From August 2013

CCR75 – Tip to Tip reinforcement

The next and most difficult step is the tip-to-tip reinforcement. I’m using Aeropoxy PR2032/PH3660  with 2 layers of 5.7oz Bidirectional plain weave Carbon Fiber purchased from Aircraft Spruce.

I created a paper pattern using a printout of the fin and a lot of math. I made 3 copies of each pattern, and taped them to the cloth.

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The first layer is half the size of the second layer, which creates a flutter resistant reinforcement.

All 6 pieces were then cut out of the cloth.

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We encountered some problems here, since the original idea was to cut around the cloth and leave a bit of tape to prevent fraying. However, we soon discovered that trying to take the tape off resulted in more fraying than would have happened if we had just cut on the inside of the line. The next 5 pieces were carefully cut about 1/32″ inside the paper pattern.

 

Here’s what the top pattern looks like when all is said and done.

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The first step was to clean the airframe and fin surfaces with IPA. After this point, the airframe was not touched with bare hands until the process was complete.

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The next step was to mix up some Aeropoxy ES6209 for fillets. I made a 1 and a half gram batch, which ended up being almost perfect for what I needed.

 

After waiting about 2 hours for the Aeropoxy to start to cure, or “go green”, we started the layup.

I painted some PR2032/PH3660 on the surface and then laid the cloth.

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I then painted enough epoxy to wet out the first layer, and then laid the second layer.

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After the entire 12.7 gram batch had been used up, I laid some 0.0005″ mylar over the surface. Mylar is a great peel-ply material because it doesn’t stick to epoxy when the epoxy is cured, but it sticks down when the epoxy is wet. It also leaves an incredible shiny surface which looks great and requires minimal finishing.

We put about 33lbs of lead weights onto the mylar to compress the layup. This technique is essentially the best way to do a layup without having to use a vacuum bag.

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After waiting the ~6 hour cure time, I peeled back the mylar.

The resulting finish is pretty good. It’s certainly not the best I’ve ever seen, but it’s extremely strong and that’s what’s important for this project.

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Now, repeat all of that twice more, and the most tedious step can begin – SANDING!

CCR75 – Fin Bonding

The first step in bonding the fins is to sand off the gloss coat of epoxy on the carbon tube, to expose the cloth. I first cleaned the section of tube that was to be bonded with Isopropyl alcohol, and then sanded the tube with 150grit sandpaper until the shine was gone. I then cleaned the surface again with IPA.

You can see a few scratches here, you want it to be about like this; not too much more. It’s important to remember when bonding composites that you want chemical bonds, and not mechanical bonds. Thus, if you rough up the tube too much, you end up breaking the fibers instead of preparing the surface for bonding. 120grit is the absolute maximum that you should use for composite bonding.

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I then mixed up a batch of Aeropoxy ES6209. Apply some to the root of the fin, and fit the fin into the alignment jig.

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The entire surface is sanded here because i’ll be doing tip-to-tip carbon fiber reinforcement.

Here’s an overview of the alignment jig:

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The tube is clamped in the Workmate table, with the jig resting on top. The jig has adjustable spacing between the two blocks to allow for different tube diameters.

The fin is clamped to the side of the jig as shown, and this allows for the fin to be perfectly straight in all vectors.

 

ES6209 takes about 5 hours to cure with this setup. The other two fins are repeated as shown.

CCR 75

I have on order an all-carbon-fiber CCR 75 from Carolina Composite Rocketry. It has been modified to fit my 75/7680 case, to fly on an Aerotech M685W. RASAero simulations put it at just over 40,000′ from Black Rock. Hopefully it’ll be ready for XPRS 2013.

I’ll be doing an experiment to see if a tailcone will help the drag coefficient. Using the Rouse-tech aft tail cone closure on one flight, and a normal aft closure on another flight.

These tests will probably happen after the maiden voyage at XPRS.

CCR 75MM

asdad