Tip to Tip Carbon or Not?

[sailmike]

I am currently building a 3" diameter 5' tall rocket with a 54mm MMT. The body tubes are cardboard mailing tubes laid up with 2 layers of about 5 oz carbon cloth. The fins were laid up with 3 layers of unidirectional carbon and are extremely stiff. I cannot bend them at all with my hands. These fins are not going to flex in flight. The fins will be mounted through the wall. I'm trying to design this rocket to withstand the most powerful 54mm motor there is.

Now, I'm thinking of laying a sheet of carbon just up to the inside diameter of the body tube rather than all the way to the tips of the fins. My thinking is that I just need a good solid fin to MMT joint. If the fins weren't stiff enough then the tip to tip carbon lay-up would be necessary. Or will chopped carbon in the filets be sufficient?

What are your thoughts on this?

Thanks a lot,
Mike

 

[troj]

I think there are a lot of tip-to-tip layup jobs out there that just aren't necessary. In your case, a good, reinforced internal and external fillet is going to be more than sufficient.

If you want to go a little extra beyond that, then put a strip of CF on the fin joint -- no need to go full tip-to-tip.

-Kevin

 

[ben_ullman]

I think there are a lot of tip-to-tip layup jobs out there that just aren't necessary. In your case, a good, reinforced internal and external fillet is going to be more than sufficient.

If you want to go a little extra beyond that, then put a strip of CF on the fin joint -- no need to go full tip-to-tip.

-Kevin

I agree completely. There is already alot of reinforcement on it already it sounds like.

Ben

 

[DMcCauley]

I agree. Way overkill already IMHO. For that size rocket, epoxy fillets are all thats needed. No need for fiberglassing, even if you were using high peak impulse motors such as Warp-9s.

I don't fiberglass until at least 6 inches in diameter and only fiberglass less than that if i'm doing minimum diameter with surface mounted fins.

 

[MarkM]

I agree with the others. You've already overbuilt it. At most, 1 layer of 6 oz fiberglass would've been fine and without the need to glass the fins (unless you are using extremely thin ply) - and even that is usually not necessary. CF is expensive and use of it is rarely needed unless you're flying minimal diameter rockets with high thrust motors.

 

[sailmike]

That's what I thought. I used carbon for the fins because I wanted them to be thinner, besides, a friend of mine gave me the unidirectional carbon for free. I got good amount of the stuff, so I might as well use it to make fins. So, thanks a lot guys.

Mike

 

[troj]

That's what I thought. I used carbon for the fins because I wanted them to be thinner, besides, a friend of mine gave me the unidirectional carbon for free. I got good amount of the stuff, so I might as well use it to make fins. So, thanks a lot guys.

Uni carbon? Okay, I'm going to verify something, which is likely obvious, but I'll do it anyway...

You're adding multiple layers of this, when you want reinforcement, correct? Otherwise, it's adding "grain" like you get with balsa.

-Kevin

 

[sailmike]

Unidirectional carbon is not weaved like the cloth in your shirt is. Instead, all the carbon fibers go in the same direction. Mine has another layer of carbon fibers going in a 45 degree angle. All of that is sewn together with a yellow thread, which I'm guessing may be kevlar thread. So, three layers of this cloth actually puts 6 layers total, three at right angles to each other and three at 45 degree angle. Got plenty of this stuff left for many more rockets. It's way too stiff to roll into tubes, so it's just fins and bulkheads.

Mike

 

[Adrian A]

I agree with the others. You've already overbuilt it. At most, 1 layer of 6 oz fiberglass would've been fine and without the need to glass the fins (unless you are using extremely thin ply) - and even that is usually not necessary. CF is expensive and use of it is rarely needed unless you're flying minimal diameter rockets with high thrust motors.

I don't know why people keep saying that do-it-yourself CF is expensive. At Soller Composites, 1 yard x 50" of CF is enough for 3 3" rockets + tip-to-tip, and it costs $26. The equivalent amount of fiberglass is $17.49. So for a price difference $8.50, or a whopping $2.83 per rocket, you can have a CF-reinforced rocket rather than a FG-reinforced rocket, and it will be noticeably lighter, stronger, and stiffer. If you want to fly it to the same altitude as a FG rocket, it will pay for itself on its first flight because you can use a smaller motor. Frankly, I don't know why anyone bothers with FG when they can get a much more satisfying result with CF.

