8" Wildman Goblin 4x75mm Cluster Build

This project is a customized 8” diameter Goblin from Wildman Rocketry. It uses the normal 8” airframe and nosecone and substitutes the ¼” thick fiberglass fins with ½” thick carbon fiber honeycomb fins. It is also replaces the standard 98mm motor mount with a 4 x 75mm cluster. The rocket has been designed to support airstarting any of the motors in the cluster.

In late 2020, I began a discussion with Tim Lehr from Wildman Rocketry about building a large Goblin with a 75mm cluster. I had built smaller Goblins previously – in particular, I have built a couple versions of Wildman’s 5” Goblin and they fly at almost every launch I go to. It really is a fun and wonderful rocket to build and fly. Endemic to Goblins, though, are fin flutter issues due to the oversize fins the rockets use. These large fins contribute to incredibly straight flights and keep the bird stable, even at low launch velocities, but the fin flutter can cause the fins to rip off at relatively low speeds. Conventional wisdom on the 5” Goblin is that is susceptible to fin separation on motors above small “L”s. I had the experience of seeing this happen on one my 5” Goblins while flying it on an Aerotech L850.

This is a long winded way of getting to the point – Tim was immediately wary about the use of fiberglass fins on an 8” Goblin with a 75mm cluster. Even with a relatively short booster section (which limits the length of motor one can fit in the motor mount), this rocket would be capable of hitting Mach 1.5 and travel to altitudes above 20,000 feet. Tim was confident the fiberglass fins would fail long before that. He had been using a carbon fiber honeycomb material on some custom projects and suggested that material would stand up to the speeds this rocket would generate. The material itself is extremely expensive, which is the main reason, at the time, he wasn’t offering it as an option on the website (he has since found a similar alternative to the aramid honeycomb I used that is cheaper and is now offering it as an option on the website). I decided to go with the carbon fiber honeycomb.

With that decision out of the way, we had to determine if there was going to be enough room in the 8” frame to fit these very large fins, 4 x 75m motor tubes and all the hardware needed for building structure, recovery attach points and airstarts. After trading CAD designs back and forth for a while, and discussing what material to use for the centering rings, we determined it would be tight, but it could be done.

Tim proceeded to manufacture the necessary parts and got them to me in the summer of 2021.

I flew the rocket as test at Red Glare (MDRA) in March 2022 and those flights revealed some small flaws that needed to be rectified. The changes were made and the paint and decals were applied in April 2022 with an expected “production” launch scheduled for June 2022 at URRG in Potter, NY.

Build date: August 2021 – April 2022

Electronics: Primary: Eggtimer Proton

                     Backup: Missile Works RRC2L

Dry weight: 43.8Lbs pounds (19,870g)

Length: 97.5 in (2476mm)

CP: 83.5 in (2120mm)

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Documentation & Rocksim File

Rocksim File:

Parts List and components

A number of parts for this build needed to be custom made. I fabricated them on my Shapeoko 3 CNC. The aluminum used was .19” thick 6061 aluminum. The fiberglass is .25” thick FR4/G10. All design work was done in Fusion 360. Part number 13 in the picture below is a set of “Carbon Fiber Core Edge Trim” from DragonPlate. These are used to clean up the ragged edge of the honeycomb material. Tim provided two of these channels with the parts he sent me, but they were not long enough to cover all the fin edges, so I purchased another one directly from DragonPlate. 

  1. 8” Diameter slotted body tube (booster – 52” long)
  2. 8” Diameter 5:1 Ogive nosecone
  3. 4 x 75mm Motor Mount tubes (22” long)
  4. 4 x Carbon Fiber Honey Comb Fins. ½” thick
  5. ¼” thick FR4 fiberglass centering rings
  6. 75mm retainer
  7. 54mm retainer
  8. 8” Ebay Coupler (18” long)
  9. 8” Diameter switch band
  10. Ebay Bulkheads
  11. 8g charge wells
  12. 3/8”-16 U-bolts (McMaster-Carr)
  13. Carbon Fiber Core Edge Trim
  14. 5/16”-18 All-thread (McMaster-Carr)
  15. Onebadhawk Kevlar attach point with 5/16”-18 forged eye-bolts and 3/8” swivel

Initial Build Notes

Clearly, this is a more complex build than a normal Goblin. The cluster motor mount, in particular, presented challenges in both design and engineering in order to get everything to fit and be secure enough to handle the high thrust of 4 x 75mm motors. It also was a challenge to ensure the fins were anchored to something internally since they had to fit between the motor mount tubes. Tim and I did our best to ensure the length of the fin tabs was correct to just touch the motor mount tubes, but I was reluctant to rely on just this small area of contact. I played around with a number of ideas in the design phase and thought about CNC’ing a platform of sorts between each mount tube, but, in the end, I decided to keep it simple and formed a platform internally with epoxy.

