Scarab 54 vs Wild Thang Jr build challenge

Still building?

I'm betting school started.

Maybe its some die that didn't get mixed in epoxy when tube was made??????????????

Pardon the interruption on the build - I've been preoccupied for the last number of weeks as my wife went though a difficult surgery that has had some complications. We are hoping to have her back home this week and get things back to a more normal schedule. When that occurs, I'll be able to get back to the build. I also have a deadline - October Skies is just over 4 weeks out! So look for something soon.

No pardon necessary, take care of those closest to the heart. We'll be here. I hope that she gets better soon.

Ok, back to it.

Started free hand with the Scarab fins and, after looking at them, took a more measured approach to rough up the area of the Wild Thang fins where glue would be applied. Both booster tubes were sanded on the inside where the centering rings and fins would attach. With no plan to paint over the body or fins, any scuffing would show through so I tried to be careful.


The Wild Thang was first for tacking up the fins. The slit in the body tub was a bit longer than the fins so there was a question about where to seat the fins. I selected the low end but by the time things started to harden on the last fin after adding epoxy for the upper centering ring and then the other fins, enough effort hadn't been put into holding everything in place. So, although the fin alignment tool assured proper spacing between fins, not all fins were equally level relative to the base of the booster tube.


Once the Scarab has its fins tacked up, internal filets will be applied using Rocketpoxy. I'm a little worried that the hardener has become even more thick over time. Is there a way to thin it down or is that a good idea?

leave it.... it's cool. mix on flat cardboard, and let it sit a few minutes before applying. less bubbles.

Mr G said:

Ok, back to it.



Once the Scarab has its fins tacked up, internal filets will be applied using Rocketpoxy. I'm a little worried that the hardener has become even more thick over time. Is there a way to thin it down or is that a good idea?

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When older, it gets that way. No problem, after you start mixing it with resin, it will become silky smooth again.

Actually holds fillet shape better when aged, fillets won't settle out nor do you need to wait as long after mixing to use.

As David states..best to mix on flat surface , spread out....leave for 15 minutes or so, till bubble surface & dissipate, then use.

After a day, it was time to take off the Wild Thang retainer and rear centering ring but the retainer didn't budge at first. I had been careful to not put any epoxy on the last quarter inch of the fins just in case but apparently some little bit must have made it down there. Thankfully, a light tap on the bench top broke the retainer free. The screws were then inserted in the previously drilled centering ring holes and with a twist and some wiggling, the ring came out as well - whew!


As the Scarab gets its fins next, there is a question on the best method to use for internal fillets. When first attempted on a Vulcanite (see post #34 https://www.rocketryforum.com/showt...zon-build-challenge/page2&highlight=vulcanite), slipping epoxy through the narrow passages between fins was messy and difficult at best. There is two part foam or maybe gorilla glue that could be an alternate way of doing it. But would that be as strong? Anyone have suggestions here?

Got the fins tacked up on the Scarab with enough epoxy left over to secure the vent band for the altimeter bay.

Mr G said:

After a day, it was time to take off the Wild Thang retainer and rear centering ring but the retainer didn't budge at first. I had been careful to not put any epoxy on the last quarter inch of the fins just in case but apparently some little bit must have made it down there. Thankfully, a light tap on the bench top broke the retainer free. The screws were then inserted in the previously drilled centering ring holes and with a twist and some wiggling, the ring came out as well - whew!
View attachment 301982

As the Scarab gets its fins next, there is a question on the best method to use for internal fillets. When first attempted on a Vulcanite (see post #34 https://www.rocketryforum.com/showt...zon-build-challenge/page2&highlight=vulcanite), slipping epoxy through the narrow passages between fins was messy and difficult at best. There is two part foam or maybe gorilla glue that could be an alternate way of doing it. But would that be as strong? Anyone have suggestions here?

Click to expand...

Injecting is easier than you might think. If you're going through the aft end (removing the rear centering ring), I use a tube attached to a syringe to get it all the way to the top CR. West systems+ chopped carbon.
Or, the 2-part foam works well, too. It'll be strong enough. Most of the fin strength has to do with the root edges and external fillets on these rockets, anyway.

Equal parts of RocketPoxy were mixed on wax papered foam board then black dye was added in to match the Wild Thang fin color making up a gooey tar-looking substance.


While waiting for the RocketPoxy to firm up, the vent band was glued on to the e-bay. Had I been thinking, I would have held off mixing in the black dye until after doing this. One downside of translucent tubing is that any mistake is going to show through.


