4" Airframe x 75mm Motor Mount Scratch Build Thread

[GaryDean]

My original thought was a 2-Stage. Then, got hung-up on a transition piece and decided to pursue a single stage rocket. Design goals are;
1. Rocket will accept a CTI 75mm 6XL-Grain motor.
2. Incorporate a Thrust Plate to ensure thrust loads are focused through the airframe, not centering rings, epoxy, motor mount, etc.
3. 4 fins to satisfy my perception of improved flight stability.
4. Keep weight to a minimum.
5. Create and update a RocSim file to accurately represent the finished product.
6. Stability Ratio of 1.5 calipers minimum. At the expected length, this should not be an issue.
7. Dual electronic deploy, with redundancy. This rocket will NOT use motor ejection at any time.
8. Final height (length) less than 110", when sitting on a 1" high base, so it can stand upright in my man-cave. Ha ha.

My accumulated parts list includes;
- 60" x 4" diameter fiberglass airframe
- 24" x 4" payload section
- 4" 5:1 Ogive fiberglass nosecone (black), with metal tip
- 30" 75mm fiberglass motor mount tube
- 1 each 7.5", 8", and 12" coupler tubes for nosecone and E-Bay
- Aeropack 75mm motor retainer (flanged version)
- SC Precision 4" x 75mm thrust plate, anodized red
- 4 1/8" thick G10 fiberglass fins from Giant Leap Rocketry, design # 15, beveled
- 8' 2,500 lb kevlar, for booster tether
- 30' 2,000 lb kevlar, for drogue / streamer shock-cord
- 30' 1,500 lb kevlar, for main parachute shock-cord
- 30' 100 lb kevlar thread, for sewing shock cords, chute protectors, etc.
- 5 4" OD x 75mm ID 1/4" plywood centering rings
- 1 4' OD x 54mm ID G10 centering ring
- 1 4" diameter 1/4" plywood bulkplate
- 2 E-Bay coupler G10 end plates
- 36" 1/4" x 20 threaded steel rod
- Rocketman 60" 2.2 CD main parachute
- Rocketman 3 ft Pro-Experimental Drogue chute
- Rocketman 6" x 30' streamer
- 6 Rocketman 1,200 lb kevlar soft links
- 1 each 12" and 18" kevlar chute protectors
- 2 Missileworks RRC2L altimeters
- Missileworks Dual 98mm Modular Sled
- Missileworks supplied rotary switches, bulkhead terminals, and battery connectors
- 2 9V Alkaline battery's
- 4 3-gram ejection charge cannisters
- 2 Bindery Design / Dog House Rocketry locking connectors (1 orange and 1 red)
- Various nuts, washers, welded eye-bolts, wing nuts, screws, etc.
- 1 1,500 lb swivel
- 2 1010 Rail Buttons
- 2 Wildman Rocketry 4" airframe fin alignment guides

With all of that documented, off we go ......

 

[ThePlmbr]

Very nice set of parts. Good-luck on the build.

 

[GaryDean]

My first action is of course to wash all of the fiberglass parts in warm soapy water, and dry throughly. Then, I sand EVERY surface (inside and out, front and back) with clean 220 grit sandpaper, using a mask. Then, use Acetone outdoors to clean all surfaces. From my past experiences, this process provides some back-up, on the chance that I am proceeding too quickly through a process, and might forget to sand. Having written that, in every process that involves epoxy, etc. I will sand and clean again, ensuring that the surface has been correctly prepped within 1 hour of applying any adhesive.

Now, let's tackle the hard part first, the fin slots. I start by wrapping paper around the 60" airframe, and drawing straight lines along the airframe at each of the equally distanced 4 final locations. Then, knowing that I want the bottom (aft end) of the fins to be exactly at the end of the airframe tube, I draw guide lines to represent the location, length (13.5" long fins) and width (1/8") for each fin slot. I accept that some error on the width is acceptable because of the future use of J-B Weld to fill gaps, etc.
Then, drill 1/8" holes at the end of each of the slots. Reference picture below.
Will holes drilled, proceed to cut the fin slots with a Dremel diamond circular disk, attached to my FOREDOM rotary head, at 14,000 RPM, with of course a MASK! Used a straight aluminum bar as a guide to prevent getting too far off course. Pictures below.

