Dipartimento di Ingegneria Civile e Industriale
Master’s Degree Thesis
Photobioreactor
-Structural Analysis and Optimization
of an Experiment Payload for Manned
Space Missions
Professors:
Govanni Mengali
Mario R. Chiarelli
Company Supervisor:
Luca Briganti
Student:
Francesco Stortini
Accademic Year 2015/16scientific calculation. Ultimately, fulfilment crowns the dream.
This Master thesis is the result of a six month internship carried out at Air-bus DS GmbH, in Friedrichshafen, Germany. The main topic is the structural analysis and assessment of the Photobioreactor (PBR), a soft stowed experiment fitted into a standard National Aeronautics and Space Administration (NASA) locker, which will be tested on board the International Space Station (ISS) in 2018. The PBR is a biological life support system which is operated in parallel with the physiochemical one on board. It will produce oxygen by means of the photosynthesis of Algae stored in it. An introduction to the experiment and its scientific objectives is reported first. The environmental conditions, to which it will be subjected to, are then described, together with the definition of the over-all requirements. A detailed description of the developed Finite Element (FE) model, performed with MSC Patran, follows and the implemented procedure of the structural analysis, completed with MSC Nastran, is summarized in detail. An optimization campaign, regarding the natural frequency of the structure as well as its stress strength, is performed and all the results of the experiment strength assessment are presented at the end.
Abstract iv
List of Figures xi
List of Tables xii
List of Acronyms xiii
Introduction 1
1 Experiment Concept 5
1.1 Scientific Background . . . 5
1.2 Experiment Goals and Objectives . . . 6
1.3 PBR Hardware and Functional Block Diagram . . . 7
1.4 PBR Operations . . . 12
1.5 Development Status . . . 13
2 Requirements and Environment 14 2.1 Payload Requirements . . . 15
2.1.1 Launch and Accommodation . . . 15
2.1.2 Interfaces . . . 15
2.1.3 Functions and Performances . . . 16
2.1.4 Physical Characteristics . . . 16 2.1.5 Life Limits . . . 16 2.1.6 Handling . . . 17 2.1.7 Logistics . . . 17 2.1.8 Design Guideline . . . 17 2.2 Environmental Conditions . . . 17
2.2.1 Ground Handling and Transportation Loads . . . 17
2.2.2 Launch Mechanical Environment and Load Factors . . . 18
Soft Stowed Configuration . . . 18
Factors of Safety . . . 19
Quasi-Static Launch Loads . . . 19
Random Vibration Environment . . . 19
Belt Loading . . . 22
2.2.3 Crew Induced Loads . . . 22
3.1 General Approach . . . 24 3.2 Geometry Definition . . . 26 3.3 Mesh Definition . . . 28 3.3.1 Mono-dimensional Elements . . . 29 3.3.2 Bi-dimensional Elements . . . 29 3.3.3 Three-dimensional Elements . . . 32 3.3.4 Connections . . . 34 Fasteners . . . 34
Reactors Fixation Strategy . . . 35
Experiment Compartment Modelling . . . 36
3.4 Properties Definition . . . 36
3.4.1 Materials . . . 36
3.4.2 Surfaces Thickness . . . 37
3.4.3 Non-structural mass . . . 37
3.4.4 CBUSH Elements Properties . . . 37
3.5 Total number of Elements . . . 38
4 Model Checks 39 4.1 Patran Verification Tools . . . 39
4.1.1 Verify Normals . . . 39
4.1.2 Equivalence - Free Edges . . . 41
4.1.3 Verify Element Geometry . . . 42
4.1.4 Unreferenced Nodes . . . 44
4.2 Nastran Checks . . . 44
4.2.1 Free-Free Check . . . 44
Element Geometry and Nodes Singularity Checks . . . 44
Constraints Check . . . 45
Mass Check . . . 47
Eigenvalues Analysis . . . 47
4.2.2 Quasi-Static Check . . . 48
5 Soft Stowed Configuration 49 5.1 General Approach . . . 49
5.2 Modal Analysis . . . 50
5.3 Limit Load Factor Definition . . . 51
5.4 Strength Computation Procedure . . . 52
5.5 Pressure Application . . . 54
5.6 Initial Results . . . 57
5.7 First Optimization Iteration . . . 57
5.7.1 Increasing the First Natural Frequency . . . 58
5.7.2 Reducing Local Loads . . . 63
5.7.3 Results . . . 65
5.8 Second Optimization Iteration . . . 65
5.8.1 Remarks on Deformations . . . 65
6.2.1 Constraints . . . 71
6.2.2 Applied Loads . . . 71
6.3 Results . . . 73
6.3.1 Load Case “distributed pressure” . . . 73
6.3.2 Load Case “line force” . . . 74
7 Conclusions 76 A Mass Budget 78 B Meshed Surfaces 81 B.1 Experiment Compartment . . . 81 B.2 O2Absorber . . . 85 B.3 Outer PBR Structure . . . 86 B.4 E-Box . . . 89 B.5 rhAbsorber . . . 89 B.6 Superabsorber . . . 90 C Element Properties 91 D Nastran Input Files 94 D.1 Free-free Check Input File . . . 94
D.2 Quasi-static Check Input File . . . 97
D.3 Static Analysis - Unit Pressure . . . 101
D.4 Static Analysis - Crew Induced Loads . . . 105
E Mode Shapes 109
1 International Space Station - NASA . . . 1
2 Photobioreactor cycle . . . 2
3 Photobioreactor at U.S. Lab rack . . . 3
4 Example of soft stowed configuration . . . 3
1.1 The Photobioreactor, consisting of two Middeck Lockers . . . 5
1.2 Combined Life Support System (biological and physiochemical) . 6 1.3 Experiment Compartment section view . . . 8
1.4 Reactor exploded view . . . 8
1.5 Functional block diagram . . . 9
1.6 PBR cut view, right side . . . 10
1.7 PBR cut view, left side . . . 11
1.8 PBR rear view . . . 11
1.9 Liquid Exchange Device, Flight Model . . . 13
1.10 Liquid Exchange Device, working principle . . . 13
2.1 Standard NASA Middeck Locker . . . 18
2.2 Standard M01 Bag . . . 18
2.3 Belt Secured Bags . . . 18
2.4 Unattenuated PSD Envelope . . . 20
2.5 Foam Attenuation Factor Envelope . . . 20
2.6 Attenuated and Unattenuated PSD Environment . . . 21
3.1 Final PBR Design . . . 25
3.2 PBR Finite Element Model, view of elements by property . . . . 25
3.3 Coldplate Cooling System, Exploded View . . . 27
3.4 Frontplate, CAD Solid Part . . . 27
3.5 Frontplate, Surface Cut into Basic Shapes . . . 27
3.6 Example of Bolt Head Geometry . . . 28
3.7 Detail of the Cylindrical Parts on the Lower Shell of the Superab-sorber . . . 29
3.8 Detail of the CBAR Elements on the Lower Shell of the Superab-sorber . . . 29
3.9 Example of a region with both CQUAD4 and CTRIA3 mesh, detail of the rhAbsorber . . . 30
3.10 Patran window for splitting a quad into two trias . . . 30
3.11 Patran window for Tabular mesh seed . . . 31
3.12 Example of hole Element modelling, detail of the Coldplate . . . 32
3.13 Three-dimensional Elements on the frames of the main plates . . 33 3.14 Experiment Compartment section view, different units outlined . 34
4.1 Tool to check and reverse surfaces normals . . . 39
4.2 3D Plate Thickness utility . . . 40
4.3 Verify Element Normals tool . . . 40
4.4 Verify Free Edges - Equivalencing tool . . . 41
4.5 Verify Quad Element geometry . . . 42
4.6 Skew angle . . . 42
4.7 Taper . . . 43
4.8 Aspect ratio . . . 43
4.9 Warp angle . . . 43
4.10 Summary table of Element geometry test . . . 44
4.11 Inception of Grid point singularity table . . . 45
4.12 Matrix KGG check . . . 45
4.13 Matrix KNN check . . . 46
4.14 Matrix KFF check . . . 46
4.15 Matrix KAA check . . . 46
4.16 Weight check output . . . 47
4.17 Real Eigenvalues table . . . 48
