Super Big Bertha Build Advice?

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techrat

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I'm starting a Super Big Bertha build. I'm on the fence regarding building the 3-ply balsa fins versus buying the Plywood upgrades. While the plywood is likely to be MUCH stronger and durable, I feel like I want to take on the challenge of building the 3-ply balsa fins, as it looks like those are going to take a week of time onto themselves. I can then paper them, soak 'em in CA and do whatever it takes to make them as durable as I can.

Any advice? Also be aware that I'm going to re-purpose an older BT-80 project nosecone/payload bay, so my "SBB" is going to be about 6 to 8 inches taller than stock. And after that it's going to be Q-Jet 29mm all the way, not even going to bother with black powder "F" (which incurr Hazmat fees anyhow).

Also, if anyone who has built the balsa fins can recommend adhesive, chime in -- I think wood glue will warp the fins, but I guess wax paper and some heavy books are going to be needed tools. Other things I'm thinking involve 2-stage epoxy thinned with a little acetone to make it more spreadable and soak-able. But I'm all ears and open to advice as I want to build this thing to be as durable as it can be without going overboard on the weight.
 
Wood glue and heavy books will keeps the fins flat. Don't be in a hurry to take them out of the press. When sandwiched in waxed paper, they can take a good amount of time to fully dry. I leave mine for several days after having one build where I took them out too soon and they warped. None warp if left for 4 days+. Those sandwich balsa fins will be light and strong. I've seen them fly on punchy Gs stock. The Q-Jets aren't massively powerful at the start, so keeping it light will help with enough rail speed. Unless you're going to shove high power motors in it, building it light and feeding it smaller motors are cheap ways to be happy.
 
I used wood glue when I built my Mega Mosquito which has the same three piece fin design. Worked great with no warping. I just let them under a a couple of phone books for a day and let them dry. When I built my Super Big Bertha I decided to to use some BSI 15 mins epoxy as I didn't want to wait for the glue to dry. Again, I put them under a book for about two hours and they looked perfect.

The one downside was the sanding. I wanted a tapered leading edge and epoxy made it slightly harder to sand but at the same time added strength to the fins. If I did it again I would use epoxy.
 
I built the fins for both my Mega Mosquito and SBB using PVA and papered them too - nothing high-tech, just the usual pile of old books and some plastic wrap. My SBB flies at nearly every club launch though the biggest motor so far was an H135.
 
OK, I am building the fins. While they seem to be coming out OK (unwarped), my complaint is that the thinner outer pieces are slightly larger than the thicker inner piece, which is making it difficult to line everything up. I guess they assume you'll be sanding the crap out of these fins. Which is what I may have to do to avoid overhang. Then I'm going to add a little wood filler, sand again, and then coat with some thinned-down epoxy. And yes, the fins by themselves are taking a week, at least. I just hope that after all this, the wood grain isn't all in the same direction.
 
OK, I am building the fins. While they seem to be coming out OK (unwarped), my complaint is that the thinner outer pieces are slightly larger than the thicker inner piece, which is making it difficult to line everything up. I guess they assume you'll be sanding the crap out of these fins. Which is what I may have to do to avoid overhang. Then I'm going to add a little wood filler, sand again, and then coat with some thinned-down epoxy. And yes, the fins by themselves are taking a week, at least. I just hope that after all this, the wood grain isn't all in the same direction.

Enjoy the process. You'd hate to see the time I've spent so far on my Super Orbital Transport and very little is glued together. 😃
 
I used wood glue, and the built-up fins are very strong! And yes, the balsa is oversized a bit to allow sanding to a very nice rounded edge.
 

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OK, I am building the fins. While they seem to be coming out OK (unwarped), my complaint is that the thinner outer pieces are slightly larger than the thicker inner piece, which is making it difficult to line everything up. I guess they assume you'll be sanding the crap out of these fins. Which is what I may have to do to avoid overhang. Then I'm going to add a little wood filler, sand again, and then coat with some thinned-down epoxy. And yes, the fins by themselves are taking a week, at least. I just hope that after all this, the wood grain isn't all in the same direction.

Yes. The assumption is that you will sand to fit.
 