 

[Hobie1dog]

Wow, what a great start to a build thread. real rockets have 4 fins.

 

[pugachu]

Looks like a quality bill of materials and plan. What altitude will you expect? Do you think it's too high for a standard BP charge?

 

[GaryDean]

Hmm, going to need a Switch Band. Now's a good time with the FOREDOM out to cut a 3" section off the 4" airframe. This jig allows me to make the initial cut by rotating the fiberglass straight into the wheel, and then rotating the fiberglass tube against stops toward the wheel and the end of the tube, to maintain decent straightness and accurracy. Hope that makes sense. It works for me. With mask of course.

 

[GaryDean]

Looks like a quality bill of materials and plan. What altitude will you expect? Do you think it's too high for a standard BP charge?

Thank you! My preliminary RocSim shows almost 17,000 feet with a M1545GR. So, should be OK.
I have thought about this rocket as potential for my L3, but with our waiver at 13,500 feet, the lowest thrust M motor would put it at 13,474 theoretically. I could just put some weight up front to keep it lower, but, I already have something a bit bigger in the planning stage.

 

[pugachu]

Sounds good. I thought the 6g 75mm M would push it too high for standard charges

 

[David Schwantz]

Looks good, but you don't always need a transition for 2 stager.

 

[GaryDean]

Have labeled the fin slots 1 - 4. Pushed in a "forward" wood centering ring to approximate position. Now to dry fit the fins to find the "best fit" and then to do any modifications which are necessary. Proceeded with each fin separately here.
My design requires the use of a wood centering ring on the aft end of the motor mount, into which I can attach the three (3) screws for the thrust plate. So, I put the centering ring into the aft end, and then use the thrust plate to position the centering ring correctly. It's not truly the optimal position for the centering ring, because with this process the thrust plate will be in contact with the centering ring. Some chance of thrust being delivered into an unintended path! So, my plan is to proceed with this process, as it is an easy path. Then, in the final assembly of the thrust plate, will remove maybe 0.015" of material (sanding) from the backside of the thrust plate. This should ensure a small gap between the centering ring and thrust plate. The screws are not a "problem" because they will only be screwed into the centering ring, with no physical connection to the thrust plate, other than holding it snug against the end of the airframe.
So, from the picture below, there is an interference between the fin tab and aft centering ring, as expected. I individually marked each fin tab area to be removed as shown, removed the material with my rotary tool and dremel blade, and then checked the fit. Continued the fitment process as needed, until all fins seemed correct. Final picture below shows completion of this stage.

 

[GaryDean]

Now to prepare the motor mount tube for installation. Seems this might be an opportunity for some weight reduction, as the area of the motor mount above the forward centering ring will not be taking any load. The booster section tether will be connected to a platform high in the tube, at a position to allow that CTI 75mm 6XL-Grain to fit, but just barely. I am not going to assemble a centering ring on the forward end of the motor mount.
Disassemble everything, and mark the location of the aft centering ring for all 4 fins. This might be helpful when installing the motor mount.
2nd picture below shows my effort to introduce some weight reduction on the motor mount, then the finished product. Finally, the right-hand picture indicates that I did not cut the "windows" to far down the tube, as there appears to be room for the centering ring on solid motor mount tube.

