Big Bertha Engine tube variation, stuffer tubes, and ejection pressure

[ZoomieG]

The other day I opened a new Big Bertha kit (#1948), and noticed it had a standard engine tube. I had seen online plans/instructions, and thought for sure BB was supposed to have a stuffer tube. I took a look at an older kit (black/white Beta series #1948), and it did have a BT-20B stuffer/engine tube.

Not content to stop there, I looked for plans online, and found the following:

Before it was a kit, BB was a free plan in Model Rocket News in 1963 (MRNv03n02). In that plan BB has a standard 2.75" MMT, with a coupler between the centering rings, and no engine hook.

Big Bertha was released as a kit in 1965 (K-23). At some point, the engine tube was changed to a BT-20B (8.65") Motor Mount/stuffer tube. This kit changed to #1223 when the K number system was retired.

In 1985, Big Bertha was revamped, with a new model number (#1948), and a new look. It seems the stuffer tube was maintained in the update.

When Estes moved builder's kits to the Beta Series in 1993, the look again changed (B/W scheme), but the engine config still contained a stuffer tube.

For 2012, the BB reverted back to the 1985 Black and Yellow scheme. At that point, the BT-20B stuffer tube was replaced with a standard BT-20J (2.75") engine mount.

With the background laid out, this brings me to my two questions, which is the point of this post:

1 - I realize that changes to long running kits happen over time, for various reasons. Nose cones can change with balsa availablility, or a mold being damaged/worn out. Fins have moved from templates, to die-cut, to laser cut. However, can anyone shed any light in particular on why BB has switched back and forth from using a stuffer tube style mount?

2 - Does anyone have a good calculation method for pressure volume to determine when a stuffer tube is warranted? BB has had one for much of it's life, but apparently it isn't necessary. Indeed, many other 18mm, 18" BT-60 kits never had one (Centuri Centurion, Fliskits Rhino, Semroc Vega, Estes Raven/ESAM-58/Dark Star/Sentinel). I could assume that it was determined unnecessary for the volume and removed to save a bit of weight, but then Estes re-issued Citation Patriot in 2018, with a BT-20B stuffer tube just like the original.

Thanks.

 

[Rob Campbell]

The other day I opened a new Big Bertha kit (#1948), and noticed it had a standard engine tube. I had seen online plans/instructions, and thought for sure BB was supposed to have a stuffer tube. I took a look at an older kit (black/white Beta series #1948), and it did have a BT-20B stuffer/engine tube.

Not content to stop there, I looked for plans online, and found the following:

Before it was a kit, BB was a free plan in Model Rocket News in 1963 (MRNv03n02). In that plan BB has a standard 2.75" MMT, with a coupler between the centering rings, and no engine hook.

Big Bertha was released as a kit in 1965 (K-23). At some point, the engine tube was changed to a BT-20B (8.65") Motor Mount/stuffer tube. This kit changed to #1223 when the K number system was retired.

In 1985, Big Bertha was revamped, with a new model number (#1948), and a new look. It seems the stuffer tube was maintained in the update.

When Estes moved builder's kits to the Beta Series in 1993, the look again changed (B/W scheme), but the engine config still contained a stuffer tube.

For 2012, the BB reverted back to the 1985 Black and Yellow scheme. At that point, the BT-20B stuffer tube was replaced with a standard BT-20J (2.75") engine mount.

With the background laid out, this brings me to my two questions, which is the point of this post:

1 - I realize that changes to long running kits happen over time, for various reasons. Nose cones can change with balsa availablility, or a mold being damaged/worn out. Fins have moved from templates, to die-cut, to laser cut. However, can anyone shed any light in particular on why BB has switched back and forth from using a stuffer tube style mount?

2 - Does anyone have a good calculation method for pressure volume to determine when a stuffer tube is warranted? BB has had one for much of it's life, but apparently it isn't necessary. Indeed, many other 18mm, 18" BT-60 kits never had one (Centuri Centurion, Fliskits Rhino, Semroc Vega, Estes Raven/ESAM-58/Dark Star/Sentinel). I could assume that it was determined unnecessary for the volume and removed to save a bit of weight, but then Estes re-issued Citation Patriot in 2018, with a BT-20B stuffer tube just like the original.

Thanks.

It may also be a cost savings move. Shorter motor tubes mean more motor mounts from a single full length BT-20 tube.

 

[Back_at_it]

As a huge fan of the Bertha style rockets, I noticed this inconsistency as well. I have built and flown a half dozen Big Bertha's and the only one that ever gave me a bit of a problem was the one with the standard motor mount. Granted, It flew twice successfully before the first failure to eject. Thankfully it landed in tall weeds and didn't hurt anything. On the very next flight I had a second failure where it came down and crunched about half the tube. During the repair I added about 8 inches of stuffer tube and have flown that rocket countless times since without an issue.

