EXPjawa
Well-Known Member
Since I seem to have nothing better to do (like half a dozen unfinished rockets), recently ordered BT80-upscaled Big Bertha decals from Gord in the Pre-D 1965 (USAF) style and set about building a rocket to put them on. I could've easily built a Super Bertha clone, but that's not what this is, not really. That is where I started out here, but I soon realized that the Super Big Bertha isn't really properly upscaled. Estes adapted the design (I suspect) to use the existing BT80 standard lengths. Also, the PNC80BB cone isn't really as long as an upscaled BNC60L would be (4" vs 5"), so between those two things, the SBB was only 36.5" long, whereas a true-to-scale BT80 sized Bertha would be 38.118" long. That might be close enough to not split hairs, I don't know. But since I'm building from scratch anyway, I can make it however I want, so I might as well make it accurate as possible (within reason). For this design, I rounded to the nearest 1/8", so my upscale Bertha will be 38.125" tall. The other area that's distinct in my design from the SBB is the motor mount - SBB was 24mm, but a "proper" upscaling of the mount to match the profile makes the motor mount approximately 28.5mm, so this design will use a 29mm mount. My RockSim file is attached.
One thing I did lift from the SBB was the fin pattern, just due to easy of availability. I cut out two sets of fin from 1/8" balsa sheet, the second so I can build a BT80-sized Astron Ranger later on (I know the fins should be slightly different, but I don't have enough hair left to split them). I papered one set for now over the weekend, so last night they were suitably dry. Starting with a 34" segment of BT80 tubing from BMS, I cut the body tube to 29.625". That leaves me with a 4.5" long BT80 chunk (since the tube was slightly overlong) that I'll have to find a creative use for later. Anyway, on Sunday evening I put together a drilled-plate style baffle, which I already had, with a eye bolt to mount the recovery harness.
This leads me up to last night, where I sanded the fin edges and began attaching. I used the baffle to keep the tube round, and using my fin guillotine, set about gluing the fins on one at a time:
Tonight, hopefully, I will get the other two attached. One thing that I've learned using this jig was not to be in a hurry. Its important to make sure that the glue has set up sufficiently before moving to the next fin...
I have yet to build a motor mount, however. I wanted to have a 14" long stuffer tube per the design I deemed best, and I only have 11" segments on hand. So, I ordered a longer length from BMS, as well as some other things, yesterday. So MM to come; when I do build it, I will use an Estes plastic threaded motor retainer. At least with the guillotine, the mount is not necessary to align the fins. So, that's where I am for now...
One thing I did lift from the SBB was the fin pattern, just due to easy of availability. I cut out two sets of fin from 1/8" balsa sheet, the second so I can build a BT80-sized Astron Ranger later on (I know the fins should be slightly different, but I don't have enough hair left to split them). I papered one set for now over the weekend, so last night they were suitably dry. Starting with a 34" segment of BT80 tubing from BMS, I cut the body tube to 29.625". That leaves me with a 4.5" long BT80 chunk (since the tube was slightly overlong) that I'll have to find a creative use for later. Anyway, on Sunday evening I put together a drilled-plate style baffle, which I already had, with a eye bolt to mount the recovery harness.
This leads me up to last night, where I sanded the fin edges and began attaching. I used the baffle to keep the tube round, and using my fin guillotine, set about gluing the fins on one at a time:
Tonight, hopefully, I will get the other two attached. One thing that I've learned using this jig was not to be in a hurry. Its important to make sure that the glue has set up sufficiently before moving to the next fin...
I have yet to build a motor mount, however. I wanted to have a 14" long stuffer tube per the design I deemed best, and I only have 11" segments on hand. So, I ordered a longer length from BMS, as well as some other things, yesterday. So MM to come; when I do build it, I will use an Estes plastic threaded motor retainer. At least with the guillotine, the mount is not necessary to align the fins. So, that's where I am for now...