Bonding CF and Fiberglass

I'm going to be building the Giant Leap Rocketry "T-Bolt" minimum diameter 38mm rocket. I'm going to substitute it's fiberglass airframe for a carbon fiber airframe instead. The kit also comes with a Acme fiberglass fin can, which I'll be attaching to the carbon fiber tube.

My questions are very basic--as this is my first CF build. I have done a few fiberglass rockets with success:

A.) Do I need to give the CF tube a Dawn soap and water bath like we do with fiberglass?

B.) With a fiberglass airframe, I would normally sand the entire tube with 220 grit sandpaper, and wash the tube in alcohol when finished. Then I'd sand the area where I planned to attach the fin can with 80 grit sandpaper, and use a hobby knife to etch a "cross hatch" pattern into this area to ensure good adhesion of the epoxy. What would be a good procedure for prepping the CF?

C.) I plan to use Rocketpoxy G-5000 to bond the fin can to the CF tube. Any reason to change this?

D.) I'll be using JB Weld to epoxy a stainless steel "hardpoint" retainer inside the CF airframe. The shock cord will attach here. Any caveats here?

Any other issues or suggestions?

Grateful as always for the expertise of the TRF community.

Personally I'd save the the CF airframe and pair it with CF fins. Those Acme fin cans aren't light or aerodynamic; at least in terms of what can be achieved with some CF fin stock and a better fin design.

Beyond that the only comment I'd share is that the JB Weld for the hardpoint is probably not necessary. If I were you I'd use the RocketPoxy for that as well.

Not sure why you are using a stainless hardpoint for the motor fixing. If you can get it light enough then it is a reasonable substitute to aluminium. If you have the capabilities, go for it. Make sure you have enough surface area to withstand the required design forces. Anyway, I would skip the JB and use something rated better for shear forces.

I have bonded some G10 isogrid cores with CF plate skins for fins on my Apache. Only had it to Mach 1.2 or thereabouts so far, but no issues at all. Here, FYI if you want a look https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019&start=30

Sorry--sleep deprivation kicking in. The hardpoint retainer is aluminum, not SS. It's affixed with JB Weld because it has to withstand ejection gasses. This one allows ejection gases to pass through for motor ejection.

Using CF for the toughest possible airframe, not performance.

It's affixed with JB Weld because it has to withstand ejection gasses. This one allows ejection gases to pass through for motor ejection.

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I would still not bother with JB. The heat capacity of the aluminium will keep it from any significant heat rise due to ejection gases.

lowga said:

Sorry--sleep deprivation kicking in. The hardpoint retainer is aluminum, not SS. It's affixed with JB Weld because it has to withstand ejection gasses. This one allows ejection gases to pass through for motor ejection.

Using CF for the toughest possible airframe, not performance.

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Fiberglass is plenty tough. I would save the expense and go with fiberglass.


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As for bonding CF to FG, both composites use similar epoxies to make them. Usually whatever you would use to bond FG to FG will also bond CF, so System Three T88, Rocketpoxy, ProBond 4500, US Composites 150 etc.

And yes the CF needs a bath too.

And if you sand the bonding area with a coarser sandpaper, such as 120 grit or even 80 grit, crosshatching with a hobby knife has no value. Just be sure to clean the dust off, regardless of the material.


Steve Shannon

I've used sixty grit before for roughing up CF fins for fillets.

plugger said:

Personally I'd save the the CF airframe and pair it with CF fins.

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The fin flutter Mach goes up with fins of CF. And you don't have any RF transparency issues for electronics with a fiberglass tube. And with rocketpoxy I've had this combo with rocketpoxy survive 5400ft falls. This was good enough materials for nationals at URSC SEDS. We tried all carbon fiber, it was expensive and not really needed actually below Mach 2. Put the money saved into more reloads or a better tracker, because you won't see it. Like others have said fiberglass is generally tough enough. And if somebody wants an attitude of Fiberglass isn't good enough. Explain to them how basic HPR comps use it. Very hard to beat economics of fiberglass and it's not that phenolic stuff that isn't Mach rated.

Experimenting with the CF Airframe for several reasons:

1.) Wanted to gain some experience working with carbon fiber--and I got a good deal on the airframe.
2.) The rocket is called "Blackout" and I liked the looks of the material. Will be painting the fin can and nose cone flat black, and leaving the CF unpainted with just a few layers of clear coat.
3.) The tracker is going into the Ape-RC nose cone modification, so no concerns about RF with the airframe.
4.) It's a hobby--and it sounded like fun.

I'll prep and sand as normal with fiberglass, and appreciate the advice as always.

Sometimes we do things just for fun. Go for it

Yeah. Carbon fiber is real pretty when 1200 grit is used. I bet some people can make it look even nicer. Oh if your gonna sand a bunch of it do it outdoors or get a N-95 paint respirator from harbor freight or better. The dust will hurt lungs for short/moderate exposure in decent quantities when inhaled. I preferred running water outside and sanding wet. If your just roughing up the fins no bigger but on long tubes for lots of sanding you personally may have problems with particles.

Andrew_ASC said:

The fin flutter Mach goes up with fins of CF.

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So are you saying that a CF fin show less flutter at the same thickness and speed a fiberglass fin does? Because that's how I'm reading that statement.

Andrew_ASC said:

And you don't have any RF transparency issues for electronics with a fiberglass tube.

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Agreed. But that's why I house trackers/transmitters in FG nose cones.

Andrew_ASC said:

And with rocketpoxy I've had this combo with rocketpoxy survive 5400ft falls.

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And I've had hand rolled CF airframes survive a flat spin fall (as in no deployment charge fired) from 12k'.

Andrew_ASC said:

We tried all carbon fiber, it was expensive and not really needed actually below Mach 2.

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CF fabric is definitely more expensive than fiberglass. That said neither are "needed" above Mach 2. But CF will get you to Mach 2 faster as you vehicle will be lighter and therefore faster on any given motor when compared to a FG airframe.

Andrew_ASC said:

And if somebody wants an attitude of Fiberglass isn't good enough.

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I don't understand this at all.