# Caliber and stability

#### Lt72884

##### Well-Known Member
Ok, reading some documentation in OR and a few books, i have seen the same term used over and over, and that is caliber. In units of measurement, that is inches, IE 50 caliber is 0.5 inches in diameter. So if my rocket has a 2.35 caliber, which according to OR basic rocket design documentation is over stable, does this mean 0.02 inches from CG to CP?

" Ensure at least 1.0 airframe caliber of separation between the (forward) center of mass and (aft) center of pressure. This is a rule of thumb, not a hard-and-fast stability solution."
what is deemed as "airframe caliber" ? is that meaning the diameter of the body tube in a measurement of caliber rather than inches? so a 3 inch diameter is a 300 caliber body tube?

I know QFactor gave me a tip of keeping the CP about 1-2 times the body diameter behind the CG.

thanks.

#### Antares JS

##### Professional Amateur
TRF Supporter
One caliber of stability is one body diameter of distance between the CG and CP.

#### Lt72884

##### Well-Known Member
One caliber of stability is one body diameter of distance between the CG and CP.
interesting. where are they getting that measurement from since caliber is inches? i have never seen caliber used this way.

#### mh9162013

##### Well-Known Member
TRF Supporter
i have never seen caliber used this way.
I haven't either, but who am I to tell Romans what to do in Rome, right?

But the term "caliber" can refer to the diameter of a gun barrel, right? So using it to refer to the diameter of a rocket seems logical...or at least somewhat reasonable.

#### Antares JS

##### Professional Amateur
TRF Supporter
interesting. where are they getting that measurement from since caliber is inches? i have never seen caliber used this way.
Calibers of stability are dimensionless.

Calibers = distance between CG and CP / diameter of the rocket

#### Lt72884

##### Well-Known Member
Calibers of stability are dimensionless.

Calibers = distance between CG and CP / diameter of the rocket
That is very helpful to see that math. The wiki article above also states the same thing. Now things make more sense. I can see how it all relates now.

thanks for this

#### UhClem

##### Well-Known Member
But the term "caliber" can refer to the diameter of a gun barrel, right? So using it to refer to the diameter of a rocket seems logical...or at least somewhat reasonable.
It also has another meaning. The main battery on the Iowa class battleships for example are 16" 50 caliber. The 50 caliber means that the gun barrel length is 50 times the shell diameter.

You have to be careful as seemingly simple words can have multiple different meanings depending on context.

#### Charles_McG

##### Ciderwright
I forever get it mixed up with 'calipers' - I image an ancient architect using an outside caliper (or compass) measurement of the body tube diameter and comparing that to the Cg-Cp distance as a ratio.

Which makes sense in -my- head - but apparently no one else's.

I doesn't help that there's only 1 letter difference.

#### mh9162013

##### Well-Known Member
TRF Supporter
I forever get it mixed up with 'calipers' - I image an ancient architect using an outside caliper (or compass) measurement of the body tube diameter and comparing that to the Cg-Cp distance as a ratio.

Which makes sense in -my- head - but apparently no one else's.

I doesn't help that there's only 1 letter difference.
I get them confused, too. And "caliper" make as much sense to me as caliber, given how calipers can be used to measure the width of things.

#### Lt72884

##### Well-Known Member
I get them confused, too. And "caliper" make as much sense to me as caliber, given how calipers can be used to measure the width of things.
EXACTLY!! i had to edit my post before posting it because i kept putting caliper haha.

#### mtnmanak

##### ™
As with many other things, this is all explained in Harry Stine's Handbook of Model Rocketry, specifically, chapter 9. All rocketeers should have a well-read, dog-eared copy of this book with plenty of notes. Stine also provides all the math necessary for the Barrowman equations, should you really want to understand rocket stability (at subsonic speeds). And, if you are doing high power rocketry, you really should understand rocket stability. Stine's book is not the only reference for this subject, but it should be considered a minimum.

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#### Lt72884

##### Well-Known Member
As with many other things, this is all explained in Harry Stine's Handbook of Model Rocketry, specifically, chapter 9. All rocketeers should have a well-read, dog-eared copy of this book with plenty of notes. Stine also provides all the math necessary for the Barrowman equations, should you really want to understand rocket stability (at subsonic speeds). And, if you are doing high power rocketry, you really should understand rocket stability. Stine's books is not the only reference for this subject, but it should be considered a minimum.
i have another book about rockets but not this one. If this book goes over the barrowman equations, then i think ill pick this book up. I understand the stability to a point based off of the open rocket documentation as well as modern rocketry book, but the term in the open rocket documentation did not explain what the caliber was in reference too.

i get that cp needs to be 1 caliber aft of CG, but what IS caliber was what i was trying to figure out. When i searched on google, caliber always came up as a diameter, even in the standard wiki article, until someone above showed a different wiki article which was very helpful.

i highly appreciate your reply and reference to that book. im going to get it now. One thing i did learn today was the 5:1 thrust to weight ratio. that changed A LOT of my design

#### Scott_650

##### Well-Known Member
As with many other things, this is all explained in Harry Stine's Handbook of Model Rocketry, specifically, chapter 9. All rocketeers should have a well-read, dog-eared copy of this book with plenty of notes. Stine also provides all the math necessary for the Barrowman equations, should you really want to understand rocket stability (at subsonic speeds). And, if you are doing high power rocketry, you really should understand rocket stability. Stine's books is not the only reference for this subject, but it should be considered a minimum.
Exactly! The Handbook plus Mark Canepa’s Modern High Power Rocketry are two must haves - those two plus all the online stuff on the NAR and Tripoli sites, the how-to info on the Apogee site and John Coker’s site (John’s videos are very organized and thorough ) are all great references.

