P19127: XHab Deployable Crew Lock Structure
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Preliminary Detailed Design

Table of Contents

Team Vision for Preliminary Detailed Design Phase

During the Preliminary Detailed Design phase, the team focused on breaking down the whole system concept for the Crew-Lock, into subsystems. These subsystems would then be better researched, defined, designed, and analyzed for feasibility and acceptance throughout this phase. The goal of this phase was not necessarily to come up with a concrete design and Bill of Materials (BOM), but to let ideas for each subsystem be discussed and then come up with enough analysis on the subsystem to properly present to the customer and our guide for critique.

Subsystems Design Chart

Due to the complexity of the project, at the beginning of this phase the team created a diagram documenting the way subsystems would be broken down. Each subsystem is connected to one another to form the whole Crew-Lock; however, each subsystem has components and focuses that required more detailed research and analysis before inclusion into the whole Crew-lock concept. Below is a Subsystem Design Tree which breaks down the subsystems further into detailed components or mechanisms that need further design consideration during this phase. The tree was created using both the Functional Decomposition Tree and team discussions on the subsystems at the beginning of this phase. The Subsystems Design Chart can be seen below:
Subsystem Design Tree

Subsystem Design Tree

Phase III Technical Changes

Change Management Documentation

Below is a spreadsheet used to track major changes to the project. From design to requirement changes - this document will allow the team to document items for future reference and accountability. This Excel sheet can be downloaded the Design Review Materials Section at the bottom of this page.
Change Management - Phase III

Change Management - Phase III

Engineering Requirement Changes

After further analysis of the original NASA project memo, it was discovered that the NASA requirements for the deployable crew-lock only had a maximum operating pressure of 0.1 - 0.5 psig. This significantly reduced both the pressure stresses in our design, and allowed us more room to work on the deployment mechanism, ergonomics, and the air reclamation and pumping system rather than the mechanical stability at the original 14.7 psig. Below are the changes made to reflect this. The updated Engineering Requirements can be downloaded the Design Review Materials Section at the bottom of this page.
Customer Requirements - Phase III Removed

Customer Requirements - Phase III Removed

Engineering Requirements - Phase III Modified

Engineering Requirements - Phase III Modified

Project Requirements Confirmation

Below is a document that outlines the fulfillment of Engineering and Customer Requirements by Subsystems and Test Plans. This Excel sheet can be downloaded the Design Review Materials Section at the bottom of this page.
Engineering and Customer Requirements Confirmation Matrix

Engineering and Customer Requirements Confirmation Matrix

Feasibility: Prototyping, Analysis, Simulation

Many of the analysis are theoretical calculations using conservative material properties and assumptions. Software to perform analysis like FEA, require years of experience to be correct in terms of items like contacts, meshing, tolerances, etc. In addition, FEA is not accurate or over-estimates stresses when complex geometries and holes are present. Calculations provided by internationally recognized standards with factors of safety can provide a reasonable and conservative estimates for manufacturing guidelines. The team would rather over-design for safety than under-design, so these calculations are more assuring than a poorly run FEA simulation.

Bulkhead Material

The bulkhead will be made with wood, due to its relatively lightweight properties and easy work-ability. The specific type of wood the team will be using is Medium Density Fiberboard (MDF). It is made up of separated fibres, but can be used as a building material similar in application to plywood. It is stronger and much denser than particle board and plywood. MDF does not contain knots or rings, making it more uniform than natural woods during cutting and in service. MDF has the following benefits relative to other wood types:
  1. Some varieties are less expensive than many natural woods
  2. Consistent in strength and size
  3. Shapes well
  4. Stable dimensions (less expansion and contraction than natural wood)
  5. Takes paint well
  6. Takes wood-glue well
  7. High screw pull-out strength in the face grain of the material
  8. Can be epoxy-painted to resist moisture and air ingress

The mechanical properties of MDF can be seen in the table below.

MDF Mechanical Properties

MDF Mechanical Properties

The epoxy paint used will have to cover 50-60 feet squared of area. The epoxy paint will have a 10 mm coat. This approximates the amount of epoxy paint we will need to 1/2 a gallon. The epoxy paint and MDF sources can be seen on the Very Low and Low Budget BOM.

