P20129: Flight-Ready Heat Switch
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Detailed Design

Table of Contents

Team Vision for Detailed Design Phase

Our team planned to:

Our team actually did:

Prototyping, Engineering Analysis, Simulation

Unmodified Solenoid Testing

To ensure the solenoid would work under cryogenic conditions, the solenoid was placed in a cryostat and cooled to 77K. As it cooled, power was applied to solenoid at various to see if or when it stopped actuating. The process was repeated as it warmed back up from 77K. Tabular results can be seen below.

Modified Solenoid

Modified Solenoid

The solenoid stopped actuating between 227 and 240K. Which put it near the manufacturer specifications of operation. The solenoid was then taken apart and it was determined that an internal fabric pad was freezing the solenoid together.

Modified Solenoid Testing

The fabric pad and outer housing which contained the fabric pad were removed from the solenoid. The solenoid placed back in the cryostat and the test was repeated. Tabular results can be seen below,

Modified Solenoid

Modified Solenoid

The solenoid stopped actuated down to 87K at which point it stuck in the actuated position. It remained this was until the solenoid was removed and manually reset. The cause of this failure has been determined to either be related to cold welding, CTE mismatch and or errors in solenoid reassembly.

Modified Solenoid Temperature v Resistance

Modified Solenoid Temperature v Resistance

The solenoid coil resistance changes very drastically with temperature. As a result the required voltage to achieve the same power output is also drastically decreased. The target power was 12 watts for this test because that is the normal power applied when operating in room temperature.

Mechanical Design

Overall

Preliminary Mechanical Heat Switch Design

Preliminary Mechanical Heat Switch Design

Preliminary Mechanical Heat Switch Design

Preliminary Mechanical Heat Switch Design

Solenoid

A Ledex 3E H-2711-033 45 degree clockwise rotary solenoid in being used to actuate the system. The solenoid is to be modified with the outer casing and fabric pad removed to prevent the solenoid from freezing and to allow for cleaning and application of cryogenic powder.

Modified Solenoid

Modified Solenoid

Removed Solenoid components

Removed Solenoid components

Support Structure

Solenoid Housing

The solenoid housing is very similar to the previous design. It allows the solenoid to be mounted slightly above the cryostat base plate and the torque from the solenoid is reacted into the mounting on the housing. Cross-drilled hole allows for ease of assembly with lower ratchet mechanism.

Solenoid Housing

Solenoid Housing

Arm Support Plate

The arm support plate has two functions. It allows for the mounting of the flex pivots to be located relative to the centerline of the solenoid, and it compresses the compression spring which resides in the ratchet assembly.

Ratchet & Cam

Ratchet & Cam System

Ratchet & Cam System

Upper and Lower Ratchets

The teeth geometry of the ratchets allow only for clockwise rotation of the peanut cam. Counter-clockwise rotation of the solenoid will result in the teeth skipping and the peanut cam remaining in its current position. This will make it so the solenoid only has to be pulsed to open and close the system.

7075 Aluminum was selected for both ratchets for its increased strength, wear-resistance and availability compared to the more common 6061 Aluminum. The teeth geometry promotes design maximum engagement area during actuation and better surface finish over alternative designs. The Lower Ratchet connects to the solenoid by a M2-0.4 set screw. Two designs of these ratchets exist. One is to be made on a manual mill or 5-axis CNC, the other is to be made on a 3-axis CNC.

Upper Ratchets

Upper Ratchets

Lower Ratchet

Lower Ratchet

Peanut Cam

The bearings by default rest in the recesses of the peanut cam, holding the system open. When the peanut cam is rotated, it pushes the bearings apart, causing the arms to close on the salt pill. The force exerted on the peanut cam by the bearings should hold the arms in this state until the solenoid actuates again.

The peanut cam is connected to the upper ratchet by a M2-0.4 set screw. 7075 Aluminum will be used for its increased wear-resistance compared to 6061.

Peanut Cam

Peanut Cam

Ratchet Housing

The ratchet housing contains the top of the return spring and assists in keeping the upper and lower ratchet in alignment. Since the load it sees is negligible and it shall not see much wear, it will be made out of 6061 Aluminum.

Ratchet Housing

Ratchet Housing

Return Spring

A return spring acts upon the upper ratchet keeping it in contact with the lower ratchet. The spring allows for the the upper ratchet to move vertically as the lower ratchet is rotated by the rotary solenoid.

A 9.5 mm long, 5.5 mm OD, 0.5 mm wire diameter compression spring with a spring rate of 0.29 N/mm was chosen as the return spring. The spring allows for a sufficiently large diameter on the shaft of the upper ratchet, while maintaining a downward force that is not too large for the solenoid to overcome.