 

[gizza2]

I second Adrian's opinion. I buy 60" wide 5.8 oz CF at aircraft spruce for $38.50/linear yd:

https://www.aircraftspruce.com/catalog/cmpages/bigraphite.php

I used fiberglass on a number of rockets and, despite similar area densities, carbon fiber yields much greater strength and just feels less dense than glass fibers. Git sum.

 

[Chrisn]

You really need to hold a peice of carbon fibre to realise how light it is. We have a 75mm airframe and you could pick it up with your pinky and not even know its there if it wasnt for your sense of touch.

 

[sailmike]

I agree with Adrian also. Mine didn't cost anything! :headbang: The carbon I have came from friends dad who uses it to build aircraft and he had a lot of scrap left over.

On another note, it depends on which weight of carbon you get. I've seen 3.5 ounce carbon for about 150 bucks a yard.

Mike

 

[troj]

On another note, it depends on which weight of carbon you get. I've seen 3.5 ounce carbon for about 150 bucks a yard.

Also the fiber. Not all CF is created equal -- 1K vs 3K vs 6K vs 12K. The higher numbers are stiffer, and will cost more.

To give you an idea of where I'm coming from when I say it's expensive, I had some CF left over from a purchase, years ago. It cost me $12/yd, including shipping. That exact same fiber/weave was $58/yd, last I looked, without shipping. That's a significant price jump.

I don't use it often, because I rarely need carbon fiber.

-Kevin

 

[DMcCauley]

I used to use carbon fiber initially because i thought it was "cool", but i have found unless you are doing a super duper high performance minimum diameter rocket milking every last inch of altitude out of it, carbon fiber isn't needed.

Fiberglass is both cheaper and easier to work with and works great for 99.99% of those rockets!

 

[hardinlw]

I flew a Hawk Mountain Basilisk (2" rocket with 38mm motor mount and TTW 1/16" FG fins) at LDRS on a Loki J-1000. Peak thrust is 1400N with a 6.5 pound rocket yielding aroung 50 g's. No problems at all except that someone else's rocket was announced and then they pushed the button for mine and I didn't even get to see it leave the pad because I was looking the wrong place.

 

[Adrian A]

Also the fiber. Not all CF is created equal -- 1K vs 3K vs 6K vs 12K. The higher numbers are stiffer, and will cost more.

To give you an idea of where I'm coming from when I say it's expensive, I had some CF left over from a purchase, years ago. It cost me $12/yd, including shipping. That exact same fiber/weave was $58/yd, last I looked, without shipping. That's a significant price jump.

I don't use it often, because I rarely need carbon fiber.

-Kevin

I can see where you're coming from; CF had big price jump and avaialbility problem over the last few years because fiber production wasn't keeping up with the new 777 airliner, etc. But it's not nearly that expensive at the moment.

The K numbers refer to how many thousands of fibers are in each yarn of the weave. Higher numbers will result in a thicker, coarser weave, sort of the opposite of threadcount in sheets.

There are different types of carbon fiber with different modulus (stiffness), and the stiffer grades are somewhat more expensive.

The only time I use fiberglass instead of carbon fiber is when I need to use fiberglass, because it's non-conductive (for example couplers for av-bays, places where I want my transmitter to radiate out). Otherwise, even for a low-performance rocket I would prefer carbon fiber because fewer or thinner layers are required, and the final product turns out further away from being a brick. Reinforcing with fiberglass or CF takes about the same amount of effort either way, so to me, CF is just a much better value. Everyone has their own value judgement, but very few people use paper mache to reinforce their rocket for the cost savings. I'm just encouraging people who already go to the trouble of fiberglassing anything to consider using CF instead, because I think they'll be happier with how they spent their money when they get done. And don't forget, a little carbon fiber goes a long way, so don't overbuild!

 

[troj]

And don't forget, a little carbon fiber goes a long way, so don't overbuild!

The last bit of that is the single most important part.

10 years ago, composites weren't used much, at all.

Now, there are people out there (you know who you are) who think they need to use composites on any high power rocket. I once had someone tell me that he does, because he had a rocket where the spot it broke is the one place that wasn't fiberglassed. In that case, fiberglass was likely a band-aid to bad design.

There's a need for composites in some rockets, but nowhere near as many as you find 'em....

-Kevin