it s possible specifying the time history of the X-values of the above table. In fact, referring to Figure 10, in the SPOINT a proportional SLOAD between time and the displacements of the VSS separation trend (Figure 5) has been imposed. Following the previous described approach the dynamic load has been applied without convergence problems. A&C - Analisi e calcolo. Num. 65, novembre/dicembre 2014, pagg. 24-33 - Consedit sas 4

Figure 11: Local Model where the forces and moments have been extracted. Figure 12: Linear Transient Analysis with dummy model. 2.4. Element Forces and Moments Calculation Element forces are a typical MSC Nastran output but it is not available for nonlinear elements. Therefore, a dedicated methodology has been introduced in order to recover such output. This methodology is similar to the Modal Stress Recovery available in Adams and MSC Nastran. In fact, in this case the results obtained for a flexible body in an Adams simulation in terms of modal contribution are imported in MSC Nastran in order to be used with the corresponding modes to rebuild the transient solution. In this way it is possible to calculate the stress in the finite element model. The transient nonlinear analysis SOL 400 is not on modal basis and this procedure has been modified by importing in a direct transient linear analysis (SOL 109) accelerations, velocities and displacements. The elements are linear in solution 109 and the forces can be calculated. By using an appropriate DMAP Alters, the first step of the above-mentioned methodology requires the export from the transient nonlinear analysis of: Figure 13: Breathing Cable length variation. a) Acceleration/velocity/displacement vector (UGS); b) Vector of reactions in multipoint constraints (QMGS); c) Vector of reactions in single point constraints (QGS); d) Load vector (PJ1). A&C - Analisi e calcolo. Num. 65, novembre/dicembre 2014, pagg. 24-33 - Consedit sas 5

In the second step these vectors must be imported in a linear analysis executed on a finite element model that consists of only the elements for which the force output has to be requested (i.e. the local model to be analysed by non-linear static analysis, see 3). Substantially a linear transient analysis has been executed on the above model to which a dummy finite element model (a simple oscillator) has been added. This dummy model has been loaded in order to satisfy the program requirements. A further requirement is that the time step must be the same than that one used in the previous SOL 400 analysis. Just before the SOL 109 outputs are computed, in a specific SDR2 module, the solution vectors of the dummy model have been substituted by the ones generated in the first step (through another DMAP Alters). Finally the desired element forces have been extracted. 2.5. Transient Analysis Loop Results In order to validate the procedures presented in the previous sections, the following outputs have been obtained for the analyses: 1. Breathing mode time history; 2. Forces and moments per unit length (fluxes) at the local model contour. Figure 14: Time history force of an element belonging to the local model. Figure 15: CCD local vs. global FE models. 2.5.1. Breathing Mode Time History Once the displacement for which the separation force is zero has been reached, the two PLF halves continue to move due to their inertia. This phase of the motion is dominated by the breathing mode, whose amplitude is very large compared with the PLF dimensions. Figure 13 shows the time history of the breathing mode. The different curves are relevant to different modelling of the HSS load. The first HSS distribution has been taken into account considering the total HSS load as a concentrated one. The calculation has been executed with and without damping and the results show, in both the analysis, an initial rapid decrease breathing motion due a local effect of the concentrated forces. To verify this conclusion a limited analysis has been executed considering more nodes where distributing the concentrated loads. The results (green curve) show a reduction in the initial negative trend. As an extension of the distributed load condition, a pressure load distribution has been considered for representing the effect of the release in the horizontal PLF connection. With this load condition, the initial negative peak disappears confirming again the previous conclusion. Figure 16: Details of CCD local FE model. Figure 17: Bolts Preload and Fixed displacement of springs nodes. A&C - Analisi e calcolo. Num. 65, novembre/dicembre 2014, pagg. 24-33 - Consedit sas 6

2.5.2. Forces on selected elements The forces output are not available for SOL 400, but using the procedure described in 2.4and choosing an element (or a set of elements) the time history of the fluxes relative to the element forces can be obtained as per Figure 14. The max level of the forces (per unit length) has been considered as dimensioning load for the Cable Connector Door in the subsequent local non-linear static analysis. 3. CCD Non-Linear Local FEA The structural verification of the Cable Connector Door has been performed by a detailed nonlinear local finite element analysis taking into account the outcomes of the transient analysis, in terms of fluxes on the CCD section. Figure 15 shows the CCD local vs. global FE models. 3.1. CCD local FE model description As shown in Figure 16 the CCD is completely modelled with MSC Mentat [2] and consists of the following components: Aluminium honeycomb core Face-skins Doublers (door aluminium reinforcement) Adhesive layers (between the face-skin and the doubler) Bolts Aluminium Potted inserts. The analysis performed is a static non-linear analysis (MSC MARC code is used). The analysis has been performed in two steps (i.e. two load cases): i. Preload on bolts: application of preload in order to achieved the correct load status of the bolted connection before applying the external loads; ii. External fluxes on the boundary of the model: the fluxes on the contour of the local model are applied on the composite face-skins. Such fluxes are the gross fluxes resulting from the separation analysis performed at the previous paragraph. 3.2. Load Cases and boundary conditions All the boundaries of the local FE model are not constrained. In order to avoid convergence problems due to model rigid movements, the four corners have been constrained by elastic supports (springs) with a very low stiffness ( 10^5 N/m). A sensitivity analysis with Figure 18: Fluxes from global FE model applied on top face-skin boundary of local model. respect to the spring stiffness has been also performed demonstrating no impact in terms of stress. 3.2.1. Bolts Preload The first step of the non-linear analysis consists in the application of the bolts preloads. The load case is applied through the built-in tool of MSC Marc called MPC69 [2]. Figure 17 shows the FE model loaded with bolts preload. 3.2.2. Loads from global PLF separation analysis The loads computed by the PLF global FEM model (separation analysis performed at 2.5.2) are applied at the boundary of the local model. In particular, the gross fluxes computed by the global model in the composite face-skins are applied at the boundary of the face-skins modelled in the local FEM. This is justified by the fact that the local model is very detailed and contains all the discontinuities of the PLF in the CCD area (namely CCD hole, CCD doublers, adhesive). 3.3. Material properties and allowable The applied material properties are based on Avio heritage. For the honeycomb core, the following schematization has been considered [3]: E x E y 0; ν xy ν xz ν xz 0; G xy 0; G xz = G L (Ribbon or longitudinal direction); G yz = G w (Direction perpendicular to the Ribbon); E z (Compression Modulus). A&C - Analisi e calcolo. Num. 65, novembre/dicembre 2014, pagg. 24-33 - Consedit sas 7