 

[GaryDean]

Now to install the motor mount tube. But first, just as a safety factor after all that work on the motor mount tube, I will reassemble the fin set, just to be 100% confident of not introducing some error into my efforts. From attached picture is seems OK.
Disassemble everything, and pull out the forward centering ring. Attach a 100 grit piece of sandpaper to the end of a wooden dowel and sand the ID of the airframe in the area where the forward centering ring will be positioned. Used alcohol to clean that surface.
Lightly sanded the OD of the centering ring and cleaned it with alcohol also.
Mixed up some J-B Weld and used that wooden dowel to apply the epoxy to the area just slightly aft (1/8") of the final centering ring position, but ONLY in the area between and away from the fin slots! Do NOT want the epoxy to block the fin slots, as that mistake could be challenging to overcome.
Then, push the centering ring in using a coupler tube for most of the way. At the end I use a PVC pipe to push the ring forward, in small increments, moving carefully around the ring. When the ring is very close, but not quite in place, I carefully install the motor mount, add the rear centering ring and thrust plate, and insert the fins. I insert the fins into their respective slots forward end first, and use the fin to push the centering ring into its final position. When happy with the position, I pull out the fins and let it cure.
After cure, I mixed another batch of J-B Weld and used a wooden dowel to apply the epoxy to the OD of the centering ring just installed. With J-B Weld now on the OD on both sides of the centering ring, I'm confident that interface will be adequately strong.

 

[GaryDean]

Might be a good idea to focus my effort now on a couple of rail buttons, BEFORE installing the motor mount. Using 1010 rail buttons as I expect the finished dry (no motor, no casing, no motor adaptor) weight to be in the range of 12 - 13 lbs.
The aft rail button will be installed immediately in front of the aft centering ring. The forward rail button will be installed at a position exactly 1/3 of the length of the rocket, from the aft end. Used my long aluminum angled straight edge to mark the positions in alignment. Drilled holes in the noted locations, cleaned up the holes with a needle file, and sanded (100 grit) the ID of the tube in these positions. Cleaned with alcohol. Put a slight "bend" in the rail button backing plates with channel pliers, so I have to be aware of the correct "direction" during installation. I also used a grinder to remove some of the material on the aft rail button backing plate. This allows the rail button to install directly next to the centering ring. Sanded the backing plates. Used that wooden dowel again to apply some mixed J-B Weld to the ID of the airframe tube.
The aft backing plate was a snap, obviously. Got it rotated correctly and installed the button and screw to keep it in place. Applied a bit more J-B Weld to the ID. Used a 1" PVC pipe to install the forward backing plate, by laying it on the end of the pipe in the correct orientation, held in place by a dab of J-B Weld. My partner used a flashlight to follow the progress as I inserted the pipe further into the airframe from the forward end. When in place I just lifted the pipe to insert the backing plate, they confirmed orientation, then attached the button and screw.
Easy peasy........

 

[GaryDean]

With the aft rail button in place, reassemble the motor mount tube, aft centering ring, and thrust ring to confirm fit. Seems OK.
Determine final intended position of the thrust plate and insert screws to mark the centering ring, and put tape on the tube and thrust plate to confirm intended position. Not as much flexibility in this positioning as I anticipated, due to 3 screws and 4 fins! Apply a piece of tape on the motor mount to indicate the intended final position in reference to fin # 1. The centering ring orientation was marked previously. These markings should allow me to assemble the motor mount tube and rear centering ring in the correct final positions.

 

[GaryDean]

This one's going to be tricky...... Sanding the motor mount tube and the airframe, then cleaning with alcohol is the easy part. I just keep going over the actual process of putting J-B Weld on the Motor Mount, inserting the motor mount (but not ALL the way initially) without smearing the epoxy, orientated correctly, holding it in place while placing epoxy on the aft end OD of the motor mount (between fin slots) AND aft end ID of the airframe tube (between fin slots), before installing the aft centering ring, orientated correctly. OMG, even with my jig to stabilize the motor mount tube, J-B Weld went everywhere!!! Mostly my hands and shirt. If you check out the last 2 pictures you might be able to spot the J-B Weld in spots on the OD of the motor mount tube that were not planned. Although super messy, I did manage to get everything orientated as planned. It's gonna work after it cures.

