98mm N5800 MD rocket for BALLS

SilicdyneTRF
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You might want to test whether a thicker coat is better than a thin one. I have one NC with thick paint on it. The paint can flake off under the right circumstances. The paint is bonded stronger to itself than to the nosecone. In such a situation, under sufficient pressure and thermal stress, the paint might come away in sheets - once the first flaw is exposed. A sheet of paint ripping off and blasting into the fins would not appeal to me. The resulting asymmetry in the nosecone would not appeal to me either.

Gerald
 
Slightly off topic, but maybe not. For my antique tractors, you can now get mufflers and manifolds covered with a ceramic coating. I don't know if this would work, but thought I'd throw it out there. I have not purchased any of these items yet, because my tractors work and are not show queens. I don't mind manifold and mufflers that turn color from the heat. I can give out the company info that does this it you think it would be helpful.

Adrian
 
New Ocean said:
I always wondered about this stuff:

https://www.cotronics.com/vo/cotr/pdf/6105sp.pdf

The 350 cure keeps this out of the range of normal composites though.
Click to expand...

I had a conversation with one of the Cotronics engineers a few months back. We talked about the application and the suitability for it of many of their products. Their main concern for this and other of their ceramic-based products was damage by moisture. I don't recall the specifics of the conversation, but I got the impression this engineer was concerned about degredation of the products at ambient conditions over time. The concern was not related to landing in a lake, rain, or something like that. He recommended their 4525 epoxy, which is what I have used for many years and continue to use. I've taken that up to Mach 2.6 or so with no ill effects. Beyond that, I don't know.

Jim
 
You don't need a steady hand. You can make a jig. A carrier can stabilize the fin in a fixed location, or it can even be temporarily attached with 3M77 (temporary, if you don't use much). Tapered wood strips can form sanding guides. A board with sandpaper in the middle region can glide over the sticks. Sand down until it is gliding over the sticks. Swap fins, repeat...

Fairly good repeatability can be obtained with this sort of method. It is a low-tech approach. I've used it for making pretty accurate airfoils on cores for vacuum bagged tailfeathers for RC gliders, some years ago. Same principle and methods, just slightly different materials.

Gerald
 
cjl said:
Chemical bond, possibly with T2T as well (that's still up in the air at this point, depending in part on how sturdy I feel that they are after the surface mounting). I'll be significantly scoring the bonding surfaces, and the fin root is quite thick (I didn't take much material off near the root, so at the root, it's pretty much just a biconvex airfoil with the thickest point at 0.44"), so there should be a lot of bond area for it to grab on to. My biggest concern is fin alignment - if aligned properly, the forces on the fin aren't bad, but with even a slight misalignment, they become enormous.
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What about just reinforcing the joints with CF? That way you can still keep your airfoil and have added surface area to keep them on to the airframe.
 
Tominator 2 said:
What about just reinforcing the joints with CF? That way you can still keep your airfoil and have added surface area to keep them on to the airframe.
Click to expand...

Well, that's happening regardless - they're getting fairly solid fillets with chopped cf and colloidal silica. The t2t is only if that feels inadequate.
 
In reading through these threads, I've wondered a few things:

1) Ceramic. Seems to me, for places sensitive to heat --NC, fin edges-- ceramic would be desirable. No? Too heavy? Too brittle? Doesn't dissipate heat, thereby causing a degradation of underlying structure? What?
2) Fins. I dig that more fin= more drag. but wouldn't fins with a longer root make for a better attachment in minimum diameter? This applies, too, to cans. This seems like one of the challenges of a build of this type (not just for the N5800 attempts, but in general).
3) NC shape. Seems the gravitation is to long, pointy NCs. Wouldn't these be more susceptible to heating at mach through the denser air, thus, creating a potential failure point? Seems that a less aggressive conical cone would actually be of benefit, no?

Forgive my ignorance. A lot of the discussion on these build threads is over my head. I've no experience with minimum diameter, supersonic flight, or anything close to the power of the N5800, but I am fascinated by the idea, and have much respect for those up to the challenge --because I ain't.


Later!

--Coop
 
Bare Necessities will have a 28" (7:1) conical nose, which not only helps drag but also helps with stability. The blunter tip of the Von Karman nose shape seems to cause more forward shift of the center of pressure at high Mach numbers, according to RASAero.
 
