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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.
Description of the Part
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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
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Requirements:
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Needs to be able to withstand heat from flying up to mach 3.5 (for stage 1)
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Cannot break or flutter too much as the rocket flies
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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.
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.
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| Fin 1 | Fin 2 | Fin 3 | Fin 4 | Extra Fin (1) | Extra Fin (2) | |
|---|---|---|---|---|---|---|
| Average thickness x (in) | ||||||
| Y-value of first cut = (x - tab thickness*)/2 | ||||||
| Y-value of second cut = Diameter of cutting tool + x - tab thickness* |
*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 passesAfter 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
- Clamp your phenolic piece horizontally (the 1/16" cutting bit will come in horizontally). It does not matter which side you select for the slot, as when you flip over the piece, it is symmetrical.
- Zero the cutting piece at the center of the phenolic.
- 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.
- 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.
- 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.
Use oil so that phenolic doesn't heat up too much
Part Five: Cut Phenolic Taper
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