 

[pugachu]

Looking good so far. Gonna leave it fiberglass red, or do you have a paint scheme in mind?

 

[Handeman]

Why not just cut the MMT shorter? You don't want to use the fin to MMT joint as a thrust load, so you only need 5" or 6" of motor tube to keep the motor on axis.

 

[GaryDean]

Now to find out if my efforts on the MMT really did work. How, with a dry-fit of the fins of course, AGAIN. Forgot to take the picture with all 4 fins in position, but the pictures included below show a very good fit both front and rear for the first fin installed. The fin tabs fit snug between the centering rings.
So, on to installation of the fins. Cleaned the fin slots with alcohol again, after all my physical contact in the MMT installation.
Cut small grooves in the tabs of the fins (done earlier) to provide a greater area of contact for the J-B Weld, then sanded the fins and cleaned with alcohol.
For installation of the fins, done individually, I began with a liberal application of J-B Weld along the fully exposed fin tab (attached picture). Then inserted that fin into it's respective fin slot, and pressed downward while sliding both the front and rear fin alignment guide into place. Tried to place the forward fin alignment guide in a manner that it helped lock the fin against the MMT.

Hoping to speed up the cure process by using a light bulb heat source below the airframe, with the assembly covered with a towel. Low tech for sure, but it helps. Goal is to have the last fin installed before calling an end to the night. It's gonna be a LATE night.

 

[GaryDean]

Looking good so far. Gonna leave it fiberglass red, or do you have a paint scheme in mind?

Picked up a can of Rustoleum Metallic Pure Gold about a month ago, with intention of using it on this build. Because I plan to have a tracker (Eggfinder) in the nosecone, I will not leave the nosecone a polished black due to the heat generated in our Florida sun. Might fry the tracker. So, probably the nosecone, fin can area, and the switch band will be metallic gold, while the red fiberglass will stay as-is, after I wet-sand to a polish with 2000 grit. I would like a kind of "wet" look on the red, and might practice (spare parts) with clear polyurethane, etc. Any suggestions?

 

[GaryDean]

Why not just cut the MMT shorter? You don't want to use the fin to MMT joint as a thrust load, so you only need 5" or 6" of motor tube to keep the motor on axis.

That thought did go through my mind! But, it was after I had removed the MMT material. I still thought it would be the best idea, but then went through the mental gymnastics of how to pull off installing the MMT. My situation was having to initially install the MMT only part way into the forward centering ring, to allow me space to apply J-B Weld to the aft end of the OD of the MMT, along with the aft end of the airframe ID. That is when I realized if I cut off that forward section of MMT, whatever ended up as the "end" of the tube might not willingly slide into the forward centering ring. That would be a nightmare. So, I decided to leave it that way, and make better plans in the future. Your observation is 100% correct. I just painted myself into a corner from which I could not take the best course of action.

 

[GaryDean]

The fins came out really well! It was too late (early morning) when the fins were completed to proceed with injecting fin fillets for all interfaces between the fins, motor mount, and airframe.

But, my one last effort before bed was to mix up some more J-B Weld, and using a razor blade and back side of a metal scale, insert the epoxy into all gaps between the fins and the fin slots. That effort ensures that I do NOT have epoxy leaks when doing fillet injections on the fin to airframe ID intersections. After the effort to carefully fill those gaps, I ran a small bead of J-B Weld along each exterior fillet, and formed a crude radius with my finger.

 

[GaryDean]

I can now confidently inject epoxy fillets for the internal interfaces between fins, motor mount, and airframe. The fins are 13.5" long. I divided that length into 3 sections, and drilled holes 4.5" from the aft end of the fins, and also 4.5" from the forward end of the fins. The holes are 1/8" diameter, as that allows a snug fit for the nose of the syringe I use. All the holes are drilled and cleaned as necessary with my needle file before mixing any epoxy. Reference the 2nd picture attached.

