Is There Any Logic To Body Tube Designations

[lakeroadster]

I saw this on Facebook and it sent me down a rabbit hole trying to find a decoder ring for body tube sizing...



At the risk of asking a question that has an obvious answer that I am missing...

Is there any logic to the sizing? Does the designation mean anything?

BT-50: 0.976 O.D. x 0.950 I.D. (0.013 wall thickness)

BT = Body Tube​

50 = ???​

I've found various articles, one even entitled "The History of Body Tubes" but it doesn't address the gorilla in the room? Maybe I'm the gorilla?
The article states:

The "BT" designation was developed by a rocket designer named Bill Simon. According to legend, he didn't have any set plan when he started naming tubes. He jumped around a bit, leaving gaps in the numbering system to give room for future tubes.​

​

Come on Bill.... No logic and proportion?​

This article states "The larger the number, the larger the diameter of the body tube."

 

[neil_w]

The only sort of logic I recall was that BT-60 was so named because it could fit three BT-20s inside (in a cluster arrangement). That may be apocryphal, but it sounds about right.

 

[CalebJ]

Therre's certainly nothing to it that I could ever make sense of.

 

[samb]

No logic, just an historical artifact from the worlds biggest hobby rocket company. Bill Simon provided some insight in a 2007 Launch magazine interview:

"Launch: I read somewhere that you actually came up with the body tube designations (BT-50, BT-60, etc.) that, in general, became the hobby’s standard way to measure the airframes of these rockets. Is this correct? And if so, how complicated (or simple) was it to come up with this system?

Simon: The body tube designations were my fault. I really didn’t know anything about good practice in part numbering, so I just tried to use numbers that would let us add in-between sizes later on. BT-60, of course, accommodated 3 BT-20 tubes inside, and that was the entire basis for the system. The complexity came a few years later as we added special tube sizes for specific scale models. By that point the system was a huge dinosaur, but we were stuck with it. When we switched to electronic inventory systems we assigned purely numerical part numbers, and the old BT number became just a part of the description. About those body tubes… Vern put George Miller, our purchasing agent, on the task of trying to come up with a source of better engine tubes. George sent inquiries to every company listed under paper tubes in Thomas Register. One reply, from Euclid Spiral Paper Tube Company included samples of a polykraft/polyglassine construction that struck us immediately as perfect for body tubes. They also sent mylar/polyglassine samples that were eventually used for the Streak."

http://vernestes.com/images/LAUNCH Articles/Launch - Bill Simon Interview-Correccted.pdf

 

[Grant_Edwards]

Is there any logic to the sizing? Does the designation mean anything?

BT-50: 0.976 O.D. x 0.950 I.D. (0.013 wall thickness)

BT = Body Tube​

50 = ???​

AFAIK, the 50 means "larger than BT-N for N < 50 and smaller than BT-M for M > 50"

 

[jqavins]

There's more on the subject in this old thread. And there's some more information here.

Using data from this page, I condensed out the table attached. (The page gives dimensions in inches. The sheet of millimeter values has conversions therefrom.)

 

[boomtube-mk2]

It gets even worse when you consider that body tubes get larger dia. with larger numbers but shotgun barrels get larger Dia. with smaller numbers.

And then there's the whole wire gage meshugana.

 

[user 35280]

https://www.erockets.biz/body-tube-sizes/
https://www.erockets.biz/body-tube-centering-ring-measuring-guide/

 

[Scott_650]

…Vern put George Miller, our purchasing agent, on the task of trying to come up with a source of better engine tubes. George sent inquiries to every company listed under paper tubes in Thomas Register..."