Well, the good news is that Bertha is coming along! Anyone following my build threads will notice that I'm using the nosecone and payload bay from "Drag Queen" at the top -- Drag Queen had a CATO in early December, and the surviving piece is being used here, so, may I present "Queen Bertha", as this build is going to be named. The little extra makes the rocket well over 3 feet tall, I'm not even sure how I'm getting this into my car to take to a launch when it's ready to fly. I have a small car, but, it's a hatchback and the seats fold, so I'll find a way.
 

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Hey, I am just about to start my build on the SBB and was wondering if you had any further insights into the build, or anything that, in hindsight, you might do differently?

Just tasking a quick look at the motors available, frankly, I'm not seeing much difference in price between the BP and the Q-jets, if anything the Q-jets are quite a bit more. This will be flown at a local park field where they have rocket fly-ins, and noting the suggested motors it looks like the Estes E16-4 is pretty anemic for something like the SBB, but I don't want to go hiking a mile and a half to recover, either. What motors gave you a nice flight but reasonable recovery? Was the chute an appropriate size, in your opinion?

Lastly, how did the built up fins work for you? I'm a little concerned about balsa for larger impulse F-motors, but if it worked for you I'll go with the built up fins to keep it light.

Thanks!

C
 
Hey, I am just about to start my build on the SBB and was wondering if you had any further insights into the build, or anything that, in hindsight, you might do differently?

Just tasking a quick look at the motors available, frankly, I'm not seeing much difference in price between the BP and the Q-jets, if anything the Q-jets are quite a bit more. This will be flown at a local park field where they have rocket fly-ins, and noting the suggested motors it looks like the Estes E16-4 is pretty anemic for something like the SBB, but I don't want to go hiking a mile and a half to recover, either. What motors gave you a nice flight but reasonable recovery? Was the chute an appropriate size, in your opinion?

Lastly, how did the built up fins work for you? I'm a little concerned about balsa for larger impulse F-motors, but if it worked for you I'll go with the built up fins to keep it light.

Thanks!

C
I built mine a few years ago but I don’t recall any “gotchas”. The built up fins work fine for me, some people don’t like them and go with one piece plywood and centering rings, I papered my fins (I paper all my fins - too lazy for all that sanding and filling) and stuck with the original cardboard rings - it’s survived several hard/crash landings with nothing but upper body tube damage. I did the Kevlar leader attached to the motor mount style recovery gear rather than the big trifold and elastic plus I probably tripled the total length (if not quadrupled it - long shock cords save fins!). I added rail guides too so I can use my club’s 1010 rail.

Before I extended mine after a crash by adding a payload section it flew on adapted Estes 24mm E12s just fine as long as the winds were calm - not very high (little under 400’) but high enough for a good recovery, now it’s too heavy but works great on either of their BP 29mm motors. But it does fly even better over a G79 or one of the single use Econojet 29mm F motors, though IIRC the last time I launched it was over my 29mm Hobbyline case with a G64 reload.

I guess my advice would depend on how you plan on flying it - over Estes BP motors build it straight from the box, keep the weight down and it’s a great and impressive rocket on a smaller field. If you plan on using bigger G motors or a lot of Hs then you’ll probably at least need plywood rings (or doubled up cardboard ones) and papered balsa fins, if you’re looking to really “overclock” it the plywood fins are probably a good idea…but then it’ll most likely be too heavy for smaller/slower motors. The SBB looks awesome launched over a long burning Estes BP motor so for me I’d want it light enough to at least use an Estes E16 or F15 - off a rail in light winds it flies just fine on those motors.

I fly mine at nearly every club launch, definitely one of my favorite rockets even though I’m not really a Big Bertha fan or anything - I like because it just works so well even after repairs from my mistakes 😏
 

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I put in an stuffer tube, leaving about 10" of the body tube in the front for parachute, etc. I ran kevlar (first #200, then reduced it to #100) from the rear of the rocket, making an loop and putting it around the end of the motor mount, put in a plastic tube that was wrapped in aluminum duct tape for heat protection and ran the kevlar out the front of the centering ring that secured the front of the stuffer tube.

I usually use E12-4 engines, though an occasional D12-3 flight happens. It flies well on an E16-6 as well.
 
