Upscale Big Bertha

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Well-Known Member
Apr 21, 2011
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Well I now have everything gathered to make a 2.5X upscaled Estes Big Bertha.
So now it is time to make this thing.
The first part was planning.
I used Rocsim for all the size conversions.
Below is the Rocsim file
The next step was to gather all the materials.
I already had most of what I needed with the exception of a nosecone.
The nosecone was supplied by our very own Sandman.
(wonderful job)
Here is a pic of that

I then printed a fin template on posterboard cut to fit my printer and upscaled the decal sheet (found at JimZs website) using Adobe Photo Delux
Here is a pic of the template and decal sheet
Fins are cut out on a band saw and leading edges slightly rounded with a palm sander
Through the wall fin slots are cut on a router table with a strait flute 1/4" bit and using a fence to guide the cut.
Body tube is 3.9" PML phenolic.
2 pieces are used to make the 44.5" length.
1 at 36" and a payload section at 8.5"
After cutting slots I pre-assembled the fins to test fit , see attachment below (my daughter is next to rocket to show scale)
Looking really good!!!!........ I noticed you had a 38mm motor mount........... What motor are you planning to use?
I would like to start with an I211 but made the motor mount long enough to accomodate perhaps a J350 in the future.
That brings up a question I have.
I am using PML construction methods with fin and motor mounting ie...
Do not epoxy aft CR in place until after applying fillets to internal fin roots.
The question is should I add PML expanding foam to the fincan?
On the one hand, yes things are stronger, on the other, that adds more aft weight.
Anyone have an opinion on that?
Here is a view of the coupler and motor mount.
Notice the piston strap epoxied to the side of the motor tube.
Here is a look at the business end.
The pic is to show the inside fillets but they are kind of hard to see.
A layer of 3.5 oz. glass on the fincan area.
Fins are 1/4" birch ply purchased at the local hardware store.
looks awesome, I am gonna have to stop by my local harware store and ssee if I can get my hands on some of that fin material
Great job!

Love the classsic paint job.

But really should you be building rockets bigger than your kids?

I finally got to fly Bertha today.
It went up on an I195J for a perfect flight to 2,500 ft.
Here is a launch pic.