12" Bullet Bobby Upscale 4.6

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PSLimo said:
Surface mounted 1/4" plywood should be fine. If the build stays under 10 lbs I can fly AT 54/1280 motors like the J-415W for a low and loud flight, if not it's the J-800T just to get it off the ground.
Click to expand...


Eeek, id be concerned about landing, better go big big chute on calm days
 
bjphoenix said:
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

.
Click to expand...
why not invert the angles, use smaller grooves for the fins to fit through the body tube, and keep all hardware INSIDE the body tube (assuming you can epoxy the angle/body tube joint, which I think you are going to do anyway)? Gives you cleaner lines and less drag externally
 
PSLimo said:
Fillets are done and it's looking more like a 4.6x Bullet Bobby.

View attachment 540834
Click to expand...
Fingers crossed.


Looks like

big rocket,

big fins,

small surface area for surface mount tube fin joints.


how strong is the body tube Outside paper liner the fins are attached to?



not sure if adding a few thin Layers of fillets would help, sure wouldn‘t hurt.
 
BABAR said:
Fingers crossed.


Looks like

big rocket,

big fins,

small surface area for surface mount tube fin joints.


how strong is the body tube Outside paper liner the fins are attached to?



not sure if adding a few thin Layers of fillets would help, sure wouldn‘t hurt.
Click to expand...

The outside paper liner on Sonotubes is very flimsy but not waxy.

I wet out some drywall fibertape with Westcoast on the inside of both ends of the tube to strengthen it up and keep it in round (post #30). I also wet out an inch or so on each side of the fillets to strengthen that up before primer.

Overall, the build is better than I expected on strength while staying light. The original goal was an easy to transport, low and loud 12" diameter flyer that's under 10lbs to be able to fly on 54/1280 motors like an AT J-450W.
 
Last edited:
BABAR said:
why not invert the angles, use smaller grooves for the fins to fit through the body tube, and keep all hardware INSIDE the body tube (assuming you can epoxy the angle/body tube joint, which I think you are going to do anyway)? Gives you cleaner lines and less drag externally
Click to expand...
The original objective was to remove the fins so the rocket would fit in the car for transport, then put the fins back on at the launch site. With the hardware inside the airframe it would be hard to get to.
 
DigBaddy said:
base drag cone looks narrow from the screenshots. I use length of pi * diameter, conical, with a diameter same as the rocket. Guessing it might be showing low as configured.
Click to expand...
Don't use conical. Use a transition so that the trailing cone is a triangle.
 
Mike Haberer said:
Don't use conical. Use a transition so that the trailing cone is a triangle.
Click to expand...

ok, I tried that and get the same stability results as the conical nosecone; which it should be since it's the exact same shape. I'm not seeing how that would be any different.

1666090570141.png

Hell, even openrocket says a conical nosecone has the profile of a triangle.

1666090633579.png
 
I was reacting to this pic...
1666096266961.png

...where the tail looks like an ogive nose cone vs. a triangular (conical) nose cone (and the base does look narrow).

The model you used above is for a Big Daddy and not the upscale Bullet Bobby, so it's comparing apples to oranges a bit. BB will have a lot of drag based on the shape of its nose cone vs. a Big Daddy. Your Big Daddy file also show a caliber of 1.26 with the transition and motor in place, so it's actually very stable for a short fat rocket.

I pulled up your .ork file for BB. I used a transition instead of a nose cone. The other thing you have to do is use a mass override of 0 grams. I can't see if you did that with the Big Daddy file, it's cut off.

The transition without the mass override has a cal of 1.2. With the mass override the cal is 1.37, a significant difference. You want no weight for the transition adjustment, since it is not a real part of the rocket, it is only added to model base drag accurately. Without a mass override, it shifts the CG towards the back end of the rocket. Pics below.

cal 2.jpg

cal 1.jpg

Then I modeled it with no nose weight, which comes to .72 cals, which is plenty for a short fat rocket. I shoot for .75 cals on mine just to be safe, knowing that they would fly OK at .5 cals.

cal 3.jpg
 
Mike Haberer said:
I was reacting to this pic...
View attachment 542240

...where the tail looks like an ogive nose cone vs. a triangular (conical) nose cone (and the base does look narrow).

The model you used above is for a Big Daddy and not the upscale Bullet Bobby, so it's comparing apples to oranges a bit. BB will have a lot of drag based on the shape of its nose cone vs. a Big Daddy. Your Big Daddy file also show a caliber of 1.26 with the transition and motor in place, so it's actually very stable for a short fat rocket.

I pulled up your .ork file for BB. I used a transition instead of a nose cone. The other thing you have to do is use a mass override of 0 grams. I can't see if you did that with the Big Daddy file, it's cut off.

The transition without the mass override has a cal of 1.2. With the mass override the cal is 1.37, a significant difference. You want no weight for the transition adjustment, since it is not a real part of the rocket, it is only added to model base drag accurately. Without a mass override, it shifts the CG towards the back end of the rocket. Pics below.

View attachment 542241

View attachment 542242

Then I modeled it with no nose weight, which comes to .72 cals, which is plenty for a short fat rocket. I shoot for .75 cals on mine just to be safe, knowing that they would fly OK at .5 cals.

View attachment 542243
Click to expand...

You quoted my post but were reacting to his incorrect cone shape; so I was confused. I definitely missed the mass override on the 12" BB sim...oops. The BD sims I posed are definitely zero mass for the cone or the transition and the results are the same using either. I went and corrected the 12" BB sim and tested cone vs transition - same result that you get.
 
A very minor point, but I normally just set the wall thickness to zero to force the mass to zero.

I don't think using conical vs. ogive is going to change your results much, but conical is the more "correct" approach, where "correct" means what was specified in the original article that described the base drag hack.
 

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