The next step was to glue the centering ring into place.
Decisions
I decided to use 30min Epoxy glue from AusRocketry. I didn’t want to go for 5 minute and I didn’t want to for 24 hour.
Preparation
While every effort was made to ensure glue/mess did not make its way on o the surfaces where the Centering Ring is to be glued, some does make its way and so I had to clean it off. There wasn’t much. I used some Methylated spirits to clean the surfaces as well.
Next I wanted to tape up the motor mount exterior, so I would not get any epoxy glue. I also taped up the exterior of the airframe, just to help reduce chance of getting Epoxy on there.
I also decided to stuff some paper up the Motor Mount tube to ensure nothing got up there.
Insert centering ring in ~6mm and mark on the motor mount three places with pencil. This helps us know how far we have to push it in.
Gluing the Centering Ring
This was a carefully thought out job.
Internal and External ring is sanded with Grit 60
Sand internal/external with Grit 60 sand paper
Mix the epoxy resin and hardener
Apply some of epoxy to the external surface of the centering ring. Need to be liberal in application, but not so much it drips too much or make a mess.AS YOU DO THE JOB, rotate the rocket so the glue doesn’t drip down to one area of the rocket!
Apply liberal amount of epoxy on inside of air-frame (a ring)
Apply liberal amount of epoxy on outside of the motor mount (a ring)
Remember, keep rotating the rocket.
Slide the centering ring on
Double check the depth the centering ring has been push in
Remove the wire holding the centering Ring
Take the rocket out of the Jig and attach to pole and cable-tie up
Here are some pictures.
Taped off areas to prevent Epoxy spoiling the rocket.All tools and materials out, ready to go.
Never used the Syringe in the end. Used chopstick instead.
Mixing 24 Epoxy.
‘Wetted’ the Centering Ring with Epoxy.Applied Epoxy to Motor Mount and Airframe. (Might be hard to see)
After this I slid the Centering Ring on. This requires two hands, so no photos.
Then I erected the rocket against the workbench, so that Gravity would bring the Epoxy on to the top of the Centering Ring.
Rocket strapped to bench with cable tiesPhoto of bottom of rocket while gluing. I removed the bottom tape about 2 hours later.
Next I wanted to attach the Rail Buttons. Rail Buttons are what we use to guide the rocket up the rail. A few decisions had to be made:-
What type of button to use – 1010 or 1515
Where the buttons should be installed
How to affix the buttons – to ensure sturdy install
Deciding What Type of button to use
I’ve heard that using 1010 on the rocket I’ve built has worked well for the creator/designer of the rocket. So why would I not want to go for a 1010?
Firstly, I know that this rocket can get quite heavy with certain motors, and the heavier the rocket, the more I want to be sure that the Buttons are appropriately sized – for the weight. But I wanted to err on side of caution. I’m responsible for the rocket and I have to feel comfortable with the build.
Secondly, I know that the QRS club 1010 launch rail is 1.7 metres and the 1515 launch rail is 3.5 metres. When I do simulations using OpenRocket, I get some rail exit velocities that are below 15ms-1. (15ms-1 is approximately 50feet/second). 50 feet/second is considered to be a good around min safety speed. Yes some people have managed to get away with lower speed rockets, but I want to err on side of caution.
Below is a simulation of some motors using a 1.7 meter launch rail.
Simulation of a set of 54 mm CTI motors using 170cm Rail.
A second restriction I have at the QRS Cedar launch site is that the Ceiling height is 5000 feet. This translates to 1524 metres. With a 10% safety margin, this comes down to about 1372 metres. This means the only real contenders ar:-
K1720ST-P
K515-SK-13
1633-K940-WT-18A
By having a 3.5metre rail, all Velocity off Rod are acceptable.
Simulation of a set of 54 mm CTI motors using 350cm Rail.
The downside of having larger buttons is increased drag – and slightly less sleek looking rocket. Also, if I take the rocket to a launch site without a 1515 rail, I’m going to have problems!
Where the buttons should be
I decided to accept the button placement suggested in the Katana 4 Build instructions, which is 3cm and 45cm from the rear of the airframe. Having the button at the rear means less chance of taking paint off the rocket while loading on to the rail. The 45cm is not the most ideal position (not around CG), but it has to be here because we don’t want the button screw snagging the Drogue parachute. (At 45cm, it is within Motor Mount/Airframe cavity.
How to affix the buttons
Decided to screw into 4.5mm holes that were very carefully drilled. I added Selleys non-drip superglue (CA glue) into the hole to give the whole arrangement a little more strength. Don’t just want them popping out. I was careful here to ensure that no CA glue was on the exterior which might glue the button to the air-frame. I read that it was important to allow the button to rotate freely.
