Baby Bertha stability

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EXPjawa

EXPjawa

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OK, decided to create a Rocksim file for the Baby Bertha, mostly to support moving to upscaling the design a bit. But I wanted to have the standard "base" design right first.

The trouble is, being a short, squatty design, I can't seem to get the BB to have anything better than marginal stability with an 18mm engine in it. Some of the posts I'd read when I searched it indicated that Baby Bertha was inherently overstable, but I frankly can't see how. I've got the sim model made up as near as I can tell correct, per the Baby Bertha instruction sheet and part list. I've checked the sizes and materials of all the parts, so the calculated masses should be correct, though I don't have an actual Baby Bertha on hand to weigh actual parts. One assumption I did make was to pull in the fins from the Big Bertha sim model, but maybe they're not the same? Related to that, Rocksim seems to think that it should be about 1/4oz heavier than Estes claims, so something is inconsistent here. Here's the model as I have it:
Baby Bertha 1261.jpg

Assumptions: Rocksim equations used for 2D drawings & simulation, static reference dimension is set for max frontal diameter, PNC-60MS has a wall thickness of .0425", so nose cone mass is .3908oz. It has a 0.72 margin with an A8-3, and a 0.61 with a C6-5. By all accounts, it looks it should have nose weight added, but the kit comes with none. What am I missing here? Does Baby Bertha just inherently have marginal stability, and that's the way Estes released the design? Thoughts?
 

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EXPjawa said:
OK, decided to create a Rocksim file for the Baby Bertha, mostly to support moving to upscaling the design a bit. But I wanted to have the standard "base" design right first.

The trouble is, being a short, squatty design, I can't seem to get the BB to have anything better than marginal stability with an 18mm engine in it. Some of the posts I'd read when I searched it indicated that Baby Bertha was inherently overstable, but I frankly can't see how. I've got the sim model made up as near as I can tell correct, per the Baby Bertha instruction sheet and part list. I've checked the sizes and materials of all the parts, so the calculated masses should be correct, though I don't have an actual Baby Bertha on hand to weigh actual parts. One assumption I did make was to pull in the fins from the Big Bertha sim model, but maybe they're not the same? Related to that, Rocksim seems to think that it should be about 1/4oz heavier than Estes claims, so something is inconsistent here. Here's the model as I have it:
View attachment 258958

Assumptions: Rocksim equations used for 2D drawings & simulation, static reference dimension is set for max frontal diameter, PNC-60MS has a wall thickness of .0425", so nose cone mass is .3908oz. It has a 0.72 margin with an A8-3, and a 0.61 with a C6-5. By all accounts, it looks it should have nose weight added, but the kit comes with none. What am I missing here? Does Baby Bertha just inherently have marginal stability, and that's the way Estes released the design? Thoughts?
Click to expand...

The Baby Bertha file on the RocketReviews.com RockSim library page looks even more marginal. I don't know enough to explain it, but it is strange.
 
From several comments on this board, stability rules don't really work for short, stubby rockets; on designs like the Baby Bertha, marginal stability, even less than 1 caliber, is just fine.

From my own experience, the Baby Bertha flies just fine as designed without nose weight.

Edit: It would be an interesting science project to build a set of identical rockets with different lengths, and figure out how the minimum stability changes with aspect ratio.
 
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It's like the LOC Mini Magg, or Estes Big Daddy in that it can fly with marginal stability. These short rockets are stable, although they can go off course in wind. Adding nose weight can help but may not be needed depending on motors.
 
So, then how would I know when I'm dealing with something exempt from normal rules? Is there an established cut off for, say, length to diameter ratio that is known to be safe? At what point is what stability margin acceptable?
 
I'm thinking there was an article about this in Apogee's newsletter a few years back. It explained the inherent stability of short squat rockets as something to do with base drag. I don't remember which issue it was in, but a quick scan of the issue contents should mention it - it was the lead article in that issue. Just go to Apogee's website, click on the newsletter link and you should be able to find it.
 
I’m guessing that those large trailing fins have something to do with the stability of the BB.
Kind of like the stick behind a sky-rocket.
Or is this what people are referring to when they say “Base Drag”.

Make the BB’s fins stick straight out and I’d bet you’d lose your stability.
 
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I've flown a Baby with a 4x13mm motor cluster (more weight toward the tail) and 3 fins (vs 4 in the 'stock' design) and it flew fine.
 
boomtube-mk2 said:
I’m guessing that those large trailing fins have something to do with the stability of the BB.
Kind of like the stick behind a sky-rocket.
Or is this what people are referring to when they say “Base Drag”.

