Hermes II Fin Design & Manufacturing

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. Phenolic was used as the leading edge because it is an ablative, which forms a char layer as the leading edge heats up from the airflow of the rocket, and burns away, getting rid of some of the heat generated. This is to protect the rest of the fin from the heating that we expect at supersonic speeds (Mach 3-4). 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. 

The G10 is referred to as the "fin core" because it will eventually be covered with layers of carbon fiber during the fin can layup process to increase strength. G10 was selected because it is relatively inexpensive, and we think it is strong enough for our purposes (Hermes 1 also used G10 as the fin core material and the fins were still intact after the flight), but in the future, more analysis should be done to find stronger materials that are suitable for higher speeds.

Below is a table summarizing the process, as well as the approximate time that each step took.

Things to improve for next time:

• Make a jig to bevel the fins or figure out some better way of doing it than by hand
• Consider using honeycomb for the fins- would save mass while maintaining strength

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)

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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)
• End mill (0.3125") for cutting G10 tab
• End mill for cutting off excess phenolic
• West Systems epoxy (CHECK)
• Machine tape 
• Machine oil
• File
• Calipers

Part One: Waterjet G10 Fin Core (~3 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 (~3 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"

After we cut the G10 tabs, we noticed that certain areas were thicker than others, particularly in the middle of the part. This could be because we only used one pass on the mill when cutting either side, and since the tab is so thin, it probably moved back and forth slightly as the mill was passing over it. We used 60 grit sandpaper to sand the thicker areas afterwards, but in the future, it would be more ideal to use several passes, and to consider decreasing the tab thickness so that it fits more easily into the phenolic slot. 

Part Four: Cut Phenolic Slot (~4 hours)

1. Clamp your phenolic piece horizontally (the 1/16" cutting bit will come in horizontally) on either side of the phenolic. It does not matter which side you select for the slot, as when you flip over the piece, it is symmetrical. 
2. Zero the cutting piece at the center of the phenolic. 
3. Before turning the mill on, have some machine oil ready, and set the mill to low speed because the phenolic heats up while it's being cut.
4. When you turn the mill on, make sure it's set to "Forward" and cutting in a clockwise direction (to the right). Cut out the 1/16" slot.
5. After cutting, use the air hose to remove the phenolic dust and use ethanol* to remove the machine oil. 

*After some brief research, we found that machine oil and ethanol doesn't have an adverse effect on phenolic. However, this could be incorrect, so more research should be done to determine whether a better procedure for cutting the slot exists, and whether either of these materials decreases the strength of the phenolic somehow. 

Part Five: Cut Phenolic Taper (~4 hours)

1. Since the phenolic piece is long (11.66"), you'll need to use two clamps to clamp it vertically.
2. Zero the end mill at the middle of the part. 
3. Use a 15 degree end mill to cut a taper on either side.

Part Six: Mill Down Phenolic (~2 hours)

This step was required because since the phenolic is twice as thick as the G10, some material had to be taken off each side so that it better matched up with the G10. You  may wonder why we didn't simply start out with 1/8" thick phenolic: this is because the phenolic has to be slightly thicker than the G10 so that when the carbon fiber plies are added during the fin can layup, the carbon fiber is flush with the phenolic on either side. Therefore, we took 0.020" in off either side to account for the thickness of two plies of carbon fiber. Ideally, the sides would be milled down before we cut the taper, but this worked out fine. 

***Important note: you may or may not need to mill down the phenolic depending on how thick the carbon fiber is. So to calculate how much to mill down the sides so that the phenolic is flush with the carbon fiber, use the following formula (and see the image below for reference): z = 1/2*(t_{phenolic sheet} - t_{fin preform (G10)}) - (t_CF x Number of layers CF on one side of the fin)

1. Set up a 90-degree angle block on the mill and, after wiping down the top surface, apply machine tape and attach the phenolic such that the non-tapered edge lines up with the edge of the angle block. 
2. Select an end mill that has a diameter that is larger than the flat section of the phenolic (the part that you want to take off). 
3. Touch down on the surface and zero the mill. 
4. Move the mill off the part (to the left) and set it to Z = -0.020". Slide one of the G10 fins into the slot and hold it so that when you mill down the excess phenolic, the slot is supported and the ends don't break off. If you don't hold the G10 fin as you're cutting the part, the weight of the fin will cause the phenolic to pull up the tape and ruin your cut. 
5. Set the mill to "Forward" and cut such that the end mill moves to the right.
   
6. Vacuum the phenolic dust, then carefully remove the phenolic piece from the tape.
7. Lay down a new piece of tape and flip the phenolic piece over and attach it to the tape. 
8. Repeat step 4 except this time, to account for the 0.020" that you just took off and the extra 0.020" that you want to take off from the other side, set Z = -0.040". 
9. Repeat step 5-6, then repeat the whole process for the rest of the phenolic pieces. Make extras in case something goes wrong now or later. 

Part Seven: Assemble Fins (~2 hours)

1. Make sure that each G10 fin core fits into the phenolic slots. 
2. Use West Systems epoxy (do NOT use 5-minute epoxy) to attach the phenolic leading edge to the G10. In reality, this step was done after we bonded the fins to the fin collar because the phenolic leading edge did not fit into our fin can jig, but this step could also be done before the fins are attached to the fin collar if the jig is designed to account for this. 
3. Wait a bit and wipe away the excess on the sides and oozing out of the slot. 
4. Let cure for 24 hours. 

Part Eight: Bevel Fins (~4 hours)

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.