4.18 Maximum displacements values for unit gravity loading . . . 48
5.1 Modal analysis, first natural mode deformation, 90.30 Hz. In blue the undeformed shape . . . 51
5.2 Foam Attenuated PSD Environment . . . 52
5.3 Unit pressure load, 1 kPa, acting on the right side of PBR . . . . 55
5.4 Unit pressure load, 1 kPa, acting on the rear surface of PBR . . . 55
5.5 Unit pressure load, 1 kPa, acting on the top side of PBR . . . 56
5.6 Unit pressure load, 1 kPa, acting on the bottom side of PBR . . 56
5.7 Unit pressure load, 1 kPa, acting on the front surface of PBR . . 57
5.8 Deformed shape and displacement fringe plot, first mode, 90.30 Hz 58 5.9 Detail of the fixation points between EC and Frontplate . . . 59
5.10 Deformed shape and displacement fringe plot, second mode, 94.09 Hz . . . 59
5.11 Detail of the Frontplate rear side, two vertical and two horizontal ribs added in the lower central area . . . 60
5.12 Deformed shape and displacement fringe plot, fourth mode, 115.59 Hz . . . 61
5.13 Detail of the Superabsorber Supporting bracket improved design 61 5.14 Deformed shape and displacement fringe plot, fifth mode, 125.74 Hz 62 5.15 Detail of the Backplate improved design . . . 62
5.16 Detail of the Lateral plates improved design, pink Elements have a 3 mm thickness . . . 63
5.17 Detail of one of the Backplate corner, original design . . . 64
5.18 Detail of one of the Backplate corner, improved design . . . 64
5.19 Unit pressure load, 1 kPa, acting on the Backplate with foam cut through . . . 66
in the direction parallel to the cylinder axis of the Experiment
Compartment (EC)-Window, cross section . . . 70
6.2 Load Case “line force”, equivalent to the force acting laterally w.r.t. the cylinder of the EC-Window, cross section . . . 70
6.3 Constrained configuration of the PBR for crew induced load static analysis . . . 71
6.4 Load Case “distributed pressure”, pressure acting on the EC-Window, FE model . . . 72
6.5 Load Case “line force”, force acting on the EC-Window, , FE model 72 6.6 Fringe plot of maximum Von Mises stress, Load Case “distributed pressure” . . . 73
6.7 Fringe plot of maximum displacement, Load Case “distributed pressure” . . . 74
6.8 Fringe plot of maximum Von Mises stress, Load Case “line force” 75 6.9 Fringe plot of maximum displacement, Load Case “line force” . . 75
7.1 Design hypothesis for the EC front side fixation . . . 77
B.1 Experiment Compartment Assembly . . . 81
B.2 Experiment Compartment Housing and Serviceport . . . 82
B.3 Experiment Compartment Back Insulation Frame . . . 82
B.4 Experiment Compartment Backplate . . . 83
B.5 Experiment Compartment Window . . . 83
B.6 Experiment Compartment Window Frame . . . 84
B.7 Experiment Compartment Reactors Assembly . . . 84
B.8 Experiment Compartment Reactors - Grid Detail . . . 85
B.9 O2Absorber . . . 85 B.10 Backplate . . . 86 B.11 Brackets . . . 86 B.12 Coldplate . . . 87 B.13 Frontplate . . . 87 B.14 Lateral plates . . . 88
B.15 Superabsorber supporting bracket, the right one displayed . . . . 88
B.16 E-Box . . . 89
B.17 rhAbsorber . . . 89
B.18 Superabsorber, top view . . . 90
B.19 Superabsorber, bottom view . . . 90
E.1 Modal analysis, second natural mode deformation, 94.09 Hz -Frontplate . . . 110
E.2 Modal analysis, third natural mode deformation, 113.57 Hz - Front-plate . . . 110
E.3 Modal analysis, fourth natural mode deformation, 115.59 Hz - Su-perabsorber Supporting Brackets . . . 111
E.4 Modal analysis, fifth natural mode deformation, 125.74 Hz - Back-plate . . . 111
2.1 Launch and Accommodation Requirements . . . 15
2.2 Interfaces Requirements . . . 15
2.3 Functions and Performances Requirements . . . 16
2.4 Physical Characteristics Requirements . . . 16
2.5 Life Limits Requirements . . . 16
2.6 Handling Requirements . . . 17
2.7 Logistics Requirements . . . 17
2.8 Design Guideline Requirements . . . 17
2.9 Ground Handling and Transportation Load Factors . . . 17
2.10 Factors of Safety for Express Rack Payload Analysis . . . 19
2.11 Launch Load Factors Envelope for Pre-Determined Orientation . 19 2.12 Unattenuated and Foam Attenuated Envelopes, Foam Attenuation Factor . . . 21
2.13 Crew Induced Loads . . . 22
3.1 List of materials . . . 37
3.2 Total Number of Finite Element . . . 38
5.1 PBR natural modes . . . 50
5.2 Margins of Safety results, original PBR design . . . 57
5.3 Margins of Safety results, first optimization iteration . . . 65
5.4 Margins of Safety results, second optimization iteration . . . 68
7.1 Margins of Safety for the analysed Load Cases . . . 76
AFE Agency Furnished Equipment CAD Computer Assisted Design DDR Detailed Design Review EC Experiment Compartment FE Finite Element
FEA Finite Element Analysis FEM Finite Element Method H/W Hardware
I/F Interface
ICC Integrated Cargo Carrier ISS International Space Station
ITAR International Traffic in Arms Regulations LiED Liquid Exchange Device
LSR Life Support Rack LSS Life Support System MDL Middeck Locker
MPC Multi Point Constraint
NASA National Aeronautics and Space Administration NSM non-structural mass
OD Optical Density PBR Photobioreactor
PDR Preliminary Design Review PFM Proto-Flight Model
POM Polyoxymethylene
RVL Random Vibration Loads SAPs Superabsorbent polymers SPC Single Point Constraint
Figure 1: International Space Station - NASA
The ISS (figure 1) orbiting around the Earth at an altitude between 330 and 410 km, represents a unique human outpost in space. It helped mankind to set the record for the longest human presence in low Earth orbit as well as the one for the largest artificial body in orbit. It is also an extraordinary example of collaboration between different nations and an unrivalled engineering piece of work.
This artificial satellite, enabling investigations in real environments, i.e. real cabin air, microgravity conditions and increased radiation loads, is most of all an exceptional testing facility for a wide range of technologies and experiments. During the years it has welcomed a succession of astronauts and cosmonauts, that have conducted biological observations and assessed technical applications. The PBR is one of the experiments that will be tested in this particular frame-work. Its main purpose is to prove a technology that could be then further developed and used on a larger scale to allow an improved recycling of the re-sources on board, as outlined in figure 2. This is clearly the objective for the
upcoming manned exploration missions, involving a reduction of the required mass of supply and an increase of the operational autonomy.
The experiment has the task to convert part of the carbon dioxide, produced by the crew members’ respiration, into oxygen. This is performed by means of pho-tosynthesis of algae Chlorella Vulgaris stored in the reactors of the experiment. The PBR will be operated in parallel with the main physiochemical life support system running on the space station, the Life Support Rack (LSR). It will be able to absorb the carbon dioxide, which is at present released in outer space, and it will be able to convert the CO2 into breathable oxygen.
