D powered Big Bertha?

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Mini 14

Active Member
Nov 30, 2004
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Hey all,

Just wondering if anyone here has modified an Estes Big Bertha to take a D engine. I'm going to end up doing it, if for any reason that the first time I launched it decided to wait until it was on the ground before the parachute deployed. Besides, it will be cool.
If the chute popped out on the ground you must have used an A83 engine or something. A c6-5 is quite impressive in this rocket.
I'm building a big bertha right now so this thread caught my attention. You had better make sure that you have a large launch area, cause that bertha's gonna go darn high!!!!!!! You might want to consider going to a smaller chute or at LEAST cut out the spill hole in the 18" chute. I'm building mine stock cause I dont really have a huge launch area, and like the other guy said, the berth is pretty awesome on a C6-5. good luck-keep us posted.
Brandon wrote:

> Just wondering if anyone here has modified an
> Estes Big Bertha to take a D engine.

Hi, Brandon,

I think this is a pretty common upgrade. I
would make the motor tube long like the
standard Bertha's MMT (ie, make it a stuffer
tube). Also, leave out the engine block and
use tape to make an aft thrust ring on the
motor. This will let you fly longer 24mm
motors (Estes E's, AT F's).

I built my last one with interchangeable mounts.

Like others have said, it really gets up on
a D. I've flown my on the E9. It was a long
walk, but stayed in sight. Haven't had the
guts to use the F32 I have stashed.

I fly my Big Bertha (a survivor of my first rocketry days) on C6-5 or B6-4 motors. It's great on either. The flight is just the right level of stability. It rides up like it's on rails, and it doesn't tend to weathercock overly much.

Doing some quick and dirty measurements, my Big Bertha is approximately 23 1/2" tall with a center of gravity as follows.
Empty: 11" from the nose tip
Loaded with a B6-4: 12 3/8" from the nose tip
Loaded with a C6-5: 12 3/4" from the nose tip

If you decide to go with a mount for D motors (24mm), just install it as you would for a mount for A-B-C- motors (18mm), placing the rear end of the mount in the same position as the stock directions specify. [On mine, the rear of the motor mount tube is flush with the rear of the body tube, and the rear centering ring is recessed about 1/4".] Then add nose weight (if necessary) to bring the center of gravity back up to about 12 1/2" from the nose. The Bertha has plenty of extra room in the body tube for the extra thickness (and length? - See below.) of the 24mm motor mount. You'll need new or modified centering rings, of course.

While you're at it, why not build it to the length for E motors (assming you have the space to fly a Bertha on an E or the eyes to track it)? Then cut a short piece of a spent 24mm motor as an E-to-D adapter (Just slip it into the motor mount before the D motor.), and make or buy a 24mm-to-18mm adapter. Then you can fly that baby on anything from a B to an E! I love rockets with wide motor ranges.

Be careful to weigh the rocket when finished to make sure the B6 can still loft it safely, and pay special attention to the fin attachments if you want to fly Es. Glue rivets come to mind, and proper tube preparation is important. Do a double glue joint. If you're really ambitious (for a Bertha, that is), cut your own fins and include tabs for through-the-wall attachment.

Micro: Will a Bertha fly high enough to recover properly on an A8? I can see it better with a pair of As.
I made a scratch Bertha with a standard Estes D motor mount. It flies very well on C11 and D12 motors. I use a 12 inch chute on both of my Berthas. I have high grass in my field and too many trees all around. The rocket is fine on a 12 inch chute.
Originally posted by Micromeister
I made one with a changable motor insert so it can fly on any motor A8-E15 at the time:)

I'm actually getting one delivered soon and the thought of being able to go either route is intruiging, any tips on how to make an 18/24 mount?
The Big Bertha works great on an E9! I'd definitely recommend it.

To avoid those long walks, I'm working on a BT-60 based dual deployment rocket now...

-- Jim
With regards to getting it back you should pay attention to how the wind and breeze behaves in your area. I learned this the hard way when flying rc planes. I have a decent sized area to launch in but I have hazards in and around me. If I have any breeze at all I will launch the rocket with enough of an offset from center angle to carry it outside of my launch area but into the wind. This has always made it drift back down into my launch area. It can be nerve wracking because it's way the hell outside of where I am but it always comes back. Sometimes I think it's almost better if I do have a slight breeze from a given direction that is steady. This way I can launch it with angle towards that direction and be pretty certain where it will come down. I have lost 3 rockets. Two were found but unrecoverable(trees), one was taken by the wind (a heatseeker) and was never seen again.
I built my Big Bertha with epoxy in order to fly it on Aerotech 18mm D21-7s. Man, it really screams!!

