Carbon Fiber Newbie Questions

Discussion in 'High Power Rocketry (HPR)' started by lowga, Oct 17, 2018.

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  1. Oct 17, 2018 #1

    lowga

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    I've attempted in vain to research the answers I'm seeking on this forum, Google searches, etc. So here goes nothing. Pardon the newbie questions.

    I'm attempting to build my first ever rocket with a carbon fiber airframe. The desire isn't based on flight profile, but rather aesthetic reasons. I want to build an entirely black rocket using a mixture of elements (nose cone and fin can) that are painted flat, matte black, and an airframe that has a polished, carbon fiber black color.

    It will be a minimum-diameter bird, build from parts that I had laying around (except for the airframe which I purchased.) 38mm, using a fly away rail guide instead of launch lugs.

    My questions are many:

    1.) Any special precautions when working with CF? Should I bath the airframe in Dawn dish soap like fiberglass?

    2.) Recommendations on how to sand or prep the airframe? I want to cover it with a gloss clear finish after decals are applied?

    3.) Best way to bond the fiberglass fin can to the airframe? I'm assuming sand and bond with epoxy as normal? Rocketpoxy is my adhesive of choice.

    BTW, the rocket is called "Blackout." I was inspired by a hot rod build that I saw at a local speed shop. The car was a Dodge Charger (late model) that had been extensively modified with carbon fiber elements. Every inch of the vehicle was either CF black, or a matte flat black. The result was striking. I immediately thought that an MD rocket would also benefit from this treatment.

    Thanks in advance for any advice or recommendations.
     
  2. Oct 17, 2018 #2

    djs

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    Following this as I'll be doing the same over the winter. I know this gets into a religious argument, but does anyone have a feeling for the "speed" of Rocketpoxy? My specific goal is to break Mach 2.
     
  3. Oct 17, 2018 #3

    Andrew_ASC

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    I trust Rocketpoxy to M1.4 based on theoretical calculations. After M1.7 I get thermally worried and start picking cotronics 4525IP or 4700. That’s just me. Most people don’t do adv compressible fluid flow calc on an epoxy. And yes my fillets aren’t as nice as you alls.

    As far as safety concerns wear a mask with N95 filter if sanding large amounts. Do not sand indoors. Wear eyewear if going to cut it or especially grind it on a bench grinder. Disclaimer, I’ve only done two minimum diameter carbon fiber rockets. One multistage, one single stage. The faster M1.7-2.4 multistage got Cotronics 4700.

    Sand and bond is fine for joining fiberglass to CF.
     
  4. Oct 17, 2018 #4

    rharshberger

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    Treat it similar to FG ( the fabric is the main difference, the epoxies are pretty much the same for CF and FG), just don't sand CF to deep and hit the weave or it fuzzes (not like kevlar though).
     
  5. Oct 17, 2018 #5

    lowga

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    What about in terms of finishing the CF? I want to retain that "CF Look" but coat with clear gloss after the decals are applied. Should I just lightly sand with #400 or so before applying decals? Or should I leave it alone entirely except where I'm joining the fin can?

    Thanks for the interest in the topic. While I'm sure this will be a fast bird, that isn't really my goal. Just want it to look cool.
     
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  6. Oct 17, 2018 #6

    Andrew_ASC

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    I’ve sanded CF with 1200 grit for finishing. I wet the fine grit sand paper with water. I’ve used as low as 220 for the places where fins join. You could try a 1000 grit, clear coat then 1200 grit wet. I’m no finishing expert.

    In my rockets we left the CF bare but went to 1200 grit wet.
     
  7. Oct 17, 2018 #7

    lowga

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    Thanks Andrew. Let me order some #1000 and #1200 grit sandpaper. Exactly the kind of advice I was hoping for.
     
  8. Oct 17, 2018 #8

    Andrew_ASC

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    3E53C775-85EA-4477-814A-FC50579C4F68.jpeg 775DEE9B-157F-405E-B76A-9B27434C0C07.jpeg This is for finishing results demo only on a 24mm MD. I didn’t even clear coat it. At the aft end near fins I roughed the entire cylinder with 220 grit you can see it’s duller, then went over that with wetted 1200 grit after the fins were glued on. Just to show what bare CF can look like.