#### Lt72884

##### Well-Known Member
Exactly! The Handbook plus Mark Canepa’s Modern High Power Rocketry are two must haves - those two plus all the online stuff on the NAR and Tripoli sites, the how-to info on the Apogee site and John Coker’s site (John’s videos are very organized and thorough ) are all great references.
Who is john? I will look him up on the YouTube and see what i find. I have Marks book, and im in the middle of getting Steins book.

#### mtnmanak

##### ™
i have another book about rockets but not this one. If this book goes over the barrowman equations, then i think ill pick this book up. I understand the stability to a point based off of the open rocket documentation as well as modern rocketry book, but the term in the open rocket documentation did not explain what the caliber was in reference too.

i get that cp needs to be 1 caliber aft of CG, but what IS caliber was what i was trying to figure out. When i searched on google, caliber always came up as a diameter, even in the standard wiki article, until someone above showed a different wiki article which was very helpful.

i highly appreciate your reply and reference to that book. im going to get it now. One thing i did learn today was the 5:1 thrust to weight ratio. that changed A LOT of my design

#### Charles_McG

##### Ciderwright
You're going to find that [hobby] rocketeers work with a weird mix of rules of thumb alongside detailed calculations. For insurance/safety reasons, both NAR and Tripoli impose a 3:1 minimum thrust to weight ratio. Most people use 5:1. I use 10:1 for my two-stagers. A more complete answer comes from a deeper 'rule of thumb' - that most flat plate airfoils stall at about 15 degrees angle of attack. You want to leave the rod/rail with the fins flying - working backwards, that means the exit speed should > 4X the cross wind speed. If the lowest detectable wind (by lay people, not sailors - it's been studied) is 5 mph, then for typical rod lengths, it takes 3:1 TTW to get to 20mph. So, two rules of thumb (stall angle and max undetectable breeze) and some trig and algebra get to a minimum rule.
That's one example, and rocketry is full of them - especially low power and the low end of high power.

Also consider that most hobbyists don't have access to CFD tools, nor the background to use them. (They are out there, though.)

, personally, wish for a second dimensionless number to add to Calibers of stability. I'd like an indication of restoring force - maybe as a ratio to moment of angular inertia. I've seen rockets that sim stable do some wilely things in flight. I think that measure could help explain/predict it.

#### Lt72884

##### Well-Known Member
You're going to find that [hobby] rocketeers work with a weird mix of rules of thumb alongside detailed calculations. For insurance/safety reasons, both NAR and Tripoli impose a 3:1 minimum thrust to weight ratio. Most people use 5:1. I use 10:1 for my two-stagers. A more complete answer comes from a deeper 'rule of thumb' - that most flat plate airfoils stall at about 15 degrees angle of attack. You want to leave the rod/rail with the fins flying - working backwards, that means the exit speed should > 4X the cross wind speed. If the lowest detectable wind (by lay people, not sailors - it's been studied) is 5 mph, then for typical rod lengths, it takes 3:1 TTW to get to 20mph. So, two rules of thumb (stall angle and max undetectable breeze) and some trig and algebra get to a minimum rule.
That's one example, and rocketry is full of them - especially low power and the low end of high power.

Also consider that most hobbyists don't have access to CFD tools, nor the background to use them. (They are out there, though.)

, personally, wish for a second dimensionless number to add to Calibers of stability. I'd like an indication of restoring force - maybe as a ratio to moment of angular inertia. I've seen rockets that sim stable do some wilely things in flight. I think that measure could help explain/predict it.
Just added all of this to my notebook. Im thinking of printing a logo of the forums in vynl and sticking to our rocket since i have received so much help and information here. its been AWESOME!!!

#### Off Grid Gecko

##### Well-Known Member
i have another book about rockets but not this one. If this book goes over the barrowman equations, then i think ill pick this book up. I understand the stability to a point based off of the open rocket documentation as well as modern rocketry book, but the term in the open rocket documentation did not explain what the caliber was in reference too.

i get that cp needs to be 1 caliber aft of CG, but what IS caliber was what i was trying to figure out. When i searched on google, caliber always came up as a diameter, even in the standard wiki article, until someone above showed a different wiki article which was very helpful.

i highly appreciate your reply and reference to that book. im going to get it now. One thing i did learn today was the 5:1 thrust to weight ratio. that changed A LOT of my design
Little off-topic, but good to keep in mind these are rules of thumb and not hard facts. Just like CP is determined by angle of attack and velocity, that 5:1 ratio is just a helpful suggestion that's usually good to go by. Before doing a cert flight especially, but with any unproven rocket/motor combo, it's a good idea to figure up the speed it will be traveling at when the forward lug leaves the rail. I have one rocket in particular I can launch on an E16 and it flies great, but with that motor I put it on one of the taller 1010 rails at our launch site, because it won't be as stable off a 6' rod and 4' is a no-go. Of an 8-10' section it makes for a beautiful slow liftoff.