I also originally played around with the idea of putting commercial retainers on the end of the motor mount tubes, but this proved to be impossible. There just wasn’t enough room to fit them. I realized the easiest thing to do was fabricate some retainers and use the all-thread from the motor mount assembly to provide secure attach points. That worked out very well.

Motor Mount Build

In general, Goblins are usually one of the easiest rockets to build. The motor mount on this rocket definitely presented some challenges. 

The first challenge was getting the centering rings fabricated in such a way as to house the 4 x 75mm motor tubes, the ¼” aluminum tubing for airstarts, the 5/16”-18 all-thread for structure and retainers, and jamming the two 5/16”-18 eye bolts in the forward CR – all while keeping enough structural integrity of the CR that they could do their job. I prototyped everything with plywood and, once I had all the spacing and sizing correct, I had to decide whether to use aluminum or fiberglass (or, something else?). The issue is that the tubes needed to be spaced a minimum of 1/8” apart in order to accommodate the Aerotech aft closure ring. This left just over 1/32” left of material between the tube and outer edge of the CR. I tested it in aluminum and it was too fragile. That little strip of material wanted to bend and break off. The ¼” thick FR4 fiberglass was much better. FR4/G10 has just a slightly smaller tensile strength than 6061, but the FR4 has almost twice the flexural strength.

With the material chosen and the prototypes proven, I fabricated 4 CRs. I planned to use 3, but I wanted a spare in case I broke one. I can cut two rings out at once on the machine, so it was easier to make 4 while the machine was setup for the cut.

With the CRs done, I cleaned up the motor tubes. They were all slightly different lengths and had some ragged ends. I found the shortest one and ground the other three to size on my belt sander.

In determining the location of the fore and aft CRs, I knew I wanted to pour epoxy dams on both ends, I wanted the epoxy to come right the to edge of the tubes, but I didn’t want to add excessive weight. The motor tubes and hardware take up a lot of the surface area and I wanted to have about a 100ml epoxy dam on either end, so I calculated that the forward CR needed to be 15 mm back from the edge of the tube to accommodate that volume. The aft end was a little easier  in the sense that there was very little room to work with. There was only a ½” between the end of the tube and the fin and the CR was going to take up half that, so the CR would need to be about 5mm from the aft end of the motor tubes. 

I marked all the lines on the motor tubes and started dry-fitting the whole assembly, using the all-thread to make fine adjustments to the CRs. I dry-fit it into the body tube many times to ensure spacing was correct and check that the fins would fit and I hadn’t inadvertently blocked one of the fins with the internal hardware. Once I was satisfied I had a good initial setup, I tacked the forward CR in place with CA.

If you ever struggled with threading a nut on a long piece of all-thread, here is a trick I picked up along the way that makes it easier. Chuck a decent size bolt (in this case, it is a 3” long ¼”-20 bolt), put an elastic band around the bolt and the hex nut and use the drills direction switch to thread the nut. One key is remembering to put the elastics in place as you slide on each part. After the motor mount is complete, I just cut the bands off.

It took some adjusting to get the rear CR exactly right and I made sure to do all the alignment with motors in place and the retainer holding things steady (note, in the photos below, I was using a prototype retainer made of plywood. I would later mill them out of aluminum). Key here was not only ensuring the rear CR was positioned correctly, but also that the aft ends of the motor tubes were perfectly aligned and that the ends of the all-thread rods were also aligned and at the proper distance. They had to be long enough to accommodate the motor closures, the thickness of the retainer and the thickness of any 54mm adaptors while still having enough thread to securely install wing nuts to hold the retainers on. Yet, I didn’t want them to be so long that they would touch the ground if the rocket was standing upright. Once I had the spacing right, I locked in the rear hex nuts with Loctite. With the forward CR fixed and the rear hex nuts fixed, the assembly was still adjustable. But the tubes would be in the proper alignment and the whole assembly would remain fixed.

I dry fit again in the body tube and then roughed up the aluminum tubing (used for airstarts). I used JB weld to lock all the hex nuts and tubing in position. Once the JB weld had cured, I cut the rear aluminum tubes to length with a dremel.

I installed the attach-point eye bolts with JB weld and trimmed the forward end of the aluminum tubes to size. At this time, I also put a coat of epoxy around all the motor tubes on both sides of the CRs to tack them in place and to seal them against leaks when epoxy gets poured later.

The next step was to build small shelves between the motor tubes for the fins to sit on/adhere to. I measured where each fin touched the motor tubes and drew a line down each tube at those points. This would be the top of the shelf. I lined each gap with some masking tape, ensuring the edges of the tape were below the shelf line. I sealed all the tape edges and seams to prevent the epoxy from leaking.

