Team Vision for Subsystem-Level Design Phase
- Review SDR Action Items, Update Requirements, and
Identify Subsystem Resources
- Begin Subsystem Design Layout
- Identify All Critical to Performance Interfaces
- Analyze Possible Alternative Designs
- Begin to Develop a CAD Package with Guides
- Follow Labeling Scheme from the Nomenclature Page
- Relate All Subsystem Component Specs to Parent Requirements.
- Begin Subsystem Design Layout
- Proof of Concept
- Analysis and Simulation
- Deployment - Simulation
- Vibration - Analysis and Simulation
- Inertial Concept - Analysis
- Thermal Environment - Simulation
- Possibly Create a Full Launch Cycle Simulation - Thermal, and Mechanical Stress Simulation
- Analysis and Simulation
- What did our team actually accomplish during this phase?
Feasibility: Prototyping, Analysis, SimulationTo build P16102 successfully, three major subsystems have been identified, all of them relating back to primary functions of the final deliverable. With this functional decomposition, the elements with the red border were developed by the morphological chart and prototyped out.
From this breakdown, a refined functional decomposition was created.
Archetype : Hinged
Archetype : Exploded
For our final concept, the exploded structure was chosen. This structure takes advantage of the material required for the solar panels, and uses it as a structural member. Reducing the required mass of the base structure. This method is novel, and allows us to use our mass budget more efficiently.
Deployment MechanismWe have defined the deployment mechanism to encompass everything that takes the CubeSat from its undeployed state to its deployed state. It does not include the solar arrays themselves or the frame that supports them as those fall under the category of deployable structure. Deployment mechanism includes, rather, things such as springs, hinges, restraints, shape memory and hot knives.
Hot KnifeAfter preliminary testing, the decision was made to use the hot knife method as our deployment mechanism. Hot knives take advantage of the fact that high resistance wires generate heat when a current is passed through them. In a hot knife, a high resistance metal such as NiChrome is heated in this way and used to melt through objects. Hot knives are often used in CubeSats where a line holds a deployable in place and the hot knife cuts the line, allowing deployment. This method can reliably release a deployable without requiring undue mass, power, cost or mechanical complexity that would have to be delt with in a system using motors, servos or similar mechanical parts.
Our current hot knife design requires the following equipment:
- Springs/hinges (to push the array into their deployed configuration)
- Fishing Line (to hold the arrays in their undeployed configuration)
- NiChrome Wire (to cut the fishing line, allowing deployment)
- Power Source (to heat the NiChrome wire)
Experiment/ProcedureTo get an idea of the effects of fishing line type and NiChrome configuration on power consumption for our hot knife, bench tests were conducted, using a hot knife in a variety of these scenarios. The NiCrome wire would be placed in contact with the fishing line and different currents were passed through the wire. For each fishing line - NiChrome setup, the minimum current required to break the fishing line was determined. To simulate the presence of springs in the final setup, light tension was applied to the fishing line by hanging a clip from the end of the line. Hot knife testing in action
Our testing showed that required current can vary between fishing line types and will decrease as the number of coils increases. We also showed that even in a scenario where an unrealistically long NiChrome wire is used, the total energy and power required are manageable for what we expect our battery to be able to supply. Testing also showed that higher currents result in less total energy being required to break the fishing line.
System DemoWith the knowledge gained from the above hot knife testing, we were able to construct functioning prototypes of the entire hot knife deployment mechanism. These prototypes include not only the hot knife but a boiler plate model of the CubeSat and springs to force the deployable open.
- Shape memory alloy
- Not chosen due to weak deployment force and low activation temperature
- Electric motors
- Not chosen due to increased mass and complexity
- Magnetic latches
- Not chosen due to potential interference with magnetorquers in ADCS.
Mathematical ModelThe torque supplied by the torsional spring can be described as a function of its geometry, material, and deflection based on the following analysis:
- n = number of turns
- L = length of wire
- D = wire diameter
- E = modulus of elasticity
- R = radius of coil
- M = supplied moment
- r = effective radius of curvature
- I = moment of inertia
- m = spring mass
- sigma = stress in spring
- Theta = rotational displacement in spring
- rho = spring density
To calculate the torsional stiffness of the spring, consider the spring in its unwound state, as a beam in pure bending.
This effective radius of curvature can be translated to a rotational displacement in the spring by:
Resulting in the following equation for applied moment:
If we assume that we want to maintain a holding force equal to half of the deployment force, it can be shown that in our application the spring will be displaced by 180 degrees before deployment, and 90 degrees post deployment. Knowing this, we can calculate the moment produced by the spring, the maximum spring stress, and the mass of the spring based on the following equations.