Blue painters tape outlined the area along the fins where the epoxy was placed. A scrap piece of 1/2-inch diameter oak dowel dipped in rubbing alcohol was used to evenly spread the epoxy along the joint. After waiting about a half hour, the tape was carefully removed leaving the fillets to set up overnight.

Seeing how much extra epoxy there was, a smaller batch will be prepared for the Scarab.

Went to add external fillets to the Scarab fins only to have them snap off where they had been epoxied to the motor tube. As they were all about the same to dislodge, I suspect the 30-minute epoxy used had had enough time to start to set up and therefore did not fully adhere to the motor tube. The fin slots in the booster tube were also really narrow so maybe not enough epoxy got through to create enough surface area for a strong bond. I may try first sliding in some epoxy from the bottom of the booster tube to coat the areas on the motor tube below the fin slots.

Mr G said:

Went to add external fillets to the Scarab fins only to have them snap off where they had been epoxied to the motor tube. As they were all about the same to dislodge, I suspect the 30-minute epoxy used had had enough time to start to set up and therefore did not fully adhere to the motor tube. The fin slots in the booster tube were also really narrow so maybe not enough epoxy got through to create enough surface area for a strong bond. I may try first sliding in some epoxy from the bottom of the booster tube to coat the areas on the motor tube below the fin slots.

Click to expand...

Oh man.....that sucks.

The Scarab fins have been re-epoxied into place after a bit of sanding. They are much more solid this time around so I will go ahead with the external filleting.

After the painful and ugly process of internal filleting with the Wild Thang, I am contemplating using two-part foam to fill between the Scarab fins.

Kinda neat how the light shines right through the red tubing to highlight the mess created in an attempt to make nice even internal fillets. Hopefully there was enough contact along the fin-tube seam to provide the intended support.

On a rocket of this size the best option for the internal filets of to inject them. It is much easier, and will provide a more consistent result.

markkoelsch said:

On a rocket of this size the best option for the internal filets of to inject them. It is much easier, and will provide a more consistent result.

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Ditto.

Inject, or just skip. Good externals are going to be fine.

This week I received these two kits. Holding my breath for updates.

The decision was made to experiment with foaming the Scarab fins to see what that process was like compared to epoxy fillets. The 2-part foam I ordered from Wildman ended up being Public Missiles Adjustable Density Foam. A test batch of 2g each of Parts A & B resulted in a 10-1 expansion making the little paper cup look like a muffin holder.


Not knowing how hard it would be to get the dried foam off the fiberglass, painters tape was applied where the motor retainer would fit and along the inside of the booster tube where the foam would be poured.


After stirring parts A & B together, you have about 15 seconds to pour the liquid. Then you wait and see how the foam expands in the cavity. I did one fin section at a time and learned that it took more than I thought to fill the void. You can add a drop of water to a batch to increase the expansion ratio but the strength decreases with each drop. Once the foam is dry, it is relatively easy to break off and dig out as necessary.


Expanding foam has enough pressure to push through small voids and holes as evidenced from this escape through the booster fin slot. No problem, just slice off and sand and maybe color to match the fin.

After picking at the foam with an Xacto knife, the Dremel tool was put to use to clean out any excess. A number of dry fits (using the two screws in the CR holes to have something to grip) made sure the centering ring sat flush against the fins. 15-minute epoxy was dabbed on the fins, the centering ring was set in place, and then the remainder of the epoxy was carefully placed along the seam between the centering ring and the inner booster tube.

Next, a thin coat of JB Weld was smeared on the outside of the motor tube and the Aeropack retainer was "screwed" on assuring good distribution into the grooves on the inside of the retainer. An alcohol wetted Q-tip was swept along where the top of the motor mount and retainer touched to be sure no excess glue would block the way.


Centering ring and retainer glued in place.


Not much room between booster tube and retainer cap.


Retainer cap extends past end of booster tube for easy grip when screwing on or off.


Looking up the business end of an empty 38/240 case held in place by the retainer.

Instead of homemaking a couple of sleds, the decision was to order a couple of Additive Engineering 54mm Smart sleds. With they arrived, dimensions were confirmed and the first task was to drill holes for the threaded rod that would hold the sleds in place inside the electronics bay. After sliding in a short 1/4" bolt and taping the edges of the bulkheads or -as Wildman calls them- "lids" to keep everything aligned, 3/16" holes were drilled 8mm in from the edge.

