Baby Bertha kit bash

[bjphoenix]

This all started when I found a dozen C6-3 in my stuff. It takes a narrow combination of rocket characteristics for this motor to work well and I didn't have that so after a lot of playing with designs I settled on a kit bash based on a Baby Bertha. (I'm not into spools and things like that.) I chose the Baby Bertha because it had a good amount of parts to start with and they are low priced at HobbyLobby. I had previously kit bashed a MeanMachine into something shorter so I had leftover BT60 tubes. I played around with various designs in Autocad and came up with the one below, essentially adding 3 external pods. Using the dimensions from Autocad yesterday I started cutting tubes and gluing together. The tubes were supposed to meet at the back and they weren't so I realized I had pulled the wrong dimensions off of my drawing. In the (second?) picture below the desired configuration is on the left, with cuts for a BT20 motor tube, the configuration with the cuts that I made is on the right. You can see how the outer tubes were supposed to touch. By chance what I built would work with a 24mm motor tube but I wanted this to fly on C6-3. The frontal area of the 4 tubes combined is about equal to a 3" diameter airframe such as a Big Daddy but I think overall this will be lighter than a Big Daddy. I had also planned to use the original Baby Bertha fins, but obviously only 3 fins. Now I'm thinking I will play around with other fin designs first. On the side pods I was going to create the front fairings out of cones rolled from cardstock. I've figured out a way to create the measurements using autocad.

 

[Fattbank64]

Did you make a jig to cut the tubes?

 

[bjphoenix]

Did you make a jig to cut the tubes?

I did not make a jig. I wrap a stiff piece of paper around the tube and mark where it overlaps, then I unroll it and measure that length. I put marks on that tube to match the lines of cut then wrap it back around the tube and tape it. I transfer the marks to the tube and then extend them with a piece of 1" x 1" aluminum angle from the hardware store. In this case it is 4 parallel lines, but I marked another line along the opposite face of the tube for the centerline of the fin. Then using that angle as a guide I start the cuts with a sharp XActo blade. After I've made the first light cut about 2 or 3 more light cuts go all the way through the tube and these cuts follow the first light cut pretty easily without using the straightedge.
And BTW this was where I made the mistake in measurements. In my autocad drawing I had measured the straight distance that would exist between the cut edges, and I also calculated the distance along the curve. I accidentally took the straight distance (chord of a circle) and used it as the distance around the curve, so my slots were not wide enough.

I should add this about the original construction- when I cut the slot out of the tube it actually opened up a little bit. I had made marks on the center tube to show where the edge of each tube would hit. I put a tiny bit of glue on one edge of the tube and put it on the line, holding it in place for a few minutes until it would hold itself. After half an hour I put on a full glue fillet and let that dry. Now the other side of the tube is out side of its line so I squeeze the tube and put a couple of light bits of glue on the center tube then hold the edge of the outer pod on that line for a few minutes until it would hold itself. After that dries 30 minutes I put on a full fillet. When that dries I go to the next pod. This took most of yesterday, going out in the garage for the next operation whenever I thought about it.

 

[cls]

Husky design! I appreciate your construction details. I'm wondering about Krushnik effect, with the motor nozzle recessed so far inside the pods?

 

[bjphoenix]

Husky design! I appreciate your construction details. I'm wondering about Krushnik effect, with the motor nozzle recessed so far inside the pods?

That would wipe out those pods. I haven't installed the motor tube yet but the plan was for it to glue to the pieces of pod that project below the main body tube, and just have one centering ring inside the body tube. So the motor tube when installed would extend to the ends of the side pods. I'm still thinking about what to do about the motor tube but I'll probably install a BT50 and then either use an adapter or install an 18mm tube inside that. The next time I go out to mess with it I'm going to try a BT50 in there to see how it fits. I might have to come up with a new plan. I'm trying to decide if I need bulkheads at the back ends of the pods to help stiffen the fins or if I extend fin tabs in there., fin tabs would need to be a little over 1.5" to reach the motor tube.

