P18101: CubeSat Solar Sail
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Systems Design

Table of Contents

Team Vision for System-Level Design Phase

Planned:

Accomplished:

Functional Decomposition

Document Owner: Michael Berezny

Starting with the overall project assignment of deploying a solar sail on a CubeSat, and using customer needs, the following diagram was made to outline the necessary functionality of our product. Moving down the chart elaborates how the above task is to be accomplished, while moving up the chart explains the reason for sub-tasks.

Functional Decomposition Chart

Benchmarking

Functional Decomposition Chart

The existing solar sail solution which most closely resembles the desired state of our customer is the LightSail 1 created by the Planetary Society. LightSail 1 is a fully functional research CubeSat and not a dedicated solar sail module like we intend to develop, but it has a satellite volume and sail area comparable to our targets. Additionally, it has a deployment mechanism and sail storagewhich will be useful for reference and comparison. The LightSail deployment mechanism had dimensions of 10cm x 10cm x 5.5cm in addition to a motor of unknown size, shown below.

LightSail Deployment Module

Concept Development

From initial brainstorming sessions, the group came to a consensus on four different concepts from which rough ideas for system-level components were generated (and eventually, combined into a final conceptual design). In all cases, the booms are assumed to be attached to the sail, and the sail will be folded in a pattern that allows the sail to unfold as the booms extend. Concept sketches, starting with Concept 1 on the left and Concept 4 on the right, can be seen below.

Concept 1Concept 2Concept 3Concept 4

Concept 1 (pink) relies on a central motor housed within the frame. Tape-measure-style booms are attached to a central spindle which is rotated by the motor, and the booms extend in turn. The spinning motor may need some sort of device to counter the torque it creates during deployment.

Concept 2 (green) also utilizes a motor and tape-measure-style booms, however instead of attaching all four booms to a central spindle, this design gives each of the four booms its own spindle which is rotated via an attached gear which is reliant upon the motion of the motor. This design provides a more predictable and coordinated deployment of the sails, but would require more moving parts which could present potential for failure.

Concept 3 (orange) again relies on a central motor for deployment, however the booms for this idea utilize a telescoping mechanism similar to one found in a collapsible antenna on an old radio. These telescoping booms would extend via centripetal force generated from the rotation of the motor. This concept would most likely create the greatest moment out of any of the designs, and there would be no way of making sure the booms deploy at an even rate.

Concept 4 (blue) entirely ditches the concept of traditional booms, and instead proposes a mesh made of memory metal wire which is merged with the sail. This creates a larger volume sail due to the thickness and bending radius of the added wire, but room is also freed up within the frame due to the lack of a driving force such as a motor. Cost may also be prohibitive for this design.

Feasibility: Analysis and Prototyping

Volume Requirement Feasibility

Analysis

Target Sail Area: 32m^2

LightSail Sheet Thickness: 4.5*10^-6m

2 Mil Commercial Mylar: 50.8*10^-6m

0.5 Mil Commercial Mylar: 12.7*10^-6m

Assuming perfect folding and neglecting seams, our target sail area would require the following volumes from the above material.

LightSail: 144 cm^3

2 Mil: 1625.6 cm^3

0.5 Mil: 406.4 cm^3

The internal volumes of three standard CubeSat frames are lsited below for reference.

1U: 1,000 cm^3 1.5U: 1,500 cm^3 2U: 2,000 cm^3

Results

Considering sail material alone, with very generous assumptions, the 2 Mil mylar sheet will not function in any framer smaller than a 2U. Once deployment components are considered the 2 Mil mylar will not fit in a 2U either, this rules out any material thicker than 2 mil for our sail material. The LightSail deployment mechanism mentioned in the benchmarking section had a volume of 550 cm^3. Assuming we achieve a similar deployment mechanism volume, this and a sail made from the thinnest commercially available mylar sheet will barely fit in a 1U frame. This does not include a motor, powerbus, or space for controller components. Our project appears feasible from a volume perspective, but we will have to either invent a novel, much smaller method of deployment, or build within a frame of 1.5U or larger.

Mass Requirement Feasibility

Assumptions

Pumpkin CAD model was used for the analysis of the frame and rails which was found on their website.

The frame and rails are made from Aluminum 7075.

The SolarSail is 32m^2 with 2 thou thickness.

