Building my Super Big Bertha

February 27, 2022

This kit was purchased in 2000 and designed to fly on D12-5 engines (only).  It's 36" tall and 2.5" in diameter with an 18" parachute.  The current version of the Super Big Bertha is designated a Pro Series II model and designed to fly with bigger 29 mm engines, projected to fly to 1200 feet on an Estes  F15-6. (https://estesrockets.com/product/009719-pro-series-ii-super-big-bertha/).  Not wanting to be embarrassed flying a "wimpy" Super Big Bertha on D engines, I ordered 29 mm engine tubes from Estes and plywood engine mounts from Apogee.  Here are the modifications I'm planning to upscale my 2000 vintage Super Big Bertha:

1. Use a 29 mm engine tube to expand what kind of engines I can use.
2. Use an Estes Pro 29 mm motor retainer set instead of hook & engine block.
3. Hand cut engine mount for 29 mm from extra 2.6X 18 mm cardboard mount.  —><— Decided to buy pre-cut plywood engine mounts from Apogee instead!
4. Cut the 24 mm stuffer/engine tube to remove 29 mm length (8 of 18”) THEN mate end to end w/ mounts glued together.
5. Make an ejection baffle of the last pair of mounts (as suggested by https://www.rocketreviews.com/estes-super-big-bertha-lpr-mike-goss.html) to cut down on having to use so much flame retardant wadding and prevent damage to the parachute.
6. Make a Kevlar no zipper mount like my DE’ FatBoyKevlar no zipper mount like my DE’ FatBoy instead of Estes trifold with the short elastic.
7. Use a swivel to attach parachute so I can easily swap if needed.
8. Use an Apogee 1/4” x 3” launch lug (cut in half) instead of 3/16” so I can use the more stable 1/4" Pro launch pad.
9. Use a red 18” nylon chute instead of the provided 18” plastic.
10. Use Apogee plywood centering rings for 29 mm & 24 mm stuffer tube instead of the provided 24 mm cardboard rings.

My Super Big Bertha is predicted (OpenRocket) to fly almost 800 feet on an E16-6, 1200 feet on an F15-6 and over 1600 feet on an F50-6.  I will modify these estimates based upon the actual weight as construction progresses.  Incidentally, a new beta version of OpenRocket was just released that allows me to simulate different colored fins.  I'm toying with using an all black throwback to the earlier Big Bertha look with a single red fin.

I could even buy upscaled "black bertha" decals to fit my rocket from stickershock23.com (check out the larger than life Bertha in one of their pictures!).

My plan is to use a 29 mm engine mount, then cut the 24 mm tube that serves a engine mount and stuffer tube and just use the left portion to reduce the ejection volume in the body.  I've laid it out with the plywood rings (24 mm hole on the left and 29 mm hole on the right), with a 29 mm engine retainer in place at the right.  This should leave a manageable amount of space for the 18" nylon parachute and some flame retardant wadding.

It's time to start cutting (24 mm engine tube; I just slid the excess into the 29 mm above to check the layout) and glueing on the centering rings.  I'll glue the two together where the two tube sizes meet for strength and to prevent gas escaping.  I need to drill baffle holes in the upper (far left) centering ring and smaller holes to attach the Kevlar string.  I'll use the white disks to block the top of the 24 mm tube to redirect the ejection gasses as described in Mike Goss's Rocket Reviews posting linked above.  I'll keep updating this page as construction progresses.

March 5 update:

I used my new Estes tube cutting guides to trim the original 24 mm engine tube.

I'm going to use the upper portion as a "stuffer tube", which decreases the volume that has to be pressurized so that the chute is ejected properly.  I sanded the cut end to remove some slight imperfections (hey, I said it was my first time using the new cutting guide!) then test fitted the plywood centering rings.  Following Mike Goss's recommendations , I drilled holes with a 7/16" drill bit in the upper centering tube, at the end of the stuffer tube, and poked holes near the top of the tube itself using an awl.  I'll block the end of the stuffer tube by glueing some disks to force the exhaust to go through the holes and avoid sending hot particles directly into the parachute.

