Stock Size 29mm Baby Bertha!?

[86mustang408w]

Idk where this is going. I have one already built for an E motor down to A but the fins are warped and it doesn't fit for crap. Now this will probably fly on a F something eventually, but probably a B4-4 first. I'm going to cut the nosecone shoulder off, then drop in a eyelet bolt in with epoxy. The fins will all be covered in poster board and glue, with tabs to the motor tube. Definitely going to airfoil the fins and I'm not sure if I'll need to beef up those centering rings. I'll be able to put glue on each side of them with the tube slots I'm going to cut in sometime today, and that will basically be 2 great centering rings themselves once they dry. Probably going to use 550 lb kevlar shock cord, I'll wrap that around the motor tube and its going to be longer than usual. I'm not sure if I'm going to make a chute protector for this or not. I might have plenty of room for wadding and dog barf woth using the nosecone to stuff the chute into. Stock plastic one with a spill hole. Pretty good chance of this going up and not being found, so I'm not using anything nice or nylon in it yet. Ill get an eggfinder mini too before the composite flights its gonna go high enough buts its super small. Thats another thing I'll watch and do flight simulations as not not break the rules or see what it could do. Its not going to launched on anything composite unless its at a launch day. I'm not sure how high even say a F42-4T will take it up. Once I finish it, weigh it, OpenRocket it, then ill know what motors will do what. But the motor retainer cap will screw on right up the the aft body tube. Those markings and measurements I'm working on right now. And a few more fin skins and templates. I'm going to be pretty concentrated on just this build.

 

[86mustang408w]

Mocked up and messed around with the motor retainer. I have a spent motor to use for mock ups. I'm going to make sure when the cap is tight it butts right up to the body tube. I need to choo that noseckne shoulder off too, but waiting on a new hacksaw and blades. So for the time being I'll keep working on the fins/tabs and tube slots.

 

[86mustang408w]

Once its all line up and glued it will be centered.

 

[SolarYellow]

I would be concerned about flutter with those long Bertha fins at high airspeed.

 

[Back_at_it]

I've flown a normal Big Bertha on an F44 with the stock Balsa fins to a touch under 1700ft. Not something I'd recommend doing often but it worked out for me once. I typically fly that rocket on D12-7's and E20-7's

I do also have a Baby Bertha that has flown on everything from C11's, D12's up to E30-7's. Great flier but I needed to add nose weight. I can't recall exactly what I added but the nose cone now weights 1.2oz.

 

[DAllen]

Our club did a Baby Bertha challenge once where the goal was to see who could build a BB and have it survive a H195. I don't recall any of them shredding but we never saw most of them again. The motor was so long that most of us had to cut out the bottom of the nose cone to get it to fit. I glassed the fins, body tube and then went tip to tip glass. I think RockSim put it at around mach 1.3 and 5k...?

 

[86mustang408w]

I've flown a normal Big Bertha on an F44 with the stock Balsa fins to a touch under 1700ft. Not something I'd recommend doing often but it worked out for me once. I typically fly that rocket on D12-7's and E20-7's

I do also have a Baby Bertha that has flown on everything from C11's, D12's up to E30-7's. Great flier but I needed to add nose weight. I can't recall exactly what I added but the nose cone now weights 1.2oz.

Thats about what this will be flown like too, from B to F's, occasionally though on the last one.

 

[86mustang408w]

Our club did a Baby Bertha challenge once where the goal was to see who could build a BB and have it survive a H195. I don't recall any of them shredding but we never saw most of them again. The motor was so long that most of us had to cut out the bottom of the nose cone to get it to fit. I glassed the fins, body tube and then went tip to tip glass. I think RockSim put it at around mach 1.3 and 5k...?

I'm definitely going to have a tracker in this with a big field, ill probably wait until I go up to URRG in NY. I'm also cutting out the nosecone but to just give me more room..

 

[86mustang408w]

I would be concerned about flutter with those long Bertha fins at high airspeed.

I'm working on that one...lol..

 

[86mustang408w]

Well sat down and have been marking the body tube, and fin skins. Ill add a piece of wood in the tab spot on the fin. I've built a few like this and they are strong once its all done. Right now I just started cutting out a random fin and once I get that thickness I'll measure out, mark, and cut a tube slot. Then once I find out what the tab width needs to be ill set the template and finish the other fins on the poster board. Each side getsba skin and glue. I'm working on that right now.

 

[86mustang408w]

Got one fin thickness figured out, so cut one fin slot. Ill cut out the other 3 it fits nice, then cut the fin tab down on #3. It will be this process for a while yet...

 

[86mustang408w]

More progress..fins now..