I mixed up a batch of West Systems epoxy and used a lot of their 410 microlight filler until the mix was the consistency of peanut butter. I spread it in the gap, smoothed it with a flattened craft stick and cleaned up the edges to make sure nothing would prevent the fin from sliding in neatly.

I then did a water test to determine if there were any leaks and to also measure the volume the basin would hold. I wrote the volume on the aft CR so I wouldn’t forget later.

I had to install the rotaloc nuts for the rail buttons before I could install the motor mount assembly (since the rear CR was fixed in place). I figured out the location I wanted, drilled a clean hole (by coating the inside of the tube with CA and tape) and then JB welded the rotaloc in place. At this point, I was not concerned with getting JB weld in the threads. 

I then JB Welded the top of the nut and built up a nice platform of JB weld for the large unistrut buttons to sit on. When the JB had cured, I sanded it smooth, shaped the sides and re-tapped the hole. Now I can switch between Unistrut and 1515 Airfoil buttons easily with a ¼”-20 screw.

Last step before installing the Motor Mount Assembly was to grind off a small section of the CR to get past the rotaloc install. I sanded the JB weld down a bit and then marked off each CR for the width of the rotaloc. I used a Permagrit sanding rod to grind out the section and dry fit to make sure it was all good.

Finally, I coated the inside of the booster tube with a healthy smear of epoxy and slid the motor mount assembly in place, ensuring the shelves between the motor tubes lined up under the fins slots. This epoxy is just to tack the assembly in place and to seal the edges for an epoxy dam I am going to pour later.

Booster Section Build

Getting the motor mount assembly complete and accurate was the bulk of the work on this build. I planned to pour epoxy into the fin wells and then slide the fins into that pool. In order to ensure some of the epoxy got into the interior of the fins, I drilled some holes in the fins tabs.

Then, I calculated the volume each fin tab would displace (it came to about 50 ml) and subtracted that from the basin volume I had derived earlier using the water test. It meant I needed to use about 70 ml of epoxy per fin.

I made sure the rocket was plumb and level, mixed up 70ml of epoxy and poured it into the fin slot. I carefully slid the fin in place, made sure it was plumb and taped it in place until the epoxy cured. I repeated this step for all four fins.

With the fins in place, I poured an epoxy dam on the aft end. I poured epoxy in until it just reach the edge of the motor tubes. I used left over epoxy to paint the sides of the booster tube (mostly for aesthetics). 

Just ahead of the forward CR, I drilled and tapped holes for 10-32 screws. These screws will be imbedded in the forward epoxy dam, giving it a little more mechanical hold. In order to ensure epoxy wouldn’t leak around the screws, I costed the threads with medium thick CA. I then used a syringe attached to rubber tubing backed with a wooden dowel to pour a forward epoxy dam that covered all the hardware on the front of the CR.

In order to get a finished edge with this honeycomb material, I used a ½” wide carbon fiber channel. Wildman provided me with enough of the channel material to cover the top and outside edges of the fins, but not enough for the bottom edges. I purchased an additional 4 foot section directly from DragonPlate. At about $22 a foot, this is not cheap material.
DragonPlate recommends using 3M Scotch-Weld Flexible Epoxy to bond the channel to the edge of the honeycomb material, so I also purchased a 3 ounce kit of that epoxy. As with the channel material, the Scotch-Weld is eye-wateringly expensive at almost $90 for that small amount, however, I figured I was already in for a lot of money on this project, no reason to skimp on the final step and have the fin edges break.

I measured out the length needed for each edge and found that cutting through the sides of the channel with a pair of straight aviator snips and then finishing the cut across the material with scissors was the easiest way to get a nice clean cut. I slightly beveled the edges close to airframe to match the shallow angle there, but, for the corners where two pieces of channel intersected ,I needed to cut a about a 45 degree angle and match them as closely as I could. This worked out well and any gaps I ended up with were filled in with epoxy.

I was unsure what the best epoxy to use for the fillets would be, so, before the test flights, I tried four different fillet materials. I tried rocket epoxy, west systems with colloidal silica, west systems with high strength filler and 3M Scotch-Weld flexible epoxy. After the flights, I looked at the fillets and all but the 3M Scotch-Weld showed signs of cracking. Unfortunately, the 3M product is stupendously expensive ($87 for a few ounces), so it wasn’t practical to use it more broadly. I had bought it for adhering the carbon fiber channels to the edge of the fins (it is what DragonPlate recommends), but had some left over, so wanted to give it a try. 

At the end of the day, I dremeled all those crappy fillets out and used West Systems with cut up carbon fiber. I know there has been a lot of discussion about whether chopped carbon fiber adds any strength, but, for this application, I wasn’t looking for added strength, I wanted added flexibility. I believe the carbon fiber mix gives me that on huge fillets like this. When you mix the CF in, you end up with a mix that has the consistency of wet hair. I laid down the fillet, sanded them as smooth as I could, filled them with Bondo and got a nice finished product. 