#### JoePfeiffer

##### Well-Known Member
interesting. where are they getting that measurement from since caliber is inches? i have never seen caliber used this way.
"Caliber" is diameter. When people say the CG should be 1 to 2 calibers ahead of the CP, that means if the body diameter is 4 inches, CG should be 4 to 8 inches ahead. If the diameter is 1 inch, it should be 1 to 2 inches ahead.

This is a rule of thumb, which is most appropriate for reasonably normal designs. Designs that are either super-long or super-stubby end up needing different stability margins.

#### Scott_650

##### Well-Known Member
Who is john? I will look him up on the YouTube and see what i find. I have Marks book, and im in the middle of getting Steins book.
John Coker is an experienced high power builder and flyer, he has an extensive website with tons of info http://www.jcrocket.com/

#### boatgeek

##### Well-Known Member
, personally, wish for a second dimensionless number to add to Calibers of stability. I'd like an indication of restoring force - maybe as a ratio to moment of angular inertia. I've seen rockets that sim stable do some wilely things in flight. I think that measure could help explain/predict it.
The other rule of thumb of distance between CP and CG >= 10% of length is another good option that is closer to a ratio of restoring force to angular inertia. The challenge with calculating a true angular inertia is knowing where the weight is. Rockets with the same CG could have wildly different inertia values if one has a heavier motor and lots of nose weight to bring the CG to the same location.

#### rharshberger

##### Well-Known Member
John Coker is an experienced high power builder and flyer, he has an extensive website with tons of info http://www.jcrocket.com/
And I believe I recommended Johns video for plastic nose cone mods in one of the OP's other threads (noseweight maybe?) and recommended he peruse the rest of the site too.

#### Lt72884

##### Well-Known Member
And I believe I recommended Johns video for plastic nose cone mods in one of the OP's other threads (noseweight maybe?) and recommended he peruse the rest of the site too.
If you did and i didnt see it, im sorry. There is just so much awesome information i have written down. I have 65 tabs open in my browser and pages of written notes for this project from you guys haha.

wait, i did watch the videos you posted, is that John?

#### Lt72884

##### Well-Known Member
The other rule of thumb of distance between CP and CG >= 10% of length is another good option that is closer to a ratio of restoring force to angular inertia. The challenge with calculating a true angular inertia is knowing where the weight is. Rockets with the same CG could have wildly different inertia values if one has a heavier motor and lots of nose weight to bring the CG to the same location.
ok, let me make sure i understood the way you wrote it. so CP should be 10% of the rockets length behind CG? ie, if rocket is 3 feet, cp should be (0.1)(36inches) behind the CG?

#### Off Grid Gecko

##### Well-Known Member
ok, let me make sure i understood the way you wrote it. so CP should be 10% of the rockets length behind CG? ie, if rocket is 3 feet, cp should be (0.1)(36inches) behind the CG?
Rules of thumb, it's something you will start to figure out flying different designs also. That's why L1 kits are usually similar aspect ratios, weights, overall design. You might also look up swing test. As others have said, go to Hobby Lobby if you get a chance this week and check out some little rockets. Heck take them with you to the flying field. Model them on OpenRocket and look at the numbers, specially on the weird ones. Swing test them (smaller rockets are safer than 4' models for that). Actually if you have access to a wind tunnel.... hehehe.

#### Lt72884

##### Well-Known Member
Dang, watching that youtube video from Barrowmen, i was just able to connect the dots from my current automatic controls class with linear systems to rockets.

#### rharshberger

##### Well-Known Member
If you did and i didnt see it, im sorry. There is just so much awesome information i have written down. I have 65 tabs open in my browser and pages of written notes for this project from you guys haha.

wait, i did watch the videos you posted, is that John?
That is John Coker

#### Lt72884

##### Well-Known Member
Rules of thumb, it's something you will start to figure out flying different designs also. That's why L1 kits are usually similar aspect ratios, weights, overall design. You might also look up swing test. As others have said, go to Hobby Lobby if you get a chance this week and check out some little rockets. Heck take them with you to the flying field. Model them on OpenRocket and look at the numbers, specially on the weird ones. Swing test them (smaller rockets are safer than 4' models for that). Actually if you have access to a wind tunnel.... hehehe.
I have a few estes rockets and a 3D printed rocket for a D motor. Ill re-search the threads of mine because i think i missed the swing test.
Ill also model the estes rockets i have in OR to see what i get. back in the day i used to tape a stick to just a rocket motor and launch it. they didnt go as high as my rockets and when i was younger, that puzzled me haha

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