I was a little concerned that the glue might dry very quickly before I had completely screwed the screw in. So I did a test run. No issues occurred during the test run.
Doing a practice drill/screw run into some perspex.
I wanted to make sure the buttons were mid-way between two fins.
Ensuring button install axis is equi-distant from each two adjacent fin-sets.
The first part of install was to mark the places to drill. I used Right Angle aluminium length and pencil.
Angle Aluminium to help mark drill holes for buttons.
I drilled holes very carefully making sure the drill bit was perfectly vertical. I drilled the initial hole at 2mm. Then used 3, 4 and finally 4.5mm drill bits.
Drilling holes into Airframe.
As you see, I carefully labelled every area I was to drill, double checking measurements before drilling.
Here is how it looked at the end.
Rear button is installed.
Forward button is installed.
I was careful to make sure the buttons could spin,
After the buttons were installed, I put a dab of 105/206/403 epoxy on the inside on the screw. Here is how the rear screw looked. Not pretty, but it helps to take some of the load.
Eventually I applied the final External Fillet Epoxy to the air-frame. They all looked reasonably good, all quite similar in profile and size to the photo below.
View from back of both fillets.
Preparation
I wanted to avoid sanding the air-frame and fins at all costs. I decided that if I employed masking tape, I’d have to sand through the masking tape before sanding the fins/air-frame and I’d notice in time that I was sanding the wrong thing. Below is a picture of it all taped up.
Taped up and ready for sanding
Sanding
The next step was to sand them. I had gone to extraordinary lengths to glue Sandpaper to a long stick to sand the fillets. I realised pretty quickly that this was overdo and I did not need along sheet and I did not need a long stick. In-fact, having a long doweling with long strip of sandpaper was counter-productive because the fins, even though they were VERY well aligned, the slight differences in epoxy between Fore/Aft fins meant that the sand paper would not reach all along the whole length of the fillet. The other fin was “in the way”.
So I pretty quickly went for a smaller doweling with about 10 cm of Sand paper just wrapped around it with some masking tape to hold it in place at one end. This allowed me to sand each fillet individually. No glue, it was so much easier.
I used the following sandpaper in the order shown:-
Grit 60
Grit 240
Grit 600
Grit 1200
I used water for Grit 600 and Grit 1200. i.e. Wet and Dry.
I had to be very careful to keep the doweling parallel to surfaces, to ensure I produced a curved profile along the whole length of the external fillet.
The results
The results were good, but not as impressive as I had hoped. Near the edges of the fillet there were craters. I couldn’t risk sanding into the air-frame, so I had to think about how to deal with them.
I didn’t keep a photo of the cratering, so I’ve retrieved a photo of cratering in some of early practice fillets.
Cratering on bottom of external fillet. Red arrow pointing to the cratering.
Preparation is key to the success. I made sure we sanded the areas to be filleted with Grit 60 Sand paper, that they were then cleaned with Methylated spirits and then taped up.
Sanding with Grit 60 Sand paper.
Finished sanding – ready for cleaning.
All practice external filleting I had done so far had neglected the ends; it was more focused on the the main stretch. The width of the fillets needs to converge to zero at both ends of the fin on the air-frame. I did a simple plot y = 2x^2 on my printer and printed it out and used it to create masking tape pieces to stick at the fin ends. What resulted looks as follows:-
Pointing to the curved masking tape end. This should ensure a consistently shaped fin fillet.
Rocket all taped up, ready for filleting.
The Glue
We made a normal ‘peanut’ consistency batch of Epoxy. We decided to use the new syringes as the old ones are too sticky and hard to push.
We originally made 20ml/4ml of epoxy for one one side of fins (lower and upper fins). We found that there was probably about 6ml of epoxy left over – so wastage. In future Epoxy runs, we’ll make it up using 15ml/3ml of 105/206.
Epoxy in new syringe, already to go.
The result
Back fin.
Front FinView from back of both fillets.
Fairly happy with it. I am going to need to build up the back/front of fins to ensure smooth transition. Not quite as good as I had hoped there.
I decided after much internal debating to create a fillet between Fin and the air-frame. (Though it is more from the previous fillet to the air-frame). I decided to do this because the instructions for Katana 4 mention Internal fillets to the Air-frame. I know some people don’t bother with these fillets and I can probably get away without having them for smaller motors. But if I want to fly larger motors or lots of flights I might experience issues with fins/air-frame.
So much focus has been on attaching the fins securely to the motor-mount and air-frame that it is easy to forget other possible modes of failure (other than fins coming off). I was a little concerned that the air-frame, being as thin as they could possibly fail due to loads. So I thought it would be prudent to strengthen the air-frame around the fins with this fillet.
