"Short and stubby rule" -- can someone explain it in pseudo-scientific terms?

[Bat-mite]

Building a MAC Performance 3FNC. This is the very epitome of short and stubby (4" d. X 29" tall).

My ORK with motor shows a stability margin of .23 cal. And yet the sim shows me 2200' of altitude on a Loki H160.

By the CP/CG rule, this rocket is not stable; but by the short & stubby rule, I believe it is.

I know base drag has something to do with the stability, but I'd like something a little more concrete. Thanks.

 

[mpitfield]

Someone more qualified can help with an explanation, however here is an interesting Apogee newsletter that I used to help better simulate the base drag of short stubby rockets.

https://www.apogeerockets.com/education/downloads/Newsletter162.pdf

 

[mpitfield]

BTW here is a RS file for my Tembo, the rocket in my Avatar, with the base drag applied as per the Apogee newsletter I linked above.

 

[Bat-mite]

I assume this trick works in OR?

 

[Bat-mite]

Nice article here, but he ends up saying "There obviously still needs to be some CP-CG margin but what minimum value is acceptable is not clear."

 

[ThirstyBarbarian]

I've used the zero-mass cone method to calculate stability on my fat, stubby rockets before too. Does it really work? I don't know. It did give more reassuring numbers, and the rocket flew just fine. But the rocket was also a known reliable design (LOC Warlock), so as long as it wasn't heavily modified, you'd expect it to fly the same for me as for everyone else.

One thing to mention, this technique is only for calculating stability. I was told it's not for predicting altitude or calculating delays for motor ejection.

 

[emckee]

I used this method on a Polecat 10" Fat Boy (Man?) in Open Rocket, and stability wasn't an issue. (the trick works in OR)

However, I found that the altitude was off by about 50%. The rocket sim'd to ~4500' on an M1297, but actual altitude was closer to 6500'.

 

[Bat-mite]

Nice article here, but he ends up saying "There obviously still needs to be some CP-CG margin but what minimum value is acceptable is not clear."

Sorry, here is the article: https://argoshpr.ch/joomla1/articles/pdf/sentinel39-galejs.pdf

 

[ColumbiaNX01]

I thought we can't talk about things like this? I thought the rule was no politics, sex, or religion. Frankly, the title breaks one of those rules. LOL I dont care. I am laughing to myself all the way home. Have a good weekend guys. Its Friday. I think everyone is in a better mood on fridays.

 

[Mr Rocket]

@mpitfield

"BTW here is a RS file for my Tomach, the rocket in my Avatar, with the base drag applied as per the Apogee newsletter I linked above."



https://flic.kr/p/NP8WEP"]

Tomach or Tembo?

 

[rstaff3]

Sorry, here is the article: https://argoshpr.ch/joomla1/articles/pdf/sentinel39-galejs.pdf

This is the course article for all the rest. I have used it many times but generally am a little generous with the nose weight, especially on the HPR bird(s).

 

[Bat-mite]

I have an e-mail out to Mike Crupe to see what kind of margin he looks for with his own 3FNC.

 

[tmacklin]

I have a Monday appointment with my urologist and I'll run this by him. Perhaps he can shed some light on this stimulating topic. :wink:

 

[BuiltFromTrash]

Uhg.

 

[ThirstyBarbarian]

I used this method on a Polecat 10" Fat Boy (Man?) in Open Rocket, and stability wasn't an issue. (the trick works in OR)

However, I found that the altitude was off by about 50%. The rocket sim'd to ~4500' on an M1297, but actual altitude was closer to 6500'.

This is what I was saying about using the cone for your stability simulations, but not for altitude estimates or delay calculations. The cone is just a trick for "hacking" the stability calcs for short, fat rockets, but the rocket is not actually pulling a real cone along through the air, so it does not actually have as much drag as if it was. You need to remove the cone to do the altitude and delay sims.

 

[Bat-mite]

I have an e-mail out to Mike Crupe to see what kind of margin he looks for with his own 3FNC.

Mike says it is stable as-is with the motor, but did not give me a number. I'll try the RockSim trick.

 

[Nytrunner]

From my readings, a separation between CP and CG 8-15% of the rocket's length should be a good design point for passive rocket stability (What we sport flyers use....Jim Jarvis and company excluded).

