Super Big Bertha - Prepping the Payload for Altimeters

 

To fly an altimeter in the payload section of the #estesrockets Super Big Bertha #modelrocket, the payload really should be vented using static ports. This will allow the payload section to depressurize as the rocket ascends, so the altimeter can correctly read the air pressure outside the rocket and determine altitude.

Some people, including some very knowledgable rocketeers with experience in using altimeters for competition rocketry, claim payload sections don't necessarily need venting. There is often leakage of air pressure through places like the nose cone shoulder. But this one is quite airtight - the nose cone pops quite loudly when I pull it off. So I'm going to assume altimeters aren't psychic, and add static ports - it can't hurt.

I usually put three static ports on three-finned rockets and four on four-finned models. I don't know why - I just prefer things to match, I suppose. Longtime readers know I can be a fussy builder. It's your hobby - build it the way you like.

After consulting several static port calculators online, I determined I would need four 1/8 inch static ports on the Bertha. It's best to have them far from the nose cone. The joint between the nose and body tube can increase turbulent airflow over the ports if they are near the nose, and this may affect the accuracy of the reading the altimeter takes. So I drilled them near the bottom, through both the tube and the coupler.

I marked the spots first in pencil, then with a push pin, and finally drilled them out.

The drill will leave some jagged bits around the hole. Static ports should be smooth and round on the outside to reduce turbulence.

To smooth the holes, wick some water thin CA glue into the drill hole. Once it's cured, you can sand the opening smooth and clean up the hole with the drill bit.

After sanding the outside smooth, there's a bit of jagged paper left in the static port.

A second pass of the drill bit should clean up the static port leaving a clean, round hole.

Final smoothing and rounding may happen after primer.

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Super Big Bertha - Fin Filling, Fillets, and F- er, uh, Rail Guides

 

I moved on to writing up the Estes V2 build and nearly forgot to finish the Super Big Bertha series!

After the fins were assembled and the edges sanded square, I moved on to filling in the wood grain.

Brodak sanding sealer is my favorite way to fill balsa wood grain. Traditionally, you apply sanding sealer after gluing fins on. But I often apply a few coats and sand beforehand. It's easier to sand them smooth without the body tube in the way.

Before anyone chimes in, yes, you do need to take care to keep the sealer off the root edges, as they can impede a good glue bond. But I've literally never had a fin break off because of using this method.

If I accidentally get some sanding sealer on the root edge, I quickly wipe it off with a paper towel and give the edge a quick sanding. And you know what? It's fine.

Furthermore, with these fin tabs, there's no problem. And you can hold the fin by the tab and seal the whole fin at once, without getting sealer on your fingers.

Brodak is expensive, but I find the results worth the price. It's more cost effective to buy a large can, rather than several smaller ones. You can buy it online HERE (Note: Looking at it as I write this, Brodak sanding sealer is a lot more expensive today than it was when I bought two cans two years ago. Inflation is pretty high right now. That said, if I needed some, I would probably still spring for it. That's how much I  like the stuff. But there are definitely much cheaper options for balsa filling out there!)

If you're worried about getting a large can, because you've had sanding sealer quickly go bad once you open it if you don't use it all in a short period of time, don't worry. I've had a can open for a year and it's still as fresh as new. This is quality stuff.

I brushed three good coats on, then sanded them smooth before attaching the fins to the rocket.

Once the fins were glued on to the Bertha, fin fillets were applied.

These are wood glue fillets, using Titebond Molding and Trim Glue, 3 layers deep at this point. The molding and trim glue is less prone to sagging and shrinking, and makes a great fillet.

I put an Elmer's pointy cap on the bottle for better fillets and more precise gluing - the bottle necks are identical, and School Glue is always on sale for 99 cents. Not good for rocket building, but the caps are perfect replacements for the wide Titebond caps. I got this tip from Chris Michaels' Model Rocket Building blog, and I wouldn't do otherwise now.

The Bertha will get conformal rail guides instead of the 1/4 inch launch lugs it comes with. Since I almost always fly with a club, a launch rail will almost always be available when I feel like flying this rocket. A 1010 high power rail is less likely to whip when a rocket lifts off, and less likely to get bent.

A friend gave me a tip that I tried. Take a rail guide from Aerospace Specialty Products and cut it in half with a razor saw and miter box. You get 2 for 1 that way. The original rail guide is a couple inches long, and I decided this would be a good rocket to try that with.

It worked, but I probably won't do it again. Holding the guide still while cutting was difficult, because while the rail guide is long, half a rail guide is not. The saw kind of shredded the ends of the 3D printed part, which required a lot of cleanup. And a short segment of this guide is kind of hard to get on the rocket perfectly straight. While the guide does conform to the BT-80 body tube, the curve is very slight, and with a layer of epoxy between the guide and the tube, there's a bit of play, and I had to keep checking to see if I really got it on there straight. A short piece is harder to get on straight than a long piece, and I found lining up the second little bit of guide 11 1/2 inches away more difficult.

