Origin of motor sizes?

As I was coming to consciousness this morning I had a strange question pop into my mind: Where did the standard motor O.D.s come from? 13mm, 18mm, 24mm, 38mm, 54mm, 75mm, 98mm? They aren't close to traditional fraction imperial measures (except 13mm) and they aren't really rational metric numbers. Does anyone know?

Good question! I've often thought about that myself. I'll take a stab at an answer and say to fit in a standard size MMT tube??

enderw88 said:

As I was coming to consciousness this morning I had a strange question pop into my mind: Where did the standard motor O.D.s come from? 13mm, 18mm, 24mm, 38mm, 54mm, 75mm, 98mm? They aren't close to traditional fraction imperial measures (except 13mm) and they aren't really rational metric numbers. Does anyone know?

Click to expand...

Oh, there was an explanation of all that somewhere... either here or elsewhere on the web. I'd tell you to do a search but someone would flame me out of my frakkin' mind for even suggesting it... LOL

Ummm... yer best bet would be to ask over on YORF... if nobody in here chimes in.

IIRC, it started out with the 18mm motors using casings that Orville Carlisle (basically, the "father of the solid propellant model rocket motor) used for making skyrockets (he was a pyrotechnics guy and developed small solid propellant rocket motors that could eject a parachute-- he sent some to G. Harry Stine who saw a market for it, and Model Missiles Inc. and soon after the NAR were formed...) 13 mm "mini-motors" came later, and 24 mm motors came about because you couldn't get enough black powder in an 18 mm casing to make a full "D" motor. Not all manufacturers stuck with the common dimensions, either... FSI used to market their motors in 19 and 21mm diameters IIRC... (again, the Ninfinger site or YORF could really explain it better). FSI also made a 29mm black powder "F" motor (which IIRC) was based on the old "Coaster" design) and which allowed more BP to be packed into the casing. So that got us 13, 18, 24, and 29mm casings. FSI went under in the 80's, and with it their "odd-sized" casings. Centuri and Estes had used common sizes for years and were interchangeable in the mounts... Using FSI motors in Estes or Centuri kits or vice-versa, using Estes/Centuri motors in FSI kits, required some careful forethought in building to make sure the motor tube was large enough to accept either type of motor, or else the motor had to be "shimmed up" with wraps of tape or an adapter had to be used.

When composite propellant motors first came about, they used the standard casing sizes available at the time, but as power levels increased, they soon ran out of room in the casing for additional propellant, leading to the 38mm and larger casings.

Anyway, that aught to get you started.

Later! OL JR

The 75 mm is a misnomer it should be either 76 mm or 3"

The story I heard about 98 mm idea was that AT got a "good deal" on some out of spec tubing...
and used it to develop the 98 mm rather than a true 4.0" or 100 mm.

JD

The 18mm story is close to what Luke said, but not quite....

The original Carlisle motors were closer to 13mm (half inch), but when Model Missiles first went to get motors made somewhere besides in Orville Carlisle's basement, the fireworks maker they were working with said that he could make the motors cheaper if he used the existing cases he had for something called a buzz bomb helicopter. Those cases were 18mm in diameter.... This story is at the bottom of page seven here: https://www.questaerospace.com/imag..._rocket_museum/Stine_Memoirs/Stine_Memoir.pdf

The whole thing is a great read for anyone who wants to get a look back at the beginnings of this hobby through the eyes of one of the key people to make it all happen, G. Harry Stine himself. I'm so glad Quest has been willing to keep it available.

18mm is also the exact diameter of a 12 gauge shotgun... also a 1/2" rocket or 4 oz rocket used by the firework guy is also that size. I made a few but it felt more like a A motor even though it was stuffed with as much as blackpowder as a C motor. No idea how Estes got so much powder into it.

Ed Brown (Retired Estes motor guru) told the story of the development of the Estes D13/D12 motor.
According to Ed, Vern wanted a 'D' performance black powder motor but with the following restrictions. The motor could not be longer than 70mm so it could be used with the 18mm motor hooks the company was already using in their kits. The second limitation was that the motor had to fit inside BT-50 body tube i.e.: 24mm.

While I was reading this I wondered, why the choice of the international system of units over the US customary system was adopted to measure motors, when the US has been so stead fast on not moving toward metric.

BEC said:

The 18mm story is close to what Luke said, but not quite....

The original Carlisle motors were closer to 13mm (half inch), but when Model Missiles first went to get motors made somewhere besides in Orville Carlisle's basement, the fireworks maker they were working with said that he could make the motors cheaper if he used the existing cases he had for something called a buzz bomb helicopter. Those cases were 18mm in diameter.... This story is at the bottom of page seven here: https://www.questaerospace.com/imag..._rocket_museum/Stine_Memoirs/Stine_Memoir.pdf

The whole thing is a great read for anyone who wants to get a look back at the beginnings of this hobby through the eyes of one of the key people to make it all happen, G. Harry Stine himself. I'm so glad Quest has been willing to keep it available.

Click to expand...

Thanks for the clarification there...

I've read it all before, but it's been awhile back, and I read A LOT OF STUFF... sometimes it's hard to keep the finer details all completely straight and accurate, which is why I suggested they look it up "straight from the horse's mouth" so to speak...

Thanks for filling in the details and correcting my oversight there...

Later! OL JR

mpitfield said:

While I was reading this I wondered, why the choice of the international system of units over the US customary system was adopted to measure motors, when the US has been so stead fast on not moving toward metric.

Click to expand...

I think that came along later... I'm SURE NOBODY went around looking specifically for "18mm motor casings" or "13 mm", "24 mm" or whatever when things started out and these decisions were being made... It all came about largely by happenstance.

