Kevlar tube

[Matt]

Hey guys

I've just starte construction of my new rocket, it's 75mm diameter rocket that is 2.2 meters tall, and will eventually run on a 8,000Ns experimental engine to 25,000 ft, I'll have pics up soon

Now i i just started and i used kevlar on the airframe (MAN IT'S STRONG!) but because it was 4.1 oz cloth it was rather thick and so the point in which it over layers is quite a big bump
I was wonder if anyone had any suggestion on what i cause do to prevent this on the next half of the airframe that i have to kevlar.

I was thinking taking out a few horizontal threads on each end and then having the thread that goes around the airframe kind of intertwine that wway theres no lose in strength and no bump, but i was wondering if anyone else ahd a simpler idea, thsi one could get a bit messy.

thanks heaps
Matt

www.nzex.tk

 

[Missileman]

I don't know how available they are in your neck of the woods but have you considered using a kevlar sock?

 

[shockwaveriderz]

Most people don't like to use Kevlar as opposed to fiberglass as kevalr when ytou sand into it has the tendency to produce "fuzzies"........

 

[daveyfire]

Originally posted by shockwaveriderz
Most people don't like to use Kevlar as opposed to fiberglass as kevalr when ytou sand into it has the tendency to produce "fuzzies"........

You... aren't... SUPPOSED... to... sand... the... laminate!

By sanding it, you're just destroying the structure you just created!

Wrap it with a layer of glass to give it some more strength, then fill it with a good high-build primer to smooth out the dimples! DON'T sand the laminate!

Though if you are sanding over straight Kevlar with just a high build primer, you'll probably nick the fibers anyways with the sandpaper... yeah that's no fun, takes LOTS of application of primer and filler and CA to make them go away!



I'll repeat one last time: DON'T SAND THE LAMINATE!

 

[Matt]

I use a 25g fibreglass veil for the laminate i do sand it very lightly with 250 grit sand paper just to smooth it off a bit then use the primer, then i sand the primer and then put a coat of paint over, then sadn that, and then a last coat of paint, yea there a bit of sanding to do but it's strong.

i just want to get rid of that bump from the layering, because it creates extra work and extras drag.

pic up soon

Later
Matt

 

[Loki]

I know this doesn't help, 'cause it's too late now, but don't use kevlar layups on rocket airframes. It's a waste of effort and money.

Kevlar is strong, but only in tension, which is exactly what you don't need in rocket tubes. Glass and carbon are both better alternatives. They are both easier to work with and the resulting tube will be stronger (stiffer). There are a few places in a rocket where I might used kevlar (e.g. recovery harnesses) but not for laying up airframes.

 

[Clay Kremer]

I agree in part, fiberglass is the most economical and easiest to work with but can add lots of weight and isn't nearly as stiff as carbon fiber. For most applications plenty strong though. Carbon fiber adds little weight and is extremely stiff. Great for laminating fins, airframes, mmts etc, but expensive and I wouldn't recommend sanding it without a respirator, nasty stuff. I use carbon in everything I build. Kevlar is pretty easy to work with if you have a good set of Kevlar shears but you can't sand it. It doesn't add alot of stiffness but it is almost impossible to tear. I like to use it on the ends of the airframe to reduce the possibility of zippers and on fin roots in conjunction with carbon in case of a hard landing. When using it on airframes I laminate a layer of 3oz glass for a sandable surface and smooth everything out with a lightweight body filler. Good luck with your project.

 

[nichanderson]

In response to the original question, use a braided sleeve (such as this ) https://design.sunsetrc.org/tubing/images/tube.jpg
to do away with the bump from overlaps. Plus it is much easier to lay-up, has no seam (and hence stronger) and can be tailored to fit different size tubes, along with more, or less strength in one direction. (by changing the angle of the braid in relation to the tube.

Kevlar is in fact very useful in most rocket applications. I agree a full Kevlar airframe is a waste of time and money. Rarely is a full aramid or carbon structure worth the cost and time involved. But you have to look at where rockets get damaged, and subsequently how they get damaged. I'd say more than half of rocket related damages occur on landing, more than likely from a) non opening main in dual deploy, b) undersized chute to begin with, c) hitting something much harder than the normal ground (rock, pavement, etc..)

All these are "impact" related damages, with the majority consisting of point loading during impact. Hence the use of composites is such a benefit, because they tend to excel in both these areas. Aramid (or Kevlar) especially, because it is so "strong" (regardless of direction). Again the benefit of composites. Any fiber+epoxy is defined as being a continuous fiber Polymer Matrix Composite(PCC).

As a result, the fibers take the brunt of the loading, from whatever direction it comes from. The epoxy (or Polymer used) serves as a medium to transfer the force to the fibers, along with keeping the general shape of whatever the part might be. How does this relate to rocket airframes ? Kevlar has a much greater tensile strength than that of glass or carbon. So ideally, all the force applied perpendicular to the airframe, will be transferred into a parallel force down these fibers. Ideally, being if your laminate is strong enough to resist the initial loading without simply breaking.

So Tensile strength needs to be in your vocabulary when dealing with rockets.
I was under the same impression until a few months ago, when I was involved in doing a finite analysis of composite structures for a small UAV project here at school. Plugging in all the relative strengths and modulus for the particular fiber we were using (Toray T700) and in relatively low speed, frontal crashes (10-15 m/s) we discovered we were significantly overloading our desired combination of 2 layers of 0-90 lay-up carbon. Hence we added a “safety layer” of kevlar in-between our two layers of carbon.. And all was good.

We ended up crashing this UAV, and our extra effort in finding a worthwhile kevlar fabric to use and integrating it into the airframe paid off wonderfully. Our airplane crashed due to tip stalling on high speed, low level high g turn. A wing hit, the uni-carbon on the wing buckled in some areas, and broke in others. Forward fuselage then hit, and our telemetry said hit with a force of 14 Gs.

F=MA Our plane weighed around 100 N, (22 lb) so ~140 m/s2 (14gees) times 100N is a pretty hefty impact force.. Much greater than we had ever planned for (and we planned to crash). About 2 inches of the forward fuselage “cracked”, and after that the carbon layers on both sides of the kevlar cracked completely off the kevlar for a good 12”. The kevlar stayed intact during the crash, and hence saved our motor and expensive prop from damage, and made repair extremely easy.

So after all that “jibbersh”, Kevlar is really good material to use, although not on its own. It’s a low modulus material, so isn’t “stiff”. So use it as a reinforcing aid, not as a structural material. There are some good charts, and data here too:
https://www.azom.com/details.asp?ArticleID=984#_Fibre_Type_Comparison

As for sanding the laminate, because composites distribute the stress effectively over the top and bottom layers, sanding is usually not a good idea.. Although for rocketry applications, airframes are usually so “overbuilt” as it is, sanding the laminate may be a worthwhile idea because it reduces the weight.. Plus hand lay-ups have a large layer of excess epoxy on the surface lessening the chance you’ll ever get to the fibers.

Nick Anderson
https://www.calpolydbf.com
https://design.sunsetrc.org