Arms & Bearings

Arm & Bearing System

Arm & Bearing System

Arms

The profile of the arms was reduced as was the material needed for manufacturing. The design to hold the contact pins into the end of the arms was changed from a screw clamp to a set screw. Finally, the holes for the braided copper cables were doubled and moved towards the outside of the apparatus.

Copper Arm

Copper Arm

Bearing Assembly

The bearing assembly is attached to the arms in order to translate the rotational motion of the cam to linear motion of the arms. The bearings are ceramic in order to limit heat transfer into the cam. The size was reduced from the previous design to allow for a more compact design.

Bearing Assembly

Bearing Assembly

Exploded View

Exploded View

Flex Pivots

The flex pivots were sized up from the preliminary design to a diameter of 0.1562 inches to increase the load that can be applied to each end and increase the amount of cycles before the flex pivots break. They are going to be press-fit into the arms in order to mitigate the chance of the arms rotating off the flex pivots, which was an issue with the past teams design.

Heat Path

Heat Path

Heat Path

Copper Pins

Machined copper pins will connect to salt pill when closed, transferring heat away from the salt pill into the arms. These pins are held in arms by M2x0.4 8 mm set screws.

Copper Pin

Copper Pin

Braided Wire

Flexible braided copper wire is being used to transfer heat from the arms to the base plate. The wire is currently 10 gauge braided wire, with 3 braids per side. The gauge, length and number of wires can be changed if necessary.

Wire Clamps

The braided wire is clamped to the arms and base plate with 0.100” thick copper plate. M2x0.4 screws and nuts are used the provided the compression on the arms, and 4-40 screws and nuts on the base plate.

Clamps

Clamps

Analysis

Static structural analysis was conducted on the system to assess structural compliance under the acceleration loading given from the customer requirements.
Static Structural Setup

Static Structural Setup

Static Structural Results

Static Structural Results

As you can see in the above figure, the structure passes with high margin.

Thermal analysis was conducted using 5W and 2.5W thermal loads at a variety of temperatures as seen in the table below. An example of the temperature plot and total flux plot can be seen in the figures below.

Thermal Analysis Results Table

Thermal Analysis Results Table

Temperature Plot

Temperature Plot

Total Heat Flux Results

Total Heat Flux Results

Modal analysis was conducted to assess the frequency response of the system. The results can be seen in the figures below.
Modal Analysis Setup

Modal Analysis Setup

Plot of First 6 Modes

Plot of First 6 Modes

First Mode Results

First Mode Results

Electrical Design

Lab Heat Switch Test System

Lab Heat Switch Test Block Diagram

Lab Heat Switch Test Block Diagram

Lab Heat Switch Test Schematic

Lab Heat Switch Test Schematic

Lab Heat Switch Test PCB

Lab Heat Switch Test PCB

Lab Heat Switch Test Board Rendering

Lab Heat Switch Test Board Rendering

Flight Ready Heat Switch Control System

Heat Switch Flight Ready System Schematic

Heat Switch Flight Ready System Schematic

Heat Switch Flight Ready System PCB layout Rev A

Heat Switch Flight Ready System PCB layout Rev A

Bill of Material (BOM)

The bill of materials shown below lists all of our purchased parts, including raw material for fabricated components. All items will be sourced from Digi-Key or McMaster-Carr when possible. .
Bill of Materials

Bill of Materials

The link to the live document is here

Test Plans

After performing more benchmark testing on the modified solenoid and further discussion with our sponsor, Dr. Zemcov, the need test requirements have been changed since the PDDR.

Benchmark Test

The new modified solenoid will be tested inside the cryostat after we make the same modifications to it as the previous solenoid without taking it apart to see if it can actuate at 77K.

Vibration Test

The largest risk and failure mode for this design is the apparatus vibrating itself apart inside the sounding rocket. A vibration test will be performed to make sure that the heat switch meets NASA’s Vehicle Level Two standard. As discussed with Dr. Zemcov, a new test fixture will need to be manufactured for the heat switch. A new customer requirement was added to aid in vibration testing. The design of a white noise amplifier is required to test for NASA level 2 random vibrations.

Heat Conduction Test

A stand with a mounted 5 Ohm resistor was attached to a piece of copper to simulate the salt pill. This test also doubles as a test to measure the effectiveness of the contact pins without the use of e-beam welding to see the effects of contact resistance. This can be tested in both the lab and inside the Cryostat.

Other Tests

These tests will be performed throughout MSD II.

Design and Flowcharts

This section should continue to be updated from your systems level design documentation.

Risk Assessment

Risk of solenoid failure has been reduced as a result of solenoid testing. However, modal analysis yielded a new high risk item.
Risk Assessment

Risk Assessment

The link to the live document is here

Design Review Materials

Include links to:

Plans for next phase

The team wishes to achieve the following for next phase:

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