 

[BABAR]

Husky design! I appreciate your construction details. I'm wondering about Krushnik effect, with the motor nozzle recessed so far inside the pods?

Alternative, leave nose cones off the pods, so they are scoops. Sort of a modified Lil Augie. As a bonus, you can Chad stage it.

 

[neil_w]

I should add this about the original construction- when I cut the slot out of the tube it actually opened up a little bit.

I’ve been bitten by this a few times. I’m not sure there’s any way to accurately predict this, you just have to try and measure and adjust.

 

[bjphoenix]

Here's an update- all the parts are in place except for outside decoration that I'm still designing. It weighs 3.7oz with C6-3 installed. I've been trying to think of a new name for it to use at launches. So far I've come up with "Berthapods". It is intended to look a bit retro so I was thinking about painting it silver but I might paint it white.

I rolled some cones for the pods out of card stock. I took measurements and laid out the shape in autocad, then cut some printer paper templates to check the shape. I put some small strips of balsa under the card stock to stiffen it a bit, in some of my real old rockets with cardstock transitions I've noticed that they can get caved in if they hit something hard enough during transport or storage. It took a long time to get to this point just because of all of the Titebond II fillets I did, inside and outside.

The fins are the original BB fins, just slightly restyled. I added a bit of width halfway down the outside face of the fin and then cut it back more at the bottom to get that retro shape. The fins are papered as you can see. Any bits of brown in the photos are Elmers wood filler.

In the last photo you can see the BT50 intermediate tube I used to overcome my earlier measurement mistake, and the BT20 inside of it with some balsa strips. There is a full BT20-BT50 centering ring at the front of the motor tube to hold the ejection charge.

 

[PDawg]

Would it fly without fins if you left the pods open instead of having nosecones?

 

[BABAR]

I like the name “Triceratops”, but it doesn’t got with the Big Bertha Theme.

 

[bjphoenix]

Would it fly without fins if you left the pods open instead of having nosecones?

It might but probably would work better if the pods were shorter. The CG is about midpoint in the side pods.

I like the name “Triceratops”, but it doesn’t got with the Big Bertha Theme.

TreyBertha, TriBertha

 

[BABAR]

It might but probably would work better if the pods were shorter. The CG is about midpoint in the side pods.


TreyBertha, TriBertha

Assuming it flies well, Tres’ Bien?

 

[Daddyisabar]

You can always strengthen cardstock with a coat of CA (super glue). CA is also great to quickly tack on tubes so you don't have to hold them in place white Titebond sets.

 

[bjphoenix]

OK I made a little more progress yesterday and this morning. I decided that this build would be a good one to experiment on with raised panel detailing that I want to do for an upcoming build. I had some relatively thin cardstock so I used it. I cut a piece of printer paper the size of the pod with a slot for the fin, this paper would wrap around the pod from fillet at main tube to fillet at main tube, the slot cleared the fin fillets. Then I sketched raised panels on this piece of paper. I got these ideas from a video by Adam Savage. You can see sort of how this works in the first photo.
When I had the panels figured out I cut them out to use as patterns, visible in the second photo. I traced the patterns onto cardstock in the third photo, then went over the lines with red ink, visible in the third photo. And BTW by using the pattern all 3 pods are close to identical, but they are not symmetrical- the left side of the pod doesn't match the right side of the pod. If I was doing this again I would probably lay out the shapes in autocad so that the lines would all be parallel and the corners square, then print it out either on paper or maybe directly on cardstock. I had already rolled the cardstock around a 3/4" diameter dowel so there was some curve to it. This didn't come out real well, the cardstock wanted to wrinkle a little bit. This might not be necessary with thin cardstock because it soaks up some of the water from the Titebond II and becomes softer, it might have conformed to the BT60 just fine without precurving it. So the next step was to cut out the pieces with knife and straightedge and glue them down. This was a bit tedious but didn't take very long. In the fourth photo you can see the finished product. I put on a few double thicknesses here and there and cut some small "greeblies" out of thicker cardboard to stick on. Overall the rocket looks like you left it out on the streetcorner and people stuck paper notices all over it. The last photo is the finished effect after a layer of primer.
It's at 4.4 oz total weight including C6-3 so I'm not going to add anything more to it but a coat of paint.
I'm trying to decide what color to paint it. At first I was thinking it is some form of space vehicle so it should be white like most of what NASA does, but I thought if it is supposed to be retro then solid silver would be best.

