Bertha Build Techniques

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Art Upton

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Ok,
Our club had a Big Bertha Tri-Athalon contest last year. It was a great time.
Parachute and Streamer Duration plus spot landing.

These are fun by the way, especially if you make it Iron Man rules.

So I took a baby bertha, tossed the little tube and used a 18" bt-60 tube from red arrow. I built the engine mount with a kevlar shock cord and I am letting it dry.

I put aero-gloss sanding sealer on the fins thick, so I can sand it all back off

I picked the sealer all off my fingers, while feeling a littel dizzy, so I had a beer.

I sized up the body tube and nose cone and sure enough, they fit
 

Art Upton

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Ok,
here is the picture of the motor mount.



Notice I used the plastic ring thingy to hold down the kevlar cord I tied a knot in. It is also important to notice I draped the kevlar cord over the glue bottle, to keep the cord from getting glued to the mount at places it should not.

Also I used yellow glue. Not wanting to start a glue thread, but this is the glue you should be using on this rocket, and in very small amounts.

The rocket will need to weigh very little to get good times in duration and saving weight by not using epoxy even though it will build quicker will result in a better performing rocket.
 

KDRaven

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Hey Art why dont you just stuff the kevlar cord in to the motor tube while the glue dries and then when you stuff the unit up into the rocket the cord can hang out the back of the motor mount tube and you dont have to wory about getting it in to the glue in the BT.
Just my .02 worth.
 

Micromeister

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Art:
I've also found taking the Kevlar to the OUTER edge of the centering rings once installed in the bodytube extends the kevlar life by a factor of about 5.

KD is also correct about taking the Kevlar out the back of the model while the glue dries on the motor moung and during installation in the model.


Heres a scratch built Bertha with a changable twist-lock motor mount for A thur D12 flying:)
 

SwingWing

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Waitaminute??!?! You skipped forward a few steps and I'm lost.
What did you use to open the bag?
Any pictures?
 

JRThro

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And where did you toss the short tube from the Baby Bertha kit?
 

RimfireJim

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Art:
I've also found taking the Kevlar to the OUTER edge of the centering rings once installed in the bodytube extends the kevlar life by a factor of about 5.
Could you supply more details on how you do that? The increased life by not being so close to the ejection blast makes perfect sense.
 

Art Upton

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Art:
I've also found taking the Kevlar to the OUTER edge of the centering rings once installed in the bodytube extends the kevlar life by a factor of about 5.

KD is also correct about taking the Kevlar out the back of the model while the glue dries on the motor moung and during installation in the model.
yes that is a nice way to move the kevlar to the edge, but this mount is built already ;)

We haven't got yet to gluing in the mount, thats a long way away yet :eek:
 
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Art Upton

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Remember I said I put on a thick coating of Aero-Gloss sanding sealer

Well, when it dried I sanded it all off

I then realised I needed my fins sealed to at least look good, even at the slight weight expense, so I put a thin coat on this time.

I didn't have to pick it off my fingers, but I still got dizzy, so I had a beer.

 

Art Upton

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Well,

Since I was dizzy and drinkin' a beer, I figured' I'd do some Cypherin' and got out Aero-Drag.

I worked out a CD of .525 (due to my aero-gloss sealing) and worked out a nice altitude plot below:

 

Art Upton

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Hey Art why dont you just stuff the kevlar cord in to the motor tube while the glue dries and then when you stuff the unit up into the rocket the cord can hang out the back of the motor mount tube and you dont have to wory about getting it in to the glue in the BT.
Just my .02 worth.
That is a good point to remember
 

Art Upton

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Warning: CFD Rocket Science :eek:

I had to flip the fins and add more Aero-Gloss to them. I got dizzy and so had a beer to feel better. Something about Beer makes me want to play with rocket computer programs.

Some time ago, I purchased from AeroRocket a program called Visual CFD. It is like having a color wind tunnel in your laptop.

