Little Big Bertha

[chuckpo]

First post on this forum, so hello everyone and here goes.

Now I'm wishing I had taken pictures during the process but hindsight is always 20/20. :bangpan:

This weekend (while it was rainy and horrible outside) I decided to mash up two standby Estes rockets.

First, let me rewind a couple of weeks. After going to the local Hobby Lobby with a fellow BAR, I saw that the Baby Bertha was only $8, before a 40% off sale. Right next to it was the original Big Bertha. Also 40% off. Noticing the body tubes were the same size I grabbed both and headed to the checkout counter. I knew I wanted to do SOMETHING with them, just not sure what yet.

After much deliberation, I figured I wanted to try a two stage Bertha. I just so happened to be in another hobby store and found a package of tube couplers. It even had 2 couplers for BT-60 size tubes, so I snagged it. Then everything sat in my project room for a week or so. That was until this weekend.

I first measured the motor mount against the Baby Bertha tube, and cut it down. I built the MMT without the spring clip installed, just like all the other multi-stagers from the past. I also started test fitting the coupler to the main body tube (Big Bertha). Once the booster MMT was done, I built up the sustainer MMT, and glued it in place.

The next morning, I started test fitting the fins, but found they would not fit neatly with each other in stock configuration. Time for Balsa Surgery!

I cut down the booster fins, then laid them on top of the main fins to see how they could go together. I drew an outline, then cut them out as well. They're not great, but also not horrible for a FrankenBertha like this.

I lined up the tubes, and glued the fins for booster and sustainer at the same time, to make sure they were straight as possible.

I also had the rest of the Baby Bertha body tube left, so I grabbed the other coupler and made up a quick payload section, using leftover balsa to construct the bulkhead.

The only problem I have is that I doubt it will lift off safely with the payload bay, since it is over the maximum take-off weight for the C6-0. I'll have to fly in two stage configuration first, to see how it handles just that.

All this jabbering and I only managed to get one crappy picture with a cell phone. I'll get more, better photos later.

 

[NjCo]

That's pretty cool! You might want to get rid of the narrow ends on the sustainer fins. Those will be the first thing that touches the ground when it comes in for a landing under parachute. And I'll be willing to bet one or more will snap on your first flight. If just that narrow piece broke off it certainly wouldn't be the end of the world but the way the grain is going the break might end up running up along the whole edge of the fin.

Anyway, let us know how the first flight goes.

 

[Peartree]

Welcome to TRF!

I'm sure you'll get plenty of opinions on this. It looks like an interesting project though.

 

[jadebox]

It appears in the photographs that you've cut the fins so that the grain of the balsa runs parallel to the root edge of the fins. If so, that means they'll be easy to break.

For maximum strength, the grain should run parallel to the leading edge of the fin (at a sharp angle to the root edge).

You can strengthen the fins you have by gluing paper or cardstock over each side of each fin. Where that little tab extends down on the upper fins, you might glue a piece of balsa to each side and round it. In addition to adding strength, it'll give it a "pod" effect that might look cool.

-- Roger

 

[chuckpo]

It appears in the photographs that you've cut the fins so that the grain of the balsa runs parallel to the root edge of the fins. If so, that means they'll be easy to break.

For maximum strength, the grain should run parallel to the leading edge of the fin (at a sharp angle to the root edge).

You can strengthen the fins you have by gluing paper or cardstock over each side of each fin. Where that little tab extends down on the upper fins, you might glue a piece of balsa to each side and round it. In addition to adding strength, it'll give it a "pod" effect that might look cool.

-- Roger

Roger, thanks. As for the the grain of the balsa, that is how they came laser cut in the packaging. I just modified the shape of them to fit. Although I should have probably cut new fins, I just glued on what was supplied and altered, and have been diligently applying coats of Sanding Sealer. So far, I've done two coats, with a third coming tonight.

I might end up either rebuilding with custom fins or removing what is already attached, since I've found a couple of other issues with them. Either way, this has been an interesting project for me. Time will tell!

 

[shreadvector]

Roger, thanks. As for the the grain of the balsa, that is how they came laser cut in the packaging. I just modified the shape of them to fit. Although I should have probably cut new fins, I just glued on what was supplied and altered, and have been diligently applying coats of Sanding Sealer. So far, I've done two coats, with a third coming tonight.

I might end up either rebuilding with custom fins or removing what is already attached, since I've found a couple of other issues with them. Either way, this has been an interesting project for me. Time will tell!