I use West Systems 205 / 105 resin and hardener, mixed in a 5 / 1 ratio. Decided on 18.0 / 3.6 grams for each pair of internal fillets, measuring by weight with a scale. I am not a fan of West System pumps. I use different color plastic spoons and dip them into the resin containers. By allowing them to drip slowly as I approach the desired quantity, I can get very close to the target weight. After mixing by hand for 60 seconds minimum, suck up the epoxy with syringe, try to remove any bubbles, and inject approximately 1/4 of the volume into each hole, for the 2 fillets rotated to the top of the airframe, and use a small level to ensure the epoxy is distributed evenly along the interfaces. I always create a cheat-sheet to keep track of what's been completed. The cheat-sheet is not so necessary on the fin to motor mount interfaces because you just leave the assembly stationary after injecting the epoxy.

It's a bit tricker for the fin to airframe ID interfaces because you have to correctly select the holes to inject into, before rotating the assembly to ensure the epoxy runs to the desired location. As an example, from the attached picture with my cheat-sheet sketch, to do what I refer to as the 2-1 and 3-4 fin to airframe ID fillets (shown in dark on my sketch);
- start with the # 2 fin at an approximately 30 degree angle from horizontal
- inject the epoxy equally into the 2 holes on the top-side of the # 2 fin
- put blue tape over the holes you just injected epoxy into. This is IMPORTANT!
- rotate the assembly CCW (from picture reference direction of view), until the # 3 fin is at an approximate 30 degree angle from horizontal
- inject the epoxy equally into the 2 holes on the top-side of the # 3 fin
- put blue tape over the holes you just injected epoxy into. This is IMPORTANT!
- rotate the assembly CW (from picture reference direction of view), until the # 1 and # 4 fins are upward, at exactly 45 degrees from horizontal.
- use the level to confirm assembly is level lengthwise (front to back)

Looking at the picture sketch, you would now have the 2-1 and 3-4 fin to airframe ID interfaces at a position that the epoxy will cure at exactly the location you desire for maximum strength. Hope that was understandable, and helpful. Of course, it is easier to do those interfaces just 1 fillet at a time, but it takes twice as much time.

Again, to speed up the process, I used a shop light positioned below the airframe with towels covering the assembly to keep heat in. This idea is not so time-saving on the fin to motor mount fillets because the epoxy will always be at the top end of the assembly. But, it saves lots of time on the fin to airframe fillets because after rotating the airframe the epoxy is down near the heat source.

Now done with that stage. Had minimal epoxy leakage when doing the fin to airframe ID interfaces. A little clean-up of the leakage and then slap more blue take on the wound. Final step here was to fill in the holes as necessary with Bob Smith Industries 30 minute slow cure epoxy, and sand them down to the contour of the airframe.

 

[GaryDean]

That thought did go through my mind!

 

[GaryDean]

Why not just cut the MMT shorter? You don't want to use the fin to MMT joint as a thrust load, so you only need 5" or 6" of motor tube to keep the motor on axis.

Kept thinking about my reply from lasy night. While working on the internal fillets, reality struck! I'm not in a corner! Sure, I had to use the extended section of the MMT for actual assembly. But, it's assembled now and my FOREDOM rotary tool and Dremel diamond wheel will fit inside the 75mm tube. So, I strapped the FOREDOM (with diamond wheel) to a 4 foot long wooden dowel, and hung this tool from an overhead door frame, setting the diamond wheel at the exact height I wanted for the cut. With my shop light it was easy to identify exactly where the centering ring is located. I cut 1/2" above it. From the top, viewing downward into the airframe, with a flashlight, it was easy to control the dowel and cutting action. After set-up, took less than 1 minute to make the cut(s), which provides another 104 grams of weight savings! THANKS!

