EPOXY SUCKS! A development thread.

[chris97rockets]

I’ve been building rockets for about 17 years, and if there’s one thing that’s remained consistent, it’s my dislike for epoxy. It’s messy, sanding is a pain, and it takes forever to complete an assembly. Growing up, I spent a lot of time in my grandfather’s tiny garage with a manual lathe and mill —he was a tool and die maker—and that’s where my love for mechanical assemblies was born. That passion still is with me today.

I believe aluminum, when properly designed, can be an excellent alternative to traditional rocket assemblies. Beyond the satisfaction of assembling the rocket, there’s a practical side too: if you break a fin, just unfasten and replace it. You don’t have to scrap the entire rocket or go through the tedious process of cutting, sanding, and essentially building a new one. Want to change the motor mount? Just swap out the centering rings. No more wondering if you mixed the adhesives correctly. Plus, it just looks cool!

Of course, aluminum components can be expensive and relatively heavy if not well-designed. This thread isn’t about a specific build; it’s a development thread aimed at creating an elegantly simple line of mechanically assembled rocket components that are both affordable and effective.

 

[MidOH]

There are no centering rings. Metal rocket would be minimum diameter due to the weight.

Just a big pipe. Couple wee screws at the angry end for motor retention.

A fancy fin can could be press formed out in one piece, like a soda can, then........

Wait for it.........




Epoxied over the end of the tube. Lol. I suppose you could tap the tube and bolt the fin can on with a dozen FFHS.

I guess you could press form a can with removable fins too.

 

[thzero]

Well, not necessarily...

Granted the Tripoli safety code, and NAR says the same, does say 'where necessary;...

"Construction; rockets shall be built using lightweight materials, such as paper, wood, plastic, rubber, or when necessary ductile lightweight metals, and construction techniques that are suitable for the planned flight."

But setting that aside..

Aluminum is what, 1.56 oz/in3 in density compared to fiberglass at 1.12 oz/in3? Thats what 39% higher density? And fiberglass tubes are about 1.75mm thick.
1" fiberglass ring is 1.34 oz.
1" aluminum ring is 1.87 oz
1" aluminum ring is 1.35 oz at 1.31mm thick.

Can find some .09" thick aluminum sheets at metal depot, so at least we're in the ballpark if you can roll your own tube. Oh and .091" wall thickness aluminum tube.

How's the strength comparison between thinner aluminum and fiberglass?

Fins on my 5" fiberglass are 5mm thick. So thats like 3.5mm to account for different in density and keeping the weight roughly the same. Is that sufficient given the differences in the materials? Certainly there is less drag!

Probably still want to use fiberglass nose cones to stuff trackers, etc. in so that its transparent to the radio signals.

 

[MidOH]

We know a minimum diameter deal is doable. You see those all over.

A low and slow thats bolt together? Well, I'd like to see one. The Al pieces would have to be artificially thick to have room and strength for the hardware.

We already see this problem in RC carpet cars. A bulkhead is too heavy. Needs to be slimmed down, but you can't make it thinner, in width, than 5mm, since it has 3mm screws in it.

Which is why you'd form them in one piece assemblies instead. Would be as challenging to assemble as an Alpha 3.

 

[chris97rockets]

Disclaimer what you are about to read was done ~15-10 years ago. I have all my fingers and I turned out ok
Early Days

The following rockets aren't show pieces. They are a ~14 yr old learning how to make things.

I built my first aluminum rocket when I was 10. Get this - it was a PVC tube, with an aluminum nose cone and motor mount and flew on a D. (what safety code?) I flew it at a literal dump. Also, some of these pictures are almost 15 years old. Please excuse the quality!




While this looks rough, it was my first introduction into machining. The motor was friction fit into the motor mount. The nose cone was hallow and I used a faster to attach the old school Loc precision elastic shock cord.

"My Grandfather's" Level 2

This was my first real large machining project. This was my first time using aluminum centering rings. This rocket was a 4"-3" rocket with a 54mm motor mount.

Before and after the transition/avionics bay.



2 Centering rings and one bulkhead for shock cord attachment.



The AV bay and bulkheads.



