Rocketry Planet

Final Steps
Well, all six of the tip-to-tip pieces are installed and they turned out great. As always, this is by far the most difficult part of the process. Now, it's time to finish the fin section to match the airframe. Most of the fin section will be finished using the same process that was used to finish the airframe. This involves applying six coats of epoxy (5 additional coats) and then sanding most of it off and ending with 320 grit sandpaper. Then, as with the airframe, two wipe-on coats of epoxy will be used to fill the scratches, and this will be followed by wet sanding to 1000 grit and polishing with 3M Finesse-It II polish. The wipe-on epoxy approach really saves a tremendous amount of work!

There are a few areas of the fin can that will take a little extra work. One area is where the carbon at the tapered parts of the fillets was slit. There are gaps in these locations (12 total) that simply need to be filled up with epoxy and then sanded smooth.

It is also necessary to blend in the top seam of the carbon to the airframe. As I apply the six coats of epoxy to the fin can, I'm going to go up a little higher on the airframe (about 1/4") with each coat. The first of the 5 remaining coats of epoxy will terminate about 1/2" above the top of the tip-to-tip carbon. Since I'm going to apply 5 coats of epoxy in total, the last coat of epoxy will end 1-1/2" above the carbon. Therefore, I'm going to lightly sand the airframe up to the point.

When the above process is completed, the tip-to-tip seams will be coated with quite a bit of epoxy, so it should be possible to just sand it flat without getting into the carbon at any point. In past efforts, some epoxy fill has been needed in this area to get things flat, either because the top seam is lying above the airframe or due to a bump resulting from the seam of the first layer of carbon. However, in this case, the seams worked out very well (the combination of the grooves and the application of the peel ply to a point above the top of the carbon pieces seemed to do the trick).

At this point, all of the tip-to-tip carbon has been applied and one coat of epoxy has been applied over the peel ply finish to fill the holes in the peel ply weave. Now I can go ahead and sand down the edges of the fins. They all turned out great with no areas where the cloth was separated from the edge.

I’ll also go ahead and lightly sand the entire area after the first epoxy coat. Note how I’ve sanded 1.5” above the seam area so that I can taper the seam into the body tube.

All 12 of the slit areas will need just a little epoxy fill.

And here is how things look after applying the second of six epoxy coats.

And here is how things look after applying all six epoxy coats. Note that each coat finished higher on the airframe, and the last coat finished just above the sanded area.

I start the sanding process by sanding the main area of each fin flat. I go far enough to remove most of the low spots on the fins (i.e., no shiny areas). Typically, there is an area near the fillet that is low for some reason.

I do not try to sand this area flat along with the fin area – that just results in sanding into the carbon.

Here, I sanded the body tube using a sanding block. It turned out completely flat. The secret to this was the light sanding of the original airframe (see pages 102 and 103).

To sand out the fillet area, I used a 0.80” dowel. Recall that the original Aeropoxy light fillet was sanded with a 0.93” dowel, so that 0.80” dowel is slightly undersized. The technique is to sand in the “crotch” of the fillet and then use the dowel to sand outward to the fin and to the body tube. The approach creates a smooth transition in each direction.

The drawing illustrates the dowel sanding motion that I have found works the best (i.e., sand the fillet area itself and then work outward in both directions). I don’t worry about the scratches in the finish. As with the airframe, these will be filled with thin coats of epoxy later on.

Sometimes, there are some low areas that can’t be sanded flat with the dowel. It is easy to accidentally sand into the carbon in the fillet, particularly at the point where the slits terminated. It is better to spot sand rather than continue to sand the entire fillet.

I spot sanded the low areas in the previous picture using the end of a slightly smaller dowel. It is better to do this then to take a chance of sanding into the carbon.

Next, I sanded the tapered areas around the perimeter of the fins. This was very easy to do and went quickly.

At this point, it’s time to sand down the seam areas above the fins. The application of the tip-to-tip carbon went pretty well, so with the six layers of epoxy added, I should be able to sand this area flat.

I used a sanding block to start the sanding process. I moved the block up and down using a rolling motion around the tube. The idea is to get the entire transition area reasonably flat.

Here is the result after a second pass with the sanding block. This is by far my best effort!. It’s the result of the grooves that were sanded into the tube, the use of the peel ply in the tip-to-tip procedure, and layering the six layers of epoxy up the airframe.

I got the tube sanded as flat as I was willing to do with the sanding block. Now, I’m going to use a thin piece of sand paper to sand out the remaining low areas.

Here is the result after the second pass with the sandpaper. There are only a few low areas (i.e., unsanded areas) remaining. Rather than continuing to sand into the seam area, these low areas can be spot sanded instead.

After a little spot sanding, there are no imperfections remaining. This is as good as I can do.

The same basic process is used to sand the aft end of the airframe below the fins.

Last, the fillet transitions need to be sanded. I used a set of dowels with 180-grit sand paper. The smallest dowel was about 1/8”. This turned out to be very easy

The secret to making the fillets easy to sand down is accurate sanding of the Aeropoxy Light fillets and accurate cutting and positioning of the tip-to-tip carbon and the peel ply. If everything is positioned correctly, it is then easy to sand flat. Here, I’ve lightly sanded the fin edges flat to help in smoothing out the transitions around the fin. This doesn’t need to be perfect because the fin edges and the transition area will later be covered with Contronics epoxy.