Shape MemoryAfter preliminary testing, the decision was made not to use shape memory as our deployment method. Shape memory is a special type of metal that can "remember" a shape that it is bent into while it is hot. If the metal is heated from a cool state to a hot one, it will revert to its "remembered" state and, while how, can be given a new shape to "remember". Shape memory controlled deployable solar arrays on CubeSats have been attempted before, though not commonly. Shape memory offers the advantages of low mass, low cost, low power consumption and mechanical simplicity.
EquipmentA Nitinol (Nickel-Titanium) alloy wire was used for the below demonstration. The wire nominally possesses the following properties:
- Length: 305mm
- Diameter: 0.30mm
- Current Required for Austenite Form: 1.5A
- Pull Force: 2.83lbs
- Resistance: 0.31 ohms/inch
Experiment/ProcedureIn order to be feasible for our use, the shape memory alloy must not deploy at a temperature below those which will be experienced in launch qualification tests (60 C at a minimum, 70C+ preferred). The wire was exposed to increasing temperatures to determine at what temperature the effect activates.
Testing/ResultsAt 170F (~77C), which is the lowest bake temperature available to us, there was some slight movement of the shape memory to its original state. This is cause for concern as any movement seen during the qualification bake test can be seen as a threat to the P-POD and the other CubeSats inside of it and would likely result in a failed test. This result makes the use of shape memory much less appealing and lead to our decision to use the hot knife method instead.
To further our understanding of what needs to go into a CubeSat design deployed by shape memory allow, a simple prototype was constructed out of cardboard and cheap Nitinol.
Due to the small diameter of the shape memory used, the wire was not able to fully deploy the mock solar array. The slow speed of deployment seen in this video would be acceptable in the final product as long as full deployment is obtained in a reasonable amount of time. This model serves as a template for us to work off of on any future shape memory controlled deployment models.
StructureStructure refers to the CubeSat component that houses internals such as batteries, PCB experiments, antennas, etc. These internals are attached solely to the structure, while the solar cells are attached to the separate deployable structure.
Metal FramesCubeSats are often are made of hard anodized aluminum. The anodized aluminum offers high strength while also reducing the weight. In addition anodized aluminum offers good electromagnetic shielding and is affordable and available off the shelf. 
These metal frames can be made with direct manufacturing through milling and shaping, or folded from sheet metal, as demonstrated with the demo, with an increase of desired properties. Additionally, the usage of laser cutting or waterjetting can allow for complex shapes and we could expect (when removing a 9cmx9cm square from a side of 10cmx10cm sheet metal) a mass reduction of 60-80% for the entire system.
3D PrintingWith 3D printing becoming an increasingly inexpensive and lightweight design alternative for light duty applications; an investigation was conducted for the possibility of 3D printing a CubeSat. Within the last 2-3 years 3D printed plastic CubeSat designs have become more common due to their high weight reduction. In addition 3D printing a CubeSat would mitigate material waste and machining/fabrication time.
A study was conducted at CalPoly titled “FEASIBILITY STUDY INTO THE USE OF 3D PRINTED MATERIALS IN CUBESAT FLIGHT MISSIONS” by Daniel Fluit. In this study different 3D printable plastics underwent CubeSat qualification tests including vibration and thermal bakeout. In addition a cost analysis was done on these plastics versus machined metal. As a result a fraction of tested plastics passed CubeSat qualification tests and were deemed a viable replacement option for traditional metal CubeSats.
PlasticsIn the study the following plastics were analyzed: In the study the following plastics were analyzed:
- ABS Plastic
- Printer/Fabricator - Cal Poly’s Stratasys Dimension 2000 Fused Deposition Modeler
- The least expensive and the most common
- Objet FullCure720 Resin
- Printer/Fabricator - Cal Poly’s Objet Eden 250 Polyjet Prototyper
- The least expensive resin
- Watershed 11122 XC Modeling Resin
- Printer/Fabricator - Stereolithography from ProtoCam
- The least expensive outsourced fabrication
- Windform XT
- Printer/Fabricator - SLS from CRP Technologies
- Highest strength & most expensive
- Prototherm 12120 Modeling Resin
- Printer/Fabricator - Stereolithography from Harvest Technologies
- Fairly expensive
More material properties
- Test - 70Celsius 3hrs
- Less than 1% mass change during bakeout.
- ABS warped significantly over 50% of the time
- Outgassing was not performed per ASTM standard of a 24 hr test at 125Celsius
- Material Properties After Bakeout
- ABS exhibited significant material property change
- All other materials did not exhibit significant material changes affecting performance
Weight ReductionAn investigation into how much weight can be saved using 3D printing was conducted. Assuming a similar structure for all materials compared a weight reduction analysis was done using SolidWorks. In addition an assumption was made that using a 3D printed plastic would require twice the mass to ensure similar structural rigidity. Pumpkin offers a skeleton structure of 5052-H32 in there standard CubeSat kit, and this was chosen as the datum for the comparisons. Also other aluminum alloys are used and these offer different strength performance characteristics.  :
- Even with twice the volume, plastics at the minimum were 20% lighter than the aluminum structure from the standard Pumpkin kit.
