P18101: CubeSat Solar Sail
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Detailed Design

Table of Contents

Team Vision for Detailed Design Phase

Planned:

• Make frame and deployment mechanism manufacturable

• Create ¼ of sail by combining the mylar sheets

• Interface sensor, motor and micro-controller to communicate with each other

• Design locking mechanism for the spindle

Accomplished:

• Frame was made easily manufacturable by making design symmetric

• Deployment Mechanism guides were simplified for manufacturing

• ¼ of the maylar sail was created

• Sensors were interfaced with motor

• Motor was determined and ordered

• Locking mechanism was designed and prototyped

Progress Report

Linked here is the Mid-Phase Review, a summary of what had been accomplished prior to Thanksgiving break and the objectives for when the team reconvened.

Prototyping, Engineering Analysis, Simulation

Boom Spool Thickness

Spooled Boom Size

Spooled Boom Size

Diameter Results

Diameter Results

The thickness of the booms when spooled together was a concern since the boom material was chosen. Each boom has to be a minimum of 13.25 feet, so a total length of at least 54 feet of boom rolled onto the spool. For testing purposes 15 foot booms were used to account for extra material fastened to the spool and extending past the sail. The full length booms were rolled onto the spool and the thickness of it was measured with a dial caliper at several locations. Two people were needed for this as one had to hold the spooled booms while the other one measured.

The results of this varied greatly on where the spool was being held, with thicknesses near where it was being held being significantly less than those away from the holding positions. This will be reduced when the booms are spooled within the guides and underneath the sail storage plat; however these were not completed at the time of this test. To find a value for our use, we averaged these. Although this is likely not accurate, it overestimates the value to give us a factor safety. Now that the booms are smaller to accommodate a smaller sail, this should be remeasured. A smaller spool diameter from this will yield more freedom for the placement of the guides within the design.

Moment from the Spooled Booms

Test of Boom Moment Setup

Test of Boom Moment Setup

Boom Moment Results

Boom Moment Results

A key component of this phase was the determination of the stored energy in the spooled booms. This issue became apparent after the small scale prototype was tested. After research into the area, it was found that theoretically the force exerted by the wound tape measure booms should be constant at any point along its length. It should only change if the diameter changes, and as the diameter increases the force would decrease. We performed this test using a prototype spindle which we no longer needed.

Eric and Victor performed this test according to the Boom Force Test Plan, which can be found under the test plan section of this page. The test was conducted using a Vernier Dual Range Force Sensor. The test beam was created by using a hot glue gun to attach two Popsicle sticks that were glued together to a prototype of the spool. Only 1 boom was attached for testing, as the force from 4 would simply be 4 times the force in 1 boom and attaching all 4 would make it very difficult to conduct the test. Forces were measured for 1, 2, 3, and 4 loops of boom around the spool. The force in each was the same within an uncertainty range for the test, confirming the theory that the spooled tape measure force is constant. The measured force for the boom was 0.2 N, giving a force of 0.8 N for all 4 booms. The length of the test boom was 0.121 m.

Frame and Deployment Mechanism Design

After completing the initial frame design for the preliminary design phase, it was determined that the frame would require a redesign due to two primary factors: cutouts with specific profiles were needed to allow rotation of the booms on the gyroscopic plate, and the previous design was not easily manufacturable without 3D printing. A Version 2 frame was constructed using already-created parts with minor edits applied. This version was primarily to visually convey ideas while discussing the design with SPEX to ensure manufacturability.

Noteworthy changes from the V1 model which was used for the 3D printed prototype were the lowered mounting point of the gyroscopic baseplate to allow for more room for sail storage, the addition of holes on each side to allow for extension/adjustment of the booms, and making the inside rails a constant thickness, instead of having one thinner portion on each rail. In addition, a Geneva Cam was added below the spindle- this was decided to be added as a locking mechanism for the booms, and will be elaborated on more within the section on the deployment mechanism's updated design.