The same basic process is then used to sand the fillet transitions behind the fins.

The entire fin can was first sanded with 180 grit paper using a sanding board for the fins, fin taper and body tube and dowels for the fillet and fillet transitions. In retrospect, I should have used 320 grit with the dowels, as some of the scratches later proved difficult to remove. Here, I am sanding everything with 320-grit paper over a sponge.

Here, I’m covering everything with a very thin coat of epoxy. I tried to wipe off as much of this as I could using lint-free towels. This fills the scratches from the dry sanding, and I applied two coats.

Here is how things look after applying the two thin coats of epoxy.

My intent was to go to wet sanding after the two layers of epoxy. However, it became apparent that more dry sanding with 320 grit would be required to remove some of the scratches. This figure shows the improvement with the two coats of epoxy followed by more 320 dry sanding.

After the 320 dry sanding, I switched to 600 grit wet sanding. I used the sponge approach for the fins and between the fins, and a thin strip of paper for the body tube above the fins. I did not use a wider strip of paper because the transition area above the fins is not completely flat.

Here is the fin can after wet sanding with 1000 grit paper.

At this point, it’s time to do some final touchups. In the process of sanding, small bubbles within the epoxy are encountered. When you sand into them, they turn white. I’m going to give them a wipe of epoxy so that they turn clear. First, I open them up a little with the tip of a knife.

Here is another area that needs a little fixing. After opening up the bubbles a little, I outline the area with tape and then wipe on the epoxy. I then try to wipe off as much of it as possible. This doesn’t fill the bubble depressions, but they aren’t white anymore. This same technique can be used if there are places where you accidentally sand into the carbon.

The final step in completing the surface finish is to polish the surface with the 3M Finesse-It II. This will really improve the gloss and appearance of the epoxy. However, as I have done in previous rockets, I also want to coat the edges of the fins with Cotronics epoxy. Since solvents in the Finesse-It II could affect the Cotronics bond, I will apply the Cotronics first and then polish with the Finesse-It II.

I am applying the Cotronics to improve the heat resistance of the edges of the fins (and the leading edge in particular). This rocket design has flown up to Mach 2.3, and I expect to fly this particular rocket at least that fast. Although the leading edges of the fins are encased in Aeropoxy, it is possible that the Aeropoxy could fail due to heat generation at high speed. This would result in delamination of the tip-to-tip carbon, and then more bad things would happen. The Cotronics 4525 epoxy has a much higher heat-handling capability than Aeropoxy. I have used Aeropoxy to cover the edges of the fins on a number of rockets, and none of them have ever suffered a problem due to leading edge heating.

Cotronics epoxy is not particularly well suited as a finish material. It doesn't go on very smoothly, and the buildup around the fin probably causes a fair amount of drag. So, I try to improve on the technique of applying this material on each fin can. This time, I'm going to sand in a groove around the perimeter of the fin, and then I'm going to try to fill in the groove with the Cotronics. If this works, I'll get a smoother transition between the Cotronics and the fin itself. I am also going to try to sharpen the leading edges of the fins because, at this point, they are relatively blunt. We'll see how it goes...

Here is a little right-angle jig that I made to sand the edge of the fin to a consistent depth.

I sanded far enough to leave a little lip. I did this so that the Cotronics wouldn’t build up too high on the flat part of the fin.

After sanding in the lip, I sanded the leading edge (only) to a relatively sharp tip. The other edges were just rounded a little.

With the edges sanded, it’s time to apply the Cotronics. The first step is to tape off the portions of the fin that won’t be coated.

I applied the Cotronics with a small brush and then smoothed it out and removed the excess with a foam brush.

Here’s how the edges look after the first coat of Cotronics.

And here’s how I terminate the Cotronics on top of the fins.

After the first coat was partly cured, I used a knife to shave off any epoxy above the groove. After it cured, I sanded the first coat and applied a second coat.

I again shaved off any epoxy above the groove and then sanded the second coat after it cured. But this time, I sanded a little further into the fin (1/4” instead of 3/16”) in preparation for a third and final coat.

Here is the third and final coat.

The final finishing step is to polish the fin can with the 3M Finesse-It II. Here’s the before and after pictures. This stuff is amazing!

Remember that area that had some bubbles? I cleared out the bubbles a little with a knife point and then rubbed epoxy on the area. Here is how it looks after the polish.

Here are the three seams ahead of the fins. In each case, the seams at the top against the airframe are, well, seamless. There is no hint of the first layer of the tip-to-tip carbon. There’s a slight gap between the side seams on the 2nd and 3rd seams, but all things considered, this worked out very well.

Just a couple of shots of the final appearance.

All that is missing is a 7600 Ns motor!

