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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.

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). 

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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. 

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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. 

Part Eight: Bevel 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.

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