New scratch build: Bomber Betty

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qquake2k

Captain Low-N-Slow
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I have thought about doing a Big Bertha/Big Betty upscale for a long time. In fact, I even cut a set of 4" upscale Big Bertha fins at some time in the past. Fast forward a few years and I turned a 5.5" elliptical nose cone for a short stubby project, that I have since abandoned. I had embedded a large u-bolt along with 12 oz of BB's in the nose cone for the stubby's stability. It took some effort, but I finally got the u-bolt, BB's and resin dug out of the cone. I have decided to build a 5.5" Big Betty upscale, that I'm calling Bomber Betty. I chose a Big Betty upscale because it only has three fins and I'm lazy; and because there are lots of Big Bertha upscales out there, but I've never seen a Big Betty upscale. The dimensions aren't exact, it will be a couple of inches longer. And I made the fins 10% smaller to save weight.

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View attachment Bomber Betty Stretch.ork
 
I chose 3/8" plywood for the fins for strength. They hang out way below the rocket. These fins are huge! I had to print the template on two pieces of cardstock, and tape them together. Normally I would arrange the template with the bottoms of the tabs against an edge of the plywood, but these are just too big. So I cut out the general shape, pinned them together, and sanded the bottoms of the tabs to make sure they were square.

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The bandsaw cut through the total of 1-1/8" of plywood relatively easily. As you can see, I miscalculated somewhere, and had to trim a little more off one edge. But I was able to sand them square. These things are really big!

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Their biggins alright!My Skil bandsaw could not cut those huge fins,table is not large enough.This is going to be a big girl!
 
I think these are the biggest, thickest fins I've ever cut.
 
Yes, I just bought a 3/16" radius roundover bit for my router. First flight probably a J350. I might throw a J575 in it at some point.
 
Not sure a J350 would take it that far But it sims at 3500' on a J510.
 
But a Loki K1127 only sims at 3600'. Not that much more than an AT J510.
 
Started on the centering rings. There will actually be two centering rings and one bulkhead.

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I was planning on using a 24" length of leftover airframe I had. But I had forgotten about the wrinkles on it. I can always put the wrinkles on the back. It will be painted satin black, so I don't know how bad they will show up. It's either that or order a new airframe.

While I was at it, I did a test fit of the nose cone. It looks good!


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I cut the center holes in the centering rings with a 1-9/16" hole saw, then sized them with the Dremel.

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I don't like the narrow backing plates that u-bolts come with, so I usually make my own. This is 1/8" x 2" aluminum bar. I normally use 1-1/2",
but this is what I had.


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You know, I was going through my motor stash today, and found a 38mm J350W, which would be perfect for a first flight. But I also found a 54mm J135W moon burner. I don't even remember buying it. The only 54mm rocket I have is 5 Finger Death Punch. 5FDP sims at 3900' with it, and that's pretty high for me. Bomber Betty sims at about 3000', which is still high for me, but probably doable with the Chute Release. I would have to redesign it with a 54mm motor mount, and order the motor tube, but I'm considering it. My biggest concern is that it sims at 44fps off the rail, which is kind of slow for such a big rocket.

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View attachment Bomber Betty Stretch 54mm.ork
 
Yeah, 44FPS is pretty slow for a big bird.

I like mine to get out of Dodge in a hurry. Especially if they're stubby. Bomber Betty has some sleek lines so you could probably get by leaving the pad at 50-60 FPS...I favor the faster speed.

Hate to tell you the initial launch speed of my Goblin 5.5 on an AT I366R but it pulls 14 g's (73 FPS) .
 
I'll probably stick with the 38mm mount, and use a J350 that I also have. It sims at 1900' and 87fps off the pad.
 
Yeah, the J350 is a bit more energetic than I would have guessed.
 
Wow! 78" chute! That's a lot of laundry! I always liked the Big Bertha (I'll have to dig one out for the next SARG launch for old time's sake).
 
Wow! 78" chute! That's a lot of laundry! I always liked the Big Bertha (I'll have to dig one out for the next SARG launch for old time's sake).

It is a big chute. It will be the same one I used in my Shadow, with a J575.

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I found a 3/16" radius roundover bit to round the leading edges of the fins with. Since the fin edges are straight, I decided to use the fence on the router table, and not just rely on the pilot bearing. I didn't have the bit completely tight at first, and had some problems. But once I got it good and tight, and got everything adjusted, it seemed to work well. By the way, I love the above the table depth adjustment. Very handy.

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They turned out very nice. Now I just need to smooth the edges a bit with some hand sanding. I can't decide, though, if I want to round both leading edges of each fin, or just the "leading" leading edges, like they are now.

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