The fins supplied have four sides. You picked the incorrect side to glue to the body tube. Most instructions are fairly clear about that, but not always. In some kits, they used to show two views with a "YES" and "NO" version. Otherwise they either indicate the grain direction and which side is which, or they provide a pattern and label each side and tell you to carefully match the fins to the pattern.

https://www.spacemodeling.org/JimZ/estes/k-23.pdf
https://www.spacemodeling.org/JimZ/estes/est1948.pdf
https://www.spacemodeling.org/JimZ/estes/est1948_BigBertha(Beta)/est1948.pdf
https://www.spacemodeling.org/JimZ/est0803.htm

 

[chuckpo]

The fins supplied have four sides. You picked the incorrect side to glue to the body tube. Most instructions are fairly clear about that, but not always. In some kits, they used to show two views with a "YES" and "NO" version. Otherwise they either indicate the grain direction and which side is which, or they provide a pattern and label each side and tell you to carefully match the fins to the pattern.

https://www.spacemodeling.org/JimZ/estes/k-23.pdf
https://www.spacemodeling.org/JimZ/estes/est1948.pdf
https://www.spacemodeling.org/JimZ/estes/est1948_BigBertha(Beta)/est1948.pdf
https://www.spacemodeling.org/JimZ/est0803.htm

Thanks for the links! The instructions I have look like the third link in the list, and according to that, it reads that the grain should be parallel to the leading edge. Obviously I missed that mark, and stuck them on in the way that looked like it fit the best. Bad mistake, since I've built about 4 or 5 kits since starting back up.

I may end up buying some new balsa sooner, rather than later, and do it up right. I just need to figure out how to take them off without destroying the tubes. Any tips for that?

Chuck

 

[Peartree]

jadebox (Roger) noted that you could paper the fins to reinforce them. I have a couple rockets that have fins with the wrong grain direction (for a variety of reasons). I have papered these and they've held up surprisingly well even to the point that they've been detached (several times) from the rocket they were on but not broken.

To do this is not difficult. Simply spread a thin layer of white glue (or yellow glue) on BOTH sides with your finger and then lay a sheet of paper (typing paper, copy paper or cardstock) on both sides as well. Using a thin layer of glue and doing both sides is important because the moisture from the glue may otherwise cause the balsa to warp.

This is a great tool. It adds a lot of strength but not much weight.

 

[jadebox]

This is a great tool. It adds a lot of strength but not much weight.

Papering the fins also makes finishing easier. I did it on my Estes Comanche III just because I didn't want to fill and sand 12 fins!

-- Roger

 

[NjCo]

To do this is not difficult. Simply spread a thin layer of white glue (or yellow glue) on BOTH sides with your finger and then lay a sheet of paper (typing paper, copy paper or cardstock) on both sides as well. Using a thin layer of glue and doing both sides is important because the moisture from the glue may otherwise cause the balsa to warp.

This is a great tool. It adds a lot of strength but not much weight.

I've just finished two projects and I papered the fins. Absolutely a great idea. You get stronger fins with almost no sanding/filling. All you really need to do is round the edges. To avoid the warping issue I just used full sheet labels instead of regular paper. The results are great. You get a nice smooth finish without all the mess and potential for bubbling that you get with gluing paper onto the fins. And the adhesive holds up really well. I have quite a few custom game pieces that I made by applying labels cut from full sheet labels to wooden discs. I did this about 5 years ago and I haven't had any of them begin to peal off. I do beef up the edges of the labels by applying thin CA around the outside edge of the paper to about an 1/8" or 3/16". This solidly attaches the paper to the fin and allows you to round the edges of the fin without pulling up the paper.

One thing you have to watch out for with papering fins is sanding. If you sand the bare paper you'll end up with a rough finish. You have to be sure to apply a coat of primer to the papered fin before doing ANY sanding. Then you can safely sand the CAed edges of the fins to get rid of any excess CA and round the edges of the fins.

You add almost no weight to the rocket when doing this. I recently tested this when I got my hands on two sets of fins for a rocket. One set I papered and the other I didn't. Before applying any sealer/filler or sanding I weighed each set of fins. The two sets were identical in weight to 0.1 gms which is the limit my digital kitchen scale goes to. These were fins for the Big Daddy as well which essentially used up most of two sheets of full page label! All that added <0.1 gms and I got a nice smooth surface without all the sanding, filling and resanding that I would normally do. It saved a TON of time. I had my fins done in hours as opposed to days.