I also have an assortment of parachutes along to ensure that I have the right size for the particular range obstcls (RETs) and the wind conditions.

You also might want to check out the Semroc Goliath too. It's extremely similar in size/shape/weight to the Estes Big Bertha except that it comes kitted so that the builder has a choice of either one 18mm MMT, one 24mm MMT, or 3x18mm clustered MMT. (I built one & went with option #3 :D ) The motor hook is for 2.75" long motors but I'm sure it could be easily modified for the longer Estes E motors... Another cool feature about the Goliath is that it has a payload section (instead of the long motor tube of the BB).

FWIW, Semroc claims 1300+ft on a trio of C6-7s... I'm flown mine with those motors & I can believe it!
I just got this idea from Thrustlines new up-dated site: You take a spent 24mm casing and hollow it out. An 18mm motor will fit right into the hollowed out casing! What a great idea! You may have to friction fit the 18mm motor with a little masking tape.
1/2 a's also fit right into 18mm cardboard casings


also did anyone say it? nose weight to counter the heavier motor...
Originally posted by Rob Fisher
You take a spent 24mm casing and hollow it out. An 18mm motor will fit right into the hollowed out casing!

Yes, this is a handy way to create a quickee adapter. It's been around for a long time and lots of folks have used it successfully.

Be sure to re-check your stability margins when you add the extra weight of a dead D motor case to the back end of your rocket.

It takes a little more time and effort, but it is much more safe to make an adapter. It only takes a bit of BT20, a new thrust block to fit inside the BT20, and a few 20/50 CRs. Much lighter (better rocket performance!)
Originally posted by powderburner
It takes a little more time and effort, but it is much more safe to make an adapter. It only takes a bit of BT20, a new thrust block to fit inside the BT20, and a few 20/50 CRs. Much lighter (better rocket performance!)

This is what he's talking about.

A couple of shots...I love it...It screams...
Got another one and I'll do the same...!!!
Bought the 24mm Fliskits mount form BRS last night as well as the 18MM adaptor. I'm all set =)
Originally posted by BobH48
This is what he's talking about.

I make those as rainy day projects since I've converted a lot of 18mm birds to 24mm. I make a lot of these because they don't last long. The 18mm motor is 2.75", and the mount is 3.75" to fit an E hook. That means 1" of the tube, which includes the top centering ring, is exposed to ejection. A BT20 will burn through or at least weaken, and continued thrust on the weak tube will make it start to buckle.

I've made them last longer by painting the inside of the tube with high-heat stove paint, and by using an inch of BT20 coupler as a combination engine block/tube protector.
What I do with that is I use a standard (2.88") adapter plus the estes orange spacer included with the E mounts. That way, it doesn't burn through as quickly. Also, I would like to see one of these on a 24mm AT G55.
Just finished the build on my D powered Bertha (with am 18MM adaptor built for it too). Got the primer on, and now have to start thinking about paint schemes. Might stick to the all black except near the top and nose cone which I might mask out for a black and white roll pattern. Can't wait for warm weather again.
While I don't have a Big Bertha,(gasp :eek: :eek: ), by converting it to 24mm that would enable one to use 24mm AT F21-4. Now that would be an awesome flight. Just my 2 cents.
I put a 38mm in mine....soaked the fins in epoxy, JB Welded them on, double-walled the airframe, and threw on a streamer. Flew it on a G69 (approx 4000') and an Ellis G35 (approx 5000'). Lost it on the Ellis. Needless to say, and as these guys have shown, a D will work fine in this rocket.
Originally posted by Lugnut
Just finished the build on my D powered Bertha (with am 18MM adaptor built for it too).

I'd like to do it this way also. Do you, by chance, have pics of the rocket and the adaptor?

Would like to try a baffle kit in one too?

My Baby Bertha flies pretty well on a B, C!

Ask and ye shall receive. You'll note the placement of the rings on the adaptor, two to position it in the tube and one to act as a retainer for the motor clip. I plan on tape friction fit for Cs in the adaptor and clips for D's.

I got the D motor mount and the BT-50 adaptor at the BRS Hobbies store on Ebay, he usually sells a lot of that stuff. I believe the BT-50 adaptor is a Fliskits piece. Both were cheap. Including the cost of the Bertha on a separate Ebay uction, the whole thing came to $11.50 plus freight. Not a bad deal.

With adaptor in place.

PS don't use Gorilla glue on engine mounts. Uugh what a mess.
That's the way I'll do it - an 18mm adaptor to a 24mm motor mount. I'll also try a baffle kit to cut down on wadding.

Now I need to decide what color scheme? I'm leaning toward the yellow, black and white look of years past.

Thanks again for the pics - it helps alot!