The whole rocket.




Unfortunately, this was with Blue Tube 1. I don't have pictures but the paint we used basically ate through the adhesive of the tube and when it landed it cracked.



But thankfully, I was able to take all of the components out, buy the brand new Blue Tube 2.0 and rebuilt it in a very short amount of time.




And just a few short weeks the rocket successfully flew.

What I learned: Machining can take a long time. You can reuse and swap out parts with this method. You can attach the rail guides to the centering rings. Using a motor mount tube with AL centering rings is silly.

 

[MidOH]

I don't see how the tripoli code applies.

A carbon fiber rocket with an AL tip, is every bit as risky as anything.

 

[chris97rockets]

Fast forwarding a bit and I am 17 and building for my level 3. Ok, listen, I know my level 3 was the definition of overkill. I didn't know better, I didn't have fancy tooling. I hadn't taken AAE 203 yet. But it happened and it’s one of my fondest memories.

Before ChatGPT and an abundance of tutorials I thought AutoCAD was the go to software for mechanical development. So attached is a screenshot of a rocket designed in the leading building design software.




The new thing with this rocket was the fin can. If you are not a fan of aluminum, just skip this post. I quickly realized the split fin wasn't going to work for the assembly I had.

This was like an internal fin can. A tapped metal coupler was inside the rocket. A bracket was then fastened from the outside. Looking back at this now, it’s comically thick. But it was still roughly the same weight as some peer builds.

The fin can from stock to final product.


The fin bracket stock and machined.



The fins were pinned and machined on the front to make a sick sweep across it.

All of the components sandblasted.



Everything assembled.



Takeoff!


The rocket flew perfectly. This was the last project pre college.

What I learned:

• Drawings and tolerance are important.
• The fin brackets don't need much, these were excessive.
• Making the fin a permanent attachment wasn't necessary.

 

[thzero]

All those parts look nice, but damn talk about overkill and overweight. I did essentially the same thing but using 3d printed parts and 3d printed fins (with fiberglass overlays) on a 5.5" LOC rocket, and on some 2.6" LOC rockets. So I get the wanting to bolt stuff together!

 

[chris97rockets]

All those parts look nice, but damn talk about overkill and overweight. I did essentially the same thing but using 3d printed parts and 3d printed fins (with fiberglass overlays) on a 5.5" LOC rocket, and on some 2.6" LOC rockets. So I get the wanting to bolt stuff together!

Keep in mind all this was before 3D printers were accessible. I remember going to the library to get my first part printed lol. They got popular right in the middle of the my next post. But yes it was.

 

[MidOH]

That whole piece could be done as an extrusion. Just need to make the die.

Then drill for the fins.

 

[rharshberger]

I don't see how the tripoli code applies.

A carbon fiber rocket with an AL tip, is every bit as risky as anything.

Carbon Fiber is not explicitly called out in the Tripoli or NAR safety codes like METAL is.

 

[MidOH]

That's my point exactly.

I can make a sword out of carbon fiber, and run around a tripoli event, lopping peoples heads off.

But it's cool, cause not metal???

Or are there safety rules for fiberglass and carbon rockets, that I just haven't witnessed yet?

Or does the "soft materials" rule get canceled, when the field is set for MPR or HPR?

 

[chris97rockets]

A lot of the collegiate efforts centering around l!quids. So I am going to skip over that... But needless to say, there wasn't much adhesive in this assembly. For all the required stainless steel (we were forbidden to use AL) plumbing, we got this thing impressively light.

 

[chris97rockets]

With that behind me, I set out to make light weight no epoxy assemblies. The first goal was a level 1 sized rocket kit. I wanted to not only share what I learned but allow everyone access to this type of mechanical assembly. The following sums up the last 3 years of on and off tinkering.

First iterations started actually back in college. Have you ever had 50 students try to build an L1 at the same time? It is an absolute mess and takes weeks.

First versions were all 3D printed. I tinkered with it but it just didn't feel right personally.

Other printed versions were all focused on clips. At one point I wanted to it to go together without any tools. This didn't work very well. It was always wobbly, broke easily despite multiple types of prints. It did fly well a few times.