Analysis

1U Cubesat:

Mass of CubeSat Frame and Rail – 0.152 kg

SolarSail mass – 0.565kg

Microcontroller (Ardunio Nano) – 0.007 kg

Total: 0.724 kg

2U CubeSat:

Mass of CubeSat Frame and Rail – 0.226kg

SolarSail mass – 0.565kg

Microcontroller (Ardunio Nano) – 0.007 kg

Total: 0.798 kg

Results

The 1U and 2U CubeSat have a max mass requirement of 1.33kg and 2.66kg respectively. Based on these constraints, the 1U and 2U would both pass the mass requirement. The 1U CubeSat could still hold 0.606kg but there are more components that will have to be integrated into the CubeSat. However, these components are solution dependent.

Sail Extension Moment Creation

Assumptions

In order to calculate the maximum moment generated by the extension of the sail, a number of assumptions were made:

-Design uses extending booms and mylar sail, similar to Concept 1 or Concept 2 in the Pugh matrix

-1 mil sail thickness (2.4*10^-5 m)

-Density of Mylar = 1380 kg/m^3

-Sail Area = 30 m^2 (with a square shape)

-Sail Deployment Time is less than or equal to five minutes, and speed does not exceed 0.5 m/s

Analysis

Assuming the booms extend from the farthest point possible from the center of the frame while still being contained within it, the moment arm for each boom is equal to one half the diagonal of the .1 m by .1 m cross section of the CubeSat, which is equal to 0.07071 m.

Multiplying the sail's area and thickness by the density yields a sail mass of 1.052 kg. Each boom will be responsible for deploying 25% of this mass.

Using the maximum allowable speed, desired deployment time, and an initial velocity of zero meters per second, one can use the kinematic equation Vf = Vi + a*t to find the acceleration (a) of the booms to be 0.00167 m/s^2.

Multiplying the mass by the acceleration generates the required force value, and multiplying that force by the moment arm above gives the moment created by the extension of each boom. The total, which can be found by multiplying by four due to the four booms, is 0.000124 N*m. Ultimately, this value is negligible, however if any motors are used in the system, their generated torques will impact the system much more, so some sort of mechanism would be required to counter the moments they may generate.

Making the Sail From Blankets

Assumptions

  1. Each sail is 52" X 84"
  2. Cutting the material will not negatively affect its performance
  3. Cuts can be made accurately and in perfectly straight lines
  4. Connecting the sail pieces together requires 2" of overlap
  5. Overlapping attachment areas to not affect performance

Analysis

The blankets will have to be combined into a sail with four main components, with one in between each pair of booms. It was previously assumed that the sail would be made from 12 blankets in order to get the correct sail dimensions (32 m^2). However, this turns out to not be the case.

The actual needed dimensions for the sail are 217.5" X 217.5". This cannot be as easily made from 12 sails as anticipated, as the height of each is only 52". In the 4 x 3 arrangement anticipated, the maximum width of this is 208". Using the 2" overlap required for attaching the sails to each other, this is reduced to 202". A 202" square would yield an area of only 26.32 m^2, and therefore be insufficient. The excess material from the width of each blanket (84" down to 73.8") would have to be used to complete the square. This makes the manufacture of the blanket more difficult, and making it from 15 blankets instead might be a feasible alternative.

The goal of the design in making the sail from blankets was to make as few attachment areas and cuts as possible, since these increase the complexity of the design and the amount of work needed to construct the sail. Two primary ideas were developed as a result of this constraint. They are shown below. Note that the second one would require a thin additional layer along the bottom.

Arrangement 1Arrangement 2

Sail Material Prototyping

Fortunately, reflective mylar sheets of the 0.5 mil thickness we intend to use are readily available as another product, emergency blankets. Our team obtained a blanket (shown below) to begin prototyping and to get a tactile understanding of our sail material.

public/Photo Gallery/Packaged Blanket.jpg public/Photo Gallery/Unfolded Blanket.jpg

Emergency blankets generally come pre-folded in a package like the photo on the left. Although somewhat time-consuming, our team found that it was surprisingly easy to fold and unfold the material. It would be convenient if our future design could utilize the existing folding pattern of the blankets. The reflective mylar was also remarkably durable for such a think material, we felt little risk of tearing the sheet while handling it manually. Examining this blanket also helped our team to grasp the size and scope of our project. The unfolded blanket shown on the right represents only 1/12th of our target sail area. We were able to test how compactly we could fold the sail fragment. With little effort we were able to condense 35.79cm^3 of mylar into a volume of 56.44cm^3, a packing efficiency of 63.4%.

