Building a Boosted Bertha.. and a question.

Recently got a Boosted Bertha kit and like it very much.

I am completely familiar with Big Bertha having built at least 3 of them. But there are some differences.

First of all the booster stage. It has through the wall fins and centering rings with baffle holes in them. The rocket is gap staged! Never have done a gap staged rocket before this is a cool 'feature'.

The launch lug location is different from the BB as it is down on one of the fins of the sustainer.

Engine retainers are the plastic thread on type in lieu of the engine hook. Which is great!

One question though. The instructions indicate to use plastic weld cement to attach the engine retainers on to the motor tubes. Would not epoxy be a better choice for this?

Also wont the plastic retainer in the sustainer get melted when the booster section burns through?

Thanks!

Tom said:

Recently got a Boosted Bertha kit and like it very much.

I am completely familiar with Big Bertha having built at least 3 of them. But there are some differences.

First of all the booster stage. It has through the wall fins and centering rings with baffle holes in them. The rocket is gap staged! Never have done a gap staged rocket before this is a cool 'feature'.

The launch lug location is different from the BB as it is down on one of the fins of the sustainer.

Engine retainers are the plastic thread on type in lieu of the engine hook. Which is great!

One question though. The instructions indicate to use plastic weld cement to attach the engine retainers on to the motor tubes. Would not epoxy be a better choice for this?

Also wont the plastic retainer in the sustainer get melted when the booster section burns through?

Thanks!

Click to expand...

I typically use epoxy for plastic engine retainers, either Estes or home-grown from bottle caps/plumbing parts. To join other plastic to paper tube I use Beacon Fabri-Tac - it has a high acetone content like plastic weld cement so I suppose it would work for Estes retainers.

As far as the retainer getting “cooked” with use somewhere on the interwebs - here, YORF, or FB, I don’t recall which - someone showed pix of their retainer after multiple launches and it looked fine.

I have 13 flights on a Boosted Bertha now.

Others than adding a Semroc cup-style baffle (https://www.erockets.biz/semroc-ejection-baffle-kit-conical-fiber-bt-60-sem-eb-60/) I built it as directed.

After the second or third flight each of the motor retainers popped loose (I had used the Testors liquid plastic cement in the rectangular bottle to retain them). Since I was at a meet, I did a field repair and put them back on with medium CA. They have held since then. When I build another of these I will use either a Beacon glue or epoxy to attach the retainers.

The ring portion of the sustainer’s retainer has gotten cooked, yes, but it is still functional. Around the 4th or 5th flight it got warped a little so that it doesn’t thread on as smoothly as it did originally, but it is still working fine.

On the 13th flight the sustainer’s motor mount blew out. It had been a little soft feeling for awhile. When it came out it was clear the upper centering ring-to-body joint didn’t get enough glue. It took some doing to get the retainer off the motor tube as I prepared to build a new mount. I may get to that in the next couple of days. I will use Beacon Foam-Tac (since I have it) to put the threaded portion of the retainer on the new motor tube.

It’s a neat model.

One thing you will want to watch out for is the fit between the booster and the sustainer. For about the first 8 flights you’ll be sanding either the base of the sustainer or the exposed portion of the stage coupler or both in order to keep the stage-to-stage fit from getting excessively tight from the residue that gets on the parts from staging.

I used epoxy for the retainers. The lug is up against a fin instead of centered because the fins don't 'nest'. Since the boosters fins land in the path of a centered lug, they probably decided it was easier for the kids to put it up against the fin rather than off-entering it by 1/8th circumference

Putting it there is also a more appropriate location for the two-stage stack than in the usual position.

PhlAsh said:

I used epoxy for the retainers. The lug is up against a fin instead of centered because the fins don't 'nest'. Since the boosters fins land in the path of a centered lug, they probably decided it was easier for the kids to put it up against the fin rather than off-entering it by 1/8th circumference

Click to expand...

For scratch builds (and non scale kits) I almost always put lug against a fin-body tube joint. ASSUMING the fin is on straight, the lug is automatically aligned with the long axis of the rocket, it makes the body tube-lug joint stronger AND the fin-body tube joint as well!

For a kit these heavy, you should use micro rail buttons instead of lugs.

BABAR said:

For scratch builds (and non scale kits) I almost always put lug against a fin-body tube joint. ASSUMING the fin is on straight, the lug is automatically aligned with the long axis of the rocket, it makes the body tube-lug joint stronger AND the fin-body tube joint as well!

Click to expand...

It seems Estes is going back to placing the lug in the fin/tube joint. Several of the newer kits I've built recently have the lug in the joint instead of centered between the fins.

JB Weld is best for heat related applications Epoxy for the centering rings will hold them better.

Used Wood Glue for the centering rings, JB Weld on the retainers.