Acknowledging that, and the fact that a L/D ratio of 10 is touted as an ideal rocket design, I think I can see where the 1 caliber (diameter) separation came from. 10% of the length of a rocket that's 10 times its diameter,....is 1 diameter. Rockets like Fatboy and Space Needle deviate from that ideal case, but people may very well apply the tribal "rule-of-thumb" and think they're ok.

 

[ThirstyBarbarian]

From my readings, a separation between CP and CG 8-15% of the rocket's length should be a good design point for passive rocket stability (What we sport flyers use....Jim Jarvis and company excluded).

Acknowledging that, and the fact that a L/D ratio of 10 is touted as an ideal rocket design, I think I can see where the 1 caliber (diameter) separation came from. 10% of the length of a rocket that's 10 times its diameter,....is 1 diameter. Rockets like Fatboy and Space Needle deviate from that ideal case, but people may very well apply the tribal "rule-of-thumb" and think they're ok.

This is what I think too. It has always made more sense to me for stability to expressed as a ratio using the length of the rocket, not the diameter.

 

[mpitfield]

@mpitfield

"BTW here is a RS file for my Tomach, the rocket in my Avatar, with the base drag applied as per the Apogee newsletter I linked above."

Tomach or Tembo?

Sorry the pic, it is for the Tembo...ah I just noticed what you were eluding to, my bad I was not paying attention, thanks!

A note to Bat-Mite. As stated by emckee and Thirsty, my altitude simulation was also considerably off on this one, and I am honestly not clear as to how accurate the base drag trick actually is, except to say that my Tembo is very stable even with big motors in it. FWIW

 

[MClark]

From my readings, a separation between CP and CG 8-15% of the rocket's length should be a good design point for passive rocket stability (What we sport flyers use....Jim Jarvis and company excluded).

Acknowledging that, and the fact that a L/D ratio of 10 is touted as an ideal rocket design, I think I can see where the 1 caliber (diameter) separation came from. 10% of the length of a rocket that's 10 times its diameter,....is 1 diameter. Rockets like Fatboy and Space Needle deviate from that ideal case, but people may very well apply the tribal "rule-of-thumb" and think they're ok.

For any length rocket I like the 10% of length rule of thumb. It is not just for short rockets, for a very long, high L/D, with one or two calibers stability is not adequate, but 10% of length it will be ok.

M

 

[Bat-mite]

Straight up, I have 0.166 stability. With the conical transition, I have 1.3 cal. Pretty sweet.

 

[mpitfield]

Straight up, I have 0.166 stability. With the conical transition, I have 1.3 cal. Pretty sweet.

I would be interested to see how stable it launches at this stage, do you plan on launching it with that or are you going to add some more margin, AKA nosecone weight?

 

[rstaff3]

Straight up, I have 0.166 stability. With the conical transition, I have 1.3 cal. Pretty sweet.

With or without motor?

 

[MikeyDSlagle]

For my Madcow Cowabunga I use the rear cone method. The nose cone has an altimeter sled with a total weight of 3 oz. It's maiden flight was on an AT H148R. .958 Cal stability and 1515' altitude with cone; .321 stability and 1795' altitude without the cone. I actually only got 1333' altitude out of it. I did add a bit of dog barf prior to flying so that would have lowered the stability a bit and added weight but doubt it would have had that much effect on altitude.

The latest flight was on a Loki H125 cocktail. I added 3 ounces of nose weight in anticipation of flying it on an AT I245, but that didn't happen. With the nose weight and rear cone, OR says 1.14 stability and 1453' altitude, actual altitude was 1463'. That flight was about as straight up as it could go, even with a stiff breeze...heck it was just plain windy. Without the rear drag cone, stability was .503 and 1735' altitude.

In all my sims, I never get to my simmed altitude. But it may have to do with some OR settings. The above flight was the closest I actually came. No worries atm, I use electronic deploy and no chance of busting the waiver for quite some time.

The rocket flew fine with .321 cal, but seemed to flight straighter with .5 - without rear drag cone. .166 seems low but the Cowabunga RS file shows .243 with a H148. Anywhere between .3 and .5 works for sure.

Mikey D

Mptifield - I'm sending you a PM about your Tembo

 

[Bat-mite]

With or without motor?

With.