This approach improves the closure of the mass recycling ratio, and has the po-tential to use the generated biomass from the PBR as additional food or nutrient supplement for the on-board crew.
Figure 2: Photobioreactor cycle
The experiment will be fitted into the U.S. Laboratory Module (Destiny), stored in a standard NASA Middeck Locker (MDL) interfacing one of the 24 Express Racks on board the module, figure 3.
Figure 3: Photobioreactor at U.S. Lab rack
The thesis work starts with a detailed description of the functions and processes inherent the PBR, followed by a detailed analysis and understanding of the re-quirements of the experiment, as well as the environmental conditions, e.g. the loads it has to survive. Due to the entity of the launch loads and the long exper-ience of the company, the experiment is foreseen to be launched in a soft stowed configuration, i.e. wrapped with foam and carried in a NASA standard transport bag (M01 bag). The bag is then fixed on board the launch capsule by means of straps (example shown in figure 4).
Figure 4: Example of soft stowed configuration
The Finite Element Analysis (FEA), after the modelling of the geometry and its meshing description, focuses on imposing boundary conditions which can be
regarded as representative of the actual launch loads the experiment will be subjected to. According to this purpose, a unit pressure is applied to the outer surfaces of the soft stowed structure, avoiding to constrain it but using instead the inertia relief parameter, directly in the Nastran input file. This solution technique allows for a free-free static analysis, in which the applied loads are balanced with inertia loads generated by the applied loads themselves. In this way the behaviour of the foam is simulated: the PBR, exposed to launch accelerations, feels the effect of the compressed foam, which exerts a pressure on the surfaces, acting in the direction opposite to the one of the acceleration.
Once the model is validated, a series of iterations are performed in order to obtain positive margins of safety. The developed improvements regard strategies to increase, on one hand the value of the first natural frequency of the structure, in order to get lower values of equivalent static loads resulting from Miles’ equation. On the other hand the positioning of ribs or a careful thickness increase are envisaged in order to prevent stress concentration or excessive deformation. Finally a standard static analysis is performed in order to verify the structure against crew induced loads, which could occur once the PBR is placed in the Destiny module.
Experiment Concept
1.1
Scientific Background
Figure 1.1: The Photobioreactor, consisting of two Middeck Lockers The experiment is presented in figure 1.1: the lower MDL includes the experiment module, the upper one includes a spare CO2 reservoir, the use of which is later clarified. The PBR is a biological system, based on photosynthesis, that can be combined with the physiochemical system on board the ISS and generate additional O2and (edible) biomass by consuming the CO2surplus. The biological
system used is algae. Algae cultivation in aquatic reservoirs reaches up to ten times higher growth rates and significant lower energy and volume investments than higher plants, and neither a hydroponic basis nor soil are needed. One main advantage is the output of the system, biomass and O2, related to the system volume. Up to about 30% of human food can be covered by algae biomass, limited by the high protein content of edible algae species such as Chlorella Vulgaris. The experiment can regenerate 1g/day of the CO2 collected by the
LSR. As a crew member produces on average about 1kg/day of CO2, it is clear that the experiment is not intended for high performances. It is developed for technical feasibility instead, i.e. to prove that in the future this technology could be improved in order to provide edible biomass and release O2, up to levels that could mean a reduction of 22% of resupplied O2, while the edible biomass could
enable a reduction of 12% of resupplied food.
1.2
Experiment Goals and Objectives
The PBR, once on board, will be part of a hybrid system formed by the experi-ment itself and by the LSR, which revitalises the cabin air (scrubbing of carbon dioxide) and regenerates oxygen, mostly by electrolysis of water. The figure 1.2 shows that the crew exhales CO2 from the provided O2. The LSR removes the
CO2 from the cabin air, and makes it available for the Sabatier process. Its out-put products are water to the electrolyser and methane (CH4) vented in outer
space. The electrolyser splits the water electrochemically and produces O2 for
the crew and H2 for the Sabatier process. The PBR uses a small amount of the surplus part of the CO2, otherwise vented as well in outer space, and
pro-duces O2 by photosynthesis for the crew and biomass, as potential food in future
development.
Figure 1.2: Combined Life Support System (biological and physiochemical) The verification of the PBR functionality, for a long term (six months), in com-bination with the CO2 supply from the LSR is of the highest priority. This
objective underlines the hybrid character of the experiment idea and the result will offer a first broad data set to evaluate hybrid systems in general, e.g. for extended exploration missions.
The evaluation of the performance in the space environment of the cultivation system, consisting of gas supply (O2), nutrients supply, harvesting and sensors
monitoring, is a next important goal.
The observation of the stability of the algae system during a long-term cultivation shall give a strong indication that the algae remain in a good healthy state and that their cultivation for Life Support System (LSS) purposes is feasible.
1.3
PBR Hardware and Functional Block Diagram
The main components and their functions are:
• EC with two Reactors and their gas and liquid control loops,
• CO2 spare supply (internal reservoir, accommodated in a separate MDL
with a connecting tubing in between, and LSR Interface (I/F)) to the EC, • Thermal cooling, water heat exchanger for electronics and Reactors, • Control and power supply electronics,
• Drawer front panel, for crew access, status information, and I/F data, power and cooling,
• Gas release I/F of EC to cabin,
• Cabin air supply to the EC to avoid O2 enrichment during long term op-eration,
• Experiment data transmission via WLAN to ISS hot spot.
The core part of the experiment is the EC, figure 1.3. It is a liquid tight com-partment, which houses the reactor chambers, shown in figure 1.4. Each of the two Reactors is characterised by a distinctive feature: a separated gas and liquid loop. The CO2 is injected in a closed compartment and then transferred to the
algae present in the water loop by means of a gas permeable membrane which covers the Reactors. In the EC there are also a thermal control and some sensors. The pCO2 and pO2 sensors are the main ones for system operation and regu-lation. By measuring the kinetics of the pCO2 reduction and the pO2 increase, the biological performance of the experiment can be monitored. The produced small amount of gas (∼0.5 L/day) from the PBR will be collected in the EC and released by flushing it into the cabin. The CO2 supply is controlled via the CO2 supply unit with a central valve to select the CO2 source: if the status of LSR is
“not ready” the system automatically switches to PBR dedicated CO2 reservoir,
Figure 1.3: Experiment Compartment section view
Figure 1.4: Reactor exploded view
Other components are:
• O2Absorber. Figure 1.6. It provides for, by means of scavenger pads, an O2 reduction of 12% in the EC during three months operation time (i.e.
algae growth and O2 (0,5gr./day) production period)
• Super-absorber. Figure 1.6. Superabsorbent polymers (SAPs) are water-insoluble, cross-linked polymers that can absorb and retain extremely large amounts of a liquid relative to their own mass, and not easily releasing it, even under pressure. The polymers are available as powder, stored inside this absorber. It is placed along the cabin air outlet path and has the capability to take the complete liquid volume of the reactor loop in case of failure (e.g. spillage).
• rhAbsorber. Figure 1.6. Since the CO2 supply of LSR has an expected
relative humidity close to 100%, some condensation can be expected in the I/F line. The absorber helps in preventing this to happen.
• Electronic-Box (E-Box). Figure 1.7. It contains the controlling elec-tronics and software.
• Coldplate. Figure 1.8. It is an integral part of the structure, inside which, for heat dissipation purposes, a stainless steel water tubing is routed through a meander form. The middle plate consists of two pressing plates, shaped for the water tube and in contact with it by means of a thermal glue. Both the E-Box and the EC are fixed to the plate, that acts as an heat sink, cooling the two subsystems.
