I need Baby Bertha stability help

I just built a 3" Baby Bertha and I'm trying to make sure I get the CG correct without adding a ton of weight to the nose. what's odd is my estes baby bertha has a CG barely in front of the CP but seems to fly well. is this typical for baby berthas? any chance you could load in a motor and see where yours actually balances for me? mine balances on the fins 3/4" back from to front tip of the fins.


here is my build on the MPR page https://www.rocketryforum.com/showthread.php?69331-3-quot-bertha-nose-cone-is-now-a-build-thread

When I first launched my Baby Bertha, I was surprised that the thing leaned over and flew off at a diagonal. I questioned this, and was told this was quite normal for Baby Bertha, as it has such a large fin area...that any wind would make it wind-cock into the wind very noticably. I relaxed and launched it on a more calm day, and found it went straight up and true. But then I moved onto other rockets, and never got back to my Baby Bertha.

I don't know if this helps you any, but I could try balancing it once loaded up at home tonight. Check back here for a report, OK? (I'm at work now... on dinner break.)

That would be great, I'm curious to see if other's balance around the same point as mine. i tend to over build and use lots of paint.

How hard would it be to add a bit of noseweight and fly again? It wouldn't have to be a permanent change, just crimp on a split shot fishing weight to the shock cord at the nose cone. You probably wouldn't need much; my 29mm BabyB uses 0.75 oz. And, as noted, the large, swept fins *will* cause weathercocking.

My moderately weather-worn (as in, it spent some time in a tree and got rained on) BB balances about 1/2 inch back from where the fins' LEs intersect the body tube with an Estes C6-5 loaded in. It has a Semroc baffle inside so the 'chute is squeezed a bit to fit in the remaining space below the NC.

Riley I would recommend simm'ing it in Rocksim/Open Rocket to determine the CG/CP for your design.
The other tried and true alternative is to swing test it, which is doable as it is not that big...

Greg Young said:

Riley I would recommend simm'ing it in Rocksim/Open Rocket to determine the CG/CP for your design.
The other tried and true alternative is to swing test it, which is doable as it is not that big...

Click to expand...

I have it in open rocket but I'm not confident in the CP it is calculating. here are a couple screen shots from rocketreview. the 1st one has the CP right about where my open rocket sim has it. that is almost exactly where my baby bertha's CG is and it flies well. the 2nd one shows it further back, which I wonder if is more accurate since the estes kit seems to fly well. I dont have rocksim so I cannot try comparing the rocksim method vs barrowman.

The 2nd version, was the dual motor mount intentional?

Also, You need to have motors loaded to accurately show the CG. It shifts back a lot when you load a motor.

I'm not at home so I just found these images on rocketreview. I wasn't concerned with the shown CG, but the difference in CP. by the way, the paper cutout method puts the CP right at the first image shown.

There's something very strange going on with the 2nd model, almost double the weight too. I'd honestly disregard it completely.

It looks to have a motor cluster but that doesn't explain why the CP is so far back. here is where my sim is. the CG is at about the same position as the original baby bertha, which gives a stability of only 0.4

Not sure what's up with the CP in your image either. I did find the open rocket file for that 'baby ranger', and the CP is identical to the baby bertha.
Odd, very odd.

with short fat rockets, what is typical for stability numbers? with a "normal" rocket I know 1 caliber is considered the minimum for stability, but with a short rocket like the baby bertha 1 caliber is a huge % of it's length.

Kirk G said:

When I first launched my Baby Bertha, I was surprised that the thing leaned over and flew off at a diagonal. I questioned this, and was told this was quite normal for Baby Bertha, as it has such a large fin area...that any wind would make it wind-cock into the wind very noticably. I relaxed and launched it on a more calm day, and found it went straight up and true. But then I moved onto other rockets, and never got back to my Baby Bertha.

Click to expand...

I had the same problem with my daughter's Baby Bertha. First flight sent us scattering. Second flight was better - straightened out shortly after leaving the pad. Probably should check the CG.

Paul said:

I had the same problem with my daughter's Baby Bertha. First flight sent us scattering. Second flight was better - straightened out shortly after leaving the pad. Probably should check the CG.

Click to expand...

In my instance, a local dealer was at the launch and remarked that the flight was not that odd, considering we had a steady breeze blowing. So I had an expert assure me that it was normal to do that.

After several days delay, I got around to taking out my Baby Bertha. I loaded a B6-4 into the motor mount, and after peaking to assure the parachute was rolled up, I assume recover wadding is below it.

If I understand how to do this correctly, I took a ruler with a sharp edge, and used it as the fulcrum, or balance point and attempted to balance the Baby Bertha as it laid down. I found it balanced about a quarter of an inch above where the fins began. However, when I rolled the rocket 90 degrees, it then balanced exactly at the starting point for the fins. Trying a third position, I rolled the rocket another 90 degrees, so that it was on the launch lug, and I was unable to get the rocket to balance. That is, the point of balance would have been behind or lower than where the fins began.

So I'm not certain what that tells you. I supposed that if I packed it with a bit more dog barf, I could move that balance point a little farther up the tube, but not by much.

thanks for the help. We had a great weekend flying. sadly we left the estes baby bertha in a tree but the upscale 3" baby bertha had two perfect flights and came home in 1 piece! I set the CG about 1/2 back from where the leading edge of the fins meets the body. check out the flight photos in the MPR section https://www.rocketryforum.com/showthread.php?69331-3-quot-bertha-nose-cone-is-now-a-build-thread