Laminate, Laminate, Laminate……

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     Sounds strange, but I like laminating things. Kevlar, Carbon Fiber, Fiberglass…. doesn’t really matter. Once you understand the process, it opens up a lot of options in both Rocketry, and the real world. In Rocketry, Laminating can provide key missing strength, stiffness, or just plain ruggedness to an otherwise undistinguished model. Combine lamination with foam, and a whole world of custom parts, or odd rockets can be created. In the photo on the right, you see what looks to be the launch of a boring Estes “Super Big Bertha” kit. In fact, the entire rocket was laminated with Kevlar, Carbon Fiber, and fiberglass. The end result – A kit that Este’s recommends launching with a D12-5 engine (max thrust of 29 N, total impulse of 16.8 N-s), that I launched with an Aerotech J570 (max thrust of 1142.5N, total impulse of 973.1N) – an engine about forty times more powerful. During this particular flight, I had packed the chute a bit too tight to fit in my electromagnetic ejection system. Ejection worked, but the parachute  never inflated. The rocket fell from 5000 ft, and didn’t have a scratch on it when I found it about 1/2mile away. You can check out the video from the flight below (you can see the camera duct-taped to the side)

Super Big Birtha Freefall

While I didn’t show the original fiberglassing of the magnaframe airframe, there wasn’t a lot of complexity involved. Just as I was leaving HPR rocketry the first time (thanks to the BATF), I had found a company called Fibersock that sold a knitted fiberglass “sock” that let you fiberglass a tube without any seams or complexity. Upon my return to HPR (thanks to Tripoli & NAR beating the ATF in court), Fibersock is gone, but the product is still around. Giant Leap sells a version they call “easyglas” which is exactly the same thing.  Basically you pull the sock over the airframe, tie it up on either side, and slather on epoxy laminate. The only modification I’ve made to this process is to use a Teflon coated release film to ensure a very smooth surface, and no air bubbles. This is unlike any peel-ply, or release film I’ve ever used. Very easy to remove, very smooth, super easy to cut and position. Frankly, it’s so well made, when doing tip-to-tip lamination on a small rocket, I’ll often use the same piece of cut release film over and over. Only place I’ve found it i s here.

     My plan was to to do tip-to-tip lamination of the fins with one layer of Kevlar for strength, and one layer of IMG_0009fiberglass for finishing. I’ve found Kevlar almost impossible to sand to a smooth finish once it is cured. a thin layer of fiberglass over the top of the Kevlar provides a great base to sand and smooth. I first laid some scrap newspaper over the fins to create the base outline of the fin area. I then cut out the newspaper, and used it as a base to build a cutting template out of foam board. The newspaper gave me the base dimensions, but I needed to add about 2 inches of cloth in each direction to ensure complete coverage, and allow for the Kevlar and fiberglass to wrap around the thick fin edge.IMG_0013 A typical rocket fin made out of g10 fiberglass would be about .093, to .125 inches thick. My fins and wings were about five times thicker, requiring some creative solutions down the road. My main goal in creating the laminating fin can is to ensure the fin’s don’t rip off during flight, and increase their strength in case the rocket lands funny. I plan on putting a LOT of time into the finish (I have an airbrush, and like to play….), and would be crushed if a great flight ended with a broken fin.

My normal method for tip-to-tip lamination is to use a fairly standard vacuum bag sealing process. Because of the size of the rocket, I could not create a bag that would encapsulate the entire lower sustainer with fins and keep a good seal. This may be possible, but I spent a LONG, LONG time on it. The primary advantage of vacuum sealing is it creates a lot of pressure on the lamination, squeezing the excess epoxy out through the rIMG_0021elease film (reducing weight), and creating a good mechanical connection between the epoxy, fabric, and fins. In my case, I was less concerned about weight, but wanted to make sure I would have a good mechanical bond, including the thick edge of the fins. There are lots of great tutorials on the web on how to do tip-to-tip lamination without vacuum sealing, but all of the ones I have seen assume you have a fairly thin fin. The best I’ve seen is here. In my case, needed ballast on the lamination to keep it tightly bonded to the fin, but I needed pressure on the edge of the fin as well. Using a fin jig as John did in the link will almost guarantee there won’t be enough pressure on the edge of the fin. I needed some kind of Ballast that would mold around the fin keeping pressure over the entire surface area during curing. I settled on using a  liquid as the weight, in a waterproof bag that could hang over the fin.

 

I should probably mention I spent a LOT of time sanding down the fillets. I also installed the boat tail, as I decided I wanted some lamination over the edge to ensure a smooth transition between the airframe and boat tail for

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finishing. That added even more sanding. Once complete, I ‘painted’ on a layer of epoxy (US Composites), put down the Kevlar, Wet the Kevlar, then added the fiberglass. Wet the fiberglass, then added the Teflon release film. Wet the release film, then added a Polyester breather cloth on top to suck up the excess epoxy, and provide a surface for me to put down my ballast. I filled three zip-lock bags about 3/4 with water as my ballast. One bag 

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per fin, and one for the airframe & fillets.  I know it’s kind of hard to see, but the bags on the fins hang over the side, providing positive pressure on the lamination during the cure. I left everything for 24 hours. Removed the water bags, then the Teflon release film.

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As you can see, it came out really, really well. I was a little worried with the lack of significant weight on the wings, I would get a lot of air  bubbles in the lamination over the fillets. big curves like this are notoriously difficult to make sure the Kevlar and fiberglass stay pressed against the fillet, especially when the water bags were pulling on the edges of the weave to secure them around the fins. . There wasn’t one bubble tip-to-tip. Happy with the process, I then repeated it twice more for each side.IMG_0045 I only had two issues with the process, neither was too critical. First, on the edges of each Fin, I had doubled the lamination. The second layer of lamination didn’t have the pressure of the first, which you can see looks like an air bubble along the leading edge of the fin. This isn’t a big deal, As I will be sanding down the second layer, which is mostly fiberglass. The second issue you can also see in the picture on the left is a few epoxy runs. This is a run of epoxy over a side of lamination that has already dried. Again, not too big of a deal, just requires sanding. As you can see from the picture above, I had to laminate over the pre-cut slots for the wings to get a true tip-to-tip fin can. The pre-cuts were already too small, so the next phase will be to sand the fin can, re-cut the wing slots, and install the wings. Should start to look like a real rocket soon.

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This entry was published on January 31, 2011 at 8:39 pm. It’s filed under Rocket related and tagged , , . Bookmark the permalink. Follow any comments here with the RSS feed for this post.

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