Boom Spindle Prototype

The two images below show a rough prototype of the boom deployment mechanism discussed as Concept 1 in the following sections. This prototype proved the mechanical validity of the concept. It also highlighted several features that will be required in later stages of design such as curved slots and rigidity and concentricity of the cylinders.

public/Photo Gallery/Furled Boom Prototype.jpg public/Photo Gallery/Unfurled Boom Prototype.jpg

Morphological Chart

The morphological chart below was generated by combining the ideas generated by the entire group. Although there are not many subsystems required for this design, there are a wide gamut of possible ways to achieve the functionality of each subsystem. The concepts in bold text in the chart are ones that went on to be included in some capacity within the four main concept ideas that were generated for Pugh matrix analysis.

public/Systems Level Design Documents/P18101MorphologicalChartV1_0.PNG

Concept Selection

Concept Screening

Pugh Chart Comparison Between Four Concepts and the Planetary Society LightSail as a Benchmark

Pugh Chart Comparison Between Four Concepts and the Planetary Society LightSail as a Benchmark

Pugh Chart Evaluation with Concept 1 as a Datum

Pugh Chart Evaluation with Concept 1 as a Datum

Concept Selection

Based on the Pugh Charts above, it would seem that the Memory Metal Mesh (Concept 4) is the best choice for a system design. After discussion as a group, though, it was concluded that this design could be very prohibitive due to the cost, manufacturing time, workability, and thickness of the memory metal mesh. Concept 3, which utilized telescoping arms, was deemed an inappropriate design for the application in both Pugh Chart evaluations.

As a result of the Pugh Chart evaluations and group discussions, the core design will be based on a combination between Concepts 1 and 2, with Concept 1 serving as the primary inspiration. These designs are very similar in their core nature, but vary in the number of spools on which the booms are stored. The reason for choosing Concept 1 despite it performing worse than Concept 2 in both evaluations is because Concept 1, theoretically, will have fewer parts to design. Because of this, it will also have less moving parts, and less room for error in terms of tolerancing interfaces between parts. In addition, Concept 2 is reminiscent of the design used by the Planetary Society, meaning that if the design leans more toward that of Concept 2, it will be based more off of an already proven mechanism. It should be noted that during the detailed design phases, Concept 1 will need some sort of device to counter the torque created by the motor's rotation.

Systems Architecture

The system architecture defined below describes the transfer of power and data within the system. Because the system is designed to interface with other CubeSats, it is assumed that these other CubeSats will be providing the power and commands necessary to engage the deployment mechanism for the sail. Types of communication between systems are noted with two different color arrow types, with one representing power, and the other representing data.

Original Systems Architecture Flowchart|

Original Systems Architecture Flowchart|

Up-to-Date Systems Architecture Flowchart

Up-to-Date Systems Architecture Flowchart

Risk Assessment

public/Systems Level Design Documents/Updated_Risk_Assessment_V1_1.PNG

Design Review Materials

Below are links to documents related to and generated from the Systems Level Design Review:

Plans for next phase

For the next design review we plan to have the following completed:

- Decide on sail folding pattern

- Spec out motor drive mechanism

- Design deployment mechanism inside 2U frame provided by SPEX club

- Create prototype with boom material and motor

- Updated test plan

- Updated risk assessment

Eric Pareis:

Design full sail shape with Mike

Research sensor to track distance of boom with Victor

Test method of combining mylar sheets with Mike

Design sail folding patterns with Mike

Pick microcontroller with Victor

Test sail folding patterns with Mike

Andrew Lewis:

Design basic Deployment mechanism

Print CAD model

Create prototype with boom material with Victor

Calculate weight and volume deployment mechanism

Victor Braescu:

Research sensor to track distance of boom with Eric

Pick microcontroller with Eric

Create prototype with boom material with Andrew

interface micro controller with motor with Mike

Mike Berezny:

Design full sail shape with Eric

Test method of combining mylar sheets with Eric

Design sail folding patterns with Mike

Spec out motor drive mechanism

buy microcontroller, motor and sensors

interface micro controller with motor with Victor

Test sail folding patterns with Eric


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