• Pulse chamber. 1.8. It is the CO2I/F to the EC, an aluminium structure with an inner volume of 100 ml and integrated valves at the inlet and outlet, which allows for pressurized CO2 release inside the EC.
Figure 1.7: PBR cut view, left side
1.4
PBR Operations
The experiment mission concept consists of three phases: Inoculation of the algae, Operation of the biological system (Experiment Session) and Shut down. For safety reasons, it has been proved that the shut down of the system corresponds to a state in which volumes inside the reactors do not change: on one hand algae stop growing and producing oxygen, without LED light and CO2 supply. On the other hand the metabolism of bacteria which decompose the algae do not produce any gas.
The algae culture is launched into space in a safe environment, cooled down to a temperature of 4◦C . This culture is inoculated into the system, which is already filled with nutrient medium. During 1-3 days, the algae cells are adapted to the environment (pH, temperature, pressure, movement, etc.). As soon as cell density, i.e. the optical density, increases, the nominal operation (feeding, harvesting, etc.) continues.
The experiment can run into two functional modes: “hybrid run” and “self-sustaining run”. “Hybrid run” means that the PBR is supplied with CO2 coming from the LSR. In case of “self-sustaining run”, the experiment consumes CO2
provided by its own CO2 stock. The selection of the “hybrid run” requires an operational LSR. The mode selection is performed automatically. After a suc-cessful operation phase, the experiment is shut down and either downloaded for refurbishment or disposed of.
During the cultivation, the biomass of the algae increases. When the concen-tration of bio-material in the fluid raises, the O2 production rate and the CO2
consumption one vary, until they go to zero when the density is too high. Due to this process a periodical planned harvesting of one third of the liquid loop and replenishment of fresh nutrients is necessary. Since there is a stoichiomet-ric relation between the produced amount of oxygen and the generated mass of carbohydrates (C6H12O6, glucose), during these intervals the biomass volume is monitored by the oxygen level, and the experiment can automatically regulate it via the CO2 level input. In addition the biomass is also on-line measured by a
photometer (Optical Density (OD) sensor) in the pump loop.
The experiment gets the timing signal/data from the internal clock and downlinks periodically the experiment and the housekeeping data via WLAN or via USB stick to crew laptop. The operational status of the EC is monitored by a set of pO2 and pCO2 sensors at the inlet and outlet of the O2Absorber, as well as the temperature and humidity in the PBR chambers, the pressure of the EC and the pressure of the Pulse chamber. The status “hybrid run” or “self-sustaining run” is indicated both on the front panel and in the housekeeping data.
Figure 1.9: Liquid Exchange Device, Flight Model
The algae harvesting and medium exchange is done manually by a crew mem-ber using a device with two syringes, figure 1.9, called Liquid Exchange Device (LiED). The empty one is used for the aspiration of the algae culture (harvest-ing), the other one is used to inject fresh medium. The two syringes allow for a volume neutral exchange. After use, the syringes are stowed for disposal. As an option, some harvest syringes may be stowed for probe return. The complete harvesting and feeding principle is shown in figure 1.10.
Figure 1.10: Liquid Exchange Device, working principle
1.5
Development Status
At the time of writing (March - August 2016), the project had just entered in the Phase C/D of its development, with the Preliminary Design Review (PDR) oc-curred in April 2016. The Detailed Design Review (DDR) is planned in December 2016.
The model philosophy has followed an initial Bread Board development, in Phase A/B, for testing and first technology assessment, and a Proto-Flight Model (PFM), that is currently under realization. The PFM will be subjected to qual-ification test and will actually be the final model to be carried on board the ISS.
Requirements and Environment
2.1
Payload Requirements
In the following paragraph all the requirements related to the experiment, and which it is compliant with, are listed, in form of table, for easy reference.
2.1.1 Launch and Accommodation
Launcher PBR Hardware (H/W) shall be launched with Dragon-X,
Orbital, Progress or Soyouz, in a soft-stowed configur-ation (foam envelope packed and strapped down to the capsule floor)
In Orbit
Accommodation
PBR is accommodated in the MDLs of the Express Rack, which are provided by NASA, as Agency Furnished Equipment (AFE).
Download It is not foreseen to bring back any H/W to Earth,
ex-cept for the external USB stick used for data transfer and possibly for the LiED containers, where exchanged algae are stored.
Table 2.1: Launch and Accommodation Requirements
2.1.2 Interfaces
External I/F The PBR is connected to the following services of the Express Rack I/F:
• Water cooling thermal control subsystem, • Power, via external 28 V DC supply.
External I/F The PBR is connected to the LSR CO2 I/F via flex line
and to the LSR “CO2 ready signal” on the LSR Laptop
test connector.
Thermal I/F The PBR is cooled by heat rejection to the ISS water cooling loop.
Touch
Temperature
The outer touch temperature of the system is limited to 37◦ C.
Electrical I/F The PBR has:
• an average power consumption of less than 160 W, • a maximum power consumption of less than 240 W, • a maximum input current smaller than 9 A. Command and
Data I/F
The PBR provides for a USB interface for a data storage device, and WLAN for data downlink.
2.1.3 Functions and Performances General
operation
The functional composition of the experiment is according to the block diagram in figure 1.2.
General operation
The function is demonstrated by measuring relevant data: O2 generation, CO2 consumption, pH, algae density,
bio-mass, sampling etc.
Sampling Sampling (each two weeks) of about 30 % of the reactor
volume (i.e. algae and water mixture) is manually per-formed accounting for the resupply of liquid.
Automated operation
The PBR runs autonomously as unattended payload. No ground control commanding is foreseen.
LSR The operation of LSR shall not be affected by the
con-nected PBR. General
operation
The PBR automatic operation is independent from the LSR CO2 supply for at least one month total operation time with biological material injected.
Table 2.3: Functions and Performances Requirements
2.1.4 Physical Characteristics
Facility Size The experiment fits into MDLs (i.e. glspbr experiment insert in one locker and CO2 supply assembly in second
locker).
Facility Mass The total packed mass in MO2 transportation bag must not exceed 80 kg.
Table 2.4: Physical Characteristics Requirements
2.1.5 Life Limits
Storage life
time
It is maximum one year and it starts after acceptance.
Service life
time
Until retirement, it is not less than two years, starting after integration into space vehicle within the permissible storage life time.
Operational life time
It is six months of continuous operation, as design aim, while one month is the minimum operation and processing time.
2.1.6 Handling
Handling The possibility to intervene manually must be safe and
viable.
Handling The product must be designed to be installed at the
launch site within two days.
Table 2.6: Handling Requirements
2.1.7 Logistics
Logistics Late access must be possible for the algae transport con-tainer.
Table 2.7: Logistics Requirements
2.1.8 Design Guideline
Guideline As far as European equipment is available, no Interna-tional Traffic in Arms Regulations (ITAR) equipment shall be used.
Table 2.8: Design Guideline Requirements
2.2
Environmental Conditions
2.2.1 Ground Handling and Transportation Loads
The PBR and its equipment shall be designed to withstand the acceleration listed in the table 2.9, according to [1, p. 3-2].
Nx (g) Ny (g) Nz (g)
(I) +/- 5.0 +/- 3.5 +2.0/-3.5 (S) +/- 2.0 +/- 2.0 +2.0/-3.5
Table 2.9: Ground Handling and Transportation Load Factors
Notes:
• The reference frame for the ground handling and transportation load factors with respect to the directions of motion is as follows:
– X: Longitudinal direction along axis of motion.
– Y: Y axis is perpendicular to the x and z axes and completes the right handed coordinate system.
– Z: Z axis is perpendicular to the x and y axis. Positive direction is vertically upward. Gravity (g) is acting in the z axis in the negative direction.
• (I) indicates that the loads occur independently in the three directions (except for grav-ity). (S) indicates that the loads occur simultaneously.