    What’s odd with CF when you wet sand it again it gets shiny again. This particular tube was from BlackCat Rocketry from the UK.
     
  9. Oct 17, 2018 #9

    Andrew_ASC

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    B229C242-A38A-41D3-A23A-32EC8308821A.jpeg FD531EE2-9241-4DBB-8722-F62F8D1CCC81.jpeg
    These were Madcow CF tubes 29 and 38mm that wound up on a university project. Again just for visuals. Some CF tube has different appearance.
     
  10. Oct 17, 2018 #10

    boatgeek

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    IMG_1732.JPG
    This was sanded with 220 then 320 and clear coated with gloss. No need to go to 1000 if you don't want to.
     
  11. Oct 18, 2018 #11

    CoyoteNumber2

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    Watch out for slivers, especially near cut edges.
     
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  12. Oct 18, 2018 #12

    RocketScience

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    Carbon fiber dust will make you itch, especially forearms. Suggest getting lab coat you can wash or throw away. We are extra careful with carbon dust and inhalation. Carbon fabric has less issues than unidirectional with sanding. We sand transitions and put a clear coat of epoxy over and sand 150, 220, 320, 400, 800 wet, 1000 wet, 1200 wet 1500 wet, 2000 wet and orbital sand with boat polish. Some stop at 400 and do a wipe-on and wipe-off application of epoxy to fill sanding grooves and then straight to polish. I would not overcoat but that's me. Unidirectional will have issues with break-out when drilling - use sharp drills/reamers and let them do the work, drilling through tape or otherwise providing support on the backside.
     
  13. Oct 19, 2018 #13

    David Schwantz

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    Afew years back, wet to ER with chest pains, gave me nitro pills and told to see regu;lar Doc in AM. Long story short, did all the heart tests and it was found that I had been grinding a lot of CF for a plane project on the belt sander, with no mask. Wear a mask!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     
  14. Nov 2, 2018 #14

    jolevich

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  15. Nov 2, 2018 #15

    jolevich

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    Just came across your post. I have flown the rockets you see in the photos between 2,000 feet/sec and up to 2,700 feet/sec which is around Mach 2.5. many times, up close to 25,000 feet. Both rockets were constructed by using 100% rocketpoxy. Never had any problems with the Rocketpoxy not holding up. I think when newbies try to calculate if an epoxy will hold up to a certain Mach speed most will just look at the writing on the side of a box and/or tube of epoxy or marketing materials for maximum temp ratings (yes even some material tech data sheets fall into this category of marketing materials) and will look at the Tg (glass transitional temp) and make assumptions from there. This is partial correct but there is more to take into account than just looking at the manufacturers max temp rating or Tg. You also will have to be realistic on how long your rocket would actually be at Mach 2+ speeds with extreme elevated temperatures, usually it is very brief and not long enough to cause a serious structure failure with Rocketpoxy.

    ROCKETPOXY G5000 was developed for using as a high strength structural epoxy with a published temperature rating of 225 Deg. F. When the Maximum temperature rating is exceeded ROCKETPOXY G5000 will not have a catastrophic failure it will just start to soften up and get “gummy” when the temperature cools down below the rated temperature it will usually hardened back up. Rocketpoxy is very conservative on the temperature rating and my experience has been that many epoxy companies do not rate so conservatively so always read with some skepticism the maximum temperature numbers ratings that you see on the side of marketing boxes/tubes they are to some extent what the marketing department allows to be published and not always based on the exact breakdown of the epoxy. Keep in mind that some epoxy companies are just rating the Max. temp as what a large brick of cured material can hold up to a 10 to 30 second spike of the so called published max temp., Rocketpoxy tests and rates for continuous use for small 100 gram mass.
     

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  16. Nov 2, 2018 #16

    flynfrog

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    1. No, The dust is less harmful than fiber glass. A dust mask and some glasses is fine. Not sure why people do the whole bathtub with dawn thing anyway. IPA Acetone wipe are more than sufficient.