3.4. Friction Coefficients Hypothesis The following friction contacts are considered: Steel/aluminium for the following interfaces: - Bolt and aluminium doubler on the external side; - Washer and aluminium doubler on the internal side; Steel/steel for the interface between the nut and the washer (PLF internal side). Concerning the aluminium/steel contact and the steel/ steel contact, a friction coefficient of 0.1 is considered since it is the minimum value prescribed by Table C-2 of [4]. 3.5. Analysis Results The structural verification has been performed evaluating the margins of safety for: Adhesive glue Composite face-skin Aluminium doubler. The model deformation is provided in Figure 20 in comparison with the un-deformed shape. As shown, under the applied external loads, the model tend to flatten in both axial and circumferential direction. 3.5.1. Stress in the Adhesive The shear stress in the adhesive is provided in Figure 21 considering a filter for the stress equal to the average allowable. The allowable shear stress is exceeded in the grey elements (which represent the adhesive failure). A further improvement to be introduced in the methodology shall be to perform a progressive failure of the adhesive. Figure 19: Aluminium Honeycomb. Figure 20: Model Deformation. 3.5.2. Stress in the Face-skin The aim of the stress analysis is to verify the robustness of the composite face skin. The presence of the hole induce a stress concentration factor which can lead to the structural failure. The FEA gives the possibility of designing reinforced holes without using conservative analytical approach which can lead to over dimensioning, in terms of thickness and mass, the structure. The structural margins of safety have been evaluated comparing the induced flux in correspondence of the hole with the composite allowable. The FEA gives also the stress concentration factor around the hole considering the part of the flux transferred by the adhesive to the aluminium doubler. Figure 22, Figure 23, Figure 24 and Figure 25 show the location of the top and bottom face-skins at which the highest stresses are reached and the value of the relevant stress. The maximum stress is induced at point D located in the bottom face-skin (inner surface of the Figure 21: Stress in the adhesive filtered below Adhesive average allowable. A&C - Analisi e calcolo. Num. 65, novembre/dicembre 2014, pagg. 24-33 - Consedit sas 8

AEROSPACE Figure 26: Top Face-skin: axial component of stress around the hole. gure 22: Top Faceaxial component of ss around the hole. gure 23: Top Facekin: circumferential omponent of stress around the hole. Figure 27: Top Face-skin: axial component of stress around the hole. e 24: Bottom Faceaxial component of ss around the hole. fact an high stress gradient is present in the area close to the hole and the elastic limit is exceeded only in one single node. No elasto-plastic behaviour has been considered in the analysis. e 25: Bottom Facekin: circumferential mponent of stress around the hole. 4. Conclusions The present work reports a methodology for designing a sandwich payload fairing structure in order to guarantee its structural integrity at separation instant (dimensioning case for the structure). The sandwich structural verification has been achieved putting in place the following activities: PLF). This result is coherent with the fact that the higher applied flux is the circumferential on the bottom faceskin. Since the driver parameter for the evaluation of the critical point is the external circumferential flux, Figure 26 reports the trend of the MoS vs External circ. Flux. The external circumferential flux is dimensionless with respect to the maximum value. 3.5.3. Stress in the Doublers The stress in the doubler is provided in Figure 27 and it is well below the elastic limit of the material with the exception some local plasticization effects close to the bolt holes. These plasticization effects are due to contact bodies, in Transient implicit analysis of the separation phase imposing its induced loads (global PLF model); Local non-linear FE analysis using as input the loads derived by means of the transient separation analysis (Local CCD model). The proposed methodology shall be validated by means of inflight measures. Further improvements have been also identified and will be implemented in future evolution of the activity. 5. Bibliography [1] MSC Nastran User s Guide [2] MSC Marc User s Guide [3] HexWebTM Honeycomb Sandwich Design Technology, December 2000 Publication N AGU 075b, Hexcel Composites, Duxford [4] ECSS-E-HB-32-23A Threaded fasteners handbook A&C - Analisi e calcolo. Num. 65, novembre/dicembre 2014, pagg. 24-33 - Consedit sas 9

A&C - Analisi e calcolo. Num. 65, novembre/dicembre 2014, pagg. 24-33 - Consedit sas 10