For those too young to know what the Thomas Register was it was a set of green books listing companies that made and distributed just about any industrial type product you could think of and it’s was an indispensable tool for olde tyme logistics, supply and purchasing folks in the dark ages before PCs and the internet. I remember when our contracting office got their first set of Thomas Register CD-ROMs (before the internet and online searches/databases made them obsolete - anyone else remember FedLog/WebFLIS and/or it’s dial-up commercial pay predecessor?). Spent enough time doing research in their office that my boss started keeping track of the time I was gone from desk

 

[lakeroadster]

For those too young to know what the Thomas Register was it was a set of green books listing companies that made and distributed just about any industrial type product you could think of and it’s was an indispensable tool for olde tyme logistics, supply and purchasing folks in the dark ages before PCs and the internet. I remember when our contracting office got their first set of Thomas Register CD-ROMs (before the internet and online searches/databases made them obsolete - anyone else remember FedLog/WebFLIS and/or it’s dial-up commercial pay predecessor?). Spent enough time doing research in their office that my boss started keeping track of the time I was gone from desk

I certainly remember thumbing thru the Thomas Register books. Stuff that you can find today within 5 minutes searching on the web used to take days...

 

[jqavins]

But there was no SEO in the Thomas Register. If you read through a part of it you know you've read all it's got for you.

 

[Buckeye]

"By that point the system was a huge dinosaur, but we were stuck with it"

When, in 1964? Seems like there has been plenty of time to fix this crummy naming convention. Thank goodness it didn't carry over to MPR and HPR.

20 lashes with a wet noodle for anybody who refers to a body tube as "BT-XX!"

 

[lakeroadster]

But there was no SEO in the Thomas Register. If you read through a part of it you know you've read all it's got for you.

Search Engine Optimization.... True, but there was also nobody tracking what you were doing, either...

Actually you could "Search" for 'Engines", the "Optimization" was up to the person reading the Thomas Register

 

[Cape Byron]

It gets even worse when you consider that body tubes get larger dia. with larger numbers but shotgun barrels get larger Dia. with smaller numbers.

And then there's the whole wire gage meshugana.

"I'll have 50 metres of 2" irrigation pipe, please," said I to a man not two weeks ago. It isn't just BTs.

 

[lakeroadster]

No logic, just an historical artifact from the worlds biggest hobby rocket company. Bill Simon provided some insight in a 2007 Launch magazine interview:

"Launch: I read somewhere that you actually came up with the body tube designations (BT-50, BT-60, etc.) that, in general, became the hobby’s standard way to measure the airframes of these rockets. Is this correct? And if so, how complicated (or simple) was it to come up with this system?

Simon: The body tube designations were my fault. I really didn’t know anything about good practice in part numbering, so I just tried to use numbers that would let us add in-between sizes later on. BT-60, of course, accommodated 3 BT-20 tubes inside, and that was the entire basis for the system. The complexity came a few years later as we added special tube sizes for specific scale models. By that point the system was a huge dinosaur, but we were stuck with it. When we switched to electronic inventory systems we assigned purely numerical part numbers, and the old BT number became just a part of the description. About those body tubes… Vern put George Miller, our purchasing agent, on the task of trying to come up with a source of better engine tubes. George sent inquiries to every company listed under paper tubes in Thomas Register. One reply, from Euclid Spiral Paper Tube Company included samples of a polykraft/polyglassine construction that struck us immediately as perfect for body tubes. They also sent mylar/polyglassine samples that were eventually used for the Streak."

http://vernestes.com/images/LAUNCH Articles/Launch - Bill Simon Interview-Correccted.pdf

Thanks @samb much appreciated!

 

[rocketlabdelta]

There's already been a lot of good responses to the BT-X sizing system but I also wanted to address the original question about motor mounts being sized in millimeters.

I saw this on Facebook and it sent me down a rabbit hole trying to find a decoder ring for body tube sizing...

View attachment 491165

The size of early Estes airframes was determined by the largest diameter nose cone that could be turned from standard dimensional balsa stock:

Tube	Balsa Stock	Outside Diameter	Inside Diameter
BT-50	1"	.976"	.95" (24mm)
BT-20	.75"	.736"	.71" (18mm)

Motors were sized to fit in those airframes; round-number millimeters are a lot more memorable than decimal inches.

If I were to guess, composite motors diameters are similar but are sized from the inside out instead of the outside in. Standard sized O-rings are used inside the motor casing so the case diameters themselves end up being an odd size. That being said, the size of 75mm—and possibly 98mm—motors were determined by what could fit in 3 inch and 4 inch outside diameter airframes; you’d have to ask @AeroTech to be certain.