2.2.2 Launch Mechanical Environment and Load Factors Soft Stowed Configuration
PBR items shall be launched passively, in a pressurized cargo and in a soft-stowage configuration. The soft soft-stowage configuration consists of an envelope of foam, of different thickness and material, surrounding the MDL, figure 2.1, [2, p. 3-11]. The envelope is then packed in a M01 bag, figure 2.2, [3, p. 4-196], which is secured to the cargo by means of strap belts, example in figure 2.3. Typical foam materials used are: Minicell, Zotek, Pyrell or Bubble Wrap.
Figure 2.1: Standard NASA Middeck Locker
Factors of Safety
PBR and its equipment shall have positive margins of safety for all load conditions using the applicable safety factors as defined in table 2.10, according to [2, p. 4-2]. These factors apply only to limit loads and not to qualification loads.
CATEGORY FACTOR OF SAFETY PROOF TEST
YIELD ULTIMATE FACTOR
Metallic Structures
Untested Vehicle (analysis only) 1.25 2.0 Untested On-Orbit (analysis only) 1.25 2.0
Tested Vehicle (analysis and test) 1.0 1.4 1.2 Tested On-Orbit (analysis and test) 1.1 1.5 1.2
Pressurized Systems
Pressure Vessels 2.0 1.5
Lines, Fittings, and Components <1.5 in. diameter 4.0
Lines, Fittings, and Components >1.5 in. diameter 2.0 2.0
Flex Lines 4.0
Line-Installed Bellow and Heat Pipes 2.5
Other Components 2.5
Secondary Volumes 1.5
Beryllium Structures Static Test and Analysis 2.0 1.4
Composite Structures
Non-Discontinuity Vehicle 1.4 1.2
Non-Discontinuity On-Orbit 1.5 1.2
Discontinuity Vehicle 2.0 1.2
Discontinuity On-Orbit 2.0 1.2
Ceramics and Glass
Static Test and Analysis (non-pressurized) 3.0 1.2 Static Test and Analysis (pressurized) 3.0 2.0 Analysis Only (non-pressurized) 5.0
Structural Bonds Bonded to Glass (Analysis and Test) 2.0 1.2
Other (Analysis and Test) 2.0 1.2
Table 2.10: Factors of Safety for Express Rack Payload Analysis
Quasi-Static Launch Loads
The experiment shall remain contained and intact in order not to present a hazard after being exposed to the launch accelerations documented in table 2.11 ([1, p. 3-3]). Due to the early phase of development of the project, the final orientation of the stored experiment on board the cargo is not yet defined. In order to guarantee a conservative approach to the load analysis, the maximum value of the acceleration, between the ones reported in the table, is assumed to act in each direction: +/- 11.6 g.
Nx (g) Ny (g) Nz (g) Rx (rad/sec2) Ry (rad/sec2) Rz (rad/sec2) Launch +7.7/-10.2 +/- 11.6 +/- 11.6 +/- 13.5 +/- 8.8 +/- 11.5
Table 2.11: Launch Load Factors Envelope for Pre-Determined Orientation
Note:
The reference frame for the ground handling and transportation load factors with respect to the directions of motion is as follows:
• X: Longitudinal direction along axis of motion.
• Y: Y axis is perpendicular to the x and z axes and completes the right handed coordinate system.
• Z: Z axis is perpendicular to the x and y axis. Positive direction is vertically upward. Gravity (g) is acting in the z axis in the negative direction.
Random Vibration Environment
The PBR shall survive to the Random Vibration environment defined in [1]. In the section of the document related to Random Vibration loads for items packed
in foam inside soft-stowed bags, different frequency Power Spectral Density (PSD) curves are defined w.r.t. all possible launchers. Since at the time of writing the launcher has not been defined, following the already prescribed conservative approach, a PSD curve is outlined (figure 2.4), enveloping all launch vehicles, for the unattenuated flight conditions.
Figure 2.4: Unattenuated PSD Envelope
Data and guidelines for the calculation of the attenuated flight environment due to soft packaging configuration are also provided in [1]. According to these data, not only the random environment is significantly attenuated in the upper fre-quency range (while it is slightly amplified in the low one), but also shock and sine environments are greatly reduced. A curve, similar to the previous one, is reported, enveloping any possible type and thickness of foam, providing the foam attenuation factor for the whole Random Vibration frequency range (figure 2.5).
Figure 2.5: Foam Attenuation Factor Envelope
The resulting attenuated environment, reported in the table 2.12, is given, for fi = [20,...,2000] Hz, by:
where:
P SDi,attenuated New attenuated random environment at fi [g2/N]
P SDi Unattenuated launcher random environment at fi [g2/N]
AFi Attenuation Factor at fi
Unattenuated Envelope Foam Attenuation Factor Foam Attenuated Envelope
Frequency Level Level
Hz g2/Hz g2/Hz 20 0.02 5 0.1 50 0.02 5 0.1 120 0.031 0.36 0.0112 142 0.031 0.22 0.0067 200 0.05 0.078 0.0039 500 0.05 0.005 0.00025 1000 0.025 0.005 0.00025 2000 0.013 0.005 0.000065 Overall: 7.5 GRMS 2.55 GRMS
Table 2.12: Unattenuated and Foam Attenuated Envelopes, Foam Attenuation Factor
Figure 2.6: Attenuated and Unattenuated PSD Environment
In the figure 2.6, the Unattenuated Environment envelope curve and the Atten-uation Factors envelope one, together with the resulting Attenuated PSD En-vironment (blue line), are represented. In the graph it appears clearly that, for frequencies below 90 Hz, the foam has the effect to amplify the Random Vibra-tion environment, while it dampens the enveloped component, for higher than 90 Hz frequencies.
Belt Loading
Any payload considered to be stored inside soft bags shall withstand a maximum belt preload of 340 N, that results in a total force of 640 N applied on the cargo bags during belts tightening for cargo securing inside the Integrated Cargo Carrier (ICC) racks. The belt preload shall be considered as compressive load acting on any external surface of the payload structural components/equipments.
Even if the effects of the belt preload shall be combined with the quasi-static acceleration load environment, due to the highly conservative approach chosen, this load is assumed to be covered by the Limit Load Factor which takes into account the Quasi-Static Loads and the Random Vibration Loads (given by the Miles’ equation).
2.2.3 Crew Induced Loads
The PBR permanent crew exposed parts and surfaces shall provide positive mar-gins of safety when exposed to the crew induced loads defined in table 2.13, according to [1, p. 3-32]. CREW SYSTEM OR STRUCTURE TYPE OF LOAD LOAD DIRECTION OF LOAD Levers, Handles, Operating Wheels, Controls Push or Pull concentrated on most extreme edge
222.6 N, limit Any direction
Small Knobs Twist (torsion) 14.9 N-m, limit Either direction
Exposed Utility
Lines (Gas, Fluid, and Vacuum)
Push or Pull 222.6 N Any direction
Rack front panels
and any other
normally exposed equipment
Load distributed over a 101.6 by 101.6 mm area
556.4 N, limit Any direction
Table 2.13: Crew Induced Loads
2.3
Safety
The MDL is qualified for all current launchers, soft stowed in M01 bag within the certified M01 bag allowable. During the experiment lifetime, both launch loads and operational loads apply, where the latter are negligible, since that no sufficient inertia loads exist in orbit.
Structure elements, in case of failure or deformation, may result in critical or catastrophic events leading to violation of the PBR MDL rack interface or dy-namic envelope, loose debris and particles which could damage the Express rack or cause injury to the crew. Potential failures are excluded by usage of standard Lockers within their certified capabilities.
