What do the BT numbers mean?

[TheSamurai]

I always see people discussing BT-55 and BT-60 and BT-80 and so forth. I understand that BT stands for "Body Tube" but what exactly does that refer to?

For instance if I need a coupler for my Estes Ventris how do I know what size coupler to get? My body tube diameter is 2.5" but that isn't how the couplers are listed.

Is there a chart or something that someone can post?

 

[mwtoelle]

The BT numbers are fairly arbitrary. The only constant is that bigger numbers have larger diameters than the smaller ones. One reference is here: https://www.rocketshoppe.com/info/Estes_Body_Tube_List_3.1.pdf . Another chart can be found here: https://www.ninfinger.org/rockets/body_tubes.html . If you look at Semroc's body tube page you will find Estes equivalent tubes listed under "Classic Body Tubes".

About a coupler for an Estes Ventris your best bet is to cannibalize another kit because Estes has never sold Pro Series/NCR by Estes/Pro Series II parts as separate items. Only the 4" body tube is compatible with other manufacturers parts (LOC). The 2", 2.5", and 3" body tubes are unique to Estes, unfortunately. HTH.

 

[TheSamurai]

The BT numbers are fairly arbitrary. The only constant is that bigger numbers have larger diameters than the smaller ones. One reference is here: https://www.rocketshoppe.com/info/Estes_Body_Tube_List_3.1.pdf . Another chart can be found here: https://www.ninfinger.org/rockets/body_tubes.html . If you look at Semroc's body tube page you will find Estes equivalent tubes listed under "Classic Body Tubes".

About a coupler for an Estes Ventris your best bet is to cannibalize another kit because Estes has never sold Pro Series/NCR by Estes/Pro Series II parts as separate items. Only the 4" body tube is compatible with other manufacturers parts (LOC). The 2", 2.5", and 3" body tubes are unique to Estes, unfortunately. HTH.

So there isn't a coupler I can buy that will work for the Ventris?

 

[GDJ]

So there isn't a coupler I can buy that will work for the Ventris?

No, but making a coupler is pretty easy.

 

[TheSamurai]

No, but making a coupler is pretty easy.

Got a link for that? Youtube or otherwise? lol This isn't the "Beginners" forum for nothing.

 

[krusty]

So there isn't a coupler I can buy that will work for the Ventris?

You could always buy a larger coupler and cut a slice out of it to make it fit. The bit you cut out can be used to help reinforce the cut.

If that doesn't make sense, let me know and I'll try to describe it better!

Krusty

 

[GDJ]

Got a link for that? Youtube or otherwise? lol This isn't the "Beginners" forum for nothing.

No need to get snippy.

Krusty described it right.

 

[JeromeK99]

Snippy? huh?

Here you go:

[video=youtube_share;iH3HesegWv0]https://youtu.be/iH3HesegWv0[/video]


Jerome

 

[krusty]

What Jerome posted!

Krusty

 

[cbrarick]

I use my hobby bandsaw. I use the fence to cut the length - because I'm too impatient to use exacto knives - then flip it on end and cut out a little piece, about 1/8, give it a test fit and adjust accordingly.

 

[samb]

A bit of history of the "BT" from Apogee:

https://www.docstoc.com/docs/2194276/The-History-of-Body-Tubes---PDF

And as you've discovered, the Estes Pro Series models are based on a new set of tube stock. Yeeehaaa!

p.s. I agree that creating custom tube couplers is a useful, fun, and relatively easy skill to acquire.

 

[GregGleason]

And yet, more "BT" history (from Launch magazine's interview with Estes designer, Bill Simon):

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.
​

Greg

 

[GregGleason]

Here is a "quickie" .jpg of the Estes body tubes:



Greg

 

[samb]

And yet, more "BT" history (from Launch magazine's interview with Estes designer, Bill Simon):

...

Good one, Greg. Launch had a short run but the content was really, really, really good. Really ! :grin:


https://launchmagonline.com/

 

[plano-doug]

About a coupler for an Estes Ventris your best bet is to cannibalize another kit because Estes has never sold Pro Series/NCR by Estes/Pro Series II parts as separate items. Only the 4" body tube is compatible with other manufacturers parts (LOC). The 2", 2.5", and 3" body tubes are unique to Estes, unfortunately. HTH.

Is the Pro Series 2.5" tube different than BT-80? I know BT-80 and LOC 2.56" tubes have the same inside diameter, so couplers, rings, etc, can be interchanged.

Also, for the Estes 3" tube (aka HBT-3000), Balsa Machining makes a coupler for it.

Lastly, I've used HBT-3000 as a coupler for LOC 3" tubing. They slip fit together nicely.

Doug

.

 

[TheSamurai]

Is the Pro Series 2.5" tube different than BT-80? I know BT-80 and LOC 2.56" tubes have the same inside diameter, so couplers, rings, etc, can be interchanged.

Also, for the Estes 3" tube (aka HBT-3000), Balsa Machining makes a coupler for it.

Lastly, I've used HBT-3000 as a coupler for LOC 3" tubing. They slip fit together nicely.

Doug

.

I would have thought somebody made one that would fit it. But who knows.

I'll have to try some of the above methods as well. I enjoy learning new things but I am a visual person and reading and trying to understand what people are saying is hard for me.

 

[TheSamurai]

No need to get snippy.

Krusty described it right.

