4" Bertha Upscale

So following my quest to launch Bertha down- and up- scale models on as many motor classes as possible I'm starting on a 4" upscale, 38mm MMT.

I already have BT-5 minimum diameter (13mm 1/4A) through 3"/29mm MMT (G class) built.

Our large annual picnic launch is on August 25th so hoping to get this one built, and all of them painted identically (simple: black nose, white body and fins with one black accent fin).

All but the original BT-60/18mm are scratch built.

I was going to stop at 3" but when I went to spectate at URRF I saw a beautiful 4" FG Bertha cone on Ken's (Performance Hobbies) table. Couldn't resist. Build has lots of self fabricated parts in addition to the cone I bought and some LOC 3.9" tubes, 38mm CRs, and 38mm MMT (ordered but not yet received).

Spent the weekend fabricating the fins and the John Coker style in-nosecone AV bay for my Eggtimer TX.

Pictures to follow but wanted to get the post started!

Comments always welcome.

For reference here is the link to my 3" Bertha.

https://www.rocketryforum.com/index.php?threads/149193/

Started by fabricating the fins. 1/4" birch ply cores, hollowed out with a set of holes to reduce weight (inspired by mega der red max), and skinned with 1/32" ply. 5/16 is more or less closest to upscale thickness and I wanted to keep the weight down.

Managed to get 3 cores on a 12x24 sheet of ply and the 4th from another sheet. As the skins do not have the fin tab, I was able to get all 8 skins out of 2 sheets of 3-ply 1/32" Finnish ply.

The 4th fin is in my "press" (2 12" granite floor tiles with weights on top), but the first 3 came put within 1g of each other!


Layout and cutting of fins (hand cut with jigsaw):




Then I drilled and doweled them together to stack sand on the belt/disc sander to make them identical in shape (don't worry about the 3rd dowel that is out in the main fin area - you'll see why that doesn't matter in this case):



Then finally, I drilled out a lot of the material to reduce weight (in the spirit of the Mega Der Red Max), and will apply 1/32" Finnish birch skins:

I will need to weigh the 4th fin as it was in the press overnight, but the first 3 came out nice, and are all within 0.8g of each other in terms of weight. Not bad.

Taking my first crack at a John Coker inspired in-nose AV bay. Since I needed a solution for recovery anchor anyway (the FG cone I bought doesn't have an anchor point or bulk plate installed), seemed like a no-brainer. And it will give me a chance to fly my Eggfinder which I'm yet to use.

Since these rings don't need to be perfect - i.e. they're not aligning the motor thrust, and they are off sizes due to the thickness of the nose cone material, I decided I would just cut these by hand and clean up on the disc sander and router table (hole for the small piece of coupler material). Overall I am pleased with how it came out for my first attempt!

Just got the shipping notice from LOC with my MMT (38mm), CR's, and bulkplate are on the way! I don't get much time to build during the week, so that's OK - looks like they'll be here by the weekend.

The plan is to use a longer MMT as a stuffer tube, ending in a CR at the top of the coupler where the booster section and top will meet. This will be a mid-break rocket with the coupler serving as a chore-boy baffle with removable lid for cleaning/replacement of the mesh inside.

For refernce, I'm basically building a bigger version of the baffle used in my Stretched Leviathan as documented in the last couple posts here:

https://www.rocketryforum.com/threads/estes-leviathan-stretch-build-thread.126591/page-2

Updates to come as build progresses ... thanks for looking!

This is cool. I just got back into rockets after 20 years and built a big Bertha followed by the baby Bertha. Gonna get the super kit for sure. Can’t wait to see the ultra Bertha.

Finally some more progress. Got booster tube cut to 26" (coupler will be installed as a baffle), and the fore tube will be cut to 18". This leaves about 11" of room to pack recovery.

Got MMT installed and fins all tacked with 5 minute epoxy. Internal and external fillets will be done with either rocketpoxy or filled US composites TBD.