The Epoxy resin
I decided I didn’t want to go through with the 105/206 again with leakages. The space is much more restrictive and so bogging up ends could be difficult. The fillets don’t need to look nice and so I decided to go for 105/206/403 – just short of peanut consistency. I decided to go a little under peanut consistency, so that the epoxy could ‘droop’ down and fill up the gaps. I wanted a droop more than a drip so that I wouldn’t get a large mess. After about 20 mins of curing, it’s viscosity increases sufficiently that it stops drooping.
The tools
It was too hard to use really thin tube – 5 mm in diameter, despite the obvious advantages; it can easily fit down into the cavity. The problem is that the Epoxy cannot be easily delivered due to the viscosity.
So I had to go for 6 mm internal diameter tube. The extra mm makes a big difference. A problem with 6 mm tube is that I couldn’t attach the normal doweling to get it down into the cavity. It just won’t fit. 6 mm tube by itself is so snug that it somewhat deformed (squashed a little). I also wanted an arc (1/4 circle bend) at the end to direct the epoxy into the gap between the air-frame and the fin and the only way I could do this was with 3 mm diameter copper rod that I bent into shape. I used narrow lengths of Masking Tape to bend the tube at the end to the copper rod.
6mm Internal diameter plastic tube on 3mm copper rod.
It worked! I could then withdraw the tube and push the epoxy in and was even able to just see the epoxy coming out, so I could confirm that it was going into the right place! Very happy with this.
Preparation
Bench ready for Gluing.
403 Filler ready for use.
Mixing 403 filler into 105/206 mix.
Internal fillets done
It is a bit hard to notice, but there are internal fillets done here.
I wanted to practice the external fillets again. I have only one chance at this , so the procedure needs to be clear in my mind.
My Test bed
I nailed two pieces of wood. Then I used a 10mm aluminium rod to accurately measure 10mm lines on each edge. Then I taped up the wood, so that fillets can’t extend past 10mm. Below is a photo of this set-up.
Test bed – to practice external filleting.
The Epoxy
I wanted a proper peanut epoxy mix – no drooping at all, no dripping. So I went for the 105/206/403 mixture.
I mixed 15/3 of 105/206. This did two 15cm lengths with a little bit left over. With 6 fins (both sides) we have 204cm of fillets. So we will need about 7 times this quantity of epoxy. i.e. 105/21. Though we are very likely to do this in 6 (possibly 3 if we are feeling extra confident) jobs.
I was careful not to introduce any air while mixing and I carefully filtered the 403 twice to ensure that I didn’t have any clumps. I had about 10 mins of 20 mins time left to apply the epoxy to the filleting area and create the required profile.
Delivery of Epoxy to Join
I don’t need to pass this epoxy through any tube (like before), just from a syringe. I just packed it in and then squeezed it into the spot. I made sure that I was putting in enough epoxy along the fillet join.
105/206/403 epoxy mix loaded into syringe, ready for filleting.Applying epoxy to join. (Sorry it is out of focus)Left over Epoxy mixing after using PCB tool to create fillet profile.
Epoxy left to dry for 12 hours. Notice how tape has been removed.
Sanding
After it had dried, I did the sanding using a specially made sanding stick. I made the sanding stick by Gluing Grit 60 Sandpaper to one end and Grit 240 at the other end. I used Selleys Glue to attach the sandpaper. It had to dry overnight. (Yes, lots of waiting).
Practicing using the sanding bar
The (almost) Final Product
Here are two pictures that compare one side (sanded) with the other side (un-sanded)
Un-Sanded FilletSanded Fillet
The finish is excellent – very smooth. There is still a line where the masking tape was. This line needs to be removed by using filler to bring it to the fibrerglass air-frame.
As a reminder, this effort is being made because we want to reduce Interference drag which can have a significant impact on a rocket’s performance.
Removing the line
My neighbor suggested the filler as a technique for removing the line. He had a go on this test fillet I did. I took some pictures of what he did. (Of course I’ll be doing this myself when it comes to the real thing).
Side on view of fillet sanded and filled in and painted with primer.Very smooth finish after building up edges to remove tape line. Painted with primer to help bring out any imperfections; if any.
Good aye? Can you tell where the fillet starts/ends?
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
See the internal fillet – glistening in the light.Closer view of the final fillet.
Rocket left alone for 10 hours while internal fillet dries. NO-ONE can touch it.
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.
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.
One fin to go. Just need to tape up before I start last fin.
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.
Squeezed out equal amounts of JB-Weld Epoxy
Very carefully squeezed out equal amounts of JB-Weld and spent 90 seconds mixing them.
Applied a strip of JB-Weld to root of fin.
Applied a healthy layer of JB-Weld on to the root of the fin.
Last fin in position and drying.
Clamping only required when doing the “Forward” fin.
Making sure both fins edges are level
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.