Coop said:
In reading through these threads, I've wondered a few things:

1) Ceramic. Seems to me, for places sensitive to heat --NC, fin edges-- ceramic would be desirable. No? Too heavy? Too brittle? Doesn't dissipate heat, thereby causing a degradation of underlying structure? What?
2) Fins. I dig that more fin= more drag. but wouldn't fins with a longer root make for a better attachment in minimum diameter? This applies, too, to cans. This seems like one of the challenges of a build of this type (not just for the N5800 attempts, but in general).
3) NC shape. Seems the gravitation is to long, pointy NCs. Wouldn't these be more susceptible to heating at mach through the denser air, thus, creating a potential failure point? Seems that a less aggressive conical cone would actually be of benefit, no?

Forgive my ignorance. A lot of the discussion on these build threads is over my head. I've no experience with minimum diameter, supersonic flight, or anything close to the power of the N5800, but I am fascinated by the idea, and have much respect for those up to the challenge --because I ain't.


--Coop
Click to expand...


The fact is, ceramics and other exotic materials are not needed, and they impose severe financial and design limitations. Remember this rocket will see high mach numbers (above mach 3) for a few moments only. With sacrificial layers of paint, or other material like epoxy, one should be fine. A team already flew an O motor with some phenolic on the leading edge, and it worked fine. (To mach 4.2 I think.)

A conical nosecone is prone to be sharper at the tip and experience more heating, and it is far from aerodynamically efficient. Again nosecone heating is less of a concern than overall dynamic pressure on the nosecone. Heating is only an issue where it may prevent the nosecone from surviving this pressure, and I understand the OP is using high temp. composites that are, one hopes, also stronger and thicker than normal.

This fins are very thick at the base, allowing for better adhesion to the airframe. Though I would personally consider adding metal rods through the inside of the airframe, drilled into the fins and epoxied there. Although that too comes with a major risk; it will better couple the fin interior with motor heat. Still, steel should conduct heat poorly enough to prevent this, imho. After seeing what happened to a partially welded fin can on a similar flight, I would worry the most about fin attachment.
 
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MClark said:
We used wood nose comes back in the 90s. With a Kosdon M3700 the wood will come apart and peel back. If the wood is wrapped with fiberglass tow it will protect it. The air gets in the end grain.
Mark
Click to expand...

Typically I cut 20-ish donuts out of wood, glue the 20 pieces together alternating the grain orientation at each layer, and cut the final shape on a wood lathe. A few coats of urethane varnish or spray paint finishes the job.

Wood is fine for typical HPR.. extreem project probably not..

-->MCS


.
 
JimJarvis50 said:
I had a conversation with one of the Cotronics engineers a few months back. We talked about the application and the suitability for it of many of their products. Their main concern for this and other of their ceramic-based products was damage by moisture. I don't recall the specifics of the conversation, but I got the impression this engineer was concerned about degredation of the products at ambient conditions over time. The concern was not related to landing in a lake, rain, or something like that. He recommended their 4525 epoxy, which is what I have used for many years and continue to use. I've taken that up to Mach 2.6 or so with no ill effects. Beyond that, I don't know.

Jim
Click to expand...

I have not tried ceramic-based coatings on a nose cone, but I did make a nozzle this way. The surface irregularities after coating were more pronounced than I expected. It was slightly bumpy to the touch. Some of this might be polished-out, but I didn't try.

The problem with the nozzle was that the coating abraded off.

-->MCS


edit: just to be clear, I sent the nozzle to the cermaic shop and let them do the coating. The coating was much thicker and higher quality than DIY paint. Still didn't work.

.
 
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tbonerocketeer said:
And how do you propose to do this? It is minimum diameter after all.
Click to expand...

You drill small holes through the airframe near the base, directly into the fin material. Two is minimum (as one would simply be a pivot point) and more than three is probably overkill.

So the hole is drilled from the inside of the airframe directly into the fin, or if you can, pre-drill everything and line it up. You then slide a thin piece of metal through that hole, into the fin, and epoxy it in place with high temp epoxy. Obviously the metal rivet should be flush with the inside of the airframe, and may have to be sanded after installation to make sure the motor can slide in. This would give me more confidence than a surface mount alone, even with thick fillets.

In a 75mm kit, I found these gave a remarkably strong bite, despite the relatively thin airframe. With this rocket, I presume the CF airframe has some thickness to it. (It will need it.) Surface mounting without T2T worries me, so I am just rambling about a possible solution I would consider. A tradeoff would be reduced airframe strength although from what I gather, airframe strength is not a major concern for this project.

I have been using brass for this, but any metal other than aluminum would be best as aluminum is a far better conductor of motor heat.
 
