Updates!
It's time to start tying everything together with shear pins and metal screws and then on to prime+paint.
First up is drilling for the shear pins and metal screws. Normally, with smaller rockets, I'll simply lay the frame out on my rack and drill the holes. However, since this is such a large and long model, when laid out, the seams between sections open up by about 1-2 mm due to all the weight trying to bend at the joints. It's not much, but I want to ensure a straight, tight fit.
To solve this, I assembled the rocket and let it sit upright. Duct tape secured everything to make sure it didn't get rotated accidentally while I was walking around drilling holes. Also, you can see in the image that I glued on short wooden tabs. These are alignment tabs to make sure I always match up the same holes when assembling on launch days. It takes out the guesswork. For smaller rockets, I'll use toothpicks. I simply epoxy it across the joint and saw the tab in half.
Here you can also see the registration marks where I'll drill.
I'm using 4-40 shear pins, so I drilled a pilot hole and hardened with a dab of laminating epoxy. After curing, I bored out the pilot with a #43 tap to cut threads for the nylon screws. I tested each, and they screwed in nice and tight. Each tube has 4 x 4-40 shear pins. Note, only tap one tube. Don't cut threads into both holes. Otherwise, you run the risk of cross-threading and having trouble getting the screw in straight. Later on, I'll epoxy in a metal plate as a cutter.
I used 10-32 metal screws to join sections that I don't wish to separate. These were drilled out with a 3/16 drill bit and also hardened with laminating epoxy. I want these to be tight and secure, so I epoxied on nuts to the insides of the tubes. These are special nuts made for epoxy attachment. They have a mesh grid to allow solid attachment of adhesive.
First, I used nylon 10-32 screws to hold the nuts in place. JB Weld was spread thinly across the mesh and the tube. After curing, the nylon screws could be easily backed out since epoxy doesn't stick to nylon.
Here's the progression series:
Before adding in the fin filets, I wanted to fix a few depressions in the tubes where the mylar didn't have good contact. It leaves a small depression where the weave is more exposed. For small jobs where I want a smooth tube, I typically use Bondo. However, this larger tube means I need more working time. That's when I prefer to use Evercoat Rage Ulta Xtra. This filler is lightweight, sands smooth with no real pinholes, and has a 40 minute working time while not sagging at all. I love it for jobs like this. It's supposedly greener chemistry, but it's still stinky as hell. I got it at a reasonable price from here:
https://www.ebay.com/itm/142060074703
1 gallon will last a LONG time.
After filling, I went over everything with an 80 grit palm sander. The tubes came out pretty smooth to the touch. I have trouble seeing if there are other imperfections in this glossy epoxy, so I'll need to wait until I shoot it with gray primer to see where else I need to smooth out.
Next up was the fin filets. I'm using a 1/2" PVC pipe to drag out the filets. To mark off where I need to tape, I use an old trick. Color the pipe with black marker and drag it across the fin and body like if I was smoothing out the filet. This leaves a black line exactly where I need to tape off. If I get it just right, the epoxy won't have a transition bump that needs sanding at all.
Here you can see the lines before I tape it all off.
And then after taping... You can also see all the sanded down filler. Trust me, it feels smooth.
I did the same marking and taping for the front strakes. After taping off, a filet of RocketPoxy was added and smoothed out. Since RocketPoxy has a honey-like consistency, it's great for filets as it will settle a bit and smooth itself out very well. The filets for the front strakes were dragged smooth using a popsicle craft stick. I didn't like the results as much as when I use PVC pipes wetted with isopropanol. I have better luck with that, but I wanted these filets to be smaller up front.
I let the epoxy cure for about 20' before pulling off the tape and giving it a final smoothing out with a gloves finger wetted with isopropyl alcohol.
Two filets curing a bit before tape removal.