 

[GaryDean]

With some free time while waiting for internal fillets to cure, started work on the Nosecone Bulkhead / Tracker Mount. Using a 4" to 54mm G10 centering ring, in conjunction with a 4" to 75mm plywood centering ring to create the bulkhead (with bolts incorporated), onto which a slightly reduced OD 4" G10 bulkhead can be mounted, with welded eye-bolt to connect to main parachute shock-cord.
First, marked the G10 centering ring for the 3 bolt hole locations, along with the plywood centering ring which must have some material removed for bolt clearance. Then marked the G10 bulkhead for the amount of material to be removed from the OD so it can be easily assembled or removed in use. 2nd picture shows the modified components.
Then, mix up some J-B Weld and apply to the bolt heads (after sanding and cleaning with alcohol).Then epoxy the plywood centering ring to the backside of the bolt carrier, as shown below.
I want 3" of shoulder for the nosecone. So, start by fitting the end of the coupler which will be epoxied into the nosecone, checking for fit, and applying tape around the diameter of the coupler to ensure that the coupler will be centered within the nosecone. After achieving a workable fit, assemble the bulkhead assembly, followed by the coupler, into the nosecone. Mark the extended shoulder to identify where to make the cut to result in the 3" shoulder in the final assembly. Cut the coupler.
Another dry-fit, then sand and clean the coupler OD and the nosecone ID. With everything looking good, mix and apply a thin coat of Bob Smith Industries slow cure epoxy onto the OD of the coupler. Insert the coupler into the nosecone slowly, while rotating to ensure good distribution of the epoxy. When in place, wipe off excess epoxy with alcohol dampened cloth. Set aside to cure.

 

[Art Upton]

Wow, what a great start to a build thread. real rockets have 4 fins.

Trip Barber , past NAR President and a contester since his 70s MIT and NAVY days told me that a properly aligned 4 fin rocket will get better altitude then a 3 fin rocket no mater what drag people say about it.

 

[GaryDean]

Next on the agenda are the fin fillets. I use RocketPoxy and their black die for fillets because this epoxy has a workable viscosity after it is applied, begins to harden quickly enough that the fillets I pull hold their shape well. I believe I do not need large fillets on this rocket because of the use of the thrust plate, and the fact that the internal fillets are applied at all interfaces of the fins, motor mount, and airframe. Have selected a 5/8" diameter PVC tube, about 4" long. Sand and clean (alcohol) all of the area to which the epoxy will be bonding. I then create the edge lines by putting black magic marker on the PVC piece, then sliding it along the fin valleys. This leaves a fine line to mark the edge of the epoxy. This action also serves as a confirmation that there is clearance between the PVC tube and the material already in the fillet. If any impacts are encountered, sand them down now. Run blue tape the full length on both sides with the blue tape just touching the scribed line. Create epoxy "dams" at the aft end of the fillets. The epoxy will try to escape, and whatever gets through just leads to a lot of unnecessary clean-up work!

At this point I use my level to confirm that the assembly is perfectly level end-to-end.

I can only make a guess for the amount of epoxy required for every 2 fillets. It should not take much on this rocket because of the small radius, and the fact that I ran a small bead of J-B Weld along the fillets when filling the gaps after the fins were installed. I started today with 14 grams on the first pair. Prepared 7 grams each of resin + hardener, along with 1 drop of the indigo black die. Mix by hand for 60 seconds minimum, and then I use a long popsicle stick to transfer epoxy from mixing container to the fillets. I make sure that every surface is covered by epoxy before pulling the fillets, and do not assume that the pulling tool will distribute the epoxy as needed. I waited about 15 minutes for the epoxy to harden before pulling the fillets, so it gains some stiffness. Hmm, seems that with age that RocketPoxy hardens a bit quicker. It was quite stiff when pulling those first 2 fillets. For the actual process of pulling the fillets, I first dip the PVC tool into denatured alcohol. Then immediately start the tool at a slight angle from the aft end of the rocket, pulling forward. After 4 or 5 inches of movement I wipe the PVC tool clean of epoxy (if present), re-dip into the alcohol, and proceed. By my experience, dipping the PVC tool in the alcohol allows the tool to move freely and the epoxy does not "clog-up" on the tool and drag epoxy out of the fillets.

