BT80 upscale Bertha

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EXPjawa

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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:
WP_003014.jpg
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...
 

Attachments

  • super_big_bertha clone29.rkt
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Cool. I've been thinking about large lightweight BT-80 rockets to fly on the low-thrust E16 and F15 motors, and an upscaled Big Bertha would probably work great on those motors.
 
Sure, but I'm just not sure that I trust the reliability of the 29mm BP motors. With the way I've designed this, its already borderline too heavy though; liftoff is so slow that it would require a full 6' launch guide to get to a stable speed. On the other hand, it lifts pretty readily on 24mm BP motors (D12 & E12). But those motors are light enough that its very overstable in that configuration and weathercock prone. I'd only fly it as such on calm days. FYI, according to the sim - FWIW - a BP E16 or a composite E23 will both put it up to about 700', but the BP F16 would put it closer to 1100' once it got going, so it wouldn't be a viable option for Moffett Field. If you were going to build something like this for 29mm BP motors, I'd leave out the baffle, attach the harness by looping around the MM (no eye bolt), use the one-piece body tube (no coupler) and maybe shorten the stuffer tube so that the launch weight was as low as possible. I was thinking more about F-sized motors when I designed this.
 
Fins on:
WP_003017.jpg

It looks like a rocket now, though its little more than a tube with fins glued to it, and a nose cone perched on top. I should have the supplies to build the motor mount today. I expect to epoxy fillet the fins soon too.
 
Cool build. I found a very heavy paper tube at work - 2" OD and 1.75" ID if I recall correctly, and I've been thinking about making an upscaled Bertha out of that with a MM for 29mm Ceseroni hardware. It wouldn't be as big as yours, but with that wall thickness I'll be able to go HPR if I want to; I'll be sure to construct it accordingly. The tube was from 34" wide paper for a large format printer.

What's that big white rocket on the table in the background?
 
I made one 8 years ago and flew the daylights out of it. I mean I was "really" inexperienced with rocketry and did surface mount 1/4" BALSA fins!:bangpan: Flew a lot of 29mm G40's but it eventually gained weight from many repairs and got knocked sideways off the launch rod its last flight. Delay was too long for the angle and the sustainer zippered in a very nasty fashion. I saved the upper tube with the coupler and nosecone. Upscale Berthas are fun flyers. Kurt
 
Cool build. I found a very heavy paper tube at work - 2" OD and 1.75" ID if I recall correctly, and I've been thinking about making an upscaled Bertha out of that with a MM for 29mm Ceseroni hardware. It wouldn't be as big as yours, but with that wall thickness I'll be able to go HPR if I want to; I'll be sure to construct it accordingly. The tube was from 34" wide paper for a large format printer.

What's that big white rocket on the table in the background?

Its an upscaled Estes Advanced Target Drone that I haven't quite finished yet:
https://www.rocketryforum.com/showthread.php?122701-New-project-BT70-upscale-Advanced-Target-Drone

Incidentally, I've got a small pile of paper roll tubes at work, from the 36" wide rolls our plotter uses. They're just under 2.25" in outside diameter and about 2" ID. Very pronounced spiral grooves. They're also pretty heavy. I've never considered using them for a rocket, because I stay mostly on the lighter side of things. But they would be useful for self defense, softball, etc...
 
You have inspired me to build a super big berth clone. Maybe a 3 in line 24 mm cluster so I can put it up on 1 motor if I so desire.

TA
 
Well, you're not going to fit three 24s side-by-side in a 2.6" tube, you'd have to go up to something like a LOC 3" before that'll fit, though it might just squeak into a thin-wall 3" OD tube. I was actually going to build this as a cluster initially before electing to go 29mm. I realized that if you cluster three 24mm mounts in a circle, then you've nearly got an upscaled Astron Ranger instead of Bertha. You'd just need to add a payload compartment with two-piece recovery and you're there. That's what what pushed me back to a single engine - I'm building a Bertha at the moment, and if I did a cluster, I'd build it as a Ranger. Its sort of splitting hairs, but I don't have much hair left, so I need to split as many as possible. :D I'm going to build that next month.

Upscaled Astron Ranger:
Astron Ranger BT80.jpg

Upscaled Bertha:
super_big_bertha clone29.jpg
 
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Looks really good, and better scaled than the SBB. Nice job.
 
Well, you're not going to fit three 24s side-by-side in a 2.6" tube, you'd have to go up to something like a LOC 3" before that'll fit, though it might just squeak into a thin-wall 3" OD tube. I was actually going to build this as a cluster initially before electing to go 29mm. I realized that if you cluster three 24mm mounts in a circle, then you've nearly got an upscaled Astron Ranger instead of Bertha. You'd just need to add a payload compartment with two-piece recovery and you're there. That's what what pushed me back to a single engine - I'm building a Bertha at the moment, and if I did a cluster, I'd build it as a Ranger. Its sort of splitting hairs, but I don't have much hair left, so I need to split as many as possible. :D I'm going to build that next month.

Upscaled Astron Ranger:
View attachment 258362

Upscaled Bertha:
View attachment 258363

Ahh man! I sent that from work, and I didnt run the numbers in me head, that would put you around 72mm just in motors. Tough to fin in a 66 mm tube. Boo booo I say, guess I will stick with just one 24mm motor. :sigh:

Great work by the way be sure to include flight pictures, remember if there is no pictures it didn't happen.


TA
 
I've toyed with the idea of building a 4" diameter Baby Bertha, if I could find a suitable nose cone. At that point, though, it has definitely transitioned out of the light-duty side of things and becomes heavy enough to require "real" motors. But, right now I have enough build projects on my plate and that one isn't a priority. I might build a standard-sized Baby Bertha just to have one though.