Iteration 2 of the all print was the most disappointing. It was looked so so cool. The assembly used Markforged Onyx carbon filament. The fin was printed with a threaded insert on the root. Inside the tube there were these x washers (similar to my L3) that provided a flat point to attach the fin. While this was super cool, the prints were expensive and putting the inserts in was time consuming.



It also didn't work well with paper tubes. It really needed Blue Tube or fiberglass to work well. The centering rings of the assembly was also 3D printed.

 

[chris97rockets]

The next iteration was back to my domain. Sheet cutting is very inexpensive, so this design was made entirely from sheet-cut parts. Out of all the iterations of the L1-sized kits, this was by far the least expensive. However, what doomed this design was the weight and the additional time required for assembly. Keeping a solid aluminum disk left too much weight on the centering ring, which pushed the CG too far back. The only way to reduce that weight with a laser cutter was to remove chunks from that section. While this made a super light part, it also created a wide open gap for the ejection gases to escape.

My solution was to place a smaller thin plate on top, but this added more threaded holes. The biggest problem was underestimating the time needed to create the threads for the fins. The idea was to have a threaded hole on the notch of the ring and a countersunk hole on the fin. The problem was, I would have to manually do this, and the time it would take would negate any savings from the laser cutting.

I would recommend this style to anyone who wants to try something new, but it was not practical for mass production.

 

[chris97rockets]

I then moved into the latest design.

Centering Rings

Obviously aluminum machine can be expensive. If I want to sell a kit I need to do whatever I can to lower this cost. One way to do this is making a lot of one sku. This will lower the per unit costs at scale. I eventually settled on this design. The elegance of this design is that the same fasteners that keep the centering ring in the tube, also are the same that keep the fin bracket in place. There is one threaded hole on the face of the centering ring. This is for an eyebolt and motor retention. A lot of material is taken out. I forget the exact thickness but it is less than a 1/16th of an inch. It is also super light. Each of these rings weigh just over 1 oz. Which is identical to a fiberglass centering ring.




Fin Bracket

The bracket went through nearly a dozen iterations. I settled on a SLS printed nylon design. The fin slides in and there was a brass threaded insert on one side of the fin bracket. A #4 fastener was then mated the two.


There were two issues with this design. The first was there wasn't a lot of meat on fin interfacing with the bracket. Sometimes when they landed funny, the fin snapped off. This wasn't common but what warranted a redesign was the threaded inserts. It took roughly 6 hours to mate 450 brass inserts in.


This (along with manufacturing issues and my day job) was so painful I stopped the project for about 8 months. However, the design was proven. About 50 of these were built and flown at Purdue.

What I leaned:

• Manufacturing with partner shops.
• Brass inserts should be avoided. (unless it’s a personal project)
• The bracket was to shallow and created fin problems.

 

[rharshberger]

Is this the same rocket that you (?) have been trying to sell on Facebook?

 

[chris97rockets]

After recovering from the threaded insert incident and getting the itch again, I started on the latest iteration in March.

The biggest improvement was the brackets. Instead of using threaded inserts, it just uses a locknut. I designed geometry into the bracket so you don't need another tool to tighten. I also made all the hardware #8 so the assembly goes together with one hex key. Now when I order from my manufacturing partner, I don't need to spend hours with specialized tools to get it ready for the user.



The rest of the assembly is the same, except that it’s now color coordinated.



This is the current and final version of the level 1 sized kit.

• Assembles using no epoxy.
• No special tooling is needed.
• Reasonably priced

After showing this to peopleI have gotten a lot of feedback on the assembly. Most people want the assembly for other rockets. I made the assembly alone available for sale. If you want one you can get it here.

What is next?

I am going to scale this system up for larger rockets. I have some clever ways to delete the bracket and replace it with something that can accept any fin. While I enjoyed designing the kit, I think this system is more beneficial if it works for a variety of designs. Instead of dumping years of stuff at once, I'll just update this thread. The first thing I need to do is determine a size.

I think the 4" airframe with 54mm motor mount would be popular choice. What is your favorite rocket diameter/motor diameter combo?