When I get to the end of the fillet at the forward end, I guide the PVC tool in a manner that it follows the fin bevel. This seems to reduce the amount of work necessary later on to create a good transition with the front of the fins. I typically pull the fin fillets 2 times, about 5 - 10 minutes apart, to ensure there is no excess material in the fillets. Any excess would have to be sanded away later. At this point I confirm everything is level again, turn my shop light on below the assembly, and cover everything with towels to make a speedier cure.

So much epoxy left over after the first set that I reduced the amount to 10 grams total for the 2nd set. Even that was too much and went with 8 grams for the final couple of sets.

After completion of the 4 sets of fins I removed the blue tape, used a razor blade for some material removal at the aft end of some fillets, and started sanding. For sanding I wrap a piece of 220 grit sandpaper around the PVC tool, and run it up and down the fillets. If the fillets are well formed, this effort usually takes very little time. Often have to use a razor blade or sandpaper along the edge where the blue tape might create a small "wall". I have never had a case where simple sanding resulted in a finished product, and today was no exception. Had to add some filler (Bondo Glazing & Spot Putty) to minor imperfections, and, as always, add some J-B Weld to the area at the front of the fins, to achieve a smooth and tapered transition from the fillet to the airframe tube.

You can clearly see where Bondo was used on the Finished Product picture. But, the important thing is ....... the fillets are FINISHED.

 

[GaryDean]

Trip Barber , past NAR President and a contester since his 70s MIT and NAVY days told me that a properly aligned 4 fin rocket will get better altitude then a 3 fin rocket no mater what drag people say about it.

That's encouraging! Thanks! We might have an overachiever.

 

[GaryDean]

Used my free time when the fillet epoxy was curing to work on what I call the “Tether Platform”. This is the connection within the airframe for the Drogue / Streamer shock-cord. My stated goal for this rocket of being able to fit a CTI 75mm 6XL-Grain Motor Casing means that there must be a minimum of 42.21 inches clearance from the motor thrust face to the bottom of my “Tether Platform”.

I start by cutting a 3/8” length of coupler tube. This piece will be used as a Stop Ring in the airframe, on the aft side of the “Tether Platform”. To create the Tether Platform, I used;

• Plywood 4” x 54mm centering ring
• 4” diameter plywood bulkhead, with OD reduced to the ID of a coupler, and
• Another 3/8”” section of coupler tube
• 2 welded eye-bolts, with nuts and washers
• 2 short pieces of ¼’ x 20 threaded rod

The use of 2 plywood rings / bulkheads has everything to do with strength. Started by setting a section of the cut coupler tube onto the bulkplate and drew a pencil line around the ID. Then used a sanding disk to remove the narrow ring of material from the OD, and confirmed it’s fit into the short section of coupler tube. lamped the plywood pieces together and located and drilled ¼” holes for the threaded rod. Unclamped the plywood pieces, and then;

• mixed J-B Weld to attach the 2 plywood sections together
• install the eye-bolts / threaded rod / washers
• install (with J-B Weld) the coupler tube section onto the bulkplate
• apply J-B Weld onto the nuts
• clamp everything together to cure