When the material coming out of the drilled hole changed from fiberglass to metal I stopped, thinking something was wrong. I hadn't even thought that one set of bulkheads could be aluminum as none of the promotional information indicated the kits came with them. But a hole is a hole, so the drilling was completed and the bulkheads fitted to both the Scarab and Wild Thang to see if the fit was different between the two. They were the same. On further reflection, Wildman has been upgrading materials in a number of his kits so the guess is that is where the aluminum lids came from. Don't know if that is now standard or just a special option for this particular kit. If it is standard, then that's another enhanced feature of the Wild Thang over similar kits.

it may be standard, I know I was a bit surprised when I found Al lids for my Comp 3 (2 years ago). my dremel drill press was none to happy to drill all of the associated holes though. I did have to change my plans for using binding posts.
Rex

After enough Dremeling of two of the three fins on the Wild Thang, the retainer fit squarely with the centering ring in place.

Without all the way thinking it through, holes for the terminal strip and charge well were drilled on opposite sides of the av bay bulkheads, perpendicular to the holes for the threaded rod. Once the strip and well were attached, the awkward geometry for attaching an igniter and placing it in the charge well became apparent. The most direct route would be right through the eyebolt/shock cord. Had I looked at previous setups, I would have recognized the value on placing the terminal strip and charge well closer together so the igniter could be shorter and out of the way.


Placement of the Schurter switch on the sled puts it up high and at one end. For a shorter av bay, there is no way to line the switch up with the vent band. I would either need to cut the sled down a bunch or drill a hole through both the sustainer and av bay tubes to get a screwdriver in and turn on the electronics before launch.

What have others done in this situation?

Move the switch band from center of coupler to a position that puts it over the switch location. I put the larger section of coupler into payload section which doesn't separate.

Mr G said:

As the Scarab gets its fins next, there is a question on the best method to use for internal fillets. When first attempted on a Vulcanite (see post #34 https://www.rocketryforum.com/showt...zon-build-challenge/page2&highlight=vulcanite), slipping epoxy through the narrow passages between fins was messy and difficult at best. There is two part foam or maybe gorilla glue that could be an alternate way of doing it. But would that be as strong? Anyone have suggestions here?

Click to expand...

I inject every non minimum diameter fiberglass rocket I build, could not be happier with the technique. People think it is going to be hard but frankly it may be the easiest.

Mr G said:

Placement of the Schurter switch on the sled puts it up high and at one end. For a shorter av bay, there is no way to line the switch up with the vent band. I would either need to cut the sled down a bunch or drill a hole through both the sustainer and av bay tubes to get a screwdriver in and turn on the electronics before launch.

What have others done in this situation?

Click to expand...

The switch band is really only useful if you plan to swap the av-bay to different rockets, or actually mount the switch on it. I think both of these scenarios are becoming more rare, so the switch band isn't needed.

Drill your vent holes/switch access hole in the body tube -or-

Ditch the pre-made sled and make your own, putting the switch wherever it needs to be.

Got the eyebolt tightened down to the aluminum tip on the Wild Thang. Looped a 12 foot length of 1/4" Kevlar cord through the eyebolt. Next step to glue on the nose cone coupler.

Worked on the threaded rod for the Scarab using Nylock nuts instead of epoxy. The booster side has a 1/4" forged eyebolt and 1.5 gram charge well whereas the sustainer side has a 1/4" unwelded eyebolt and .5 gram charge well. The thought was that there was less volume and stress for deployment on the sustainer side.

With nothing to attach to in the Scarab nosecone, an eyebolt assembly was fashioned from a homemade wood centering ring epoxied to the smaller ID supplied fiberglass shoulder centering ring. The eyebolt was was placed facing down leaving some room inside the shoulder area.


In contrast, the Wild Thang eyebolt was screwed into the aluminum tip leaving most of the nosecone open.


With the eyebolt extending down from the base of the shoulder, the Scarab added about three inches more space while the Wild Thang gained about eight inches.

The plan for the Wild Thang is to take up the suggestion of inserting the Eggfinder tracker and battery into a BT-55 tube, closing off the ends and securing the assembly to the shock cord. This should fit up in the nosecone, leaving plenty of room for the main chute, shock cord and other recovery hardware. With less room to work with, this may not fit in the Scarab. Without a tracker, the Scarab would be limited to lower altitudes and therefore smaller motors. So here is one of those finer details between the two kits that can determine capability.