Make the BB’s fins stick straight out and I’d bet you’d lose your stability.
Click to expand...


Yes, the fins sticking back past the rear of the model help to move the CP aft, but that is included in the CP calculations. Base drag is a separate thing that also contributes to the stability of this model but isn't accounted for in the calculations. That is what we are talking about when be me thin that, according to the simulations or calculations the BB is only marginally stable (less than one body diameter difference between CG and CP). A short stubby rocket, because the base is so broad compared to its length, acts like there is an extra cone behind the rocket that moves the CP further aft. The size of this "cone" depends on the diameter and length of the model and is what is described in the Apogee article.
 
RKeller said:
I went through the same thing building my 3" baby bertha. I have a open rocket file for the 3" one, how big are you going?
https://www.rocketryforum.com/showthread.php?69826-I-need-Baby-Bertha-stability-help
https://www.rocketryforum.com/showthread.php?69331-3-quot-bertha-nose-cone-is-now-a-build-thread

are you an URRG member?
https://forum.urrg.org/index.php?topic=26.0
Click to expand...

I am, but am a new member. At the moment, its more of an elementary issue; I'm not going to build it immediately, so I don't have any firm plans as to how big it would be. But I started down this path while looking for a way to use up some segments of BT101 tube that I have.

gpoehlein said:
I've attached issue 86 of the Apogee newsletter for those who are interested in reading the article.
Click to expand...
Thanks for posting that, though it seems to say that short rockets follow the same rules as long rockets, and short rockets often have to fly with less margin simply because they're too short to change that. Its just that sometimes those rules dictate that the CP shifts aft dynamically rather than forward, depending AOA if the fins are large enough. It still doesn't give much to predict where the transition is from one behavior to another. I think you can see it in Rocksim's Cd analysis, but I'm not sure that's enough to conclude anything solid...
 
I read an article somewhere that described a method that you could use to simulate the added stabilizing effect of base drag on models like the Mini Magg. It was something I was interested in since I overbuilt the heck out of my Mini Magg. Indeed, it's been my CAR L1, L2 and L3 rocket. NAR/TRA L2 flight was on a CTI J300. It was a beautiful flight. The thing is, I built this WAY overstable. It's embarrasing to say how much nose weight is in that nose cone. My first attempt at my L1 flight had a nose cone separation. It plumetted from 1000+ feet and imbedded itself in the ground after going through ice. The nose cone is perfectly intact and went on to fly again with a new attachment point.

Anyway, I cannot find the source of the article but based on what was written, I modified my Mini Magg Rocksim file to have the simulation nose cone added. I have no idea how I worked out the length or shape of the cone but here is the image exported from the RockSim file. If anyone can source the article that talks about how to perform this simulation. please post it here. Thanks!

Lens Mini Magg.jpg
 
The Estes Vagabond has a 8" and a 18" BT and the Baby Bertha has a 7 1/2" body tube. I would like to swap out the 7 1/2 for the 8 for my Baby Bertha. Trying to cram recovery wadding, longer shock cord, and parachute in the short tube took rocket science to fit just right. The first time I flew one, the parachute was packed so tight that it didn't open. Another time I flew one, the hot gases were so close to the parachute that is burned a chute line and making it streamer recovery.

The extra 1/2 inch BT and shaving off 1/2 inch from the long inside point of the nose cone would give me a whopping of 1 extra whole inch.

I want to convert it to cluster 3 B class engines for the NARTREK Silver. Any suggestions would be helpful.
 
The rule of thumb for simulating short rockets, those with a height less than 1:10 (iow, 4"dia:40"tall), is to add a 0gm (no mass) cone to the trailing end of the rocket. The cone should have a ratio of 1:3 (4"dia:12"length) where the major diameter is the diameter of the rocket. (More precisely it is 1: pi, but 1:3 is easier) The minor diameter is always 0 or as small as the sim program allows

This was discussed in Apogee Newsletter 154 ( https://www.apogeerockets.com/education/downloads/Newsletter154.pdf )
 
Simulate angles of attack above zero and watch what happens to the stability (generally decreases as CP moves forward at higher AOA). As long as it stays positive thru a plausible range of AOA values, you're in good shape. Stability in calibers is an arbitrary, directional measure at best; you really need to know if it stays positive in the worst-case scenario.
 

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