Version 2.0 of Frame Design

Version 2.0 of Frame Design

Following the creation of the V2 frame, the team met with SPEX members to discuss the required attributes to ensure the frame is manufacturable and agrees with the launch initiative's regulations. After this meeting concluded, it was decided that the frame should be designed with aluminum construction in mind- each piece will be constructed out of some type of aluminum sheet or bar stock. The proposed V3.0 design seen below, which does not include any of the cutouts required for sail unfolding, internal component mounting, or boom adjustment, is composed of ten pieces.

Of these ten pieces, there are only three individual designs- a side plate, an end plate, and a rail. All of these pieces were designed to be reusable in different orientations, so as to cut down the amount of programming time when using CNC, and to provide consistency. Holes used to join the pieces together are utilizing countersunk screws, so as to avoid exceeding the required 10cm by 10cm profile of a CubeSat. The four nubs on the top of the design are required for integrating springs into the design that aid in the launching process. The rail pieces would be constructed from aluminum bar stock, and the sides and ends would be constructed from aluminum sheets.

Version 3.0 of the Frame Design

Version 3.0 of the Frame Design

Exploded View of Version 3.0 Frame Design

Exploded View of Version 3.0 Frame Design

Following the completion of the V3.0 frame design, the previously designed components were reworked to interface with the new skeleton of the CubeSat. On the whole, the deployment mechanism itself required few major changes in terms of component additions, save for the addition of a locking mechanism to keep the spring force stored in the booms from rotating the spindle when not desired. The mechanism decided upon was a geneva cam- this device could be made low profile, and keeps the spindle locked in place via its geometry, meaning no electrical or chemical energy is required to activate the locking mechanism. The relations for this set of parts were created as editable parameters, so if an aspect such as the number of slots in the cam needs to change, the entire assembly can automatically update. In addition to the geneva cam, a plate was added on top of the boom guides to keep the booms from exiting their channels, as well as to separate the sail quadrants from the deployment mechanism. This piece mounts to the boom guides.

Geneva Cam

Geneva Cam

Version 3.1 Mounting Plate

Version 3.1 Mounting Plate

In terms of reworked parts, the rotating mounting plate is largely the same from a functional standpoint. Holes have been added to this and many other parts in order to allow for mounting of pieces with hardware, and the holes that previously allowed for rotation have increased in size to allow for the use of dowel pins as axles. The mounting point for this mounting plate on the frame was also moved in order to prevent collision between the microcontroller and motor in the lower half of the frame. The boom guide was redesigned from the ground up, but serves the same functional purpose of before. The curved nature of the previous design was shaping up to be very difficult to draw and manufacture, so the new design is designed to be manufactured from simple bar stock. The boom guides now also include windows to and mounting holes allow a reflectance sensor to monitor the position of each boom.

With all of the deployment components designed, the appropriate cuts could be made in the pieces of the frame, and hardware and purchase parts could be integrates into the assembly in Solidworks. The side plates include cutouts to allow the booms to extend and rotate on the mounting plate, and to allow the sail to deploy from the upper half of the frame. slots are also cut on the edges of these side pieces so that the cord attaching the sails to the booms can overlap each corner of the frame without exceeding the allotted dimensions specified in the CubeSat regulations. All cuts were made so that the same side design can still be used in all four orientations. Finally, holes were added into the end frame piece to facilitate mounting of the microcontroller, an Arduino Uno. When assembled together, the full Version 3.1 Design has a mass of approximately 1230 grams (not including the sail or booms), and does not exceed the bounding dimensions specified in the CubeSat regulations.

Head-on View of the Version 3.1 Frame Side Design

Head-on View of the Version 3.1 Frame Side Design

Version 3.1 of Full Assembly

Version 3.1 of Full Assembly

All machined components are planned to be constructed from 6061 aluminum, and drawings will be posted in the public directory as they are completed. CAD models for the components designed by the team can be found within SPEX's Google Drive folders. CAD models for hardware came from McMaster-Carr's website, the model for the motor was provided from the manufacturer's website, and the model for the Arduino Uno was obtained from Thingiverse.com using an open-source, creative commons attribution design. The model can be located here.