Vacuum Barrier-Bulkhead Airtight Sealing

Sealing Material
The sealing material selected was Closed-Cell High-Temperature Silicone Foam Rubber. The reason this material was selected for airtight sealing was due to several reasons:
  1. One of the mixed rubbers used by NASA for airtight sealing is a Silicone Rubber based blend according to NASA/TM—2010-216332.
  2. Silicone Rubber is a widely available and commonly used airtight sealing material that is manufactured to ASTM standards.
  3. Silicone Rubber has low compression set, low outgassing, and low creep relative to other airtight materials such as EDPM, Nitrile, and and Viton rubbers. In the graph below, the compression set, based on creep testing, is compared to several common rubbers and foams at the same compression loads:
Compression Set of Silicone Rubber

Compression Set of Silicone Rubber

Gasket Cover Material
The selected gasket covering material is 6061 Aluminum. This material was selected for airtight sealing for several reasons:
  1. 6061 Aluminum is a cheap and widely available metal manufactured to ASTM standards.
  2. 6061 Aluminum has a relatively high Young's Modulus (69 GPa) compared to Silicone Rubber's Compression Modulus (10 MPa).
  3. 6061 Aluminum is highly machinable, and resists high temperatures and corrosion.
Bolt Torque and Thread Engagement
In order to secure the sealing material and keep it under compression for an airtight seal, bolts must be correctly torqued. Correctly torqued bolts are an important aspect to any engineering design; however, due to manufacturing limitations, the team decided that the thread engagement length and torque should be provided as manufacturing guidelines. This decision is used to make sure that at least one or both of the methods are used to ensure an acceptable mating of the Vacuum Barrier and Bulkheads.

Some preliminary analysis data that must be accounted for is the Compression Modulus of Silicone Rubber. Below is a graph of the Durometer rating of 50A Silicone Rubber at room temperatures:

Silicone Rubber 50A Durometer Compression Modulus

Silicone Rubber 50A Durometer Compression Modulus

The formulas, parameters, assumptions, and results are seen in the analysis below:

Wood Screw Thread Engagement and Torque Analysis

Wood Screw Thread Engagement and Torque Analysis

Bolt Spacing
The minimal distance between bolt diameters for the Vacuum Barrier to Bulkheads gasket compression is found using Robert's Formula. This formula is a further improvement of the TEMA standard formula for bolt spacing of a gasket. Roberts provides a more realistic formula of bolt spacing based on the use of a beam on an elastic foundation to obtain gasket contact stress between bolts. The formula achieves a bolt spacing that maintains a 95% contact stress midway between bolts.

The Robert's Formula is based on the convergence of the superimposition of bolts forces and gasket stress. This relationship can be seen in the plot below:

Robert's Formula Elastic Foundation Convergence

Robert's Formula Elastic Foundation Convergence

The formulas, parameters, assumptions, and results are seen in the analysis below:

Vacuum Barrier Bulkhead Sealing Analysis

Vacuum Barrier Bulkhead Sealing Analysis

Despite obtaining a value of 1 bolt every 2.70 inches, the team added a 1.2 factor of safety to this value in order to compensate for human and manufacturing errors during the building and test phase of the Crew-Lock. The final H value is closer to 2.25 inches when factor of safety is considered.

Hatch Sealing

The airtight sealing of the EV and IV hatch under current budgetary, manufacturability, technical ability, and time constraints is not guaranteed. Analysis of various rubbers and their Durometer ratings (compression modulus) show that an astronaut would not be able to seal the hatch without forces that well exceed human capability. Please see "Bolt Torque and Thread Engagement" for the Durometer rating of 50A Silicone Rubber at room temperatures.

With this Compression Modulus, it is not possible for an astronaut to compress the silicone foam rubber to an airtight state without a considerable mechanical advantage. NASA airtight hatch designs, and even those for earth-based pressurized chambers or maritime-based vessels, are complex and difficult to manufacture or procure without considerable expertise and funds. Below is an example of the complexity of such hatches:

Current ISS Airlock EVA Hatch Design

Current ISS Airlock EVA Hatch Design

These commercially available air-tight hatches are costly, made entirely of metal, and contain complex geometries that cannot be easily manufactured by the team. In light of this, the simple geometry of the hatch and the silicone rubber bulb-seal is used for a semi-airtight seal. The bulb-seal is easily compressible, and used in many commercial applications to prevent the ingress of water, air, dust, fire, etc. The ease of compression allows for an ergonomic closing and latching of the hatch. In addition, the relatively low price and ease of installation (adhesive-based mating) allows the team to focus on the other important aspects of the Crew-Lock - the deployment mechanism and the ergonomics. The leak assessment deliverable and vacuum barrier sealing will ultimately be more important than the actual air-tightness of the hatch. The bulb seal will surround the entire perimeter of the hatch and will be dove-tailed to achieve the greatest seal at the corners. A cross section of the compressible bulb seal can be seen in the CAD rendering below as the black rubber attached to the wooden door frame:

EV and IV Hatch Bulb-Seal Design

EV and IV Hatch Bulb-Seal Design

Air Pump and Vacuum System

The analysis below is a comprehensive analysis of the required air compressor and vacuum pump size for the Crew-Lock to achieve a 0.1 - 0.5 psig internal pressure. The pictures of the analysis are labeled in a numerical order.
Air Pump System - Page 1

Air Pump System - Page 1

Air Pump System - Page 2

Air Pump System - Page 2

Air Pump System - Page 3

Air Pump System - Page 3

Air Pump System - Page 4

Air Pump System - Page 4

Soft Structure

Soft Structure Material Selection
The selected materials for the vacuum barrier were suggested by Dr. Lamkin-Kennard through previous projects using air-tight, inflatable materials. The materials in question are Thermoplastic polyurethane (TPU) and Polyvinyl chloride (PVC) film. Both can be thermally bonded (so that the cylinder of vacuum material can be made), both are manufactured in large amounts, and both are commercially available. Some aspects of these two plastics that the team compared and contrasted are listed in the table below. The properties listed were looked at subjectively through research into each product on various websites. This table was used to guide whether TPU or PVC would work best for our project (green highlighted items are what the material is better at relative to the other material). In the end, PVC film was the choice due to it's cheaper price, and similar mechanical properties.
Vacuum Barrier Material Selection Table

Vacuum Barrier Material Selection Table

Soft Structure Stress Analysis
This analysis is an update of the version proposed in the previous phase. It outlines the stress induced on the soft structure material at the updated pressure of 0.5 psi (maximum). The stress results below have been compared to PVC film - the proposed vacuum barrier and soft material. The PVC film from McMaster-Carr has a tensile strength of 1,900 psi, which is much higher than the hoop and axial stress that will be exerted on the PVC film.
Updated Pressure Stress Analysis

Updated Pressure Stress Analysis

Ergonomics

Handhold Placement
Ergonomic Analysis of Handhold Placement

Ergonomic Analysis of Handhold Placement

Crew-Lock Lighting
Ergonomic Analysis of Lighting

Ergonomic Analysis of Lighting

All analysis can be downloaded from the Design Review Materials section at the bottom of this page, and through the P19127 public directory on EDGE.

Schematics, Drawings, Prototypes, and 3D Models

Hand-drawings

Below is a hand drawing the team used to properly flesh out and show ideas to the rest of the team before further detailed design.
Final Design Hand-drawing prior to Detailed Design of Subsystems

Final Design Hand-drawing prior to Detailed Design of Subsystems

3D CAD Models of Crew-Lock

Below are various CAD model views of the 1/4th scale Crew-Lock concept. The parts have been given mechanical properties and appearances similar to the actual items to be purchased on the BOM. The CAD files can be downloaded from the Design Review Materials section at the bottom of this page.
Crew-Lock Assembly View 1

Crew-Lock Assembly View 1

Crew-Lock Assembly View 2

Crew-Lock Assembly View 2

Prototype Concepts

Below are images of the habitable volume handles idea, as well as the concept proto-type decided upon during this phase.
Concept Model from Preliminary Detailed Design Review

Concept Model from Preliminary Detailed Design Review

Habitable Volume Navigational Aids

Habitable Volume Navigational Aids

Electrical and Controls Schematics

Due to the incomplete deploy / contract system, the team was only able to create a preliminary overview of electrical and control systems. For more details on what electrical hardware were selected, please look in the BOM.

Below is an Electrical / Control System Flow Chart.

Electrical / Control System Flow Chart

Electrical / Control System Flow Chart

Below is a Programming Flow Chart for the Control Systems.

Electrical / Programming Flow Chart for the Control Systems

Electrical / Programming Flow Chart for the Control Systems

Below is an example of a Graphical User Interface (GUI) to be used for controlling the Crew-Lock.

