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My Second Build - Estes Big Bertha

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snrkl

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So I decided a week or so back that I would use a kit for my second build - figured it was a good way to refine construction techniques without having to actually construct all parts of the thing.

Chose the Big Bertha as it seems like it will be a good "show" rocket for playing field launches and perhaps taking to my kids' school eventually...

I am super keen to take my time with this one (all my kids now have rockets to fly, so there is no rush to get stuff finished now) and I am working on building good construction habits... I am writing up my builds to ensure that anyone with better methods might point me in a better direction.

On advice from the gents at my local hobby shop, I'm planning on using epoxy for fin attachment and motor tube construction. The 5min Epoxy I have used for previous rockets is a little too quick for me on this one, so I am planning on getting some 3hr epoxy just to give myself a little more working time.

I have also used medium CA to fill/strengthen the balsa fins.

With the fin filling, the first two rounds of using an ice cream stick to spread the CA left me with rather uneven coverage, I have reverted instead to using just a gloved finger to spread the CA, doing one side at a time (x4 fins) and leaving to dry then repeating on the other side. This seems to have given a MUCH better result.

So here I get to my first question: when working with balsa fins (3mm) do others usually shape the LE/TE then CA, or CA then shape? I figured that it was easier to take away then try to add more, so I wicked CA into the edges of the balsa before shaping. As it turns out, almost all of the CA ended up being sanded away. I am thinking that next time I will shape, then CA, then fine sand (600grit) before painting but just wanted to seek input...

01_Fins Drying.jpg02_Fin Shaping.jpg
 

snrkl

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And if anyone knows why the iPhone pics keep uploading upside down, I'd love to know... ;)
 

snrkl

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So the next steps are motor tube assembly. The Estes kit has cardboard CRs. A dry fit shows that they are REALLY a tight fit.. I am assuming that a little sanding will be ok, but I am looking for any hints/tips on preventing the cardboard CRs from delaminating...
 

dhbarr

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So the next steps are motor tube assembly. The Estes kit has cardboard CRs. A dry fit shows that they are REALLY a tight fit.. I am assuming that a little sanding will be ok, but I am looking for any hints/tips on preventing the cardboard CRs from delaminating...
I usually just have to get rid of the laser nubs.
 

neil_w

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On advice from the gents at my local hobby shop, I'm planning on using epoxy for fin attachment and motor tube construction. The 5min Epoxy I have used for previous rockets is a little too quick for me on this one, so I am planning on getting some 3hr epoxy just to give myself a little more working time.
Let's take a pause right there.

First of all, everything you're doing will *work*.

However, there is really no reason to use epoxy to attach fins on an LPR rocket, save for unusual situations of which a standard Big Bertha is not. Wood glue is more than sufficient. I (as do many around here) use Titebond II for attachment and Titebond Quick and Thick for fillets, but you can pick your own favorites. Wood glue is non-toxic, easy to work with and is stronger than the materials it attaches to, especially if you remove the glassine layer from the tube where it attaches to the fin root.

Next, while I've used CA to fill and harden nose cones, it wouldn't be my choice for doing fins. I've become a big fan of papering, but wood filler also works well, and many folks are still devoted to dope-based sanding sealers and fillers (of which Brodak seems to be the best available option right now). I use CA when I have to but it produces highly noxious fumes and is horrible (IMHO) to sand unless you get it before it fully hardens up.

Which brings me to the point that you might actually have an unusual situation on your hands here: if your CA drenching of the fins includes the root edges, then you don't really have a wood fin anymore, and epoxy might then be a better choice.

But I would never choose to build LPR that way.

I *have* used epoxy for installing motor mounts, because it won't seize and is quite pleasant to use for that task. You do need to make sure it is held in position while curing, otherwise it can slide out of position. The rest of the motor mount assembly would usually be wood glue, though.

Finally: going from 5 minute to 3 hour epoxy is quite a jump. I don't think there's any reason for you to go beyond 15 or 30 minute epoxy for this sort of use (I've never actually even seen 3 hour epoxy, certainly not for hobby use).

When you move up to MPR or HPR then different considerations apply, and there are different epoxies that are commonly used for assembly and fillets. But those are not really relevant for an LPR build like this.

