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.

What I wish to create

I’ve started creating some external fillets on my test fin using West Systems Epoxy. I wanted to make 1cm fillets on both sides. So 2 * 20 = 40cm of fillets in total.

The radius of curvature is 1cm.  With some simple calculations I determined that I needed about 17cm^3 of material. I decided to make a lot more, so that I didn’t run out.

Materials

So I used:-

• 25ml  of 105 Resin
• 5ml of 206 Hardener
• Approx 4 grams of 403 – to get it to peanut butter consistency, where it would not drip. This is a bit of a visual ‘job’, don’t try to measure it accurately as the scales aren’t accurate enough.

I put this mixture into a plastic bag and cut a small hole in the bag and then used this like an icing bag to squirt the epoxy material into the fillet.

The area to fillet had already been sanded with Grit-80 and cleaned. I used some clothe to ensure no dust/crud there.

After applying the epoxy material I used my new tool a 2cm side 10cm long stick with a 2cm diameter semi-circle at one end, to form the fillet profile I wished. I then used the other end to scrape off any material left over.

Here are some pictures taken during the job.

 

I did not use the whisk in the first batch, which in turn was on BOTH sides of the fin. The second batch was done with the whisk and was used to fillet up two pieces of wood. The consistency of the epoxy was significantly greater, though still not perfect.

 

Results after curing (before sanding)

I forgot to lay down tape 1cm from fin/wood interface. I will do this next time to ensure fillets are uniform along the length.

I tried using methylated spirits on my finger to smooth the paste, but this failed.

I will use a sieve next time to introduce the 403 into the epoxy mix, as I believe I’ve introduced air that is causing some issues with the quality of the fillet.

Sanding

I’ve started sanding with flexible sanding pads. I can bend them back on themselves to produce a pretty good sanding block with a semi-circle end. I have used coarse, then medium and finally the fine version. I used some water as well to get a finer finish.

 

I’ve purchased some real G10 material to practice on. What I wanted to practice was:-

1. Learning how to cut it well (even though this isn’t required for this build)
2. How to sand the Leading and Trailing edges to the profiles I’ve decided.
3. Sand the root and tang to Grit 80. Also sand other edges lightly with Grit 80 to remove any burs. (But I don’t want to change the shape)
4. Sanding back with Grit 240 to give it a good finish for painting. I sand it ALL over, except on the tang, and the root where it is glued on the motor mount
5.  Score the root of the fin (yes, on the thin edge) using a punch and a bit of elbow grease. Did this every cm, in both directions at 45 degrees
6. Score Tang of Fin that is going to be below the air-frame. I did this every 2cm at 45 degrees in both directions
7. Cleaning the surfaces thoroughly with Methylated Spirits
8. Inspect the fins to make sure there is no other dust, grime or dirt.
9. Perform the following preparation steps
10. Attaching to “motor-mount”
11. Test paints – to see how finish is.

 

Preparations Steps

Before I can begin attaching fin, I need to prepare my work area. The “main” steps are:-

• Clean desk
• Get Methylated Spirits
• Get Paper towel
• Get a clean wood board for mixing Glue and mixing stick
• Get my Timer
• Wood blocks
• Get G-Clamps.

Steps to attach the Fin

We can’t do these steps until the fins are prepared!

1. Then I mix up equal amounts of J-B Weld black and white on a clean surface for mixing.
2. I start time for 20 minutes. This tells me how long I have to go.
3. I apply this to the root of the fin as evenly as possible, and try and avoid it going over the side
4. Clean off excess J-B Weld off sides very carefully using Methylated Spirits and paper towel.
5. Carefully lower the Fin into the slot, make sure that the J-B Weld doesn’t get on to the “Air-Frame”
6. Apply a bit of a wriggling action to get the glue to fill gapes
7. Make sure the Fin is 90 degrees and use clamps and wood blocks to ensure that the fin cannot move
8. Leave it for 15 hrs minimum!