Version 3.2 of Full Assembly

Version 3.2 of Full Assembly

Once the design was largely finalized, minor changes were made to the models in order to facilitate easier manufacturing. Among multiple small changes, many holes and dimensions were changed from metric dimensions to imperial units, and the appropriate hardware was swapped in to accompany these changes. More specifically in regards to hardware changes, a shoulder screw was decided to be used to secure the spindle to the inner mounting plate, and some pieces of hardware were changed to ones that already existed in the model in order to simplify the BOM and reduce complexity and overall cost. For purposes of visualization, a yellow "dummy tape measure" model was added to the assembly in order to demonstrate how the four tape measure booms will wrap around the spindle and deploy through the guide pieces.

Sail Quadrant

Sail Components

A primary goal of this phase was to complete a prototype of one of the sail quadrants. This needed to be done to facilitate testing of the folded sail volume and unfolding. Until this task was complete, it was impossible to accurately design a finished CAD model of the CubeSat. The sail quadrant was assembled from the 3 of the aluminized Mylar blankets chosen for the sail material during previous feasibility analysis. This is shown in the image below. The numerical value on the components represents the blanket it came from and the letter describes the size of the piece, with "a" pieces being largest and "c" being the smallest. There was one very small component that was needed for the construction; it's location and dimensions are shown below the overall sail construction image.

Breakdown of Sail Quadrant Construction

Breakdown of Sail Quadrant Construction

Location of Small Component 3c

Location of Small Component 3c

Sail Component 3c

Sail Component 3c

Sail Assembly

The sail was assembled using the metalized Mylar tape that was determined as the best option from previous feasibility analyses. Each sail component was constructed using butted joints with the tape holding the sail components together. This quadrant was constructed to determine the strength of the tape bonds when holding an entire sail together and to check the volume a sail quadrant takes up in actuality compared to the calculated value. To tape the components, it was necessary for a group member to remove his shoes and sit on the sail material. This did not damage the sail or leave and permanent marks on the sail. The sail construction process is shown below. Note that piece 3c was not attached because it's small size was not significant for the things we needed to check.

Cutting Sail Component 2a

Cutting Sail Component 2a

Location of Small Component 3c

Location of Small Component 3c

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Completed Sail Quadrant

Completed Sail Quadrant

There were some issues that came up during the sail creation. It was already known that 2 people were required to cut the sail material. Firstly, due to the amount of sail being handled, 2 people were needed to hold the sail components taut while a 3rd person taped the sail. In addition, it was found that the machine cut blankets had uneven edges. In addition, the folding patterns on the blankets were not the same. This was unexpected, since we assumed that the machine cut and folded blankets would be identical. This was corrected by an additional cut and tape task to line up the fold patterns between most of the components. Another issue that arose from the machine cut blankets was that they did not actually have smooth edges, and for the final sail these may have to be cut. For this quadrant, the rough edges were used for the taped sides to solve this issue. It was also challenging to tape the seams straight, but removing incorrectly taped segments was found to be relatively easy.

Sail Folding

The sail folding prototyping was completed using the existing fold patterns on the taped blankets. This was why efforts were made to line up the existing folding patterns between the bonded blankets. Piece 3b was unable to be attached in this manner, so new fold lines were forced into it. The sail folding process is shown below, but some steps are important to note. Whenever a fold passed over the tape, which occurred at least once on most folds, time had to be taken to individually crease and fold each section of tape. In addition, the sail had to be held taut to make the folds, otherwise the folds started to come undone. In addition, once the quadrant was folded about halfway, folding began from the other side to make the process easier. The quadrant was then folded in an alternating pattern to make it take up a minimal amount of volume. At times a piece of wood was used to gold down parts of the sail to make folding easier.