- The Windform XT is the most expensive and yet has the best mass reduction.
- Using different aluminum alloys did not significantly reduce mass.
ReviewD. Cormier - Eark W. Brinkman Professor, Industrial and Systems Engineering, Director of Multifunction 3D Printing.
- Familiar with all resins mentioned in the feasibility study.
- New machine will be purchased that can produce carbon
- Increases strength
- Decreases weight
3D printing is a viable option for constructing the CubeSat. It has its advantages and disadvantages. It will be cheaper and produce parts faster but it could give up reliability in LEO. The model does not solely need to be 3D printed plastics but can have components that are plastics and others that are metal. Calpoly is further investigating this using a combined metal and 3D printed structure. Internal configuration can be easily adaptable to any mounting interface for internal components.
Deployable StructureThe deployable structure subsystem contains all of the parts that are to be moved during solar cell deployment. This includes the solar cells, their support structure, and any frame members that will move during deployment. This system does not include the springs, hinges, or hot knife mechanisms used to perform the deployment.
Solar Cell StructureA typical solar cell comes mounted on a dedicated PCB. The PCB provides support for the panels and aids in heat distribution. Usually, two solar cells are mounted on each panel. For most CubeSat providers, the mass of the panel and solar cells averages roughly 55g per CubeSat face with magnetorquers and less than 30g per face without magnetorquers. Integrated magnetorquers use a solenoid to produce a magnetic field which is used by the attitude control system to apply a torque on the spacecraft. Each CubeSat typically has three magnetorquers oriented orthogonally to allow for control in all three axes of rotation. Depending on the panels chosen we will likely have three panels with magnetorquers, and three without.
Remaining ComponentsAside from the solar cell PCB's, the deployable structure will have:
- Mechanical stops to allow for accurate fully open/fully closed positioning
- Deployment cable attachment points to interface with the hot knife system
- Mounting holes/surfaces for hinge
- Frame slides to interface with the P-POD launch system
MassThe overall mass of each deployable face will likely be 55 to 110g depending on the how the frame slides are integrated into the overall structure.
- PCB/Solar panel ~50g per panel
- Mechanical stops ~5g per panel
- Deployment system attachment points ~0g
- Frame slides ~90g per CubeSat
PackagesCubeSat panel packages are available from a variety of manufacturers. They typically cost roughly $2,000 per face. Most manufacturers also offer the option to purchase solar panels, power conditioning, and batteries as an inclusive system.
IntegrationHow do we integrate this to our customer requirements.
Preliminary Mass Budget
Variations were seen with various solar cells available through COTS suppliers such as CubeSat Shop's Solar Panels. For example, the ISIS CubeSat solar panels weigh only 50 grams per unit and lack magnetorquers, where the 63 gram ClydeSpace solar panels were would significantly reduce mass allowances. However, it shows that in both cases, well over half the mass allowance is used only for solar panels and nothing else. Thus, any marginal mass gain over time is important and helpful.
Additionally, P16104 is planning on keeping the CubeSat in a sun-facing mode which indicates the demand for a articulating system is not there for this payload, which strengthens our assumptions and choice for non-articulating system with payloads like these.
Bill of Materials (BOM)
Based on the research we have conducted to this point, we have identified items that may need to be purchased for this project. Due to the preliminary nature of this bill of materials, some of the items listed are from alternative design concepts and would not be purchased at the same time and each subsystem will likely require more parts than have been listed. This version will serve as a framework for bills of material going forward.
Plans for next phase
Phase IV Deliverables
- CAD model
- Estimated total structure mass
- Choice of material
- Internals sub-structure interface
- Deployable Structure
- CAD model
- Estimated mass
- Chose assumed solar panel
- Deployment Mechanism
- Build/Purchase Decisions
- Hot knife support
- Retaining wire routing
- Build/Purchase Decisions
- Begin simulation design
MSD I Deliverable PlanningIn the phase 2 design review, we planned our expected progress for our key deliverables. In our documentation, bill of materials, vibration test plan, CAD modeling, and risk assessments we are on schedule. In our MSD planning section, we are a bit ahead of schedule, as we have made contact with the professor from the 3D printing lab, and we have done some preliminary searches for sources of hinges and springs for our application.
- ↑ http://www.cubesatkit.com/content/structure.html, Web, Pumpkin, 2015
- ↑ http://www.cubesatkit.com/content/structure.html, Web, Pumpkin, 2015
- ↑ http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA5052H32, Web, ASM, 2015
- ↑ http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6, Web, ASM, 2015
- ↑ http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA7050T745, Web, ASM, 2015