Here are some pictures:-

 

 

 

I need to create internal fillets against the fins between the motor Mount and the Air-frame. The gap is very small – ~12 mm. So I’ve purchased some 5 mm diameter tubing with some 50 mm syringes to get the glue down there. I have used some thin doweling with masking tape to help direct the end of the tube to where the glue needs to go. Below are a few pictures:-

 

The process will be:-

1. Suck up 25 cubic cm of Araldite into the gun
2. Clean end
3. Insert gun up to the ‘zero’ mark
4. Push 1 cubic cm of araldite in
5. Withdraw 1 notch

Repeat step  4,5 will tall the way out

I’ve finally purchased glues for the rocket.

I have West Systems 105, 206 and 403 filler for the external fillets.

I have J-B Weld Epoxy for the root fillets.

For the internal fillets, I’ll probably be using West Systems 105/205 (no filler). What I will need to get the epoxy up the rocket into the area of interest is a tube/syringe.

 

Yet to purchase the pipe.

I’ve discovered that the fins are not exactly (or even close) to 120 degrees apart. The distance between two sets of fins is the same, while the other Fin/fin distance is greater than the other two by 5mm. Not terribly good. What this means is that the Jig that I made to hold the Fins in place ONLY has perfect alignment in ONE position. i.e. I cannot rotate the rocket and have the fins line up in the slots of the wood.

This has meant that I need to change my fin install procedure, by allowing me to rotate the WHOLE Jig into position, so that each fin that is being glued is vertical. I’ve done it as follows

 

 

 

We can take the wood stand off and attach it to the Jig, pointing out the other side. This allows us to install the other fin pointing down to the wooden base.

The next step is to profile the fins and to score/sand the roots and tangs. Some important decisions to make are:-

• How to evenly sand the surface of the fins (so it is symmetrical)
• What profile is required – at leading edge and trailing edge.
• How to score the fins

We concentrate on the first two here.

Fin Symmetry

When I examined the fins, the first thing I noted was that the chamfer in them is not symmetrical to begin with. This is unfortunate, but I’m just going to have to work with it.

 

Well, that is something we will need to work with. I had a vision of a semi-circle shaped leading edge, but this is probably not possible now. Will probably make them parabolic, or elliptical in shape.

 

The tools to sand down the fins

The first tool I created to sand down fins was the following:-

 

The problem with this is that the sandpaper lifts off from the surface and interferes with the sanding process. The area of sandpaper available sanding is reduced because it needs to be sandwiched between the pieces of wood. Because the longest section of fin to be sanded is 250mm, this means we can’t sand the edge down (along the whole length) in the same stroke. This is a big disadvantage, and was one of the identified issues we wanted to solve.

Some tests were done to see if we could get a nice job done using wood and it looks like it “should” work. See below:-

 

As you can see, the edges look very well curved. I did this curve by doing sanding at ~30 degrees, ~45 degrees, ~60 degrees. Then I did a ‘curving’ action to try and get it rounded. This makes a ‘semi-circle’ like edge.

I decided to build a simpler sanding plate. I needed:-

• Sandpaper
• Spray Glue
• Plywood
• Clamps

I followed the instructions very carefully. Spraying both the sandpaper and plywood, then letting them sit for 10 minutes. Then I put the two together and I clamped them over night. I decided to put 80-grit sand-paper on one side and 240-grit sand paper on the other side.

Here is the end result.

Notice how there is some bare plywood accessible on the edge of the sanding board. We have drilled some holes into this can can mount this to a large piece of plywood. This allows mounting the sanding board on a solid (non-moving) bench. See below.

 

 

 

 

 

 

One of the steps I’m coming up to is the attachment of the fins to the Air-frame/Motor Mount. For a good join and a good rocket build it is important that we:-

• Sand/score the root of the fins
• Sand the Motor Mount where they are going to be glued
• Sand/score the Tang of the Fins for the fillets later on
• Important that the Fin/rocket do not move for 3 days while it dries
• Very important that the fins are precisely 120 degrees apart and are perpendicular to the air-frame

The the last two bullet points, we have build up a “jig” to hold everything in place. Below are some pictures of it.

 

 

 

 

We want to hold the fins completely still while it is drying. For this, we have used two square pieces of timber and some G-clamps to keep the fins steady. See the photo below.