Located the install position for the Stop Ring along the airframe, measuring from the location of thrust in the AeroPak Motor Retainer. Attached picture shows the assembly sitting on top of the airframe, in the specified location. Knowing the location of the Stop Ring allows me to measure the distance from the forward end of the airframe to the stop ring location. Did a Stop Ring assembly dry-run using 2 couplers to slide the Stop Ring into place. Placed blue tape around the coupler to identify the stop point intended during the actual installation. Now satisfied I could position the Stop Ring correctly, I used sandpaper on the end of a wooden dowel to sand the airframe ID, and also sanded the Stop Ring OD. Cleaned both with alcohol. Placed tape around the wooden dowel to represent the desired location (1/8” forward from the final ring position) for the epoxy to be applied within the airframe. Mixed up some Bob Smith Industries 30 minute slow cure epoxy and used the wooden dowel to apply around the airframe ID. Set the Stop Ring into the airframe and used the couplers to slide the Stop Ring into the airframe, right to the designated stop point. Attached picture shows the result. This stop point provides ½” of safety for the length of the 75mm 6XL-Grain casing. See attached picture. That’s it for now, as I have to stitch the tether itself onto the platform before final assembly into the airframe.

 

[GaryDean]

Should be a good morning for building an E-Bay. Started with the E-Bay end plates. Clamped the end plates back-to-back and drilled a 1/4" diameter hole in the center, using the scribe mark provided by the manufacturer. Then scribed a mark for the threaded rod holes, with a 3" spread, and drilled 1/16" holes at each of these locations. See attached picture. After confirmation that these holes were located correctly, used the 1/4" drill.
Had previously cut-off a 3" section of coupler tube for a switch band. My RocSim file and a quick dry-fit indicated that the rocket length would be just over 110", which exceeds my desired length (< 110") for my man-cave. So, cut the switch band width down to 1.25". Decided that I would go with a E-bay coupler length of 3.0" to fit within the airframe, and 2.0" to fit within the payload section. Therefore, need a coupler with 6.25" total length. Got that length by cutting a piece off my 8.5" coupler to get the correct length. Components shown in picture below.
I improved the fiberglass faces on each end of the coupler by sanding. Did this by laying a large, flat piece of clean 100 grit sandpaper on a very flat surface (kitchen granite countertop), and rotating the switch band against the sandpaper. Followed the 110 grit with 220, 400, and 600 grit respectively. Did this with both ends of the switch band, and also with both ends of the payload tube. Had done the airframe tube earlier (before fins) because I am aware of how difficult (dangerous and impossible) this sanding is with that much top-heavy weight.
Then went through a fitting process with the coupler, switch band, payload tube, and airframe. Goal is to find the best combination of fiberglass faces to achieve solid contact around the circumference, and to start the coupler shimming (blue tape and scotch tape) process with the airframe and payload. By my experience the end result is vastly improved if the switch band is well centered on the coupler, along with the airframe, when epoxing the switch band to coupler.
After identifying the best fit condition, marked the parts (picture attached), and proceeded to epoxy the switch band in place. Did this by sanding the OD of the coupler, along with the ID of the switch band. Cleaned with alcohol. Mixed a small batch of Bob Smith Industries slow cure 30 minute epoxy, and applier a very thin coating onto the coupler. This coating started very near to the face towards the airframe tube, and went 2/3 of the distance towards the payload tube end. My thinking is that this interface is not load-bearing, and that my priority is more towards the final fitment, rather than strenth. Slid the switch band onto the coupler from the airframe end, rotating slowly to both distribute the epoxy evenly, but also to ensure that the switch band was centered (in radial direction) as much as possible. Rotate it to marked location. When in the final location, I pushed the coupler into the airframe section, rotated to it's marked location, and then did the same with the payload section, applying pressure from both ends to move the switch band in a manner that both ends of the switch band were "square" with the airframe and payload tubes. Immediately removed the assembly from the airframe tube, and cleaned all contact surfaces with alcohol. Reinstalled into the airframe tube, removed the payload tube, and cleaned all contact surface with alcohol. Set aside to cure (see attached picture). Yes, after the epoxy had cured, it did take some special effort to remove the coupler / switch band from the airframe tube. About 20 lbs of force from the inside against the coupler tube, along with some fear that I had created a permanent bond. The end result was nicely square interfaces, and the switch band only slightly off center in radial direction. This can be easily improved with sanding (see attached picture).