The way I see such an approach failing, is the bond to the fin failing first - the bond being whatever primary attachment method is used. After all, that will be the stiffest part of the joint. After that fails (assuming), the rivets will last about a millisecond. The rivets themselves cannot even begin to take the load, and won't take any load until the rest of the joint fails. So what is the point of the rivets?

Gerald
 
I could see one advantage of adding a series of "rivets" or rods inserted internal to the fin. If designed/placed right (and I'm no mechanical/structural engineer), and dissimilar materials with respect to stiffness/resonance, this could aid in combatting flutter.

G_T said:
The way I see such an approach failing, is the bond to the fin failing first - the bond being whatever primary attachment method is used. After all, that will be the stiffest part of the joint. After that fails (assuming), the rivets will last about a millisecond. The rivets themselves cannot even begin to take the load, and won't take any load until the rest of the joint fails. So what is the point of the rivets?

Gerald
Click to expand...
 
daveyfire said:
I suspect Chris has already done this, but I suggest you go take a look at some peel strength measurements of good bonding epoxies. You may end up reconsidering your worry.
Click to expand...

Consider also that you are reducing your effective bonding area on the tube surface while also destroying the fiber matrix of the airframe.

Hows the build going Chris? I came here for more awesome pictures of sexy carbon plate and real applied engineering and all I got was lots speculative commentary :(.
 
G_T said:
The way I see such an approach failing, is the bond to the fin failing first - the bond being whatever primary attachment method is used. After all, that will be the stiffest part of the joint. After that fails (assuming), the rivets will last about a millisecond. The rivets themselves cannot even begin to take the load, and won't take any load until the rest of the joint fails. So what is the point of the rivets?

Gerald
Click to expand...

Three 1/8 inch steel rivets in a row = what, 1000 lbs shear resistance for a second or two? Ever try bending a steel nail? By drilling into the fin, you significantly increase the bonding surface area (think of all the builds you have seen with grooves cut into the root of a fin) but you do weaken the internal structure of the fin. These fins are so thick and overbuilt, I doubt they are at any great risk from three 1/2 inch deep 1/8th inch drill holes. The fins are certainly much stronger than any bond they are likely to achieve with an airframe.
 
New Ocean said:
Three 1/8 inch steel rivets in a row = what, 1000 lbs shear resistance for a second or two? Ever try bending a steel nail? By drilling into the fin, you significantly increase the bonding surface area (think of all the builds you have seen with grooves cut into the root of a fin) but you do weaken the internal structure of the fin. These fins are so thick and overbuilt, I doubt they are at any great risk from three 1/2 inch deep 1/8th inch drill holes. The fins are certainly much stronger than any bond they are likely to achieve with an airframe.
Click to expand...

What are the rivets anchored to on the other side? :y:
 
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The nails won't help or hurt. They need to be offset from the center of the fins in order to add much resistance to the overturning moment at all.
 
jd2cylman said:
There are flat head rivets that are like flat head screws. The head ends up being flush with the surface.

Adrian
Click to expand...

Yes. And what happened to the tube wall / strength of the tube wall at the countersink...
 
Quaranta said:
Yes. And what happened to the tube wall / strength of the tube wall at the countersink...
Click to expand...

Yes reduced airframe strength to afford better fin strength. I was under the impression that the airframe is basically a minor issue, and I know for a fact that the fins and nosecone will take the brunt of the stress. If the fin attachment is strong enough to render the airframe the weak point... that sounds good to me.
 
New Ocean said:
Yes reduced airframe strength to afford better fin strength. I was under the impression that the airframe is basically a minor issue, and I know for a fact that the fins and nosecone will take the brunt of the stress. If the fin attachment is strong enough to render the airframe the weak point... that sounds good to me.
Click to expand...

That's not what I am saying at all. I agree with GT. The rivets are non-load bearing until the bond fails. And once the bond fails, the rivet system (rivets and what they anchor too) are too weak to take the load, rendering them useless. So what's the point?

Without knowing what the rivets attach too on the body tube side,... I suspect that would be the weak spot of the rivet joint. (at the end of the day it's an MD rocket,... so by definition it's a thin walled tube.) Thus my earlier comments...
 
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Quaranta said:
That's not what I am saying at all. I agree with GT. The rivets are non-load bearing until the bond fails. And once the bond fails, the rivet system (rivets and what they anchor too) are too weak to take the load, rendering them useless. So what's the point?
Click to expand...

The rivets are part of and embedded in the joint. They are like a few large fibers in a composite layup.
 

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