I was still a 'noob with it at that time, so I put the Bertha into the program to try to learn somemore.

here is one plot I did :



If you are interesting in learning more about the program, you can see it here:

https://www.aerorocket.com/VisualCFD/Instructions.html
 

Art Upton

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Some time ago, A guy named Rich had asked me how I was going to secure the nose cone on this rocket

He tends to use cement and telephone pole guy wire don't cha knows, so I had to come up with something to impress him :p





But after having a beer I realized for C Streamer duration the rocket would come in like a u-bolt was in the nose cone :eek:
 

Micromeister

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Could you supply more details on how you do that? The increased life by not being so close to the ejection blast makes perfect sense.

Jim:
It's a pretty simple change: tie the kevlar around the motor tube as usual, run the line out to the outer edge of the centering ring. I generally file a small notch in the outside edge a tiny bit larger then the diameter of the kevlar so it doesn't cause a bulge in the outer bodytube (airframe). run the kevlar back thru the motor mount out the aft end during Motor mount installation. Once you've installed the motor mount and before the glue or epoxy setup up hard, drop the shock line back through the motor mount and out the front. sometimes taping it to the outside of the body tube so it lays flat against the inside tube during drying. Heres a little drawing of a micro motor mount, all work the same way;)
the beading wire shockcord mount pics show how I run shocklines or shockline anchors out thur the back of the motor mount while inserting in the model. I do exactly the same process with 13, 18 and 24mm motor mounts.
 

Rich Pitzeruse

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Some time ago, A guy named Rich had asked me how I was going to secure the nose cone on this rocket

He tends to use cement and telephone pole guy wire don't cha knows, so I had to come up with something to impress him :p
LOL, you're a funny man, Art!:p
 

Art Upton

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Well, we weighed the parts we have so far without engine or streamer/parachute. It comes under 2 and 1/3 oz by quite a bit under that so far. I can tell you if the U-Bolt was used, well, it would come in like it had a U-Bolt in the nose cone

So for our next tasks, we sanded all that nice and smooth “Glassine” coating on the body tube. You know, that stuff that makes the finish so nice, you have to sand it off

At least we only sanded it off at the part we will glue the fins on, with Yellow Wood Glue.

Next you will see we have the BMI CAD fin jig setup with the 18mm core and the motor mount for the Bertha. The BMI CAD jig can be seen at https://www.bmicad.com/ I got the complete setup with cores from 13mm to 54mm for $129, a Bargain

At NARAM 45 (2003) , my pop-up was next to Trip Barber's during contest week. Trip has been known to have anti-gravity in his altitude competition rockets.

On Wednesday of that week, I got a few mins to talk with him. He told me his secret was using a fin jig to make fins absolutely straight and keep weight down as much as possible. Then make sure you get an absolute straight boost

here is what I did

 
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Art Upton

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Ok,
we have a problem here, can anyone tell what the problem is?

No, not the blurry picture, what else?

 

Wingarcher

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Ok,
here is the picture of the motor mount.


Is that a LiPo battery on your BUILD TABLE? Those things are super-dangerous!! You should definitely invest in a ceramic bunker for storage of those super-dangerous batteries. The worst case situation would be a LiPo fire, setting off all those sealer fumes and in the process..........cause you to spill your beer. :D

N
 
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Art Upton

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Very good answers to a wide open question ;)

Ok, the bertha fin design is not really a good one for a fin jig, so I had to extend the core as much as I could to get a close fit. And I made sure the grain direction was ok first. Good eye prowler guy

Used yellow glue, and held the fins on with masking tape to the jig.

 
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KDRaven

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I seen in the whats wrong pic that the body tube was either too far down or the fin was too far up but I did not catch the grain direction.
 

Art Upton

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Here is the part those that do not like Yellow glue do not like.
The Yellow glue fin fillets. Make fillets like this and support the rocket so no weight is on any of its fins.

Edit: Updated times

You need to do this at 9am and the flip side at 12 noon. Then do another section at 3pm and the flip side of that at 6pm.

So it takes a day to make and dry the fillets for a Four Fin Rocket.

Its the 9am part here so you can see my Green Monster Energy Drink.

 

MarkII

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Ok,
we have a problem here, can anyone tell what the problem is?

No, not the blurry picture, what else?
Is the blurriness due to the beer or to the Aerogloss? :rolleyes:

Sorry - it just seemed like more than a coincidence...

Mark \\.
 
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