Sail Folding Beginning from One Side

Sail Folding Beginning from One Side

Folding from the Second Side

Folding from the Second Side

Sail Folding Nearing Completion

Sail Folding Nearing Completion

Folded and Compressed Sail Quadrant

Folded and Compressed Sail Quadrant

Sail Quadrant Takeaways

The creation of the sail will require at least three people and around 3 hours per quadrant. It is very important during the cutting phase to ensure that the fold patterns line up between the main components of the sail. It is also necessary to check the edges of the blankets being used to ensure they are straight and free of defects. Holding the sail taut during cutting and taping is necessary to get the correct shape and dimensions of the sail. Mistakes during taping the sail can be corrected, which is very beneficial to us. Also, the sail can be safely walked on and sat on if the individual is not wearing shoes. The tape can be easily folded to match the existing fold lines, and small components can be forced to match the existing fold pattern of other pieces.

The goal of this process was to determine the final volume of a sail quadrant and see if 4 of them will fit within the CubeSat frame and to test the folding of a bonded sail quadrant. From this prototyping test, we learned that 4 of the folded quadrants will take up significantly more volume than the predicted 0.7U. This is due to decreased packing efficiency as more folds are added to sail. However, 4 quadrants will still fit within the CubeSat frame, but could take up to 1.25U depending on the sail storage position. Actively compressing the sails significantly reduces the volume taken up, so the possibility of compressed storage and a sturdy holding mechanism to keep the sails in place until deployment will be investigated.

Sail Size Decrease

After the initial size of the folded full sail was found, it was later determined that 4 of the full size quadrants would not fit within the CubeSat in an acceptable manner. There were two possible ways to address this; making the sail thinner and buying thinner material. Purchasing thinner material was investigated, bu the group decided that it would not be worth it to try to purchase a thinner sail material from the manufacturers in China. Before any cutting was done, a second folding pattern where the sail was rolled instead of folded was attempted, but this took up much more space than the folding method. Instead, it was decided to make the sail area smaller. The first reduction done was from 32 sq. m to 22.5 sq. m. This was done by reducing the size of the full size quadrant and then folding it again to see if 4 of those folded would fit. They would not, so it was reduced to 15 sq. m and 4 of those will fit within the CubeSat with little spare space.

Layout of New Sail Quadrant

Layout of New Sail Quadrant

Location of Small Component 2c

Location of Small Component 2c

Sail Component 2c

Sail Component 2c

Solar Pressure, Force, and Drag Calculations

Solar Pressure

One task for this phase was to calculate the solar pressure, force, and drag on the solar sail. The first step in this process was to determine the solar pressure. This was found using the equation:

Solar Pressure = ((1+r) X w X cos^2(theta))/c

Where:

r = reflectivity (0.9)

w = solar flux (1361 W/m^2)

theta = tilt angel (0 degrees)

c = speed of light (299,792,458 m/s)

This yields a solar pressure of 8.63 X 10^-6 N/m^2 using the above values. This value matches with the one found by the Planetary society (9 X 10^-6 N/m^2). The difference here comes from the Planetary Society's assumption of a perfect reluctance sail. Accounting for that, our calculated value falls within 1% of their value.

Solar Force

Once the solar pressure was known, the solar force is simply the solar pressure (8.63 X 10^-6 N/m^2) by the area of the sail (15 sq. m). This force was determined for a variety of altitudes and tilt angles and was then used to find the net force by subtracting drag. The net force is shown in the figure below.

Drag

The drag force was the last calculation needed to determine the net force on the sail. This depended mainly on the velocity of the spacecraft, but also the air density and drag coefficient. The drag coefficient was found from literature and research, and the air density. This values were highly uncertain, so instead a table of drag force per unit area at various altitudes for solar sails was used from NASA. This was then calculated for different sail areas and used to find the net force shown below.

The Force Varies Based on Altitude and Sail Area

The Force Varies Based on Altitude and Sail Area

The Tilt Angle Significantly Impacts Solar Force

The Tilt Angle Significantly Impacts Solar Force

Mechanical Drawings

During this phase our team achieved a complete detailed design and CAD model of our system. So far only the frame components have been drawn in detail and represented below. Other component drawings have been begun but not completed. The teams intends to complete all components, sub-assemblies, and assemblies drawings during the intercession between MSD I and MSD II.

Bill of Material (BOM)

Link to live Bill of Materials: BOM

The this spreadsheet document has now been split into two pages. One that contains a complete list of all parts acquired and purchased for this project. The second page will contain a list of components that will make up the final product.

