Modular and adhesive-free two stage

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Yoehahn

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I thought people might be interested in a new build thread, and the last thread I did was a few years back with my 2 stage 3" darkstar. This build is going to take a long time, mostly due to the fact that I intend to machine all of the parts (except fasteners) by hand instead of using CNC. So this project will also be a two stager, and the kits are also from Tim. One has been sitting in a box in my basement for a few years, the other should be arriving whenever Tim ships my order.

I remember an Issue of sport rocketry many years ago that detailed a modular rocket, and the thought of that always fascinated me. I built a few rockets with modular motor mounts, but it did not quite feel the same. I have been thinking about doing a project like this one for 5-6 years now, and with the addition of a lathe and milling machine to my workshop I decided that now is a pretty good time to attempt this.

I have been working on different designs and have finally gotten pretty close to a final after about 15 revisions, and while there will still be some tweaks (mostly to accommodate a better interstage design) I feel like I can start the thread off and use it as another place to keep a build log as well.

After I finish this project I intend to start work on the 54/4000 to 54/4000 MD 2 stage project as well, but that is something for another thread.

Anyways, some pictures of my recent renders:Fin Can Rev 2.jpgFin Can Rev 2 without MMT.jpg
 
Here is how I intend to mount the airframe to the fin can assembly, I intend to machine a lip on the outer diameter so that the screws will not be taking all of the force and some of the force (at least) will be transferred directly into the tube itself (Note: I will not be swapping the tubing out for CF, plain black just does not look as good in the renders and figured this was close enough)

tube attachment.jpg
 
Larry Mills did something similar with a Phenolic (I think) airframe and aluminum components. Fins were replaceable, segments screwed together/apart. I think he was told he couldn't fly it at a few clubs because of the amount of aluminum content.

Just sharing as a precaution.
 
I am using primarily 6061-T6 aluminum to machine everything, and I am working on it as more of a personal project than anything else. I am not too worried about getting it RSO'd but I will keep it in mind. I have done a lot of testing and simulated forces on it and I know it will hold up to anything I can throw at it, but I dont really view my assembly any differently than a rocket with an aluminum nose tip or even a motor case, So hopefully there wont be any problems.
 
I finally did a final render on this version of the fin can, I have a few tweaks to make it accept an interstage coupler a little bit better but I may just design a seperate fin can to use for the sustainer and modify this one so I can fit a nicer tail cone on it. I am going to play around with a couple ideas in the next couple of days and I have ordered the bar stock for the main assembly rings, so I will have 2' of 3.25" 6061-T6 coming in shortly (even though I only need about 4" of it, I figured I may want to make another set or two and there is always a chance I will make mistakes during the machining process) The bottom ring only measures 1.5" tall, and the top ring is .75" tall, and all in it should not weigh all that much (looks like with all the fasteners and aluminum including fully machined av bay assembly I should be adding less than one pound to the "normal" finished weight of a 3" punisher, of course actual weights will vary and I may try to reduce weight in my assemblies even more.

The other pictures were just screenshots that I took so this should hopefully give a slightly better look at the design. I cant wait to get started!



Final Fin Can Rev 2 Render.jpg
 
I also finished a version of the av bay that I am actually happy with, and I only envision making minor changes (the charge cups for example are taking up just a little too much space so I may change those out) and I would like to add wire organization and routing holes, battery mounts etc. but I will do that once I finalize what electronics I intend to use on this project. I plan to try to figure out a disconnect system for the wires going to the motor from the front av bay but Ill burn that bridge when I cross it.


Av Bay Rev 1 Final Render 1.jpg
 
Just a word of caution. I did something similar a number of years ago with ACME fin cans and Blue Tube. My plywood centering rings were held in place with adjustable threaded rod. It failed on the first flight, crumpled right above the fin can, on a fairly tame motor at that. I didn't understand how much it weakens the airframe to cut slots in it from the bottom, especially without the benefit of filets. Maybe if I'd had attachment points between every centering ring and the airframe (instead of just at the rear thrust plate) it would have been OK, but then you are weakening the airframe with a bunch of holes. I've also had a Blue Tube rocket fail right at the point that I'd created an opening for a camera to peek out.

What I learned from all of this is how well a traditional rockets works, with everything bonded well together you can't help but get all that thrust transmitted nicely from the motor mount to the fins and airframe.

Good Luck!
 
Trying new things is fantastic; love to see that.

I think your fin can itself is overkill and slotting the tubes that much will weaken the tube a lot (as Dugway pointed out).


View attachment 334260

The Fin Can is definitely overkill, Im working on making it a bit lighter and I'm getting rid of the supports after finding they only added a marginal amount of strength to an already over-built assembly. I believe the tube will be fine, But I am going to run simulations on that as well just to see. I think that the method I am using to attach the fin can to the tubes should keep it together well, and I do not believe I will be losing much in the way of strength seeing as the tube for one of my punishers was already slotted to <1/8" to the end of the tube, So it should be alright. The attachment points for the upper ring have been shifted slightly above the slots as well, so even should the lower part of the tube fail (which is very unlikely) the upper ring should support the assembly just fine.

