Better science fiction than for a funeral!
I agree 100% with the Tyvek full suit with this stuff. I have worked with fiberglass and epoxy for over 35 years now off and on on a lot of projects (I use epoxy to fix just about everything
). I use a full bodysuit with a full face forced air face mask when I have to do a lot of sanding. I can only imagine how nasty carbon fiber is to sand. Yuch!
Your fins look great BTW. I can't wait to see the finished project. Happy trails with the flying.
I picked up a tyvek suit. It was less than $10. No reason not to get it. The carbon fiber composite parts sand more easily than fiberglass (removal rate is higher with same grit sandpaper. The dust is finer and drifts further in the air. Both can embed in your skin and make you itch like crazy. Taking a hot shower makes the itching worse. The dust is bad news. Don't want to think of what it does to the lungs. Use protective gear, for sure.
I'm really happy with how my airframe turned out. Aside from a few pinholes, there will be very minimal sanding and prep required to get a really good surface.
This rocket is going up on an M1350, which has a 76 mm diameter. Turns out this is just a nominal dimension. The measured dimension actually closer to 75.3 mm, or a few thousandths of an inch less than 3". If you order a length of 3" aluminum tube, it is the same diameter as the motor casing, and this is perfect. Tube is specified by the outside diameter and pipe is specified by the inside diameter. I found that out while shopping for a mandrel. I ordered a 6 foot length of 3"aluminum tube with 1/4" wall thickness for around $60. I spent about an hour or two finishing the surface. You want the diameter to be uniform to within 1 or 2 thousandths of an inch (~50 um). If the diameter varies more, then it is OK if the tube is tapered uniformly across the length by up to 0.008" (200 um), but you definitely don't want an hourglass profile or pinched ends with a bulge in the middle. The removal in those cases is destructive to your part and can damage your mandrel and mood. Once the dimensions were corrected using 80 grit, I sanded out the scratches by working through grits 120, 200, 400, 800,1200, 2000, and then an automotive swirl removing polishing cream. Overkill, I know. But like any hobby, it is more fun when you over do it.
I set the mandrel up on wood blocks, and mounted an empty cardboard tube with 1/2 electrical conduit (use what ever is laying about the shop) behind the mandrel to spool up and feed out the carbon cloth. I did wax the mandrel with mold release wax. It shouldn't be required, but I did it anyway. Then I wrapped the mandrel with two full wraps of 0.003" clear mylar. This gives a really good fit between my motor and the airframe. Mylar doesn't bond with epoxy. When I made my first composite tube, I wrapped the mylar very tightly to the madrel. That tube never came off the madrel (in one piece). Now I wrap the mylar so that when I twist the mylar in the direction it is wound, it twists about a 1/4 turn before binding up, and will twist freely in the direction against the winding direction.
I get about 0.3 mm of thickness per wrap when I make a tube with 3k, 2x2 twill, 5.8 oz carbon fiber. To get an airframe with 1.5 mm wall thickness, I will need a 5 layer wrap, and this works out to a cloth length of 47". The cloth was cut and rolled onto my spare cardboard tube/ fixture. I also cut a piece of peel ply 18" long, to wrap the tube with two layers. I found that two layers holds the shape well while curing, and using less tends to have the layers relax on me with nasty wrinkles all over the airframe.
Fiberglass turns clear when it wets out. Carbon fiber isn't as cooperative, so it is important to pay attention to where you you have and have not yet painted on your epoxy to avoid voids and poor lamination. I like to put a layer of epoxy on the bare mylar before any cloth is placed, and then carefully position the leading edge of my fabric onto the madrel. The straighter you can align the fabric at the start, the less you have to fight the cloth to keep it straight as you roll it up. All the fighting and tugging must impart uneven tension in the cloth that starts to relax before the epoxy cures and causes an abundance of wrinkles. That's my impression of the process, at least.
I spread out some epoxy with a 1 inch disposable chip brush on the top of the mandrel. I then hit the new epoxy briefly (5 seconds max) with a heat gun. I don't want to heat the epoxy too much. I see a ton of tiny bubbles pop during this procedure, and the epoxy really starts to flow into the weave of the cloth. I turn the mandrel a quarter turn to bring new cloth to the top of the mandrel. I use the brush to slightly drag tension down and to the front of the mandrel. I don't want an overly tight wrap, just snug, aligned well, with no loose wrinkles or bubbles. Trying too hard here will cause as many problems as not spending enough attention. Repeat the process - apply epoxy, heat gun, brush the cloth lightly, rotate a 1/4 turn - until you get to the end of the cloth.
When I get to the end of the cloth, the tendency is for the cut edge to unravel. I pull one of the fibers out, a full fiber running the long length on the mandrel. This provides a nice guide to make a clean up cut on an otherwise ragged edge. I make sure the cloth is wet, then I then pull two more neighboring long fibers. Working the cloth with the brush at this point will cause a mess and a really ugly seam. I place the peel ply over the seam, and gently let the peel ply layer stick the frayed edge of the carbon cloth down to the tube. If you do this correctly, the seam of the tube becomes invisible. If you do it wrong, you have to paint a stripe on your rocket to hide the mess.
Here is the tube wrapped up in peel ply, ready for a cure at 85 deg F in my curing oven.
This is the ebay end of the tube. The peel ply leaves a woven texture that is perfect for bonding fins. The peel ply will stay on until I'm ready to bond the fins. The texture will be filled by light sanding with 200 grit sand paper, and using a rag to wipe a thin layer of epoxy across the surface.
Ok so maybe the seam is visible, but it doesn't look too bad to me, and it is better than my previous rocket.
I found the write up that Jim Jarvis compiled on
composite techniques to be incredibly helpful. I borrowed a lot of his ideas, but still have a long way to go on attaining the quality of his results.