Test Plans

Spooled Boom Thickness

Folded Sail Compression Test Setup

Folded Sail Compression Test Setup

The diameter of the spooled booms had to be determined to make sure it would fit around the spool and within the guides. If it was too thick thinner tape measure blades could be acquired. This would affect the design of the frame, so completing this test is a priority.

  1. Unroll the 4 15' booms
  2. Place the boom ends inside the spool
  3. Screw the booms into the spool
  4. Roll all 4 booms around the spool carefully
  5. Using a dial caliper measure the diameter of the spool in several locations
  6. Determine the average thickness of the spooled booms.

The location of the guides for the deployment mechanism depended on the size of the spool, as did the force in the booms. For these reasons this test is a priority for early in the phase. Eric and Victor will conduct this test.

Sail Quadrant Compression

Folded Sail Compression Test Setup

Folded Sail Compression Test Setup

The prototyping of the sail quadrant creation revealed that the folded quadrant's volume depends on the amount of compression force on it. This was not surprising, but the force required to keep it within an acceptable volume must be calculated to determine the needed strength of the sail storage area and components to hold the sail in place until deployment.

  1. Remove the restraint on the folded blanket while making sure it does not unfold.
  2. Determine the mass of the top flat plate.
  3. Place the folded blanket between two flat plate objects.
  4. Place the plates and sail on the flat test surface.
  5. Using a caliper, measure the height of the folded sail.
  6. Determine the mass of the test mass.
  7. Place the test mass on the top plate.
  8. Measure the height of the folded sail using the caliper.
  9. Repeat steps 5-7 until the compressed sail height is within the acceptable range.
  10. Determine the force exerted by the test mass and top plate on the sail in the acceptable range.

This will be completed this phase in order to determine the amount of force needed to keep the sail quadrants in place. This will then be used to design the sail storage area and the components holding the sail in place until deployment. Eric and Mike will complete this test.

Boom Moment Test Plan

Boom Moment Test Setup

Boom Moment Test Setup

The strength of the frame and deployment mechanism must be enough to withstand the moment exerted by the spooled booms. This means that the moment of the booms on the spool must be calculated. This moment also needed to be found in order to calculate the torque needed on the motor.

  1. Attach the test boom to the spool.
  2. Attach the test beam to the force sensor with a string.
  3. Have one person hold the spool so that the test boom is vertical while the other keeps the force sensor at the same height as the string on the test boom. The string should be just tight.
  4. Zero the force sensor.
  5. Roll a loop of the test boom around the spool.
  6. Make sure the spool and sensor is being held as described in step 3.
  7. Measure the force applied by the test boom.
  8. Repeat steps 5-7 for several more loops.
  9. Measure the distance from the string attachment to the middle of the spool along the test beam.
  10. Determine the moment exerted by the boom.

This will be completed this phase so the motor can be chosen. Theoretically, the force exerted by any number of spools should be the same as long as the diameter of the rolls is the same. There will be small increases in diameter size as the number of rolls increase, but the force should still be very similar. This force can then be used to calculated the moment exerted. Eric and Victor will conduct this test.

Design and Flowcharts

No changes have been made to any of the team's flowcharts since the preliminary detailed design phase. This documentation can be found Here.

Risk Assessment

There were some significant changes to the assessed risks this phase. The issue of not having enough space to work was reduced due to the smaller sail area, and the risk of the sail being unable to be sufficiently compacted reduced for the same reason. The major risks identified last phase of the sail deploying before intended and the boom mechanism failing have also been reduced. This is because the booms are stronger than anticipated and the forces in them lower than originally thought. The early deployment was solved with the creation of a locking mechanism. Finally the risk of the booms buckling has been drastically reduced since the calculated forces on the sails are much lower than those that could cause buckling
Updated Risk Assessment

Updated Risk Assessment

Design Review Materials

Include links to:

Plans for next semester

Main Tasks:

1. Vibration simulations

2. Build all four quadrants of the sail

3. Machine deployment mechanism components

4. Assemble all components

5. Write technical paper


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