Estes Industries began producing model rocket kits, parts and engines in the early 1960's. This was long before computers were much used in small to medium size businesses, so all inventory administration was done by hand.
Part numbers had meaning, because people in the stockroom had to be able to find them on the shelves and bins. There were no bar code readers or automatic part pickers. It was during this time that Estes created many of the standard model rocket body tube sizes and the legacy size-aware part numbering scheme that is still in common use.
So pervasive was the Estes influence on the industry that some other manufacturers adopted the same tube size series and naming conventions, because Estes was making most of the engines and you simply had to be compatible with the engine diameters.
In the Estes size naming system, body tube designations look like BT-10, BT-20, BT-30 etc. The "BT" means Body Tube, and the increasing numbers denote tubes of increasing diameter, but the "10", "20" etc. have no direct numeric relationship to the actual diameter.
The 1964 Estes catalog listed BT-10, BT-20, BT-30, BT-40, BT-50 and BT-60, as well as the plastic PST-20 and PST-40. The other series appeared later. BT-5 was created to hold the 13mm "mini motors", and BT-55 filled the large size gap between BT-50 and BT-60.
Here is a table of the major classic Estes tube sizes with their diameters, motor compatibility and notes.
| Name | ID | OD | Motor | Notes |
|---|---|---|---|---|
| BT-5 | .515 | .543 | Mini -T 13mm | |
| BT-10 | .710 | .720 | Standard 18mm | Mylar tube used in Astron Streak |
| BT-20 | .710 | .736 | Standard 18mm | |
| BT-30 | .725 | .767 | Standard 18mm | parallel-wound, originally hand-rolled |
| BT-40 | .765 | .825 | parallel-wound, disappeared late 1960s | |
| BT-50 | .950 | .976 | D-E 24mm | |
| BT-55 | 1.283 | 1.325 | ||
| BT-60 | 1.595 | 1.637 | ||
| BT-70 | 2.175 | 2.217 | originally for Saturn V | |
| BT-80 | 2.558 | 2.600 | ||
| BT-101 | 3.896 | 3.938 | originally for Saturn V |
Estes made some other sizes that were less commonly used, but the ones listed above are used in the vast majority of Estes kits from the start of the company to the present day, and are the sizes that were listed in the catalog and could be purchased for building your own custom rockets.
BT-40 was an oddball size. It was parallel-wound like the BT-30, but was a sloppy slip fit over a BT-20, and would not quite slip over a BT-30. It was only available in a 13.75" length, suggesting that it was wound from a legal size page (with a small allowance for trimming), and was quite thick with 0.028" walls.
Nose cones and other parts that needed to mate with the tubes had the same numeric code but with a different prefix. Examples of some of the most common part types:
- BNC-55xx - balsa nose cones for BT-55 tube, e.g. BNC-55AA
- PNC-20xx - plastic nose cones for BT-20 tube
- JT-50C - tube coupler (inside slip fit) for BT-50
- EB-20A - engine block ring for BT-20
- RA-2050 - adapter ring to mount BT-20 inside a BT-50
- NB-60 - balsa "nose block" (solid bulkhead) for BT-60
- TA-5060 - balsa transition from BT-50 to BT-60
In the BT-xx scheme, an alphabetic suffix was used to indicate the length of tubes and the shape of nose cones. Some of the suffixes were consistent - every tube ending with "J" was 2.75 inches long (motor mount tube length), such as BT-20J, BT-50J, etc. Others were not very consistent.
Because the system was manual for decades, there are many mistakes in the part numbering. Estes sometimes used the same part number again for a different part. Likewise they sometimes forgot they had previously issued a part number for a tube of a certain length and would issue a second part number for the identical item. The John Brohm Estes tube reference has a lot of information about these errors.
Numeric non-significant 4-6 digit part numbers were introduced in the 1970s. These numbers carry no systematic clues about the sizes of the parts. For many years the old and new part number systems were used in parallel, but graadually between 2000-2010, the old designators stopped appearing in kit instructions, many parts are no longer sold separately, and many were never listed in catalogs.
Most Estes manufacturing is now done in China. Per John Boren of Estes (2017), internal part numbers exist for nearly everything, but they are completely buried in the contract manufacturing process. When a customer needs a replacement part for a China-made kit, they are normally sent an entire new kit, because Estes has no packaged inventory of the individual parts that make up a kit.
There are some parts that are packaged up for consumer sale, sometimes as assortments. However, this does not always provide useful part numbers, since the assortment will get a part number but the individual parts are not always referenced.
Catalog publication has ceased as of 2017; it was reported on online forums that this happened because the person who formerly produced the catalogs was laid off. Examining the Estes website periodically will probably yield some information, but with the loss of catalogs, the amount of useful information about the parts range has declined dramatically.
This all means that there is no almost customer-facing exposure of the actual Estes internal part numbers, so they are likely to remain unknown unless Estes decides to publish its manufacturing database.
Overall we are left with no sensible way to refer to the rocket tube sizing except by the old BT-xx scheme. Documenting kits that have few or no part number references now must be done by measuring the parts and comparing to known previous parts and sizes. For previously unknown parts, the hobby community will have to create its own designations.