I did the Internal fillets very carefully and just one at a time. This gives me ample opportunity to identify issues and make improvements to my procedures.

Drying time

As a result of the , I had to do 12 fillets. Each fillet requires approx 12 hrs to cure. I could probably have got away with 6 hours, but decided against that because I wanted to ensure I get the best adhesion with the fin and motor-mount. Most of the time I just did one fillet in a day, sometimes I managed to do a second one.

Preparation

I had already roughed up the Motor Mount Fibreglass tube with Grit 80 sand paper and the Fins were already roughed up, scored and cleaned thoroughly.

Preparation included taping up fins, motor mount and airframe so that no epoxy would find its way on to these areas. I didn’t want a hard time getting the lower centering ring back on and I didn’t want to spoil the hard work done on the fins.

I had all the tools out ready to do the job:-

• Safety Googles
• Many pairs of gloves
• Epoxy 105 resin
• Hardener 206
• Cup
• Stiring Stick
• Cardboard to protect bench surface
• Lots of paper towels (some cut up into small pieces)
• Bin
• 60ml Syringe
• 6mm tube attached to 4mm doweling using masking tape
• Methylated Spirits
• Torch

Pictures

Issues encountered

Unfortunately epoxy did on occasion breach the bogs and some would seep out and run along the fins. Hence the tape. Fortunately the seeping of Epoxy was minimal and I think was often due to slightly too much Epoxy, rather than a hole in the bogs. Fortunately after about 30 mins after loading the epoxy, it would start to firm up and not seep as much.

Decided that the Forward Centering ring should have an “AFT” fillet. The “Forward” fillet was created by the initial gluing process; as the glue was pushed forward. So nothing needs to be done here.

The creation of the fillet is not straight-forward because it was all the way down. So instead decided to lay ~35 ml of Epoxy resin using tube and allow it form a ring. So not a true fillet in the sense (not shaped like a fillet), but still forms the role of a fillet.

The preparation

I taped up the Motor-Mount and Airframe where the AFT centering ring eventually needs to be glued (didn’t want this getting clogged up). I also pushed a bunch of newspaper down the Motor Mount and taped over it with Masking tape. Didn’t want ANY glue accidentally getting into the Motor Mount.

The Gluing

The Epoxy was just 105/206 mix and inserted using long doweling with 5 mm (internal diameter) plastic tubing. I secured the rocket to the bench using cable ties and let it sit there for 10 hours.

It was very straight-forward procedure

I spent considerable time debating how to do the internal fillets. I’d already done a ‘trial’ run using 105/206 on a dummy fin and this had worked well, except for the epoxy dripping at each end, despite best efforts to step the flow with some wood/tape.

Really questioned whether I should go for 105/206/403 mix instead, which would need no bogging or special care in terms of spill management.

I decided in the end that 105/206/403 wasn’t going to fit the bill, because I need the fillets between fin/motor mount to make very good contact with the surfaces it is bonding. i.e. I’m more concerned about surface adhesion, rather than the strength of the Epoxy itself. So a running Epoxy is going to get to all the gaps, cuts. So the only effective way I could think of stopping leaks/spillage of this 105/206 mix was to bog the ends. I thought about a nylon rope solution, but didn’t think I could rely upon it.

I used wood doweling with some 6mm inner diameter plastic tube to get the fin ends bogged up. The 6mm only just fit into the cavity. I choose 6mm because I didn’t want to have a hard time pushing the epoxy mix. With 5mm, it is only one mm less, but the difficulty is so much greater!I carefully marked on the Rod how far it is inside the rocket back-end. I visually confirmed where it was located and pushed thickened 105/206/403 into position.

Here is a picture of the result.

As you can see, the bog works pretty well. I wasn’t too happy with some bogs, so I added a small amount of JB-Weld to extend the bog.

You will notice that I’e shone the torch through the Fibreglass Air-frame. A very handy way to look at work.

We need to attach the fins to the Motor mount. The first step is to apply epoxy to the root of the fin and set in position to dry. Because the motor mount will get warm, we decided to use JB-Weld glue to attach them.

Like all the gluing steps, I checked, double checked and triple checked how I would go about it.

I taped up parts of the fin to ensure it would fit snugly in the slots of the wood frame. I also taped parts of the air-frame so that any accidental spillages of Epoxy would not cause problems or create a mess.

Very carefully squeezed out equal amounts of JB-Weld and spent 90 seconds mixing them.

Applied a healthy layer of JB-Weld on to the root of the fin.

Clamping only required when doing the “Forward” fin.

Checking that they were level was important because it confirmed that the fins were sitting properly (horizontal) on the motor mount.

I always glued the Aft fin first, let that dry and then did the “Forward” fin using wood/bolts to keep them aligned. I tried to keep the fins inserted with equal gap on each side.

It became apparent after painting my first external fillet test, that I needed to do a little bit more practice to perfect the fillet. The paint revealed the craters and irregularities. They could not be ignored.

Thinking about Cooking

It occurred to me that if I could make better pancake mixture, I could probably make better glue. My pancake mixture always has bump/lumps. I use a whisk to beat the bumps out. So I decided to try and whisk the glue with a ‘baby’ whisk. This helped remove the lumps, but I think it also introduced air bubbles; which are something else we want to avoid.

The outcome was a surface that was considerably better than the first one, but still had some craters.

I decided I had to try and resolve this problem without a whisk. So I decided to use a sieve to filter out all the lumpy 403 filler. I performed the filter process twice on the 403 filler.

I also decided to use a bowl, rather than a cup to mix the mixture in.

I used a ‘paddle’ like stick, rather than the chop-stick and I was very careful to keep the stick in the Epoxy all the time, and stirred very, very carefully, so as to not introduce air bubbles. When I was adding the 403 filler, I only added it in very small amounts, covering the surface.

The result was a better mixture, that when sanded down was almost perfect!

I also used masking tape to help stop the external fillet from producing a non-straight edge. This helped.

One of the mistakes I made here was that I didn’t pull the tape off BEFORE I left it alone to dry. The result was that I couldn’t pull off all the sand paper or sand it away completely.

I used 19mm dowel with Grit 80 sandpaper wrapped around the doweling. After some sanding, I then used 240 Grit sandpaper. I think I’ll try gluing the sand paper to the doweling next time, use some
Selleys spray glue and masking tape to keep it together while drying. So many improvements being thought of!

Next Steps

I’m going to do two more trial runs before I do the rocket fillets.