GUI Example

GUI Example

Testing Rig Schematic

Below is a rough schematic of how the testing rig would be set-up if the Very Low budget option (detailed in BOM section below) it selected for the project. Due to the high cost of motors to actuate the rods for the Crew-Lock, the testing rig would simulate the actuation by pushing the EV Bulkhead away from the IV Bulkhead via a motor-driven wheel platform. The rods would still move how they would if they were the ones pushing the EV Bulkhead out during deployment.
Testing Rig Schematic

Testing Rig Schematic

Bill of Material (BOM)

Below are our detailed BOMs with options of Very Low, Low, or High budget for the Crew-Lock subsystems. These documents include explanations, costs, and sources for each item. the 3 tiered budget is to be used to explain budgetary, manufactureability, and technical ability constraints to the customer and guide during the design review. The Excel sheet can be downloaded from the Design Review Materials section at the bottom of this page.

Below, green-highlighted items on the Very Low budget BOM indicate changes from the Low budget BOM. The Low budget BOM is the one the team would like to aim for; however, if funding is not available then the Very Low budget BOM will be used as a last resort. Items with "0" quantity are items that the team hopes to find for free in the MSD, ID, Construct, or Machine Shop areas. In addition, this budget assumes that team members will be able to procure cheap or free items from the scraps of companies, or from the parts of thrift store items (i.e. Using a dining room table from Goodwill as the Wood Stock for the Bulkhead). All analysis, schematics, and CAD on this page are based on the Very Low budget

Preliminary Crew-Lock BOM - Very Low Budget

Preliminary Crew-Lock BOM - Very Low Budget

Below is the Low budget BOM. This Low budget BOM allows for a cheap Crew-Lock creation that is still functional; however, some parts are cheaper and many assumptions are made about the materials in order to reduce the price. The Low budget BOM also guarantees the availability of all items, such as bolts and extra items in case of breakage or failure, for ease of building and testing of the final product.

Preliminary Crew-Lock BOM - Low Budget

Preliminary Crew-Lock BOM - Low Budget

The High budget BOM below only lists items that are considered completely out of the range of any budget increase. The High Budget BOM includes items that would allow the creation of a more technically legitimate and aesthetically pleasing Crew-Lock. All previous items in the Low Budget are still relevant in this budget.

Preliminary Crew-Lock BOM - High Budget

Preliminary Crew-Lock BOM - High Budget

Test Plans

Below are our preliminary test plans for the Crew-Lock subsystems. These test plans will be further fleshed out during the Build and Test prep phase of this project. The Excel sheet can be downloaded from the Design Review Materials section at the bottom of this page.
Preliminary Test Plans for Subsystems

Preliminary Test Plans for Subsystems

Risk Assessment

Below is an summary of changes to the risk management file. The full risk management file can be downloaded in the Design Review Materials Section at the bottom of this page.
Phase III Updates to Risk Assessment

Phase III Updates to Risk Assessment

Conclusions and Plans for this Phase

During this phase, the team was not able to complete the detailed design of the Crew-Lock. Specifically, the team was not able to finalize the deployment system, and consequentially, the electrical systems. Midterm exams, job interviews, and personal matters caused the team to intermittently lose good communication and cohesion. Furthermore, the technical abilities and financial limitations caused further research to be done in order to reduce the price and complexity of our Crew-Lock design. The bulkheads, sealing, hatches, ergonomics, vacuum seal material, test plans, and some extra research were all completed during this phase. Despite the team's best efforts, the Very Low, Low, and High budget BOMs are currently out of the $500 budget range. Next phase, the missing items must be complete and detailed so that a fleshed out BOM and CAD/Schematics may be developed and presented to the customer and the guide for further financial and technical troubleshooting.
Tasks to Complete by Phase IV

Subsystems and documents that still need refining: Electrical/Control, Deploy/Contract, Testing Rig, and Internal Ergonomics, Test Plans (metrics and failure modes addressed), and BOM.

Important line-items to resolve: Over-budget items, manufacturing space and materials storage, and MSD II Schedule Conflicts.

Preliminary Detailed Design Phase Review Notes

Starting Date Ending Date Meeting Notes
October 2018 November 2018 Design Review Notes - Preliminary Detailed Design Phase

Design Review Materials


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