 

[86mustang408w]

Got one fin skin on, ill trim that after it dries. Its under wax paper and some weight. Then I guess I'll do the other side and add in the fin tab piece. Once its all dry and together they will be plenty strong. My goal is to not destroy or lose this rocket the first time out, or use a dumb sized motor that would pretty much destroy the fins. Adding skins will help quite a bit for strength, but not enough for higher impulse motors.

 

[86mustang408w]

Went out to the garage and used a hack saw and cut the nosecone shoulder off. Debating on which eyelet to add, I was going to use the 1/4", but think the 3/16" might be better. Gonna go grab a small washer to add in. More of an anchor point. I'm gonna weigh some stuff so this will be part of the nosecone weight.

 

[86mustang408w]

I cut and sanded that nosecone a little more, I'm also going to use a shorter length eyelet bolt. But im going to do the kevlar to the motor tube first. Used a wire to mock it up. Going back the fins shortly..

 

[86mustang408w]

Got fin number 3 trimmed on one side, flipped it and put glue down on B side with the skin. Put some weigh on it, wiped up the excess glue, then put again weight on it all. I'll let that dry and make the tab.

 

[86mustang408w]

Got the shock cord cut at 5 feet, using 550 # kevlar cord. That will get wrapped around the motor tube, after a slight provision is made for it. Also cut down and mocked up a lug. I'll wait until the fins are done and its ready to fly before I glue that on, the CG will change a few times but its so small it won't matter too much. I have 3D rail guides coming I'll use too.

 

[86mustang408w]

Cut the shock cord around the motor tube. Just mocked up no glue yet. Back to fins...

 

[Sooner Boomer]

From a few years ago...

https://www.rocketryforum.com/threads/29mm-baby-bertha.59658/

 

[86mustang408w]

From a few years ago...

https://www.rocketryforum.com/threads/29mm-baby-bertha.59658/

I'll read that. I have one for E motors, but figured might as well make one 29mm. I'll mostly use B and C, maybe D with this. Once I get an eggfinder mini, more. But I'll check that out in a bit....Thank you.

 

[86mustang408w]

I was glue another skin on but I figured ill cut them out first....lol. I'm gonna cut these and thats it for tonight.

 

[86mustang408w]

Well started to cut those out last night and about 3 skins in realized I forgot the fin tabs, so went to bed. Lol, got up this morning and traced out 3 more, with tabs. Also got a piece of balsa from the board the fins come in to fill in that tab slot on fin #3. I used glue, pressed it in, wiped off the excess, and then put it on the wax paper with weight on top again. Gonna sit now and cut out the remaining skins. Then one by one get them on the fins with glue, and tabs put in like # 3 below. I'm doing them like this to prevent a lot of finishing sanding/trimming of the fins before paint. I want to make sure all the fins are exactly the same with the tab slots as I apply them with glue.

 

[86mustang408w]

Had some other things to do today, but have time now to work on this. I'm going to split the lug and do 2. I got rail guides but they might be a little too big. But going to finish up this fin, and work on the next one.

 

[86mustang408w]

Got another skin on and that last fin sanded and ready for glue. Also cut the lug in half and I like the placement of two over one for this. I got epoxy on rail guides I might try but think they might cause issues with stability and aerodynamics, or lack of aerod's. Think time, I'll keep doing the fins like this one by one.

 

[86mustang408w]

I let that dry overnight. Grabbed fin #1, marked the tab and cut it. Ill trim that fin down and then do the other skin the same way. Then to the next fin, ill put the tab piece in and let fin #1 dry with weight on it, so I can get fin #2 going. I want the fins to be identical so this works for me. I get to work a little on something each day. Two things I'm still yold off on are the eyelet bolt/expoy in the nosecone, and JB welding the motor retainer on. Oh I was messing around with the motor tube again. I have too much to mocked up and might slightly change something to permanently attach these parts yet. I don't like to glue or epoxy until its absolutely time with a build that's not time sensitive as this. I'm taking my time an md enyking the build.

 

[86mustang408w]

Brushed glue on the other side of that fin, its in a small container and it lasts for weeks. But I put weight them on the fin and then wiped the excess glue and made sure the skin was in still in a good spot. Ill let it dry with weight on it again, then after a while out in the open air. Two more fins to go after this one is sanded down and tab cut on this skin.

 

[86mustang408w]

2 fins are done, working in the 3rd still. Probably it for tonight, 2morrows a launch day so its time to prepare for that. Nothing is glued yet.

 

[86mustang408w]

Trimmed the one side of fin #2 down. I make the skins a little bigger sometime its makes it easier then cut off the excess for a good look. Going to glue that skin on the other side now.

 

[86mustang408w]

Did that fin. Dry time....

 

[Sooner Boomer]

Good work. Now you need to expand your horizons a bit...



from the top, clockwise - 29mm, 3x18mm, 18mm, 24mm. All painted in basic black (it's very slimming).