All this to say, motor sizes and airframe sizes are usually influenced by some outside factor other than being a specific number or fraction of inches in size.

 

[BABAR]

No.

 

[samb]

"... Seems like there has been plenty of time to fix this crummy naming convention. Thank goodness it didn't carry over to MPR and HPR.

20 lashes with a wet noodle for anybody who refers to a body tube as "BT-XX!"
...

Ain't hapnin' son. It's the code real OG model rocketeers use to identify at cocktail parties and online internet chat forums.

 

[prfesser]

Probably coincidental but the sizes of BT-20, -50, and 29 mm tubes are such that 1/2", 3/4", and 1" EMT conduit makes pretty accurate mandrels for those tubes.

 

[jqavins]

Search Engine Optimization.... True, but there was also nobody tracking what you were doing, either...

So that 's two advantages to the Register. By SEO I mean that the products and vendors with better SEO behind them get lots and lots of hits and sometimes one never sees some of the other options.

Motors were sized to fit in those airframes; round-number millimeters are a lot more memorable than decimal inches.

I recall reading (no time to locate now) that the original 18 mm engine diameter came from off the shelf available parallel wound tubes from the fireworks industry. And probably the same for the 24 mm engines.

If I were to guess, composite motors diameters are similar but are sized from the inside out instead of the outside in. Standard sized O-rings are used inside the motor casing so the case diameters themselves end up being an odd size.

But O-rings are available off the shelf in hundreds of rather closely spaced diameters. The available rings may have pushed them to, say, 54 mm rather than 53 or 55, but they could just as easily have gone to 51 or 57. There has to be another reason for the for the gross steps that get pretty close, even if off the shelf O-rings may have influenced the final exact values.

 

[lakeroadster]

Ain't hapnin' son. It's the code real OG model rocketeers use to identify at cocktail parties and online internet chat forums.

Body Tube, Oh Yeah?... que the disco music...

 

[rocketlabdelta]

Well, being wrong on the Internet is a fast way to learn something.

Thanks for the heads-up @BABAR & @jqavins

 

[MetricRocketeer]

Hi TRF colleagues,

I have this question, please.

Let's use the BT-50 as an example. Its outer dimension measures 0.976" and its inner dimension measures 0.950". So how can its wall thickness be anything other than 0.026"? Besides the wall, what else is there between the outer and the inner dimensions?

Incidentally, I dislike using inches rather than millimetres. The numbers would be so much easier to deal with if we were to say that the outer dimension measures 25 mm and the inner dimension measures 24 mm. But that's another story.

Thank you.

Stanley

 

[neil_w]

Let's use the BT-50 as an example. Its outer dimension measures 0.976" and its inner dimension measures 0.950". So how can its wall thickness be anything other than 0.026"?

Diameter include *2* walls. So wall thickness = (outer - inner)/2

 

[MetricRocketeer]

Oh, right. Thank you @neil_w. How silly of me to overlook this.

Stanley

 

[rocketlabdelta]

I dislike using inches rather than millimetres.

Name checks out!

 

[MetricRocketeer]

Hi @rocketlabdelta,

Your website looks interesting. I don't have time to do it now. Later, however, I will click around on it in some depth.

Stanley

 

[ThirstyBarbarian]

It all goes back to the British Navy and the early days of canons. And now we’re stuck with it.

 

[jqavins]

Well, being wrong on the Internet is a fast way to learn something.

Thanks for the heads-up @BABAR & @jqavins

This is why I love TRF; in so many other places on the internet, being challenged is a way to get into a fight. I hope it was clear that "I read it somewhere, and I can't find the source right now" is tantamount to "I'm not too sure of this, but..."

Incidentally, I dislike using inches rather than millimetres. The numbers would be so much easier to deal with if we were to say that the outer dimension measures 25 mm and the inner dimension measures 24 mm.

I, too, would rather go metric on this (and many other things, if not everything). But I must point out that the OD is not 25 mm. If 0.976 in. is correct to three significant digits then it comes to 24.8 mm. Apogee reports the same, though that may be converted from 0.976 just as my number is.

 

[DaHabes]

It all goes back to the British Navy and the early days of canons. And now we’re stuck with it.

canons? you mean cannons?