Again, what you are doing will *work*, but I wouldn't say it is the most straightforward way to build LPR.
 

smugglervt

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+ 1 on what Neil said. Carpenter's wood glue works fine for cardboard tubes and papering fins is my preferred method. CA only the leading and trailing edges after papering to seal the paper edges. Epoxy (Bob Smith 30 minute) on the MMT only because it does not seize up like wood glue when trying to get it into position.
 

soopirV

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Not to jump on the bandwagon, but wood glue is great. I've started doing a double glue joint, and the wood fails before the joint has. To do this smear some wood glue on the root of your fin, and touch it to the body tube (sanded). Smear the glue down, and let it dry. Then put another bead on, and hold in place. It grabs pretty quickly, and this joint is amazingly strong. Great choice of rocket, btw- Big Bertha was my first build with my dad when we started in this hobby back in 1985 or so. When I got back into it I made sure it was my first build with my kids too...nice tradition!
 

Larry Curcio

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Yup. Wood glue for fins and epoxy for motor mounts. (Epoxy is more heat resistant and that translates into a stronger mount.)
That's outta the way...

Would also suggest that before you glue the root edge of the fin, you make darn sure it's really the root edge.
BB fins can be confusing!
 

BEC

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HTML:
Sounds like the "gents at your local hobby shop" are more interested in selling adhesives you don't really need for this than perhaps they should be.

As Neil and others have pointed out, wood glue is more than sufficient for everything in a Big Bertha - even the motor mount. I might agree with Larry about epoxy and heat resistance in this case only if you're planning to fly lots of composite Ds in it. Otherwise, wood glue is fine there as well.

There is the possible issue of the mount seizing up before you get it into place that soopirV mentioned if you use something that grabs quickly like Titebond II, especially if the centering rings are snug in the body. For that, good old Elmer's Glue All (yes, really) or the craft glue Allene's Tacky Glue will alleviate that concern. They will also both shrink less and so you won't see where the rings are on the outside of the body after the glue cures. All of this could be an argument for sparing use of epoxy as an alternative, I suppose.

I've built a number of Berthas lately and all have had pretty firm balsa stock for the fins...so the CA is pretty much a waste of effort as well. Or at least that's my take on this.

I've flown a BB that was built only with wood glues (but with a 24mm motor mount) even on Aerotech F44s with no issues.
 

Woody's Workshop

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May I suggest a engine mount upgrade?
A 24mm will send it much higher, and will easily fit a Jolly Logic Chute Release.
And even with the upgrade, epoxy is over kill.
 

snrkl

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All: thanks for the advice - I am taking this ALL in and truly thank all of you for your inputs!! To fill in a few details:

1) translating US glue brands to AU ones left me a little concerned that I would pick the wrong one.. So, to me, epoxy being epoxy, seemed like the "I won't regret it" choice. After a bit of forum scouring after your messages, it seems that in Australia, the glue to use is Aquadhere exterior (https://www.selleys.com.au/adhesives/construction/water-based-adhesive/aquadhere-exterior/) which is a outdooor (ie: more water resistant) PVA glue. I will definitely try this for the next one.

2) As for epoxies, I have had a few "rushed" moments with the 5min epoxy (you really only get about a 30secs to a minute before it starts to gel) with the last rockets I was building - I was worried, given this was my first motor mount like this that I'd botch it. I ended up using the totally overkill super strength epoxy. (https://www.selleys.com.au/adhesives/household-adhesive/araldite/super-strength/) It gels in 2 hours and initial bond set after 8, with a 3 day full cure time. Long story short, it worked a treat for the MMT construction (yesterday) and the tube insertion (drying now) - the extra time gave me plenty of ability to get everything aligned just right and nicely filleted.. With the MMT, used a single layer of masking tape to make the CR/Motor tube fit nice and snug on the aft CR (where the engine clip hole left it a little floppy), and actually had some success with using a piece of tape behind the fwd CR to allow it to but up against it - made getting them dead straight much easier

3) I used CA to strengthen the edges of the body tube - glad I did... I laid a bead all around the inside edges and then finger spread it to about two knuckles depth..