EBay Build

I have settled in to a familiar setup with these larger Ebays that consists of CNCed aluminum bulkheads with all the holes tapped for the terminals and charge wells, holes to accommodate 4 pieces of 5/16” all-thread, and 3/8” U-bolts. I use a switch bracket CNCed out of basswood and design them to hold up to four switches or lights, as needed. One interesting thing to note on this build was that I forgot to orient the piece of basswood properly in my CNC machine and the resulting piece had the end grain pointed towards the switch area. Every time I tried to drill into it with a ½” bit, it shattered the thin piece of wood. Once I realized my mistake, I cut another bracket with the grain going in the correct direction. I doubt many people are going to use this method, but, if you do, make sure the long grain is parallel to the top of the bracket.

The sled brackets are pieces of .08” FR4, as is the sled. The switch bracket and the end brackets are designed so the sled slips through a slot in the center of those pieces. The end brackets are attached with #4-40 bolts to the sled using some ½” angle aluminum and the switch bracket is held in place by hex nuts on the all-thread on either side of the bracket.

The electronics for this build are a Proton as the primary computer. It will handle the primary deployments of the main and drogue parachutes as well as airstarts. The backup computer is a Missile Works RRC2L.

The Proton has a 2S 800mAh LiPO to power the logic and a 3S 1400mAh LiPo to handle the deployment charges. In most cases, this 3S battery would be overkill, but since it will have to do double duty as a motor start battery (for airstarts), I wanted something with a little more “oomph” to ensure those motors start quickly. The RRC2L gets a 9V lithium.
The 2S LiPo and the 9V are housed in some cool 3D printed battery holders from Additive Aerospace. I installed #6-32 PEMs in the sled to make it easier to install the battery holders and take them off. In the photos, you will see two sets of PEMs. I messed up the design on the first install because I forgot to take into account the all-thread above the holes. The PEMs were directly under the all-thread making it very difficult to get a screw driver in there. I moved the battery holders in a half inch or so and that corrected the issue. It is a reminder to think in 3D when designing portions of a rocket

Finally, I used some newer connectors on this build that will definitely be in my kit bag from now on. I installed an XT60 connector on the bottom of the sled to make it easy to connect the large 3S LiPo. The 3S LiPo is held in place with zip ties. I was faced with connecting a bunch of wires of wildly different gauges. The wire on the XT60 connector was 14 gauge and the smallest wire I could fit in the Proton terminals was 22 gauge. Other components had other various gauges. Normally, I solder connecting wires of different gauges together and step them down as I go. But, I found these 3M wire connectors that can handle any wire from 14-22 gauge:

https://www.amazon.com/3M-Scotchlok-Electrical-Self-Stripping-Retardant/dp/B001CGVB7M

I was skeptical at first, but tested them with a variety of gauges and the connections were always secure and complete. So, I used them on this build to connect all the various wire gauges and secured them to the sled with heavy duty double sided tape.

On the bulkheads, I used the heavy duty terminals from McMaster-Carr and the companion marking plates to label the channels. For the aft BH, I used an 8-position terminal to handle both the drogue deployment charges and the airstart ignitors. The charge wells are 8g aluminum from Rocket Junkies.

Nosecone

The nosecone on this build doesn’t have a separate electronics bay and I prefer to fly these head end dual deploy birds with the drogue in the nosecone, so the nosecone just sits on the ebay coupler with no connectors. To tether it to the bulkhead, I had a 2-loop piece of Kevlar from Onebadhawk that is about 3 feet long. I attached the one end to a forged eye-bolt and then used Loctite to secure the eye bolt in the aluminum nosecone tip. The drogue attaches to the other loop, as well as the shock cord going back the ebay bulkhead.

Odds and Ends

Ground testing

The charges for the ground testing were calculated using the calculator at Insane Rocketry’s site: https://www.insanerocketry.com/blackpowder.html

The calculation for the drogue (nosecone) charge was 2.5g of FFFFg black powder. That charge turned out to be way too much. You can see in the ground test video below that a 2g charge shot the nosecone off with enough force to break the small, temporary kevlar tether I had atached to the nosecone tip (the “production” tether is rated for 5300 lbs). With the drogue and shock cord installed, there is little volume in the nosecone and it has no shear pins. 1g of FFFFg kicks the NC off vigorously. The backup charge is 1.5g of FFFFg.

The charge for the main charge was calculated to be 4g of FFFFg. This also turned out to be a bit much, so the primary charge used for the shakeout flight was 4g of FFFFg and a 5g backup charge. Those charges worked well.

Pictures & Flight Videos

Shakeout flights were conducted at MDRA Red Glare 22 on 2 April 2022

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