Document: Adobe PDF of Jim Jarvis' Carbon Tutorial (9.8MB Adobe PDF)

Jim Jarvis is a high power rocketeer who resides in Austin, Texas, and flies with the Austin Area Rocketry Group. Jim holds a degree in Chemical Engineering from the University of Wisconsin and works as a project manager for a large engineering company, managing environmental control system projects on coal-fired power plants for the electric utility industry. For Jim's contribution of this article, he will receive a free Rocketry Planet T-shirt. This sponsorship is made possible by our friends at Graphix & Stuff, producers of high quality hobby apparel and vinyl signage. Want your own free gifts? Read the program details page for complete information.

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Reader comments:

#1 Re: Article: The Jarvis Illustrated Guide to Carbon Fiber Construction

Jim, absolutely wonderful work! I am so happy to see your beautiful craftsmanship fly, and I look forward to many spectacular projects from you in the future, and to flying with you again on the playa. Sexy, sexy carbon,------- faster, higher, period.........

Best regards - Dave Triano, ShadowAero

ShadowAero on 10-16-2009 09:54 PM

#2 Re: Article: The Jarvis Illustrated Guide to Carbon Fiber Construction

Great article! Thanks so much for the informative step-by-step.

Here's a pic I got of the two-stager at Balls this year:



Hope to see it make it the whole way next year!

daveyfire on 10-16-2009 10:28 PM

#3 Re: Article: The Jarvis Illustrated Guide to Carbon Fiber Construction

Wow, Jim. Just wow.

So that's how it's done when you do it right.
Your care and precision are awesome. What I've been doing is so half-***ed in comparison. Thanks for all the details. I'm definitely going to try to replicate your method for squaring up the tubes. That should be much more precise than the method I've been using. Cutting the grooves in the body tube for blending in the tip-to-tip layup makes a lot of sense, especially with your 5-layer tubes. I wondered how you got such precise butt-joints on the tip-to-tip layups, and now I know. Thanks for sharing.

Adrian A on 10-16-2009 11:03 PM

#4 Re: Article: The Jarvis Illustrated Guide to Carbon Fiber Construction

Quote:

Great article! Thanks so much for the informative step-by-step.

Here's a pic I got of the two-stager at Balls this year:



Hope to see it make it the whole way next year!

Thanks David. Great Pic!

Jim

JimJarvis50 on 10-16-2009 11:43 PM

#5 Re: Article: The Jarvis Illustrated Guide to Carbon Fiber Construction

Quote:

Jim, absolutely wonderful work! I am so happy to see your beautiful craftsmanship fly, and I look forward to many spectacular projects from you in the future, and to flying with you again on the playa. Sexy, sexy carbon,------- faster, higher, period.........

Best regards - Dave Triano, ShadowAero

Dave, it was great to finally get to meet you. Your instruction along with a few other key people is what got me started making my own stuff. I hope to be back next year.

Jim

JimJarvis50 on 10-16-2009 11:56 PM

#6 Re: Article: The Jarvis Illustrated Guide to Carbon Fiber Construction

This is an article that I will revisit often. There is too much great information in it to digest in one or even a few readings. Thank you Jim.

Steve_Shannon on 10-17-2009 01:30 AM

#7 Re: Article: The Jarvis Illustrated Guide to Carbon Fiber Construction

Jim, as always I am super impressed with the quality of your work. Your attention to detail is incredible, and it certainly shows in your results.

I do have a question or two, but they are not so much about the construction as some components used in the process. First, I would like some details about your oven- heat source, temperature controller, etc?

Secondly, within the oven do you have your mandrel attached to a motor to turn it while curing?

Thanks Jim,

Mark

markkoelsch on 10-17-2009 05:05 PM

#8 Re: Article: The Jarvis Illustrated Guide to Carbon Fiber Construction

Quote:

Jim, as always I am super impressed with the quality of your work. Your attention to detail is incredible, and it certainly shows in your results.

I do have a question or two, but they are not so much about the construction as some components used in the process. First, I would like some details about your oven- heat source, temperature controller, etc?

Secondly, within the oven do you have your mandrel attached to a motor to turn it while curing?

Thanks Jim,

Mark

I'm pretty low tech. The oven heat source is a heat lamp (or two of them) with no controller. I just prop open the lid to get the temperature I want. I prefer this to the light frequently turning on and off. During the initial cure of the tubes, i cure at 90F to prevent the peel ply from bubbling. At that temperature, or because of the peel ply cover, I haven't found a need to turn the tube. In general, I work on a 24-hour curing cycle, so I'm not in a hurry to get the epoxy cured.

Jim

JimJarvis50 on 10-17-2009 05:42 PM

#9 Re: Article: The Jarvis Illustrated Guide to Carbon Fiber Construction

Quote:

This is an article that I will revisit often. There is too much great information in it to digest in one or even a few readings. Thank you Jim.

I made another fin can over the summer, and I went back to the article several times myself. I'm kind of glad I wrote it all down.

Jim

JimJarvis50 on 10-17-2009 05:45 PM

#10 Re: Article: The Jarvis Illustrated Guide to Carbon Fiber Construction

Now that's what I call unbelievable craftsmanship, Jim!!! Your article does a great job illustrating your techniques because you made the effort to include so many details and pictures. Thanks for taking the time to document and share your awesome methods for working wonders with composites.

James L.

James L. on 10-17-2009 10:34 PM

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