4) After shaping the fins (rounded the leading and trailing edges) I have re-CA'd the leading edge and will CA the trailing edge later this afternoon. I definitely got this part back to front. If I end up using CA on balsa fins again, next time I will flat sand, shape, THEN CA the fins. I might try papering them - depends on what kind of finish I get on these after painting.
05_Fin_LE_CAd.jpg
5) Now that I have an idea of which wood glue type here in AU to use, I might try a more "standard build" next time with wood glue. Kits are pretty expensive here, so I might work to a scratch build next time...

6) Interestingly, the plastic nose cone that came in the kit doesn't seem to need filling. I just hand sanded it back with 400grit sandpaper to flatten the seams, then 600grit, washed and have the first coat painted and drying now.
Now that the MMT is in and drying, nose cone is being painted, fin attachment is next. Now: I totally suck at fin alignment, so I have a fin alignment JIG I have made from the
template creator at payloadbay.com. Printed, stuck on foam board and cut out with a brand spanking new exacto blade.

03_Fij_Jig.jpg

The plan for the fins is to use the dreaded 5min epoxy (I can't imagine having to wait 6-8hrs for each fin to dry) and spending another $15 on more glue to do it with wood glue right now is a PITA.

Once the BT is all dry, I am going to play with a dry fit run to figure out how I will actually glue these fins in with the jig (ie: where the hell to put it, and how I can get the fins in without getting glue everywhere.
One thought that I had was to slightly mod the fin jig I've cut out by cutting a small triangle out on the jig where the fin meets the body to allow a glue edged fin to be slid into the jig without getting glue on the jig (and ending up with the jig glued to the fin...). I am also toying with the idea of laying the small bit of epoxy on a flat tray in a thin line that I can dip the fin into. I plan to use a bit of balsa off cut to test this theory first before I commit to it.
I totally forgot to take pics of the assembled MMT before I put it in (what can I say, excitement got the better of me) but here it is drying now. One small pain of the super slow drying epoxy is that I have found that I need to rotate it every 30mins for the first 2 hours (figured this out with the MMT yesterday) to stop it running. Less of a problem with gluing the MMT into the BT - I learned how much epoxy was "too much" yesterday when I assembled the MMT.
04_MMT_drying.jpg
 

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snrkl

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May I suggest a engine mount upgrade?
A 24mm will send it much higher, and will easily fit a Jolly Logic Chute Release.
And even with the upgrade, epoxy is over kill.
Hehe.. totally an option for a future build. At the moment, the attraction of this Big Bertha is playing field launches with my kids and that means keeping it to 400ft or lower...

Maybe I'll build another with a 24mm engine tube for launch days out at Jimboomba... I have also been toying with the idea of another Bertha but modded for rear ejection with a nice large streamer - something about not having to walk kilometres to retrieve something that made it to 1500ft...

or are you talking about using a Jolly Logic to make it a dual deploy recovery?
 

snrkl

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Yup. Wood glue for fins and epoxy for motor mounts. (Epoxy is more heat resistant and that translates into a stronger mount.)
That's outta the way...

Would also suggest that before you glue the root edge of the fin, you make darn sure it's really the root edge.
BB fins can be confusing!
Yeah - I have made that discovery a few times when dry fitting it all together!!
 

BEC

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One thought that I had was to slightly mod the fin jig I've cut out by cutting a small triangle out on the jig where the fin meets the body to allow a glue edged fin to be slid into the jig without getting glue on the jig (and ending up with the jig glued to the fin...).
Yes, you will definitely want to do this! A fellow not too far from me (a three hour drive under ideal conditions) named David Qualman makes laser cut fin alignment guides of many sorts and they all have some material cut away where the fin meets the body tube for this very reason, and it works nicely.

(Note: for those curious, his products are here: https://www.qualmanrocketry.com/Fin_Guides.html I'm just a happy customer who used one of his guides to help with my most recent Big Bertha builds.)
 

snrkl

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Yes, you will definitely want to do this! A fellow not too far from me (a three hour drive under ideal conditions) named David Qualman makes laser cut fin alignment guides of many sorts and they all have some material cut away where the fin meets the body tube for this very reason, and it works nicely.
@BEC: I added the cutaways and it worked really well...