The Locker front panel has three openings. One of the two MDLs is modified for the CO2 Supply Module unit, in order to allow for full access to its diverse front control functions. The other Locker front panel is covered by the PBR panel, so that no potential loose parts can come out.
Structural hazards are controlled mainly by application of design rules, safety factors, material and components selection in accordance with applicable safety requirements. Vibration tests will verify proper manufacturing and assembly process. It shall be performed on PFM level. Vibration testing is intended for the mission success, and is not a concern of safety.
Finite Element Model
3.1
General Approach
The Finite Element Method (FEM) is a form of discretization of reality, a way to simplify and allow finding a mathematical solution for a physics problem. The first step consists in examining the real model, in order to find possible simplifications, which will not affect the analysis but at the same time will grant an easier translation of the geometries into FE.
Following this statement and working directly on the Computer Assisted Design (CAD) model, all the chamfers and fillets are deleted. As far as possible, the curved lines are replaced by straight or multiple broken lines. Later in the analysis particular attention is paid to these modified regions, because, clearly, every curve or fillet replaced by a sharp edge, represents a stress concentration area. In case of peaks in the stress spectrum occurring in these areas, further analysis have to be carried out.
The purpose of the explained process is to create regular shapes and geometries. This helps, during the mesh creation, to avoid generating distorted Elements, which can produce errors and unrealistic results in the analysis.
The next step is to import the simplified CAD model into the pre-processor as an assembly made out of solid components. The pre- and post-processor used for the analysis is MSC Patran. As a general guideline, in order to lighten the analysis, all the solid parts have to be represented by Shell Elements (2D triangles or quadrilateral). To satisfy the assertion, the geometry is developed from the mid plane of every single original component.
Once the geometry model is completed, the surfaces are meshed, using bi dimen-sional Elements. Where needed for specific connection strategy, also 3D Elements are employed. Fasteners, modelled with Spring and Rigid Body Elements, are developed to join the different components.
In order to guarantee mass, inertia and rigidity to be similar to the real model, properties like material, thickness, offset from the plane and non-structural mass (NSM) are then assigned to the meshed surfaces.
Checks to be performed to asses the correctness of the developed model are described in chapter 4. Once the checks are passed, the model is ready to be constrained and loaded, in order to obtain analysis results, 5 and 6.
Figure 3.1: Final PBR Design
3.2
Geometry Definition
The first aspect to define is the Model unit system, in accordance with the S.I. units:
• Newton [N] for force • Kilograms [kg] for mass • Metres [m] for length • Seconds [s] for time
• Degrees Celsius [◦C] for temperature
with the derived units:
• E-modulus [Newton/meter2]
• density [kg/meter3]
Once the CAD file is opened in Patran, all the components which constitute the experiment, imported as solids, are individually placed in groups, named “CAD_” followed by the name of the element. In the following steps each part is elaborated in a “GEOM_” group and then in a “FEM_” group. they respectively includes the geometry and the element/nodes with which the part is modelled. This procedure makes it faster and better organized the task of modelling, allow-ing to regard every part as a sallow-ingle entity.
It has to be noted that all the minor components, such as pumps, valves, sensors and tubes, are neglected in the geometry definition. They have been taken into account as NSM, distributed over the surfaces to which they are fixed.
Another assumption made at this phase is about the modelling of the Coldplate. This part is structurally and thermally relevant. It consists of a thick plate, which the EC and the E-Box are attached to, for heat dissipation purposes. The back also houses the Pulse chamber. The plate acts for the two major units as a heat sink, by keeping their temperature inside allowable ranges. The heat exchange occurs by means of a tube that winds back and forth in a proper meander milled on the plate, figure 3.3. The tube is not directly in touch with the other parts, but it is covered by a plate, with milled meanders as well, on the other side. The tube is fixed to the two plates with a thermal glue, which allows higher heat conduction. The tube is connected to the water loop of the ISS, granting heat rejection from the experiment.
The assumption made is not only to omit the tube in the modelling, but also the covering plate, as well as the meander on the Coldplate itself. The goal is to simplify the FE model, taking into consideration the omitted parts again as NSM. The particular configuration, with a plate and a tube connected to the main plate, can only add stiffness to the latter, so the basis for this hypothesis is a worst case scenario: if the analysis, as it is later showed, demonstrates that the plate alone is rigid enough to carry the loads, the real disposition can only be stiffer.
Figure 3.3: Coldplate Cooling System, Exploded View
Inside each geometry group the procedure to create the basic geometry, starting from the imported solids, follows some essential guidelines. The mid plane of each of the surfaces constituting a solid is identified first and, with different methods depending on the specific shape, a surface is formed. Once all the mid surfaces are designed, the intersections between them are identified, in order to tailor the new surfaces to the adjacent ones. The surfaces are then cut into basic shapes, figure 3.4 and 3.5, trying to keep the edges of this shapes lower than five, in order to later make it easier to mesh them with Isomesh command, to have more regular Elements.
Figure 3.4: Frontplate, CAD Solid Part
Figure 3.5: Frontplate, Surface Cut into Basic Shapes
bolt holes. From a geometry point of view the surface on which such a hole is present is intersected with a circle of the same diameter of the hole, two surfaces are generated, and the inner disc is deleted. The second step is to generate a second circle, concentric to the first one but with a slightly larger diameter, and create a new circular shape surface (intersecting the original one with the new circumference) enclosed by the two circles, figure 3.6. The reason for this choice is to define more easily the influence region of the bolt, defined by the bolt head diameter or by the washer diameter.
Figure 3.6: Example of Bolt Head Geometry
One of the main criteria with which the surfaces are created, besides the easier meshing they could allow for, is the perspective of the property that has to be assigned to them. In a region where, for example, different levels of thickness are required, the edges of the basic surfaces have to follow the boundaries of these areas. When the properties have to be defined, this philosophy makes it faster to assign them to the Elements. A Surface is in fact associated to the property, so that all the Elements belonging to it directly acquire the material, the thickness, etc., defined by the property.
Before starting to model each component completely, attention has to be paid to their symmetry. The identification of planes of symmetry between regions of the same unit, as well as between different items saves time in the modelling. As an example the Superabsorber is modelled just for one forth of it, having planes of symmetry in coincidence of the middle of its length and depth. Another case is the four brackets of the structure: it is sufficient to model one of them and then mirroring the others. It is worth pointing out that all the mirroring or translation operations are executed at group level, once the surfaces, the mesh and the properties are defined.
3.3
Mesh Definition
By the time every component is represented by elementary surfaces, their meshing is performed. The reference size of the Elements is assigned in the Global Edge Length input field, under the Finite Element form. This dimension is chosen to be 0.005 m, agreed as a good compromise between the computational cost (in terms of solving time) and the quality of the results.
Depending on the number of edges the surface has, it is possible to use the Isomesh or the Paver command. The first one offers a more regular distribution
of the Elements and can be used only on surfaces with three or four edges. It has been taken into account that this is not always the best meshing option, especially when the area is small or the corner of a surface is much smaller than 90◦, because it could originate too elongated Elements. The Paver command, on the other hand, is used in case of surfaces with five or more sides. The decision between the two alternatives is carefully made, and if the result is regarded as not satisfactory, from the distribution and distortion point of view, the mesh is recomputed with iterative attempts, slightly varying the reference size of the Elements or the Mesh Seed.
The mesh performed is pictured in detail in the appendix B. Further details about the type of Elements and strategy used are described in the following sub-paragraphs.
3.3.1 Mono-dimensional Elements
The 1D Elements used are CBAR and CBUSH. The CBAR ones are simple beam Element connections, used in the modelling of the Superabsorber. The cross section assigned to them is a ROD type, a circular section. They are employed on the lower part of the component to realistically represent the structural cylinders which house the screw needed to refer the upper shell with the lower one, figure 3.7. In order to match the mass and inertia characteristics of the component, the diameter of the ROD, figure 3.8, is the same of the cylindrical parts on the Superabsorber.
