Caliber

[swarfGoat]

New guy here.
I cant get my head around why caliber should be relative to BT diameter. Or is caliber a rule of ?
What's a minimum acceptable caliber?

 

[Titan II]

https://www.rocketryforum.com/threads/caliber-and-stability.174935/

 

[neil_w]

1 caliber is defined as 1 body tube diameter. Stability *rule of thumb* is CP is behind CG by at least 1 caliber.

An alternative is to go by percentage of rocket length, where rule of thumb CP behind CG by 10%-20%. Using percentage is a little more adaptable to rockets of different aspect ratios (i.e., long and skinny vs. short and fat). If you're using OpenRocket it'll show stability by calibers and percentage by default, so you can choose.

 

[swarfGoat]

1 caliber is defined as 1 body tube diameter. Stability *rule of thumb* is CP is behind CG by at least 1 caliber.

An alternative is to go by percentage of rocket length, where rule of thumb CP behind CG by 10%-20%. Using percentage is a little more adaptable to rockets of different aspect ratios (i.e., long and skinny vs. short and fat). If you're using OpenRocket it'll show stability by calibers and percentage by default, so you can choose.

Ahh, so calculating caliber is a rule of thumb. Yes, aspect ratio is where my brain goes when considering caliper. With a long enuff BT one could choose a caliber and adjust CG to make it so.
I built a Estes Big Daddy and am unable to achieve a caliber much greater than 0.3 without a lot of extra weight... due in part to adding a bulkhead at the aft end of the NC, greatly shortening the possible arm. With the short arm an affective moment requires a prohibitive amount of weight. Im very reluctant to fly it.

 

[neil_w]

Ahh, so calculating caliber is a rule of thumb. Yes, aspect ratio is where my brain goes when considering caliper. With a long enuff BT one could choose a caliber and adjust CG to make it so.
I built a Estes Big Daddy and am unable to achieve a caliber much greater than 0.3 without a lot of extra weight... due in part to adding a bulkhead at the aft end of the NC, greatly shortening the possible arm. With the short arm an affective moment requires a prohibitive amount of weight. Im very reluctant to fly it.

Again, "caliber" is simply a unit of measurement equal to diameter. 1 caliber is the rule of thumb for acceptable stability margin.

Unless you did something really weird the Big Daddy should be stable. What is its stability as a percentage of length?

due in part to adding a bulkhead at the aft end of the NC, greatly shortening the possible arm. With the short arm an affective moment requires a prohibitive amount of weight.

I'm not sure what you're saying here. Moment arm does not factor into stability equation.

 

[swarfGoat]

Relative to OAL then 5% (1.00")
Moment does figure into CG. As I cant alter the CP, in order to achieve a caliber near 1.0 I must move the CG forward.

 

[neil_w]

The way you're using terminology is throwing me off.

The distance between CP and CG is stability margin. What you're trying to do is achieve stability margin of 1 caliber (or 10%, or in the range 2"-4" for the Big Daddy).

Are you saying that your bulkhead attached to the nose cone is going to prevent you from adding nose weight into the tip of the nose cone? If so that is remedied easily enough with a drill (drill hole, insert weight into nose, cover hole). How much weight in the nose would you need to get the move CG forward an inch?

Unfortunately I'm not sure if the Big Daddy normally requires nose weight. Did you do anything unusual in your build to make it tail-heavy?

To get more (and better) answers, try renaming the thread to "Big Daddy Stability Margin" or something like that. Lots of folks have built them and will have good input.

 

[Art Upton]

I thought Big Daddies were like .6 or so caliber and stable due to "Base Drag" that you can "Hack" into your Sim?

 

[swarfGoat]

Again, "caliber" is simply a unit of measurement equal to diameter. 1 caliber is the rule of thumb for acceptable stability margin.

Unless you did something really weird the Big Daddy should be stable. What is its stability as a percentage of length?


I'm not sure what you're saying here. Moment arm does not factor into stability equation.

Are you saying that your bulkhead attached to the nose cone is going to prevent you from adding nose weight into the tip of the nose cone? If so that is remedied easily enough with a drill (drill hole, insert weight into nose, cover hole). How much weight in the nose would you need to get the move CG forward an inch?

Well, yeah. Gettin into the NC would greatly decrease the weight necessary to move the CG forward to maybe 0.7 caliber, around 20% OAL.
Maybe I was hoping whining about it would make it happen without me cutting into the NC

 

[neil_w]

Well, yeah. Gettin into the NC would greatly decrease the weight necessary to move the CG forward to maybe 0.7 caliber, around 20% OAL.
Maybe I was hoping whining about it would make it happen without me cutting into the NC

Nope.

A single hole in the bulkhead is not bad, and easy to cover afterwards.

What motor did you use to measure CG?

 

[bjphoenix]

Unfortunately I'm not sure if the Big Daddy normally requires nose weight. Did you do anything unusual in your build to make it tail-heavy?

I built one not too many years ago. It is built with a D motor length mount and does not require nose weight.

 

[swarfGoat]

Nope.

A single hole in the bulkhead is not bad, and easy to cover afterwards.

What motor did you use to measure CG?

F16 motor.
Took 1.3oz to move the CG 1.8" ahead of CP.
All up weight now 15.2oz

 

[neil_w]

Yeah I’m not surprised that motor pulled the CG back too far. Sounds like you’ll be in good shape.