    2. What gloss finish are you going to use. Automotive clear coat? You could sand with 400 grit and spray it. Depending on your tube you could just wetsand to 1200 or so and buff it.

    3. Read the sticky. If its a true slip on fin can you can drill some holes and inject the epoxy to bond the can in place.

    :lol:

    See above. You have a few options. It really depends what your tube looks like when it comes in.
     
  17. Nov 2, 2018 #17

    Tyler P

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    The reason that the CF is getting shiny again when you wet sand is because you're using a grit fine enough to polish it. If you wet sanded with 400 grit, you wouldn't get shine afterwards. 1000-1200 grit can be used to polish hard substances.
     
  18. Nov 3, 2018 #18

    MaxQ

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  19. Nov 3, 2018 #19

    MaxQ

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    Amen to that.
    Don't cut cured CF in the family room...and vacumn up the area you cut CF after curing thoroughly ...because those stiff little fibers will hurt.
    Removing those cured CF splinters after you get poked by them is not fun either.
     
  20. Nov 3, 2018 #20

    Barkley

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    The best way to get the 'wet' look is to just wipe epoxy on the surface, then wipe it off. What's left is just a bit to fill any voids, then progressively sand to 1000. Rinse and repeat.

    If you haven't laminated before then I'd really recommend using FG first. Practice on the cheaper stuff, then move to carbon when you have the skills to get your end result. Also, you've said minimum diameter and Mach 2+, so I'm going to strongly recommend using tip to tip support for your fins. The best way is using a vac bag, but you can get almost as good with a sandbag for compression.

    I rolled a CF tube for my BA Baracus build, and had pics right up to when we gloved up. Was going to link to it, but all the pics are red x's, so here are the ones you're probably most interested in. Here's the layup on the mandrel, 1 wrap of mylar film, sprayed with Frekote mold release, 5 wraps of 2x2 twill, peel ply, compression tape, then hit it with the heat gun.

    [​IMG]

    After the epoxy has cured while removing the peel ply:
    [​IMG]

    Here it is wet sanded to 400 wet (and it's improved since):
    [​IMG]

    In that pic you can still see a hint of the spiral from the compression tape. I got it to 400, then brushed on more epoxy and wiped it off, then got it to 1000. If you use sheet peel ply instead of the peel ply tape it reduces the spirals, but I like the additional compression you get with the taped peel ply. I'm convinced it's a stronger tube, but I have no desire for destructive testing to prove it.
     
  21. Nov 3, 2018 #21

    FalconRocketry

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    When building with CF, it is pretty much exactly the same as FG. Clean and sand and clean again before bonding.
    Be careful of the dust. Wear a mask whenever sanding, cutting, or doing anything that produces dust. A pair of gloves is also necessary. The thing about epoxies and composites like this is, they can cause you to become allergic and have serious reactions to the material the more you are exposed to it. CF is, from what I have heard, believed to be more toxic than FG, so take whatever precautions you can. A simple respirator with a VOC and particulate filter will protect you from the fumes of the paint (Unless using urethane paints) and the dust. A damp paper towel under your work area will also help with clean up.

    If you are wanting to get a really good finish, sand lightly with some 220 grit to get the major dings out, then wipe down with some acetone or rubbing alcohol. Spray on 2-3 coats of a good clear coat. 10 minutes between coats is what I usually do. I personally use Urethane based clear (Automotive paints) but I have also gotten good results from a Krylon can of gloss clear. You just have to go lightly at first then let it build up. You want it wet to the point just before it drips. Once this has cured completely (24 hours is a good rule of thumb) You go to town with wet sanding. I start at 1000 then go to 1500 and up to 2000. You can then go over it with a micro fiber buffer wheel, or by hand, and use a polishing compound. After you are happy with the results, simply polish it up with a new micro-fiber cloth and some car wax. The wax will give it a nice smooth surface, and hide whatever scratches are in the paintwork.

    Note: When using decals I usually have a glossy paint on the rocket before putting them on. I recommend a couple coats of clear prior to laying on the decals. It helps them lay smooth and without air bubbles.

    -Matthew
     

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