 

[Cape Byron]

Simply beautiful. I'm a huge greeble fan.

 

[jqavins]

I've been trying to think of a new name for it to use at launches. So far I've come up with "Berthapods".

Bertha and Betty and Belle (oh my)

I'm trying to decide what color to paint it. At first I was thinking it is some form of space vehicle so it should be white like most of what NASA does, but I thought if it is supposed to be retro then solid silver would be best.

Silver for retro is obviously good. You could combine retro with the more realistic spacecraft thing by using a silver base plus NASA-like black lettering and flag. The combination of the two influences comes out to something original. I'd also be a fan of gold trim on the silver, but I honestly don't see how to fit that in on this one.

 

[Daddyisabar]

Tacky glue! Titebond, like my salad at the high end restaurant, is way too WET!

 

[Sandy H.]

OK I made a little more progress yesterday and this morning. I decided that this build would be a good one to experiment on with raised panel detailing that I want to do for an upcoming build. I had some relatively thin cardstock so I used it. I cut a piece of printer paper the size of the pod with a slot for the fin, this paper would wrap around the pod from fillet at main tube to fillet at main tube, the slot cleared the fin fillets. Then I sketched raised panels on this piece of paper. I got these ideas from a video by Adam Savage. You can see sort of how this works in the first photo.
When I had the panels figured out I cut them out to use as patterns, visible in the second photo. I traced the patterns onto cardstock in the third photo, then went over the lines with red ink, visible in the third photo. And BTW by using the pattern all 3 pods are close to identical, but they are not symmetrical- the left side of the pod doesn't match the right side of the pod. If I was doing this again I would probably lay out the shapes in autocad so that the lines would all be parallel and the corners square, then print it out either on paper or maybe directly on cardstock. I had already rolled the cardstock around a 3/4" diameter dowel so there was some curve to it. This didn't come out real well, the cardstock wanted to wrinkle a little bit. This might not be necessary with thin cardstock because it soaks up some of the water from the Titebond II and becomes softer, it might have conformed to the BT60 just fine without precurving it. So the next step was to cut out the pieces with knife and straightedge and glue them down. This was a bit tedious but didn't take very long. In the fourth photo you can see the finished product. I put on a few double thicknesses here and there and cut some small "greeblies" out of thicker cardboard to stick on. Overall the rocket looks like you left it out on the streetcorner and people stuck paper notices all over it. The last photo is the finished effect after a layer of primer.
It's at 4.4 oz total weight including C6-3 so I'm not going to add anything more to it but a coat of paint.
I'm trying to decide what color to paint it. At first I was thinking it is some form of space vehicle so it should be white like most of what NASA does, but I thought if it is supposed to be retro then solid silver would be best.
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Great work on the details on the Berceratops (true, a weak effort, as it gets rid of the tri, which you have and keeps the 'horn face' which you don't have, but I tried. . . ). Well played on your execution of a clean scratch build!

Sandy.


Sandy.

 

[bjphoenix]

OK I put a layer of paint on, ready to launch. (I chose silver.)
The thickness of this cardstock looks good after it is painted, I was worried that it was too much at first.

 

[DigBaddy]

OK I put a layer of paint on, ready to launch. (I chose silver.)
The thickness of this cardstock looks good after it is painted, I was worried that it was too much at first.
View attachment 528598

Love it! That turned out great!

 

[lakeroadster]

Really a great looking rocket, the panelizing technique is stunning. Thanks for documenting the build.