I wasn't being snippy at all. There was some sarcasm in my post but it wasn't meant to be demeaning at all.

 

[mwtoelle]

Is the Pro Series 2.5" tube different than BT-80? I know BT-80 and LOC 2.56" tubes have the same inside diameter, so couplers, rings, etc, can be interchanged.

Also, for the Estes 3" tube (aka HBT-3000), Balsa Machining makes a coupler for it.

Lastly, I've used HBT-3000 as a coupler for LOC 3" tubing. They slip fit together nicely.

The Estes 2.5" tubing specs are OD: 2.50" ID: 2.42" Wall Thickness: 0.040".

 

[plano-doug]

I would have thought somebody made one that would fit it.

Oh, sorry for the confustion. They do indeed make couplers for LOC 3" tubes. I was just saying that the Estes/NCR 3" tube will fit inside nicely.

Doug

.

 

[JeromeK99]

The Estes 2.5" tubing specs are OD: 2.50" ID: 2.42" Wall Thickness: 0.040".

Umm.... 2.5" - 2.42" leaves .080" new math?

I mean if you are going down to a thousandth of an inch, you shouldn't round numbers..... just saying.....


Jerome :wink:

 

[JStarStar]

The Apogee link has a more detailed and more accurate explanation of the original numbering system, but it is somewhat coincidental that BT-20 is fairly close to 20mm diameter.

Of course when the BT numbering system was being cooked up in the early 1960s most US model rocketry still operated on the english system of measurement so the mm equivalents were probably not thought of as being crucial.

 

[BABAR]

Snippy? huh?

Here you go:

[video=youtube_share;iH3HesegWv0]https://youtu.be/iH3HesegWv0[/video]


Jerome

Hmmmm. I do mine much less elegantly. I just do the longitudinal cut. Roll it up so it fits inside, mark the overlap edge. Put glue on the overlap, tape it, let it dry, and use it as a coupler. Since this is INSIDE the rocket and not seen, I guess I don't see the point or importance of making the second cut Tim V.Milligan makes and snugging it up so nicely. Am I missing something? Is it significantly weaker or less effective? It isn't any less "smooth" on the inside than Tim's version. Anyway, have never had any problems with it.

 

[DavidQ]

Umm.... 2.5" - 2.42" leaves .080" new math?

That 0.080" is the change in diameter from OD to ID. But, that includes "two" walls (or one wall wrapped all the way around the front to back), meaning the wall is 0.04" thick.

I've made that mistake too many times.:wink:

 

[krusty]

Hmmmm. I do mine much less elegantly. I just do the longitudinal cut. Roll it up so it fits inside, mark the overlap edge. Put glue on the overlap, tape it, let it dry, and use it as a coupler. Since this is INSIDE the rocket and not seen, I guess I don't see the point or importance of making the second cut Tim V.Milligan makes and snugging it up so nicely. Am I missing something? Is it significantly weaker or less effective? It isn't any less "smooth" on the inside than Tim's version. Anyway, have never had any problems with it.

The only thing I can see is that there's a portion of the coupler that won't be glued to the outer body tube - small as it may be.

It will become much less smooth when you start using thicker body tubes like postal tubes (that's where I've had to do this a few times) but apart from that - as you say, it's inside the rocket so no one's going to see I don't see any problem with it.

Krusty

 

[krusty]

Oh, and I forgot! I did up this chart some time ago -

https://www.oflittleinterest.com/index.php/rocketrytoolmenu/204-bodytubesizes

Krusty

 

[JeromeK99]

That 0.080" is the change in diameter from OD to ID. But, that includes "two" walls (or one wall wrapped all the way around the front to back), meaning the wall is 0.04" thick.

I've made that mistake too many times.:wink:

Duh... my bad.. two walls when measuring. That's what I get for trying to use my brain before chowing down on my lunch break.. lol


Jerome

 

[mwtoelle]

Umm.... 2.5" - 2.42" leaves .080" new math?

I mean if you are going down to a thousandth of an inch, you shouldn't round numbers..... just saying.....

(2.500"-2.420")/2 = 0.40". Remember, you are measuring two walls between the inside and the outside diameters.

 

[dave carver]

Totally Tubular has many tubes in many different sizes. Couplers, too, in many different lengths including 34" full length.

https://www.erockets.biz/Totally-Tubular.html

 

[boomtube]

If you know the I.D. of the tube then go to Semroc’s website and check out their selection of tube couplers. They have a bunch and in sizes not found elsewhere.

As for the BT #s. A BT-20 based rocket has only a 20% chance of crashing and crunching but alas only a 20% chance of actually recovering the rocket.

Meanwhile a BT-80 based rocket has an 80% chance of crashing and crunching but a whopping 80% chance you’ll at least recover something.

These numbers were assigned in the heydays of model rocketry, back when we had to chase the Mastodons off the launch field. Improvements in materials and techniques have rendered that old nomenclature obsolete but tradition lives on.

 

[DavidQ]

As for the BT #s. A BT-20 based rocket has only a 20% chance of crashing and crunching but alas only a 20% chance of actually recovering the rocket.

Meanwhile a BT-80 based rocket has an 80% chance of crashing and crunching but a whopping 80% chance you’ll at least recover something.

I always wondered why my rockets built on BT-101 tubes always crashed, sometimes before I even launched them, yet I was always coming home with more parts than I put into the rocket.