If you want my advice, Built it with a 24mm motor mount. Make the BT50 tube about 10 inches long and don't look back. This is still plenty light enough for B6-2 and C6-5 launches and you have the option of C11 and D12 motors. Not to mention all of the composites out there.

 

[BABAR]

As a huge fan of the Bertha style rockets, I noticed this inconsistency as well. I have built and flown a half dozen Big Bertha's and the only one that ever gave me a bit of a problem was the one with the standard motor mount. Granted, It flew twice successfully before the first failure to eject. Thankfully it landed in tall weeds and didn't hurt anything. On the very next flight I had a second failure where it came down and crunched about half the tube. During the repair I added about 8 inches of stuffer tube and have flown that rocket countless times since without an issue.

If you want my advice, Built it with a 24mm motor mount. Make the BT50 tube about 10 inches long and don't look back. This is still plenty light enough for B6-2 and C6-5 launches and you have the option of C11 and D12 motors. Not to mention all of the composites out there.

As the High Power people like to say, with a given installed motor mount, you can always adapt DOWN to smaller diameter or shorter motors. Going UP is not an option, it least not an easy one.

 

[Paul Howard]

Good Question and Post! I'm now getting in the habit of putting Ejection Baffles in my rockets as I build them for safety reasons to not have burning embered recovery wadding float down and start fires. Apogee Rockets has some kit of a V-2 that has a combination Stuffer-Tube/Ejection-Baffle coming from the tail-cone that is making me think about doing something similar on rockets like the Big Bertha (I have Boosted Bertha sitting on a shelf waiting to be built).

 

[bjphoenix]

There's nothing special about the BB, these days Estes makes a lot of BT-60 kits of similar size without stuffer tubes. I've not seen ejection problems with them. Maybe the BB tube is just that much longer to cause a problem?

 

[rharshberger]

Good Question and Post! I'm now getting in the habit of putting Ejection Baffles in my rockets as I build them for safety reasons to not have burning embered recovery wadding float down and start fires. Apogee Rockets has some kit of a V-2 that has a combination Stuffer-Tube/Ejection-Baffle coming from the tail-cone that is making me think about doing something similar on rockets like the Big Bertha (I have Boosted Bertha sitting on a shelf waiting to be built).

If you are using flame retardant recovery wadding/dog barf (which you should be...) there are no burning embers to fall into the dry fuel on the ground. All Estes and Quest recovery wadding are flame retardant as is dog barf (cellulose insulation is FR by law).

 

[Paul Howard]

Yes, I use both Estes Recovery Wadding and the "Dog Barf" variety. I have had Estes Recovery Wadding burn an active ember ALL the way to the ground on several occasions in the last year, and that's why I'm putting ejection baffles in my rockets as I build them now. I either scratch build my own from toilet paper tube or couplers and cut my own lite=ply baffles or use the ones from Apogee Rockets (and experiment with modifications) and make sure I put a thin layer of epoxy on the "blast" side plus make them so I can shake the chunks out. With the fires in Oregon and elsewhere in the past, I'd rather do overkill with the ejection baffle additions.

 

[brockrwood]

I am no expert on ejection gasses, but my experience has been that the Estes “shotgun” ejection charge, even from an 18mm motor, is plenty powerful to push out the nose cone and the laundry, even with bigger diameter tubes. I have never flown the Big Bertha, however.

 

[MidOH]

Bt60's are generally more reliable poppers, than smaller Estes kits. (more surface area for pressure to exert force upon)

I've been using baffles in all of the longer ones now. Less litter. Possibly even zero if using a scrap of reusable wadding over the baffle.

 

[BEC]

I am no expert on ejection gasses, but my experience has been that the Estes “shotgun” ejection charge, even from an 18mm motor, is plenty powerful to push out the nose cone and the laundry, even with bigger diameter tubes. I have never flown the Big Bertha, however.

My wife and I have had mixed results with Big Berthas. Hers, at NSL in Alamogordo NM a few years ago, had a series of several failures to deploy that were not traceable to any error in packing the ‘chute or wadding. That one finally wound up being damaged enough that she built another. This latter one has been fine so far, but I cut an equivalent of a BT-20B to replace the motor mount tube, so it’s configured with a stuffer tube like those versions of the Big Bertha kit that had them.

On the other hand I’ve had baffle-equipped Berthas blow motor mounts rather than parachutes out on a couple of occasions, especially with Q-Jets. Either way, it’s not pretty unless you’re flying over a very soft surface.