Right now the main Issue I have been having is the shear pins on the av bay: I had originally wanted to tap holes into the aluminum bulk plates, but I wouldn't be able to get the pin out after the flight without melting it out or drilling it out, both of which are not good options. Right now I have the coupler tapped with slightly larger holes on the nosecone and airframe so that they will screw in just fine, I just never like putting shear pins near the avionics. There Is not any reason for this, I was just hoping to avoid it as a personal preference mostly.

I also have to make some better provisions for rail buttons, but I have some good ideas for that and Im generally not very worried about that.

I'm intending to 3d print a mockup of the parts once I run out of ideas for improvements to make, and have just now begun leveling and centering my lathe and milling machine again to make sure everything runs true and I am also checking my rotary table since I intend to use that to drill 90% of the holes in the assembly. Would anyone be interested in a time lapse video of the machining process? I'm planning on taking lots of pictures throughout the process to document things (I'm also building this as an engineering portfolio piece) and I am not sure if anyone would be interested in a video instead.
 
Would anyone be interested in a time lapse video of the machining process? I'm planning on taking lots of pictures throughout the process to document things (I'm also building this as an engineering portfolio piece) and I am not sure if anyone would be interested in a video instead.

A time lapse would be awesome, so long as it is close enough to show the actual work being done, not just you running around the shed... ;)
 
A time lapse would be awesome, so long as it is close enough to show the actual work being done, not just you running around the shed... ;)
+1. Any documentation of this project would be awesome. I've nothing to contribute in terms of expertise but congratulations on pushing the envelope. This is an invaluable contribution to the knowledge base of the hobby whatever transpires.
 
A time lapse would be awesome, so long as it is close enough to show the actual work being done, not just you running around the shed... ;)

I can either mount my GoPro to the lathe and milling machine near it or mount them above the machine, I will play around with camera angles and see what works best.
 
Awhile back, Harbor freight had these magnetic base flexible arms for face shields and similar. I'm betting someone handy could rig a go pro to one of them.
 
Awhile back, Harbor freight had these magnetic base flexible arms for face shields and similar. I'm betting someone handy could rig a go pro to one of them.

I went to Harbor Freight today and I found them, and I already had the clam mount for the go pro so Ill put some clear plastic on the arms and try it out, but it looks like it will work very well. Thank you!
 
I went to Harbor Freight today and I found them, and I already had the clam mount for the go pro so Ill put some clear plastic on the arms and try it out, but it looks like it will work very well. Thank you!

Cool. This was like 6 years ago, so I didn't know if they were around anymore. The machine shop in college had this big old Southbend with metal walls everywhere, so there was always somewhere to stick the base of the arm.
 
As far as removing the shear pins, you can stick the tip of an X-Acto knife in the stub and back it out. If you put a bit of beeswax on the threads beforehand it works every time.


Sent from my iPhone using Rocketry Forum
 
As far as removing the shear pins, you can stick the tip of an X-Acto knife in the stub and back it out. If you put a bit of beeswax on the threads beforehand it works every time.


Sent from my iPhone using Rocketry Forum

Im sure that would work, but I feel like there should be a better way of accomplishing that. Im still figuring that out, but that will probably work as a backup in case I cant come up with anything.

Also, My aluminum arrived yesterday, Including some to make some 38mm Av Bays as well since Im also working on a K-K 38mm two stage after this as well. Now I just have to go make friends with my local machine shop again and get them to cut my aluminum for me, if not I will have to run it through my bandsaw which will give me great cuts but take a few hours that I would honestly rather spend doing something else.
 
Don’t thread the bulk plates. Let the shear pins be a snug fit and just push them through afterwards.
 
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Well I received my second punisher a little while back, and im just getting back from vacation and about to start school again tomorrow so I figured I would go ahead and see what im working with.
Its interesting to see the changes from a few years back when the punisher was introduced to now, I have two vastly different nose cones, tim switched over to the red tubing as well, and apparently the fins changed (which im not too keen on) I may try to trade the black tube for a red one coming up, or red tube for black tube, just so everything matches.

Current observations:
The old punisher tube (black one) is a full inch longer than the red
The nose cones are COMPLETELY different, one is a good 5-6" longer and has a phenolic tip, and has parting lines on the side so it appears to be molded, the new shorter one has the aluminum tip now and is filament wound with no parting lines on the side
the fins are about .250" larger in all directions now, not really sure why that would have changed
and the slotting and cuts on the old tube are of a noticeably better quality than that of the new tube (both were bought during black saturday sales), but both tubes are of great quality and definitely workable.

I will probably give tim a call tomorrow and see what I can do about the fins, If I was going to build it "standard" with epoxy then I wouldnt worry about it; however going with my modular design I would rather not have to make two sets of rings that were slightly different so everything fits well (that sort of defeats part of the modular aspect for me)

image1.jpg
image2.jpg
 
Been making bulkheads and av bays from aluminum for quite a while now.

Shear pins... Assuming #2-56 nylon screws for shear pins, assemble the rocket, drill thru the a/f and b/h with a #51 (tap size for 2-56.) Remove the coupler/av bay, tap the a/f, and drill the inner shear pin holes in the coupler/aluminum to #43 (clearance for 2-56.)

Post-flight, unscrew the head from the a/f, and you'll still have a tiny piece of nylon floating around loose in your av bay.
 
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