12" Bullet Bobby Upscale 4.6

PSLimo

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Hey Guy's,

I'm looking to build an easy to transport 12" diameter, light, low and loud flyer. I tossed around the idea of removable fins to make it more portable but the "light" concept wasn't working. A good candidate to upscale is Bullet Bobby which at 12" diameter would only be 32" tall. So I'm going to wing it and post the build. All suggestions appreciated.

First, I picked up a 12" Styrofoam half circle and then headed to Home Depot for a matching Sonotube. A few layers of glass and a lot of primer and sanding and it's looking like a Bullet Bobby nosecone.

12nosecone.JPG

I had an extra piece of PML 11.4" body tube and it made a perfect shoulder. So far so good.

coupler.JPG

Phil
 

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PSLimo

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Hey Guy's,

I cut the 12" sonotube down to size and decided to cut a hatch out to side mount the electronics.

12bt.JPG
12bt1.JPG

Tme to cut the centering rings, just have to figure out what size motor mount makes the most sense. 75mm or 54mm?
 

techrat

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This thread answers my question as to if the Concrete form tubes get used in model rocketry. I saw these tubes in the background in Upscaler's youtube videos in his garage and it started me thinking. But Okay, now I know. (And knowing is half the battle).
 

PSLimo

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75mm and adapt down, or use the new AT K-1800 ST load coming soon... :)

This build looks like it'll come in at under 10 lbs without nose-weight so it should be able to fly on a variety of 38mm & 54mm motors.

mmt.jpg

The 54mm tube looks tiny in the 12" centering rings, lol.
 

PSLimo

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Looking nice. What fins are you putting on?
Thanks, I originally wanted to use removable fins but that large diameter, easy to transport rocket idea got caught up in a complicated and heavy design.

I'm going to put together an open rocket file but so far it looks light enough to have an upscale 12" Bullet Bobby that can even fly on 38 mm motors. If my math is correct surface mounted 1/4" plywood fins will come in under 200 grams each. TTW mounted fins will double that.

I'm up for any suggestions.
 

DigBaddy

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Thanks, I originally wanted to use removable fins but that large diameter, easy to transport rocket idea got caught up in a complicated and heavy design.

I'm going to put together an open rocket file but so far it looks light enough to have an upscale 12" Bullet Bobby that can even fly on 38 mm motors. If my math is correct surface mounted 1/4" plywood fins will come in under 200 grams each. TTW mounted fins will double that.

I'm up for any suggestions.

Thick clear polycarbonate fins? Would look good if the fins were nearly invisible. Likely pretty heavy though...
 

bjphoenix

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I was thinking a little more about an idea for removable fins and created a quick layout in CAD. I was thinking the aluminum angle could be installed inside the main airframe with legs sticking out, then slide the fins in and bolt through them. The inside legs of the angles could be bolted to the inside of the airframe including some epoxy putty and the angles themselves wouldn't require any modification. I created this drawing with approximate dimensions just as a "what if"- 12" diameter x 3/8" thick sonotube, 3/8" thick fins, 1.5"x1.5"x1/8" aluminum angles.

1664935309641.png
 

PSLimo

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I was thinking a little more about an idea for removable fins and created a quick layout in CAD. I was thinking the aluminum angle could be installed inside the main airframe with legs sticking out, then slide the fins in and bolt through them. The inside legs of the angles could be bolted to the inside of the airframe including some epoxy putty and the angles themselves wouldn't require any modification. I created this drawing with approximate dimensions just as a "what if"- 12" diameter x 3/8" thick sonotube, 3/8" thick fins, 1.5"x1.5"x1/8" aluminum angles.

View attachment 540520
Thanks, that's the solution I needed for removable fins!!!

Never thought about installing the aluminum angle inside keeping all the epoxy on the inside of the body tube with clean lines on the outside. I'm 100% using this on a build I've been putting off because of the transport problem.

For this build I'm thinking surface mount fins is the path of least resistance for a simple light design.
 
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PSLimo

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Fun build. Any idea on how much weight you will need up front?
Now that I know the weight of everything I put an open rocket file together (which I'm not very good at).

1 lb in the nose puts it at .405 calibers and 3 lbs puts it at .605 calibers, not very stable.

BB1LB406CAL.JPG
BB3LB605CAL.JPG
 

DigBaddy

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5 lbs in the nose puts it at .800 calipers. Like I said, I'm not the best at open rocket, I attached the file if anyone wants to take a look.

It's starting to resemble Bullet Bobby:

View attachment 540587 View attachment 540588

Adjusting the base drag cone to conical with same diameter and pi * length, it shows .99 with the I600R in it.

Send it!
 

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PSLimo

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With a rocket that short, 1/2 caliber is plenty.
That would be good news, reducing the total weight from 13 lbs to 9 lbs allows for more tamer motor choices.

Anyone had successful flights to share with a 1/2 caliper on a stubby rocket?
 

Off Grid Gecko

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10% of 32in length is 3.2 inches. If you are double this (1/2 cal) you are probably just fine. If you have the little bullet-boy, sim that monster out and see what it's giving you for stability margin.
 

PSLimo

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10% of 32in length is 3.2 inches. If you are double this (1/2 cal) you are probably just fine. If you have the little bullet-boy, sim that monster out and see what it's giving you for stability margin.
I did, the original Bullet Bobby fly's stable at .58 calibers but this is a 12" diameter upscale build and all the materials change as well as the thrust and duration of the motors.
 

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Mini Magg. Did you ever put one in OpenRocket? A J350 is pushing it but it flies well on any smaller motors. Do not enlarge the mount to 54 mm without a lot of nose weight.
 

Rocketjunkie

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I did, the original Bullet Bobby fly's stable at .58 calibers but this is a 12" diameter upscale build and all the materials change as well as the thrust and duration of the motors.
As long as the CG and CP are .58 calibers apart it will fly stable. ALL dimensions scale together and you are rating the stability margin in calibers.
 

PSLimo

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Mini Magg. Did you ever put one in OpenRocket? A J350 is pushing it but it flies well on any smaller motors. Do not enlarge the mount to 54 mm without a lot of nose weight.
Yes I have many moon's ago back in June of 2006 for my level 2, LOL.

LEVEL2 CERT.jpg
 

PSLimo

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I cut the 1/4" plywood fins and they came in at 180 grams each as planned. Not going TTW saved another 1 lb+ in the rear.

I think it's easier to work with larger fin's if you tape off where the fillet's will be and sand and prime before attaching them.

fins.jpg
 

PSLimo

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Before I start tacking the fins on I wet out some fiber tape to strengthen the inside top of the body tube. Simple way to keep it round and help prevent recovery damage, IMO.

Screenshot_20221006-182848_Gallery.jpg
 
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