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 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. Below were the steps used to manufacture them. All of the machining was done in Gelb (Todd's shop in the basement of the Unified Lounge).
Materials Required:
- 1/8" sheet of G10 (INSERT LINK)
- 1/4" sheet of phenolic (INSERT LINK)
- 15 degree end mill (https://www.mscdirect.com/product/details/04153326)
- 1/16" cutting bit (for mill)
Part One: Waterjet G10 Fin Core
Make DXF file
Turn on waterjet
Select proper settings, use lead in
Use weights
Make extras
Part Two: Waterjet Phenolic
Part Three: Cut G10 Tab
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)
