Big Backwards Bertha

lakeroadster

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Created as a result of an idea spawned by @BABAR on THIS THREAD

It's a stock Estes Big Bertha... with the fins mounted backwards. Requires 0.55 ounces of nose weight on a C6-5 to equal the stability of a stock Big Bertha.

I'm not sure who originally created the Open Rocket .ork file? (It wasn't me). I just used it as is and flipped the fins. So if you build this be sure to do a swing test to check for stability.

I'd build this as a 24mm capable rocket, and adjust nose weight accordingly. You can always adapt down to 18mm. Be sure to run the numbers for fin flutter, or just use basswood and make it a TTW design.

1686010694931.png
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Art Upton

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If I did that I would chop the fins off where the first black band on the body tube was... anymore makes it more unstable
 

BABAR

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If I did that I would chop the fins off where the first black band on the body tube was... anymore makes it more unstable
Absolutely true. However, violates the whole point of the design

JFK

We choose to go to the Moon in this decade and do the other things, not because they are easy, but because they are hard; because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one we intend to win, and the others, too.
 

Back_at_it

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What is the stability without nose weight? Due to the length, my guess is you will be more than stable without anything in the nose.

Looks interesting. I did the same thing with and scrap Alpha III fin can I had laying around.

IMG_8640.jpg
 

lakeroadster

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What is the stability without nose weight? Due to the length, my guess is you will be more than stable without anything in the nose.

Looks interesting. I did the same thing with and scrap Alpha III fin can I had laying around.

View attachment 584710

The Big Backwards Bertha is not stable without the nose weight
  • Stock Big Bertha Stability = 1.7 calibers
  • Big Backwards Bertha Stability without nose weight = 0.308 calibers
  • Big Backwards Bertha Stability with nose weight = 1.8 calibers
Did you fly that Alpha III yet?

I built an upscale backward finned BT-55 Alpha, it was not stable without the nose weight...
 
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Back_at_it

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The Big Backwards Bertha is not stable without the nose weight
  • Stock Big Bertha Stability = 1.7 calibers
  • Big Backwards Bertha Stability without nose weight = 0.308 calibers
  • Big Backwards Bertha Stability with nose weight = 1.8 calibers
Did you fly that Alpha III yet?

I built an upscale backward finned BT-55 Alpha, it was not stable without the nose weight...
Yes. The photo shows it on that pad up at Bong. It has flown 5 times per my log. Each was a straight flight. Has flown on B6-6 and C6-7's every flight.
 

Daddyisabar

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Oh, I do these things because I thought they were easy. Spoiler: They aren’t freakin’ easy.

That’s why they’re fun. 🤔
Rocket science is hard! That's why they had all those concrete bunkers.

This mod to a normally safe and sound Big Bertha is really scaring me. Yikes! Fin flipping is so naughty. Run, duck and cover! Don't poke your eye out.. Naughty, naughty boys! Go to your room and play in safety with your old Tinker Toys. No more rockets until you grow up! :)
 

lakeroadster

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Yes. The photo shows it on that pad up at Bong. It has flown 5 times per my log. Each was a straight flight. Has flown on B6-6 and C6-7's every flight.

But on your rocket, only the fin can is from an Alpha III, correct? It would be interesting to plug the data into Open Rocket, based on the actual dimensions and weight of your rocket, and see what it says the stability is.

A stock Alpha, with the fins mounted backwards, also has a miserable stability caliber.

1686069601381.png
 

BABAR

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But on your rocket, only the fin can is from an Alpha III, correct? It would be interesting to plug the data into Open Rocket, based on the actual dimensions and weight of your rocket, and see what it says the stability is.

A stock Alpha, with the fins mounted backwards, also has a miserable stability caliber.