My order for motor mount components has not arrived yet. USPS tracking still shows expected delivery as 3/18, and that its still in-transit. We'll see if it shows up on 3/20... In the mean time, I did start doing the epoxy fillets on the fins. I got half of them done last night, but decided to actually give the epoxy sufficient time to set up this time and didn't want to roll the rocket over too soon, so it doesn't sag or run. So, I can probably finish that up tonight if nothing else comes up.
WP_003021.jpg
 
Incidentally, I've got a small pile of paper roll tubes at work, from the 36" wide rolls our plotter uses. They're just under 2.25" in outside diameter and about 2" ID. Very pronounced spiral grooves. They're also pretty heavy. I've never considered using them for a rocket, because I stay mostly on the lighter side of things. But they would be useful for self defense, softball, etc...
I measured my tube, and that's exactly what I've got (my memory was faulty.) Yes, it's heavy, but not too heavy considering. Memory was also faulty regarding the motor diameter; with a 2" ID I'll put a 38mm mount inside, and even the smaller 38mm motors will lift that puppy easily, and with that wall thickness, the tube can take it.

From Apogee: 54mm LOC tube is not as strong and $8.09 for 34" (so a little short.) 54mm Blue Tube is $23.95 for 48", so $17.96 worth of that for 36". 54mm fiber glass is $64.65 for 48", so $48.48 for 36" worth.

From PML: Quantum Tube QT-2.1 is $16.95 for 36"; phenolic PT-2.1 is $14.99 for 36"; and glassed phenolic FGPT-2.11 is $78.99 for 36".

The overly heavy paper tube is free. Considering the weight compared to the price:strength ratio, I'll take the weight.

Also, no thread hijacking, so if I've got any more to say I'll start my own.
 
No worries on hijacking - conversations tend to take on lives of their own anyway. I have no immediate intent to move to 38mm motors myself, seeing as how I'm only just dabbling in 29s now and want to keep overall cost down. So, these tubes are free but still too heavy for my use. But I've got half a dozen of them in the corner by my desk; if you want them and, say, find yourself coming up here for LDRS, you can have them... :D
 
If you still want to build that 4 inch version, try Scot Glass for the nose cone.

https://www.scotglas.com/components2.htm

I got one of the cones for a 4inch Fat Boy and it fits nice. It's bare fiberglass and light but sturdy.

Nice build. BT-80 is a great size to play with.

See ya,
Rod
 
I got one of the cones for a 4inch Fat Boy and it fits nice. It's bare fiberglass and light but sturdy.

Nice build. BT-80 is a great size to play with.

Thanks. What does that cone weigh? I don't see that info on the site.

Over the weekend, I made a bit of progress. The tubing and centering rings for motor mount arrived on Friday, so I built up the mount:
WP_003025.jpg

Once that was installed and the epoxy cured, I filleted the inside of the mount and installed the rear ring, then the engine retainer. At the same time, the recovery system supplies I'd ordered arrived on Saturday, so I built up the harness, tied off the kevlar line to the eye bolt on the baffle, and then glued the baffle in place. Once that was set and filleted, I connected up the line to the cone, the parachute to the swivel, and stuffed it in. That lead me to this absolutely awful shot showing the filled in spirals:
WP_003027.jpg

Once that was all sanded and such, I was able to put the first coat of primer last night:
WP_003030.jpg

This thing is essentially ready to fly now, just needing paint and decals. All that, I still managed to spend about 4 hours working on my truck yesterday afternoon...
 
I brought BT80Bertha with me to work today, so I could weight it on the (calibrated) postal scale. This bird is 9.7 oz. - which is 0.8 oz lighter than RockSim predicted. I'm not sure why, but I guess I've got more room for paint that I thought...
 
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It occurred to me that I didn't have the buttons on the airframe when I weighed it, so that accounts for some of the difference. Anyway, I put a nice coat of gloss white on yesterday:
WP_003033.jpg
 
OK, thanks. I'll put that in my database.

I had cause to resand two of the fins on Bertha80, so elected to paint them black rather than white. The paint work is now finished, hopefully I'll get the decals on in the next couple of days.
IMG_3055.jpg

The last issue is installing launch guide buttons. I'm undecided as to installing 1010 buttons or mini buttons for the launcher that I plan to build. I guess its a matter of whether I want to be able to launch without the club's full size launch rails or not...
 
The last issue is installing launch guide buttons. I'm undecided as to installing 1010 buttons or mini buttons for the launcher that I plan to build. I guess its a matter of whether I want to be able to launch without the club's full size launch rails or not...
You could put on both. It's an underused option.
 
A fair point. That does add drag, not that I really care about that. But I only predrilled and reinforced one side for button holes, so maybe I could put holes for both on one side inline, and remove the 1010 buttons when I want to use the 20mm rail...
 
I've only used 1010s myself (still new to buttons) so I have to ask; do the diffferent buttons use the same size screws? If so, you could stay with the one set of holes and change the buttons as needed. Use a little thread lock (Locktite) whenever you change them and it should be fine.
 
A good thought, but no, the screws are smaller with smaller buttons. Keep in mind, too, that given what these are threaded into, more than a few repeats of screw engagement will likely wear the threads in the rocket out. Loctite won't help if the thread can't hold a load. Then you'd have to drill new holes and fill the old ones. But in this case, I think that the consensus is that for the weight of this rocket, mini buttons are fine. I may have to bring my own pad to club launches to fly it, though, but I'll have to do that for several others as well. At least until they wise up and expand their rail choices...
 
WP_003050.jpgWP_003053.jpgWP_003052.jpg

Decals are finally on. I just need to do a little touch up work and put the rail buttons on. This thing is otherwise done.
 
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