In the early days, rocket motors were rated in pound-seconds rather than newton seconds. The old motor designations reflected this... you didn't buy a B4-2 or a C6-5, it was more like a B.8-4 or something along those lines... At some point along the way, the decision was made to switch to the newton-second scale to get WHOLE NUMBERS rather than using FRACTIONAL (Decimal) numbers in pound-seconds... thus the switch the metric vs. imperial... The motor casing designations are also much easier to keep straight rather than listing them as actual "thousandths of an inch" equivalents... it's easier to remember "24 mm" rather than "0.950 in", for instance... or "18 mm" rather than "0.736 in"... That sort of thing.

Hence, the odd "mixture of systems"... remember in the late 70's, the push was on to FORCE the US to adopt the metric system, until the population pretty much simply ignored it and forced imperial measurements to stay in place for most things, and the effort was abandoned...

Later! OL JR

Also...
13mm=0.5"
18mm=0.75"
24mm=1"
29mm=1 1/8"
38mm=1.5"
54mm=2"
64mm= 2.5"
75mm=3"
98mm=4"
115mm=4.5"
152mm=6"

So they do go to relatively round numbers. Why the exact dimensions they are, well, who knows.

luke strawwalker said:

In the early days, rocket motors were rated in pound-seconds rather than newton seconds. The old motor designations reflected this... you didn't buy a B4-2 or a C6-5, it was more like a B.8-4 or something along those lines...

Click to expand...

It was exactly a B.8-4. This is from an S & T engine data sheet from 1962.

I think the standards came about based on popularity. If Estes was making motors in these sizes, then anyone wanting to make rockets compatible with Estes motors had to accept their standards. And then other motor manufacturers wanted their motors to fit in Estes rockets, and so on. Eventually you have an unquestioned industry standard.

luke strawwalker said:

Oh, there was an explanation of all that somewhere... either here or elsewhere on the web. I'd tell you to do a search but someone would flame me out of my frakkin' mind for even suggesting it... LOL

Click to expand...

I think I posted a link here a while ago to a very nice article about this which covered the sizing topic at least in part, but I made the error of not doing a PDF print of it and saving it locally and later discovered that the link is dead because the site it was on is no longer in existence.

Winston said:

I think I posted a link here a while ago to a very nice article about this which covered the sizing topic at least in part, but I made the error of not doing a PDF print of it and saving it locally and later discovered that the link is dead because the site it was on is no longer in existence.

Click to expand...

Have you tried archive.org to see if they cached a version, or whether Google cached a version?

I ask because now I'm curious.

Winston said:

I think I posted a link here a while ago to a very nice article about this which covered the sizing topic at least in part, but I made the error of not doing a PDF print of it and saving it locally and later discovered that the link is dead because the site it was on is no longer in existence.

Click to expand...

That's a shame...

Later! OL JR

blackbrandt said:

Also...
13mm=0.5"
18mm=0.75"
24mm=1"
29mm=1 1/8"
38mm=1.5"
54mm=2"
64mm= 2.5"
75mm=3"
98mm=4"
115mm=4.5"
152mm=6"

So they do go to relatively round numbers. Why the exact dimensions they are, well, who knows.

Click to expand...

Those conversions are pretty far off. Even 13 mm is 0.512 inches and it's the closest. 24mm is 0.945, almost a 16th of an inch off 1 inch. Your conversions are probably close enough for rough carpentry, but not fine woodworking and not even in the ballpark for rockets or machining. The reason I asked this question is exactly because the conversions weren't close enough to typical fractional imperial measurements.

Thanks for the history. I did search but the actual answers were buried in noise.

enderw88 said:

Those conversions are pretty far off. Even 13 mm is 0.512 inches and it's the closest. 24mm is 0.945, almost a 16th of an inch off 1 inch. Your conversions are probably close enough for rough carpentry, but not fine woodworking and not even in the ballpark for rockets or machining. The reason I asked this question is exactly because the conversions weren't close enough to typical fractional imperial measurements.

Thanks for the history. I did search but the actual answers were buried in noise.

Click to expand...

They do mostly follow imperial fractional units, though not in any regular order... CTI's website lists the Pro24 casings as 15/16'', the Pro29 casings as 1.125'', the Pro38 casings as 1.500'', Pro54 as 2.125'', and the Pro98 as 3.875''. The only one that doesn't fit is the Pro75 with a 2.965'' OD.

https://www.pro38.com/pdfs/Pro38_dimensions.pdf

As per Aerotech Consumer Aerospace FB page:

"There has been some speculation over on TRF about the origin of our hobby rocket motor casing diameters. Here is my recollection of it.
29mm (1.125"): Coaster/Mini-Max E & F motors and Enerjet E, F & G motors. Also used by George Roos/FSI with their rust colored phenolic cased composites. '60s and '70s.
38mm (1.500"): First adopted by Scott Dixon of Vulcan Systems with his fiberglass cased composites. Mid '80s.
54mm (2.125"): John Davis and Gary Rosenfield of Composite Dynamics built 2" O.D. experimental motors to fit their 2-1/8" O.D. fiberglass airframes, and later made motors using the same airframe tubing. Mid to late '70s.
75mm (2.965"): Suggested by Karl Baumann and first manufactured by AeroTech. Designed to fit LOC 3" tubing. Mid '90s.
98mm (3.875"): First manufactured by AeroTech and designed to fit LOC 4" tubing. Not based on any "off spec" tubing sizes. In fact the first motors of this diameter were phenolic cased single-use. Mid to late '80s.
Anyone with additional or more accurate information is welcome to provide it."


JD