Besides, these rail guides from ASP are cheap and lightweight. I'll probably just use full length guides in the future. You can get them HERE.

After I put the rail guides on, my eyes kept playing tricks on me. Were they straight? Were they in alignment with one another?

I got out my camera and backed it as far as I could in the Rocket Room and zoomed in to see if I could better tell. I couldn't really tell what I was looking at, because it was pretty dark.

In this picture, the two guides don't look lined up, but that may be because the camera and rocket were off-center of each other. In the daylight, they look alright.

There is a chance that the upper guide is *slightly* crooked. Like, ever so slightly. I'm hoping it's fine. I guess we'll see when we get it on the pad.

I suppose I should get myself a length of 1010 rail so I can check these things at home.

It's probably fine...

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Breaking Super Big Bertha In Two - Payload Section

Super Big Bertha is a very tall model. It's made of two lengths of BT-80 body tube joined by a coupler you glue into both tubes. I decided to turn the upper tube into a payload section, sealing one end of the coupler with a bulkhead and only gluing it into the upper tube.

This would have two practical benefits. First, I could fly a payload, like an altimeter, without worrying about the delicate electronics being fouled up by ejection charges.

Second was transportation. I take most of my rockets to launches in a large box.

A box of rockets large and small. You can lay towels between layers to protect the paint, but if
you don't jostle them too much, it's usually fine to simply lay them in and take them out gently.

It's a long box; you can see my Pro Series Nike Smoke and Ventris in there, as well as my North Coast Rocketry Archer in there. But the Bertha is just a still much too tall to fit in the box in one piece. Having it come apart in the middle makes it much easier to pack away.

I swore I had some BT-80 plywood bulkheads somewhere, but I couldn't find them.

I was taking out the recycling, when it dawned on me - these Madras lentils from Costco are not only cheap, tasty, and healthy, the carton they come in is made of very hard, finely corrugated cardboard, and would be perfect.

First step was to trace around the coupler. Then I cut the shape out with scissors, slightly larger than the finished product would be.

I then sanded the bulkhead round until it fit snugly inside the coupler. 

I reinforced one side with a bit of packing tape and punched two clean holes through the bulkhead.

A good glue bond with fillets on the inside would hold it in place. I ran some Kevlar through the holes and tied it in a loop.

Finally, I plugged the holes with some Titebond Molding and Trim Glue.

Once everything was dry, I glued the bulkhead halfway into the upper payload tube.

The finished product is very solid.

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The Estes Super Big Bertha - Time to Get Back to Business

 

Recently I realized that I haven't built a new rocket start to finish in well over a year, partly due to this funk I've been in. It's been a hard couple of years. I put the finishing touches on my Estes Mercury Redstone, yes, and slapped together a quick model from spare parts of other rockets, but I haven't started and finished a new model in way too long.

It's time to remedy that. I grabbed the Estes Super Big Bertha off the build pile and started it during my week's vacation from work.

I like the built up fins. I've done multi-ply fins on some scratch builds I've made, and they can make for very strong fins. I don't plan on beefing up anything with plywood on this one. I want to keep it light and be able to fly it on F15 black powder motors. I bet I could even get away with a D12 in this and keep the altitude really low on small fields if I watch the weight. If I want to use something thrustier, maybe I'll make a second one with plywood centering rings.

Surprisingly, this will also be my first BT-80 rocket! I have built other 2.6 inch rockets before, but they all used heavier tubing.

The fins on the Super Big Bertha are really great. 1/8 inch balsa cores and 1/16 inch balsa skins make fins which are about 1/4 inch thick when assembled. Take your time - this part can be tricky.

Instructions suggest CA (cyanoacrylate glue - Super Glue, basically) for this step. I use CA sometimes, but avoid it when I can. And in this case, I think laminating them with wood glue might make for a stronger fin. Not that it's critical - they're not likely to brake. But the wood glue, if properly applied, should soak in and make the fins one solid piece.

I spread the glue on the spars of the core, evenly and thin, then press the skins on. After holding them tight for a minute or so, I press them between waxed paper and heavy, flat books. Only one skin at a time - you want to make sure they don't slide around when you're placing them under the press! After 20-30 minutes of dry time, I flip them over and do the other skin. Then I leave them under the books for AT LEAST a day for them to dry thoroughly. Any residual moisture can still cause the fins to warp after you take them out from under the heavy weight.

The edges may not all line up perfectly when you do this - ideally, the outer skin will be slightly larger. You'll then sand everything even. Just make sure the root edges are even - that would be the hard part to fix.

Don't throw out the sheets the fins are cut from - lots of good scrap balsa there for other projects!

I really like these fins - they're solid!

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