Fins on with 5min Epoxy - I think the first one had not quite enough glue - I put a little more on fins 2-4 and I was much happier with them.. Time will tell if it falls off!

One other thing - I am getting MUCH better at estimating how much epoxy to mix!! :cool:

06_Fins_gluing_in_Jig.jpg07_Fins_gluing_in_Jig.jpg08_Fins_gluing_in_Jig.jpg
 

snrkl

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So I just laid my first pair of fillets, and I am pretty damn happy with them...

09_fillet_one.jpg10_fillet_one.jpg11_fillet_one.jpg

One question I have: where the LE meets the root of the fin, the fillet starts a few mm back from the tip of the fin. I didn't notice when I was laying them, but when I pass the ice-cream stick back and forth dry on the next one, I can see why it happened - I started the fillet at the point where the ice-cream stick has enough contact to build the little fillet channel.

What do others do at this tip? Do you take the fillet up and kinda blend it into the point of the fin? Do it the same way I did?

I am keen to hear other's thoughts on the topic...
 

lcorinth

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Big Bertha was one of my earliest builds and I wrote a whole thing about it - here.

I've only done CA on the fins of one rocket, and would never do it again. I can get as good results with other things without risk of gluing my fingers together or getting a chemical burn, either with wood filler or sanding sealer. CA is too scary for me to drizzle over a rocket willy nilly.

For fillets, I've used epoxy for a few low power builds. It can make very pretty fillets, but it adds a lot of weight, unless you fill it with a lot of filler, such as microballoons. Still, it's probably overkill for a LPR rocket. I have switched to Titebond Molding and Trim glue, a low-sag, quick-drying glue that makes nice fillets if done in two or three layers, as it's a little less prone to shrinkage.

I got the tip from here (part 1) and here (part 2).

I'm sure the hobby shop gents were well-intentioned, but probably classic overbuilders. Epoxy is usually overkill for a model rocket. White glue or wood glue is all you need. Just avoid "School Glue." Nothing washable belongs on a rocket.

Bertha's a great rocket. Enjoy!
 

lcorinth

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Next step is sanding and paint prep..

Out of interest, what is the general feeling on paint+primer all in ones? I have typically used Rustoleum Paint+primer for spraying PVC - any thoughts?

https://www.rustoleum.com/DigitalEn...er-brands/painters-touch-ultra-cover-2x/gloss
Most paints I use say they're paint/primer all in one, but I always use some kind of sandable primer first. What I've landed on as my go-to is Rustoleum Automotive Filler Primer. It's a high-build primer that will help conceal any flaws (like wood grain) that your surface prep missed. Dupli-Color has another popular filler primer. Some people prefer it, but it's a little more expensive - a couple dollars a can.

DSCN3631.jpg
 

TangoJuliet

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The Big Bertha is one of my favorites in my fleet.

For the majority of my gluing needs I use Titebond II on LPR kits, and like Daniel, I've recently switched to using the "No Run - No Drip"/"Quick & Thick"/"Molding and Trim" glue (it goes by several names, but it's all the same stuff) for doing my fillets. I only use Epoxy and CA glues when absolutely necessary, either for the type of materials being joined or for strength, but for most LPR Epoxy isn't ever necessary.

+1 on the Rusto Filler/Primer. Just be sure to give it 24 hours to dry before sanding it. I've found that if I try to sand it too soon it gums up the sandpaper, even though it appears to have fully dried. Since the majority of my builds are LPR, with a smattering of MPR thrown in, I typically use Testor's rattle cans for my painting needs.
 

snrkl

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Build update for the last few days...

Tube filled and sanded - used Timbermate wood filler. From the little I used, I guess that tub is going to last me about 1000 years... I also had more dry sprinkles on the newspaper than I think I actually used on the BT!
View attachment 319911View attachment 319913

Three very light coats of primer over an hour period - good even coverage. I had to actively hold myself back to avoid laying too much on. Very happy with the first application, but the tube got a little fuzzy in places... :(

View attachment 319912View attachment 319917View attachment 319918

Fortunately, fuzz sanded out well. :) @TangoJuliet I totally see what you mean about fully drying before sanding- I only waited 20ish hrs and the sandpaper clogged quite badly on the fins! I will have to, again, restrain myself from rushing the paint job!! (frustrating!!)

