Hermes II Fin Design

Fin Design Overview

The shape of the fins for Hermes 2 was almost identical to Hermes 1, except for the addition of a phenolic leading edge with a taper of 15 degrees. The fin core was made out of 1/8" G10 (garolite sheet), and the phenolic leading edge was cut from a sheet of 1/4" phenolic which was later tapered and milled down 0.020" on each side. 

Description of the Part

• Function of the part:

• Provide stability for the rocket as it flies

• They will be attached to the fin can and flown on the rocket

• Requirements:

• Needs to be able to withstand heat from flying up to mach 3.5 (for stage 1)

• Cannot break or flutter too much as the rocket flies

• Need to attach to the body tube of the rocket

Test Fin Can Fins

Below is a picture of the fins used for the test fin can layup. These dimensions were obtained from our OpenRocket sim, but were not the updated shape of the fins. This was acceptable for the test fin can layup, but for the flight candidate layup the shape was different.

Flight Candidate Fin Design Process

Talk about design of phenolic slot (0.3" deep, 1/16" thick)

[INSERT IMAGE]

Flight Candidate Fin Manufacturing Process

The fins for the flight fin can were made out of a 1/8" sheet of G10, with a phenolic leading edge made from 1/4" phenolic sheet. The fin core is G10 and had a tab cut into its leading edge, and the phenolic piece (with a slot cut into it) fits over the leading edge of the G10 fin. Below were the steps used to manufacture and assemble the fins. All of the machining was done in Gelb (Todd's shop in the basement of the Unified Lounge). 

Materials Required:

• 1/8" (0.125 in) sheet of G10 (INSERT LINK)
• 1/4" (0.250 in) sheet of phenolic (https://www.nationwideplastics.net/phenolic-sheet/)
• 15 degree end mill (https://www.mscdirect.com/product/details/04153326)
• 1/16" cutting bit (for mill)
• West Systems epoxy (CHECK)

Part One: Waterjet G10 Fin Core (~2 hours)

*Delamination is when the layers of your composite start coming apart. This can happen if the waterjet is cutting too quickly/strongly, and especially if your composite sheet is thin (1/8" is considered pretty thin). 

1. Make a DXF drawing of your fins, keeping in mind the size of the sheet you have and the size that the waterjet can fit. Arrange the fins such that they fit close together without being too close (leave around 1/2" space) and add at least two extra fins. You'll need one extra fin to position the fins while milling the tab, and at least one other extra in case something goes wrong. 
2. Save your fin file (.dxf) onto a USB key
3. Follow the procedure for waterjetting specific to the waterjet you're using. Use a lead-in so that the waterjet doesn't start cutting right on the part, but leads into the cut (to avoid delamination*). Make sure the cutting edge is on the outside of the part, or the fins will be slightly too small. Use the "Brittle Material" and "Low Pressure" settings when making your MAKE file so that the G10 doesn't delaminate.
4. You might have to stop and start the waterjet to weight down whatever fin you're using, because if the area around your part isn't weighted, it might move around and this will ruin the shape when it gets cut out. 
5. Did you remember to make extra fins? 

Part Two: Waterjet Phenolic (~2 hours)

1. Make a DXF drawing of your phenolic leading edge. 
2. Follow the steps for waterjetting based on the waterjet you're using. Make sure to make extras, like with the G10 fin cores. Use the same settings as for the G10- even though the phenolic is thicker (1/4" inch), this will help you be sure to avoid delamination. 
3. When waterjetting, make sure to weigh down your part. Since the phenolic pieces are long and thin, there's a higher risk than with the G10 that the cut will be uneven because the part moves around as it's being cut out. 
4. After waterjetting, file down any burrs. 

Part Three: Cut 1/16" G10 Tab on Mill (~4 hours)

In this step, you're cutting the 1/16" (0.0625 in) tab on the G10, which will be epoxied into the phenolic slot. In order to leave room for the epoxy, you want to make the tab thinner than 1/16", so we chose a tab thickness of 0.058".

1. Label your fins (1, 2, 3, 4, and extras) using a Sharpie and make a table below. Since each fin has a slightly different thickness than what the manufacturer claims, you'll have to measure the average thickness of each fin and use these measurements to determine how much to cut off each side so that you have a 1/16" tab.
2. Setup is the most time-consuming part of this process. Make sure the surface that you're clamping to is wiped down and even, because any gunk stuck to it will affect the evenness of your tab. Using the spare fin, set up your fin (by clamping the two together).
3. Before you start cutting, keep in mind that you want to make around two passes, instead of just cutting each side of the tab in one go: this will help you have a more even cut. So eventually you want to get the cut to be at the Y-values shown in the table, but not in the first go. 
4. The second row of the table is how much to cut off the first side (start with less than this at first though, as detailed in the previous step). When you turn the mill on, make sure it's set to "Forward" and cutting in a clockwise direction (to the left).
5. The third row is how much to cut off the other side of the G10 to complete the tab (start with less than this at first though, as detailed in step 3). When you turn the mill on, make sure it's set to "Forward" and cutting in a clockwise direction (to the right).

*Tab thickness = 0.058"

Use spare fin to align G10

Make sure surface is very smooth

Measure average thickness of each fin and use that to calculate how much to take off from each side

Make several passes

Part Four: Cut Phenolic Slot

Use oil so that phenolic doesn't heat up too much

Part Five: Cut Phenolic Taper

Part Six: Mill Down Phenolic

Use machining tape to stick phenolic down, edge at edge of clamp thing

Take off 0.020" inch, adjust mill when you turn it over to take off additional 0.020"

Part Seven: Assemble Fins

Lessons Learned:

• When waterjetting 1/8" G10, delamination is a concern because G10 is a composite (made of several layers of fiberglass) and 1/8" is pretty thin. Make sure to use the "Low Pressure" and "Brittle Material" settings on the OMAX waterjet. 
• Make extra fins and phenolic leading edges. That way, if anything goes wrong, you won't have to go back and make them all over again. 
• When making the tab on the G10, don't mill everything in one go- use several passes on the mill so that you take off a little bit of material each time. This way, the tab will be more uniform/even. 
• When cutting the phenolic slot, use oil because the phenolic will heat up from the cutting piece.

Software

• I used Solidworks to model the fins that were used on Hermes. Solidworks will also be used to design the phenolic - G10 interface and assemblies.

• Are there any limitations of this software? Make suggestions for how you could design the part better if we had a different program/more time

Hardware/Manufacturing

• Once you know the requirements of the part, list the materials you need to build it

• Make a rough draft of the steps required to build the part

Laser Cutting the Part:

• Create a DXF file of the part

• Using illustrator, place the drawing on to an artboard the size of the wood

• Adjust the settings on the laser cutter to appropriate settings for material

• Cut!

Water Jet:

• Transfer DXF file onto USB stick and onto waterjet computer

• Place the material on the waterjet, set the origin, and make sure the drawing is within the limits of your material and the jet.

• Cut each fin, making sure to stop in between and remove finished fins

Delamination can be avoided by:

• Using the “Low Pressure” and “Brittle Material” setting on the waterjet

• Lead into the bottom corner of the trailing edge instead of the top corner of the leading edge

• Lead in before cutting (the waterjet turns on as it’s coming in to cut instead of starting right on the part)