 

[bill_s]

The first issue here is always the subject of "calipers" of stability. To me its main use is if, in the field, you know where the CP is and check the CG, the width of the rocket is a useful scale for measuring that distance. There is no inherent relation between the diameter of the rocket and the stability margin along the length of the rocket that is required. In fact saucers show us that a wide design can increase stability. For the case of using a conventional CP calculation, an equation to calculate the typical needed stability margin using length and width of the rocket should consider diameter a negative term.

edit: calibers, not calipers, although closer in this case.

 

[Nytrunner]

The first issue here is always the subject of "calipers" of stability.

My calipers are only stable when they lie flat. Definitely unstable when I try and stand them on end or edge.

To me its main use is if, in the field, you know where the CP is and check the CG, the width of the rocket is a useful scale for measuring that distance. There is no inherent relation between the diameter of the rocket and the stability margin along the length of the rocket that is required. In fact saucers show us that a wide design can increase stability. For the case of using a conventional CP calculation, an equation to calculate the typical needed stability margin using length and width of the rocket should consider diameter a negative term.

I agree its a nice little relation, but only if your rocket is close to a 10:1 L/D ratio. I regularly hear it being said: "Make sure it has 1 diameter between them and you'll be fine!" With no caveat, constraint, or explanation. It becomes building/flying more by heresay than by understanding.

 

[ThirstyBarbarian]

I don't know what the "best" unit of stability would be for all cases, or if one even exists, but it seems like any system that reduces stability measurement to a simple ratio of CP-CG over either length or diameter, while ignoring the other measurement, is probably over-simplifying things and won't work well for unusually long, short, skinny or fat rockets. Most likely a "universal" unit for stability would have to consider BOTH length and diameter (or "girth", if you prefer :eyeroll, not just one or the other.

 

[SpaceManMat]

I tried both the 1/10th length method and a cone to simulate base drag on my scratch built X Wing. Interestingly I got the same result in OR for those 2 methods, not sure if it holds true in all cases. The X Wing would by the way not be considered stable otherwise.

 

[bill_s]

I don't know what the "best" unit of stability would be for all cases, or if one even exists, but it seems like any system that reduces stability measurement to a simple ratio of CP-CG over either length or diameter, while ignoring the other measurement, is probably over-simplifying things and won't work well for unusually long, short, skinny or fat rockets. Most likely a "universal" unit for stability would have to consider BOTH length and diameter (or "girth", if you prefer :eyeroll, not just one or the other.

I wasn't clear but I'm saying diameter is a subtraction, not an addition -- although probably not true for all cases. A wider rocket is more sensitive to imperfections of build so maybe in some cases there really is a need for increased margin, but more often not. Say you have 2 rockets that are both 30" long and have a 3" stability margin. I haven't said anything about diameter yet. If one is 1.5", making the margin 2 calibers, and the other is 3" diameter, so only 1 cal., is the latter less stable? It might or might not be but I'd definitely feel better calling the stability margins the same (3" or 1/10 of L) and saying the needed margin is different than saying they actually have different margins (2 vs. 1). But, whatever works. I'm not saying using an equation for needed stability margin that comes up with a negative number for saucers is actually "correct" either, it would be a correction for calculated vs. actual CP, but if everyone did that long enough I bet some would argue until they're blue in the face that saucers are fine with a negative stability margin. As it is, the idea of calibers of stability has been around so long that some seem to confuse the words with the actual things.

There some rockets that need special explanation of the aerodynamics to explain why they are stable, but typical short and stubby rockets just don't seem to be among them, to me. They still need a positive stability margin, just small in caliber units, because they're large caliber! In many cases, the margin as a proportion of length, is as large or larger than long and skinny rockets. If the CP calculations were just wrong in ignoring a large base drag, these rockets could fly with a negative margin (in that calculation) and that they definitely can't do. One thing is short rockets usually have their CG and CP closer to the center of their length, while some long rockets have the CP just forward of the fins. As the angle of attack increases, the CP moves towards the center of the airframe tube, because straight tube is insignificant at zero and low AoA. With OR, check it with Tools>Component Analysis. For a Big Daddy at .276 cal. stability, worst case was -.086 cal. at 55 degrees. There are some long rockets you can't make it much past 10 degrees before the large margin goes negative.