 

[chris97rockets]

Is this the same rocket that you (?) have been trying to sell on Facebook?

Yup. I am starting work on a stand alone assembly for larger rockets. I am going to use this a "build thread" as I figure what that looks like.

 

[Flare]

I think the 4" airframe with 54mm motor mount would be popular choice. What is your favorite rocket diameter/motor diameter combo?

The only diameter is minimum diameter and even there the body tube is just extra weight

 

[Neutronium95]

I think the 4" airframe with 54mm motor mount would be popular choice. What is your favorite rocket diameter/motor diameter combo?

My preferred motor mount and airframe size kinda depends on the tube material. If it's fiberglass, I'd say go one size down on the motor mount all day every day, so 3" and 54, or 4" and 98, or 54 and 38. If it's cardboard, 4" and 54 seems reasonable, although it might require glassing for some 54mm motors.

I tend to think that these nice machined parts would work better with fiberglass tubes.

 

[thzero]

After showing this to peopleI have gotten a lot of feedback on the assembly. Most people want the assembly for other rockets. I made the assembly alone available for sale. If you want one you can get it here.

Those poor Zephyrs...

 

[chris97rockets]

I tend to think that these nice machined parts would work better with fiberglass tubes.

I agree, that is what I prefer as well. Typically I'll do something like this. This used a max q fin can and a scp thrust plate with my CR's and bulkheads. I am working on a version of both of these that compliment the rings.

 

[thzero]

View attachment 653464View attachment 653461

Simple design. I've seen something like this before, and did a version of this with all 3d printed plus body tubes. The bigger difference is I had a fin tang that goes through the body tube into a slot mounted on the motor tube, so there was some epoxy. :|

With my fully 3d printed dual deploys, that go together with nylon bolts, I did original start with brass inserts. Getting them square, etc. on non-square parts was just a pain. So for the fins, I just do bolt through like you do but I just threaded the bolt hole instead.

 

[chris97rockets]

Those poor Zephyrs...

I have worked with ~600 students over the years for their L1s. This is not an uncommon site.... I coach using tape to make nice fillets, I stopped because they will on purpose epoxy over the entire piece of tape.

 

[thzero]

After showing this to peopleI have gotten a lot of feedback on the assembly. Most people want the assembly for other rockets. I made the assembly alone available for sale. If you want one you can get it here.

All that machined aluminum, and the kit comes with a non-forged eyebolt?!

Might want to check instructions, looks like there are some missing replaces. Didn't really find references to Aaa or AAA, there was also another aaaaa too.

SECTION 4: Rail Guide Assembly
4.01 Take one well nut and INSERT it into one of the {Aaa} holes.
4.02 INSTALL #8 x 7/8" Flat Head fastener with one rail button onto the well nut. Tighten until {AAA}
4.03 Repeat step {asad}
4.04 VERIFY rail buttons are aligned straight with each other and are perpendicular to the airframe.

 

[thzero]

I have worked with ~600 students over the years for their L1s. This is not an uncommon site.... I coach using tape to make nice fillets, I stopped because they will on purpose epoxy over the entire piece of tape.

engineering students?

 

[KenECoyote]

Those machined CRs look amazing and I would probably buy them if they were available as separate pieces and with a larger motor tube hole as @Neutronium95 said.

However, why include them in an L1 kit since that seems like overkill and also likely drives up the cost a lot (possibly a third of the cost or more)? Couldn't you have 3D printed CRs instead?

(Sorry if this was already explained in another thread, I've likely missed earlier threads on this.)

 

[astronwolf]

Or are there safety rules for fiberglass and carbon rockets, that I just haven't witnessed yet?

Or does the "soft materials" rule get canceled, when the field is set for MPR or HPR?

Local club and field rules can be more strict (but not less strict) than any safety code. IMHO these rockets that can survive shovel recovery, have metal-tipped nose cones, aluminum structural components, filament-wound fiberglass tubes, etc. are not appropriate for all fields under all circumstances.

 

[2-0turbo]

Interesting thread. I’ve enjoyed the iterations you’ve gone through.