I flew "Queen Bertha" on a Quest E-26 at CENJARS, and it flew well. I did not use the supplied parachute, but rather a 24" Top Flight chute I picked up at eRockets. I quasi-glassed the body tube, and coated the whole rocket in laminating epoxy, which added to strengthening the built-up fins (and thus, I also see no balsa grain), so the whole rocket came out looking great and flying well on larger E and smaller F. When I get to METRA later this month, I will fly Bertha on more aggressive motors and see what happens.
 
When I built mine I extended the motor mount so that it ends just above the coupler to reduce the internal volume.

For the fins I built them with epoxy as there was less chance of warping but I have also built these types of fins with wood glue in the past. Both worked fine but if I was going to do it again I'd stick with epoxy.

If built stock you can get away with a D12-3. Mine with the added weight of the longer motor tube and all the finishing and filling is a bit heavy and I had a really close call the one and only time I used a D12-3 in it. E12-4 is a better option and the E20-4 is my favorite motor by far.
 
For the fins I built them with epoxy as there was less chance of warping but I have also built these types of fins with wood glue in the past. Both worked fine but if I was going to do it again I'd stick with epoxy.
The one concern I have (in addition to the Estes elastic band recovery material) is the low-hanging fins being damaged. I was thinking of changing the design so it landed on the tube, but I've yet to hear anyone say the fins were weak or broke. I do like laminating fins and bagging them with a food preserver - that works really well.
 
Get some bulkheads and turn that coupler into an avbay. You can shearpin the top and keep your chute in the bottom when you don't want to DualDeploy
 
Did you find the ejection charges weren't enough as designed?

From experience I know that Estes ejection charge strength is all over the place. You might get a bunch that are "normal" then get one that is really strong then another that is really soft.

I tend to error on the side of caution and reduce the amount of volume that needs to be pressurized as much as possible. I have had instances where Estes motors would not pop the nose cone due to volume.

As for the fin design. They are very strong once built. You won't have an issue with them breaking unless the chute doesn't deploy.
 
The one concern I have (in addition to the Estes elastic band recovery material) is the low-hanging fins being damaged. I was thinking of changing the design so it landed on the tube, but I've yet to hear anyone say the fins were weak or broke. I do like laminating fins and bagging them with a food preserver - that works really well.

Forgot. I do this to every rocket now so it didn't even cross my mind. Replace the elastic with a Kevlar Leader attached to the top of the motor tube under the upper centering ring. If you replace the upper ring with plywood you can attach directly to the upper ring then attach a long length of Elastic to the Kevlar to act as a shock cord.

You can see examples of this in my build threads.

https://www.rocketryforum.com/threads/alien-space-probe-build-with-modifications-2038.175065/https://www.rocketryforum.com/threa...2188-build-rebuild-with-modifications.177401/
 
To echo a couple of others here: it works just fine built stock, especially for flying on the intended motors (E16/F15) and should be fine adapting down to 24mm Estes motors as well. The flight profile on one of these long burning BP motors is just really cool, and yet not all that high. Looking at some AltimeterThree data on my phone, my almost entirely stock SBB went to just under 500 feet on an E16-6 (with ejection 1.25 seconds after apogee) and about 850 feet on an F15-6 with similarly past apogee deployment (the actual delay on that one was almost 6.5s).

My current SBB (my second — the first got hung in a tree in winter, and what I got back later wasn't much) currently weighs 9.77 ounces (no motor, of course) with a thin mil Top Flight 'chute, a two-tube baffle (like this: https://www.erockets.biz/semroc-ejection-baffle-series-kit-plywood-bt-80-sem-eb-80t/), about 18 inches of Kevlar attached to the baffle which is then tied to the kit shock cord, and both the kit's launch lugs and rail buttons installed. Structurally it is entirely stock.

It has not seen any composite motors yet, but I'd not be too worried about the 24mm Q-Jets or 24/40 E/F loads. Not so sure about 29mm composites. But the flights are so much fun on 29mm BP motors I've no compelling reason to use others as long as I can get 29mm BP motors from on-site vendors. (I miss Hobby Lobby carrying them, even though it's been years and I understand why they don't anymore.)
 
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