Figure 3.7: Detail of the Cylindrical Parts on the Lower Shell of the Super-absorber
Figure 3.8: Detail of the CBAR Ele-ments on the Lower Shell of the Super-absorber
The CBUSH Elements, according to [4], are Generalized Spring-and-Damper Connections, and they are here used in their two coincident nodes configuration. Their employment is better described in section 3.3.4.
3.3.2 Bi-dimensional Elements
The shell Elements CQUAD4 and CTRIA3 are the only 2D Elements used, figure 3.9. They are respectively constituted by four and three nodes. As a guideline the application of quadrilateral units is pursued, because it helps to uniformly
spread the loads at a node point. When a CQUAD4 Element is showed to be inadequate, as a result of a performed check (see paragraph 4.1), the alternative is found in the triangular Elements, to be used with attention, because they add stiffness to the region they are applied to. Frequently the CTRIA3 Elements are originated by the division of a quadrilateral one, along its shorter diagonal, with the proper tool that allows to split a quad into two tria, figure 3.10 [5].
Figure 3.9: Example of a region with both CQUAD4 and CTRIA3 mesh, detail of the rhAbsorber
The strategy to properly mesh the surfaces and have coincident nodes along their sides is to create a surface starting from the edge of an adjacent one, so that the line the two areas share, is assigned to both the surfaces. This can be performed for example with the Glide command, under the Geometry form. In case the surface is created following a different procedure, the tool used is the Mesh Seed option. The selected feature of this option, Tabular, figure 3.11 [5], allows to impose on a line a seed, generated from the position of the nodes which are already present on this edge, due to the mesh of the adjacent area.
Figure 3.11: Patran window for Tabular mesh seed
In case a region includes a hole, modelled as in figure 3.6, the first action per-formed is to assign a uniform mesh seed, specifying a total number of Elements
equal to six, to both the circular lines, which form the annular surface. The use of Paver mesh option, creates a sequence of six equally trapezoid CQUAD4 Elements, as in figure 3.12. This process implies that the size of these Elements is proportional to the diameter of the hole and the number of Elements imposed. According to this statement, the number of six is evaluated to be coherent with the Global Edge Length of the model.
Figure 3.12: Example of hole Element modelling, detail of the Coldplate
3.3.3 Three-dimensional Elements
Three-dimensional Elements created in the modelling phase are CHEXA and CPENTA, respectively six and five sided solid elements. The procedure followed for their development is not to automatically mesh a solid geometry, but to extrude instead shell Elements along the desired direction. This insures a highly regular Elements modelling, because they are originated from a surface that is already meshed. It is clear that the extrusion of quad Elements creates CHEXA, while CPENTA have tria Elements as starting units.
The use of solid Elements is limited to two kinds of area. The first one is rep-resented by all the regions where fixation points are needed, between a surface and another one perpendicular to the first one. In this case, the demand for a sufficiently regular set of nodes necessary to place an MPC, see paragraph 3.3.4 - Fasteners, is fulfilled with 3D Elements frames, such as the ones on the Frontplate, the Coldplate and the Backplate, figure 3.13.
In order to guarantee a correct connection of the extruded solid Elements with the 2D ones of the component they belong to, the starting Elements used for the extrusion are not deleted. This creates a shell basis for the solid units, able to provide them with a realistic constraint. On one hand, if the starting Elements are deleted, the line of nodes that shell and solid units share, acts as an hinge, since these nodes are prevented from translating, but not from rotating. On the other hand, not cancelling them creates in the superimposition region a layer of unwanted material, a thicker (i.e. heavier) part not compliant to the real structure. To avoid this happening, the solution is found by assigning a specially created material to the shell layer property, the so called Dummy Aluminium, paragraph 3.4.1. The layer in this way gets a material with zero density, but same Elastic Modulus and Poisson ratio as the employed Aluminium has. Preserving also the same thickness as the rest of the plate, this region maintains the proper stiffness, without the side effect of a locally increased mass.
Other examples related to these connection needs, are the fixation points between the EC-Housing and the O2Absorber (red blocks on top of EC-Housing, figure
Figure 3.13: Three-dimensional Elements on the frames of the main plates
B.2), and the polycarbonate reactors, to be connected with their support struc-tures (dark blue layers in figure B.7).
The other employment of solid Elements is required to meet the fixation strategy (paragraph 3.3.4 - Experiment Compartment Modelling) for the different com-ponents belonging to the EC, figure 3.14. The development of mid-surfaces from the solids representing the real parts, involves the resulting surfaces of originally touching components not being in contact any more. According to the above mentioned strategy, boundary nodes of different units need to be exactly in the same position (to allow equivalencing of them). To avoid possible offset between the various nodes of respectively the Housing, the Window and the EC-Window Frame on one side, and the EC-Housing, the EC-Back Insulation Frame and the EC-Backplate on the other side, the middle components (EC-Window and EC-Back Insulation Frame) are then constituted by 3D Elements, figure 3.15.
Figure 3.14: Experiment Compartment section view, different units outlined
Figure 3.15: Experiment Compartment section view, FE model
3.3.4 Connections
When all the components are meshed, a connection strategy is envisaged, to actually assemble all the units, in a way that can resemble as far as possible the physics of the problem.
Fasteners
The M6 and M3 screws and bolts present in the experiment are modelled ac-cording to the following approach. At first two coincident nodes are created, in correspondence of the holes center, which is projected on a mid-plane equally distant from the two regions to be connected. A CBUSH Element is used to link the two nodes. Finally each of the two nodes is connected to the hole surfaces with a Multi Point Constraint (MPC), formed by a web of Rigid Body Elements, RBE2s, figure 3.16. The webs have the CBUSH node as an independent one, and the twelve nodes of the annular hole mesh as dependent ones. According
to [4], the independent node is the one the degrees of freedom of which remain during the solution phase, while a dependent node has its degrees of freedom condensed out of the analysis before solving the system of equations. In partic-ular the RBE2 allows the selection of the degrees of freedom for the dependent node that have indeed to be regarded as dependent. In the modelling all the six degrees are selected, granting that all the forces and displacements occurring on the independent node are rigidly transmitted to the dependent ones.
Figure 3.16: Example of MPC-CBUSH connection
Reactors Fixation Strategy
The design plans the different layers of the Reactors to be in touch one with the other, kept in place by cardlock retainers. They are placed in ten guides, five on the top and five on the bottom, structurally integral with the EC-Housing. To simplify the FE model, the retainers are not represented. The solution to properly replace their function takes into consideration the use of RBE2s. They are used to connect the nodes of the ribs of the retainer guides, one at a time, with the corresponding nodes of the underlying Reactor surface. In the same way, following the ideal load paths of this sandwich-like structure, all the nodes of a layer are connected to the successive one, figure 3.17. In the model, by means of RBE2s, also the screws fixing the Reactors polycarbonate layers with the so called Support structure (see figure 1.4) are characterized.
Figure 3.17: Connection between Reactors and EC-Housing, section view
Experiment Compartment Modelling
In a simplification perspective, as described in paragraph 3.3.3, the connection of the different EC units is performed by equivalencing their coincident nodes, which are in correspondence to the touching surfaces. The equivalencing operation can be regarded as soldering the involved components one with the other. The assumption which supports this hypothesis is the high number of screws designed to join the parts (40 on the front and 40 on the back), which grants a low margin of error, since they provide for a continuous touching frame on both sides of EC-Housing.