I glued the plywood centering rings to both tubes using 5 minute epoxy then, when dry, attached the 29 mm motor retainer to the bottom of the engine tube using Crazy Glue (cyanoacrolate).

Next step is to cut some Kevlar and assemble the anti-zipper shock mount, install the engine mount using 5 minute epoxy, glue the stuffer tube above the 29 mm engine mount and continue with the rest of the Estes instructions.  I'll use epoxy to attach the fins instead of wood glue and add some fillets to strengthen the fins in case I launch this with a really big engine!  Open Rocket is predicting that, even with the extra weight from using plywood centering rings instead of cardboard, this rocket should go over 1600 feet on an F50 (discontinued Estes) or G74 engine (Aerotech).

March 6th update:

I glued the assembled 29 mm engine mount in the bottom body tube using 5 minute clear epoxy, then glued the assembled 24 mm upper stuffer tube on top using epoxy.  I positioned the upper body tube, without glueing, to hold the stuffer tube in place until the epoxy had dried.  

Next I measured and cut the Kevlar as described.  There was 10.5" from the top centering ring to the end of the upper body tube and by body tube was 2.6" in diameter so, as described in the Apogee newsletter I needed 4 X 15" or 60" of Kevlar.  I cut that, threaded it, tied a knot (covered with glue) and tied a swivel at the center of the harness.

Next I connected the upper body tube by sliding it almost in place, applying wood glue to the inside of the upper body tube and sliding the connector into place.  Using a ~12" scrap of balsa wood I applied clear epoxy around the top edge of the upper centering ring, being careful not to get any in any of the eight baffle holes, the hung it to dry.  I've marked the positions of the fins on the lower body tube, cut and sanded the fins in preparation for applying them using my Estes fin alignment guide.

March 17th update:

It turns out you can't use the Estes fin alignment guide for rockets like the Bertha because the fins are swept back and there's no way to stabilize the body tube with the fins so high.  I tried a couple of approaches before settling on using one of the fin guide printing tools from PayloadBay.com.  You enter the diameter of your rocket, how many fins, how thick and how far out they extend ("span") then print the guide.  It took a couple of tries because the other printing tool on the PayloadBay.com site prints 11" X 17", not 8 1/2" X 11".  Once I got it printed right, I sprayed some cardboard I'd cut from a box with Fast Grab Tacky Spray adhesive, pressed the sheet down flat, then cut out the fin and body tube guides.  

Sliding the guide over the body tube, I mixed some fresh 5 minute epoxy, applied it to the bottom of one fin and slid it through the alignment guide so it was touching the body tube along the mark I'd drawn.

I repeated the process, letting the fin dry for at least half an hour before moving anything, until all four fins were in place.  Next I attached fin gussets on either side of each fin using fresh 5 minute epoxy.  Finally, I cut two 1/4" X 1" launch lugs and glued them in place on the body tube using 5 minute epoxy.  After drying, the final empty weight (with a 24" nylon parachute and shock cord inside the body) was 7.875 oz.  My OpenRocket simulation estimated weight was 7.52 oz, so I assume the extra weight is epoxy.

Next comes the sealing, sanding, base coat and paint.

May 20, 2022 Update

After a few months delay I painted one side of the fins with Rust-Oleum Carnival Red gloss enamel, waited overnight then covered the red with FrogTape before painting the rest with Rust-Oleum Black gloss enamel paint.

(and, yes, it turns out a 2.6" diameter body tube fits on top of the can of paint, used here to keep it from tipping over.). Here's the rocket with the red fins covered with FrogTape before painting with  Rust-Oleum Black:

Here's the finished rocket:

Now I just need to connect the 24 inch parachute and shock cord and buy some 29 mm engines to launch with!  The latest estimates from OpenRocket are 864 feet on an E16-6 or 1260 feet on an F15-6.  I might launch first on a D12-5 using an 24 to 29 mm adapter, which is projected to fly 394 feet.   This rocket will require my new Pro Series II launch pad with a 60" X 1/4" launch rod for stability.  I'll update with launch reports and hopefully a video. 

• November 19, 2023 Launch Report