View attachment 319919View attachment 319920View attachment 319921View attachment 319922

So after sanding, I wiped it all down very carefully then back for another 3 light coats of primer over an hour and now drying.. I am going to have to resist trying to sand it tomorrow - I might try to leave it an extra day till Sunday...

I spent some time tonight playing with colour schemes in OR tonight - finally came up with a design I like:

View attachment 319923

So the plan is to 600grit sand the current primer coat on Sunday (that will give it almost 48hrs to dry properly), then paint the whole thing orange, then mask the stripes and fins, then spray the blue.. I did a test run on an old baking paper cardboard tube - the masking tape worked quite well... Not sure if I will have to re-sand between orange and blue coats - time will tell how well I do on the orange...

One small thing I realised I forgot - the included launch lug is too small for any of my launch rods, and I have been meaning to get a suitably sized clear drinking straw to replace it.. seems I totally forgot all about it before I started paint work, so I will have to look at gluing it on post paint.

What are people's thoughts: glue it now while I am still on primer or after the paint job is finished?
 

neil_w

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What are people's thoughts: glue it now while I am still on primer or after the paint job is finished?
Now!!! Sand off the primer in the BT area where you will glue on the lug. The lug needs a good strong attachment, you don't want to glue it to paint if you can avoid it. You can also, if you like, prime the lug before you attach it, and (again) sand off the primer in the attachment area.

It is amazing how easy it is to forget the lug. I have to consciously remind myself multiple times during each build to remember it.

Oh, one more thing: you will likely find it easier to fill your spirals before any other assembly. Working around just the four fins on the Bertha is not too bad, but in general it's a much easier go while the tubes are still bare.
 

snrkl

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Now!!! Sand off the primer in the BT area where you will glue on the lug. The lug needs a good strong attachment, you don't want to glue it to paint if you can avoid it. You can also, if you like, prime the lug before you attach it, and (again) sand off the primer in the attachment area.
Point taken - I guess I have some more work to do tomorrow...

Oh, one more thing: you will likely find it easier to fill your spirals before any other assembly. Working around just the four fins on the Bertha is not too bad, but in general it's a much easier go while the tubes are still bare.
Yeah, I figured this one out myself.. I initially wasn't planning on filling them at all but I decided after the fins were on that I cared about the tube finish...

Out of interest, the glossy layer on the BT - leave it intact (except for under the fins) or sand off completely? I think the fuzzing I got was from where the tube was sanded past the glossy surface coat...
 

neil_w

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Out of interest, the glossy layer on the BT - leave it intact (except for under the fins) or sand off completely? I think the fuzzing I got was from where the tube was sanded past the glossy surface coat...
In general, I try to stop sanding once I get to the glassine layer. I've roughed it up but never gotten big fuzzies. Some folks are religious about removing it underneath all glue joints (peeling is more precise than sanding for this purpose). I've done it once, and might do it more in the future, since it's so easy. But I don't think there's much point in removing the glassine layer from the entire BT.
 

TangoJuliet

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I don't sand the glassine off before attaching fins. Never had a glue joint fail. But I would certainly attach your Launch Lug now, prior to painting. And as suggested, sand off the primer for a good glue joint.
 

neil_w

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I don't sand the glassine off before attaching fins. Never had a glue joint fail.
I had one. My Elliptic II popped a fin on landing, and it looked like the glue (including the fillet!) separated cleanly from the BT at the aft edge, where it sustained the largest force from landing. Further up the fin, the joint held, and it simply pulled the top layer of BT off with the fin.

When I did remove the glassine layer that one time, the glue joint really seemed to grab strongly and quickly. Could have been my imagination but I thought I could perceive a noticeable difference.

I think am somewhat inclined to remove the glassine in the future in situations where I can foresee the need for a bit of extra strength. In hindsight, the Elliptic II sustainer fins are a prime candidate, with a short root and large span, and only 1/16" thick.
 

Rex R

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TB original wood glue works for me. Estes has (a time or two) suggested using 600 grit to dull the glassine for the fin joint area.
Rex
 
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