3.4
Properties Definition
3.4.1 Materials
The materials used and their properties are reported in table 3.1, according to [6] for metallic materials, [7] for polycarbonate and [8] for the other polymer. The PC-Tecanat is a special polycarbonate for space use, developed by the company Ensinger GmbH. The EC-Window and the two Reactors are made out of it. The only Steel (AISI316) parts are the two Superabsorber supporting brackets and the EC-Window frame. The EC-Back insulation frame is formed by a polymer, the Polyoxymethylene (POM), for insulation purposes. All the other components are made out of Aluminium (Al7075-T7351). As mentioned before, in the table is listed also the Dummy Aluminium, with the same properties of the Al7075, but zero density.
Material Al7075-T351 AISI316 PC-Tecanat Dummy Al POM Elastic modulus [N/m2] 7.1x1010 2x1011 2.2x109 7.1x1010 2.6x109 Poisson ratio 0.33 0.265 0.3912 0.33 0.3858 Density [kg/m3] 2795 8027 1190 0 1390
Yield strenght [MPa] 393 476 - 393 -Tensile strenght [MPa] - - 69 - 71.5 Ultimate strenght [MPa] 475 855 - 475
-Table 3.1: List of materials
3.4.2 Surfaces Thickness
All the properties, with the related thickness, created for the FE model and assigned to the surfaces, are presented in the table in appendix C.
3.4.3 Non-structural mass
The tool Mass Properties is used to evaluate the mass of every group, representing each one of the components, and formed by Elements with different properties. The result is compared with the Mass Budget, see appendix A. The difference between the model mass and the real unit one is balanced by means of NSM. This is a field present in the shell Elements properties, that is used to evenly distribute over all the CQUAD4 and CTRIA3 surface the mass that is missing. Once this value is found, the Element surface for the given property is evaluated with a proper utility. The NSM voice has to be filled in with the mass per unit surface, i.e. dividing the mass by the computed surface area. In the table in appendix C, the last column presents the NSM values expressed in kg. They take into account also the mass of components not modelled with FE, such as tubes and pumps. The mass of these units is distributed over the model surfaces that are in touch or in proximity with them. This is the case of the Large membrane pump, the mass of which is assigned to the left wall of the E-Box. The Elements of the wall have the same material and thickness of the other E-Box side walls, but the NSM related to the pump is assigned only to it (this is testified by the blue colour of the Elements on the right side of the E-Box in figure B.16).
3.4.4 CBUSH Elements Properties
The CBUSH Elements, used for fastener connections, have to be carefully defined. It is common practice to define a coordinate frame which has the z-axis along the main axis of the screw. Doing this, the 100, 200 and 300 coordinate frames are created. The outlined procedure makes it easier to read and analyse bolt force data when they are listed in a report format. In the z direction is then possible to read the axial force acting on the fastener. Once the bush orientation is defined, the second important step is to assign the Spring constants to the Ele-ment. Supplementary “stiff” CBUSH Elements are required at interface points to recover interface loads. According to [9], in order to have differences on the eigenfrequencies between the original structure and the FE model less than 0.1% a stiffness of 1010 N/m for the translational degrees of freedom and 108 Nm/rad for the rotational ones is assigned, in the six Spring constant fields.
3.5
Total number of Elements
At the end of the meshing process, the overall amount of Nodes and Elements is the one listed in table 3.2.
Total number of Coordinate frames 4
Total number of Nodes 193319
Total number of Elements 177940
CBAR Elements 18 CBUSH Elements 211 CHEXA Elements 28991 CPENTA Elements 34 CQUAD4 Elements 142441 CTRIA3 Elements 294 RBE2 Elements 5951
Model Checks
Validity checks and controls to verify the meshed model are here presented. They are developed according to [9] and [10]. They are defined as checks that ensure the model is mathematically accurate. Validity checks do not ensure the accur-acy of the model in representing a physical system, just that the model will give mathematically correct results. The procedures, described in detail in the follow-ing, are internal to the pre-processor, Patran (paragraph 4.1), or involves specific input files (.dat) to be executed with the solver, Nastran (paragraph 4.2).
4.1
Patran Verification Tools
4.1.1 Verify Normals
It is advisable to control that all the surfaces created in the geometry en-vironment have the normal in the same direction, with the tool in figure 4.1 [11]. It has to be verified before modelling the mesh, in order to guar-antee that all the subsequently cre-ated Elements have also the consist-ent normals. The direction influences the offset defined for the property, i.e. the mass and inertia distribution.
Figure 4.1: Tool to check and reverse surfaces normals
Once the mesh is created the visual verification of this feature can be exploited both with the 3D Plate Thickness utility, figure 4.2, which shows offset and thickness of selected Elements, and with the Verify Normals tool, figure 4.3 [5]. The last one pictures the Element normal vectors and, differently from the utility, allows to reverse them.
Figure 4.2: 3D plate thickness utility Figure 4.3: Verify Element Normals tool
4.1.2 Equivalence - Free Edges
Element free boundaries of the model are checked to ensure that there are no unintentional gaps or disconnec-tions in the mesh or missing Ele-ments. Free edges are displayed in Patran with the tool in figure 4.4. The disconnection between ad-jacent Elements is frequently solved via equivalencing of the nodes. The creation of a mesh in following steps for two touching surfaces originates the doubling of the nodes along their boundary line. The node Equivalen-cing tool solves this problem, mak-ing the touchmak-ing Elements share the same nodes. According to [5], equi-valencing is the process of reducing all nodes that coexist at a point to a single node. This feature is em-ployed also to create bonded connec-tions, as described in the EC con-nection strategy. The only coincid-ent nodes acceptable in the model (i.e. not be equivalenced) are the in-tentional ones. It is the case of the spring elements used to extract loads (as explained in 3.3.4, such springs must have zero length).
To avoid unwanted equivalencing of the nodes, the object is defined to
be Group. The Tolerance Cube
method uses a cube and is the default method. If Tolerance Cube is selec-ted, then two node points are equi-valenced if all of their coordinates in the global Cartesian frame lie within the tolerance of each other. The node with the lower ID is always retained. It is good practice to visually pre-view the Nodes to be equivalenced, to avoid unwanted deletion.
Figure 4.4: Verify Free Edges - Equi-valencing tool
4.1.3 Verify Element Geometry
Element geometry checking is a standard feature which measures quantities such as taper, skew angle, warping, aspect ratio, Normal and Tangent offset, figure 4.5.
Figure 4.5: Verify Quad Element geometry
During the checking phase the parameters which have been carefully taken into consideration are Skew, Taper, Aspect ratio and Warp
Skew is the angle (figure 4.6) between lines that join opposite mid sides of an Element. It is recommended this angle to be greater than 30 degrees (as a reference an angle of 90 degrees represents no skew).
Taper ratio for the quadrilateral Ele-ments is defined as follows. Four tri-angles are created bounded by the element edge and the edges created by connecting the element verifica-tion reference frame origin with the two nodes at the element edge. The resulting four triangular areas (figure 4.7) are calculated and then summed. The ratio of the area with the smal-lest triangle and the total area of the element is taken as the taper ratio. Note that as the ratio approaches 0.0, the shape approaches a rectangle. It is recommended that this ratio is less than 0.5.
Figure 4.7: Taper
Aspect ratio (figure 4.8) is the ratio of the length of any two sides on a CQUAD4 element. It is recommen-ded that this value is less than 4.
Figure 4.8: Aspect ratio
Warping of shell elements occur when the connected grids are not in the same plane. The warping value is determined from the distance of the corner from the mid plane of the grids and the sum of the diagonals (figure 4.9). When the warping value ex-ceeds the defined tolerance a warning message is provided.
Figure 4.9: Warp angle
Elements that violate the above conditions may not necessarily be incorrect. The same Geometry check is performed for triangular Elements. The way the solid elements are created and their reduced quantity, allows to neglect their Geometry checking.