View attachment 584725

I was going to say there is a significant difference in metrics between @Back_at_it ‘s Reverse Alpha Fin Can bird and @lakeroadster ‘s Ahpla, but I didn’t want to start a testosterone fight.

thats said, as @Daddyisabar preaches, rules are made to be bent, if not broken, and longer body tubes can forgive a lot of sins. This is just three fins and a nose cone (so doesn’t technically fit @kuririn ‘s “avant-garde” preferences) but it flew fine for several flights. Jump to beginning of thread.


also this
 
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BEC

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What is the stability without nose weight? Due to the length, my guess is you will be more than stable without anything in the nose.

Looks interesting. I did the same thing with and scrap Alpha III fin can I had laying around.

View attachment 584710
The model in this picture clearly has more than the stock 5.5 inch body tube of an Alpha III (or the 6 inch body of an Alpha VI). I'm not surprised the pictured model is stable.
 

BEC

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But while we're talking about odd Alphas and stability (sorry to wander too far off the original topic), I had an Alpha come with a Cub Scout to the May club launch that looked like this:

Screen Shot 2023-05-28 at 6.42.30 PM.png

As you can see, it's stable enough on an A8-3. It flew perfectly twice. On a C the margin is down to about 0.4 calibers. That one would be iffy....

Now back to swept forward fins.
 

Back_at_it

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But on your rocket, only the fin can is from an Alpha III, correct? It would be interesting to plug the data into Open Rocket, based on the actual dimensions and weight of your rocket, and see what it says the stability is.

A stock Alpha, with the fins mounted backwards, also has a miserable stability caliber.

View attachment 584725

The fin can and the nose cone are stock Alpha III. The body tube was a piece of scrap I had laying around. It is 12" long.

Total length of the rocket is 17ish inches.
 
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BABAR

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The fin can and the nose cone are stock Alpha III. The body tube was a piece of scrap I had laying around. It is 12" long.

Total length of the rocket is 17ish inches.
Most of my Asymmetric fin birds are built with "stock" Estes Body tubes, which are 18 inches long, and I generally use the full length of the tube. I don't add any nose weight. Something I learned during residency on my Chest Film rotation, "If you ain't cheatin' you ain't tryin'!" I have more trouble with shorter models. "Squirrel" flew like it's name sake, which was a corkscrew flight but net straight trajectory, "Stinker" unfortunately also flew by it's name, unstable off the rod. So be careful what you name your rocket. Do a search on TRF of "Suicide King" and you will see why.
 

techrat

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And.... I have to assume after this, you are going to make a scale X-29 rocket. Or semi-scale, because that aircraft was built to be unstable.
 

OzHybrid

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Created as a result of an idea spawned by @BABAR on THIS THREAD

It's a stock Estes Big Bertha... with the fins mounted backwards. Requires 0.55 ounces of nose weight on a C6-5 to equal the stability of a stock Big Bertha.

I'm not sure who originally created the Open Rocket .ork file? (It wasn't me). I just used it as is and flipped the fins. So if you build this be sure to do a swing test to check for stability.

I'd build this as a 24mm capable rocket, and adjust nose weight accordingly. You can always adapt down to 18mm. Be sure to run the numbers for fin flutter, or just use basswood and make it a TTW design.

View attachment 584664
View attachment 584660
At least in this orientation, you're less likely to break a fin...
 

lakeroadster

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Circumference of average woman 36:24:36. So average circumf=32. divide by 3.14 = average dia of 10 ish . 10:1 ratio gives 100max =8ft 4"
You need a rethink. you'll have a crick in your neck. :)

Wait, what? What kind of math is that, Aussie New Math?

Average diameter = 32", not circumference.

32 x 10 = 320" = 26' 8" tall

..... all women are less than a 10:1 overall height to diameter ratio.

I like my rockets, like I like my women... less than a 10:1 overall height to diameter ratio... 🚀 ;)
 

OzHybrid

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Generally when you measure someone you take chest, waist and hips distance around those places. Distance around a thing is circumference. Measured at 3 points, added together and divided by 3. etc etc. Less than 8'4" is 8'3". Your neck is going to be hurting on that date night.
And I'm working in banana measurements.....
:)
 
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