Why are they that size?

This struck me the other day ...

"Standard" motor sizes are, if I'm recalling correctly ...

13mm, 18mm, 24mm, 29mm, 38mm, 54mm, 75mm, 98mm, 150mm

...Why? Was this based upon existing tooling for other applications? Have these sizes always existed? I seem to recall hearing about some oddball motors (22mm?) from the 70s, but since I was a kid in the 80s, it was always 13 and 18mm and occasionally 24....

Is this just something Vern Estes decided upon?

Inquiring minds & all that....

The 13mm and 18mm came from the fireworks world before Estes. Orville Carlisle's original hand made Rock-a-Chutes were 12.7mm. When Mr. Stine started Model Missiles Incorporated he went to the Zenith Fireworks Company for increased production. Lawrence Brown convinced him to use a standard tube that he had which happened to be 18mm in diameter. Then Vern Estes visited the MMI office with his idea for a better mousetrap (the legendary Mabel) and he just used the "standard" 18mm tube. Mr. Stine never lost his love for little motors and brought them back in 1970 with the MPC Minijets. A nice article in the July '71 issue of Model Rocket Magazine has it all.


https://www.ninfinger.org/rockets/ModelRocketry/Model_Rocketry_v03n09_07-71.pdf

ActingLikeAKid said:

This struck me the other day ...

"Standard" motor sizes are, if I'm recalling correctly ...

13mm, 18mm, 24mm, 29mm, 38mm, 54mm, 75mm, 98mm, 150mm

...Why? Was this based upon existing tooling for other applications? Have these sizes always existed? I seem to recall hearing about some oddball motors (22mm?) from the 70s, but since I was a kid in the 80s, it was always 13 and 18mm and occasionally 24....

Is this just something Vern Estes decided upon?

Inquiring minds & all that....

Click to expand...

Roughly 1/2in, 3/4in, 1in, 1 1/4in, 1 1/3in, 2in, 3in, 4in, 6in

It's military metric.

There was a very nice article mainly about Enerjet but with info about the origin of motor sizes on a site that is now unfortunately dead.

Anyway, along with all of the above info:

https://v-serv.com/usr/fx-ldrs.htm

The first "HPR" motors of LDRS-1 were Composite Distribution brand 29mm H160 and 38mm I200. Jerry Irvine popularized the 38mm size which later became an industry standard. Gary Rosenfield popularized the 54mm size starting at LDRS-3 and 98mm at LDRS-8 which later became standards. Jerry Irvine later popularized the 75mm and 66mm and 152mm sizes. Motor diameter standards made multi-brand kit compatibility practical and has stood the test of time. Estes had already standardized 13mm, 18mm and 24mm, and Enerjet standardized 29mm. The first rocket of LDRS-1 was a U.S. Rockets Hi-Test 2420.

Winston said:

There was a very nice article mainly about Enerjet but with info about the origin of motor sizes on a site that is now unfortunately dead.

Click to expand...

Did you try the Internet Wayback Machine? So often I forget it's there, but it has saved my bacon on a few occasions.

There's also the oddball 32mm rocket glider reloads in the RMS 32/60-100 case. A G12 would be cool in a rocket but would require electronic deployment. 32mm, I suppose that's 1 1/4" also but perhaps they just picked a size? Perhaps partially based on the ideal fuel grain shape for a particular impulse level?

Cabernut said:

There's also the oddball 32mm rocket glider reloads in the RMS 32/60-100 case. A G12 would be cool in a rocket but would require electronic deployment. 32mm, I suppose that's 1 1/4" also but perhaps they just picked a size? Perhaps partially based on the ideal fuel grain shape for a particular impulse level?

Click to expand...

Only Aerotech would know, but I guess the reason for the oddball diameter is so one doesn't accidentally get put in a 29mm rocket. When the motor was designed (1990-91) electronic ejection systems were very rare and had questionable reliability. If you are interested in a good flight with a G12 keep the rocket relatively light. You would still need an electronic deployment system though, assuming you are using it in a conventional rocket. An RG can be a bit heavier because of the wings actually providing lift.

mwtoelle said:

If you are interested in a good flight with a G12 keep the rocket relatively light. You would still need an electronic deployment system though, assuming you are using it in a conventional rocket.

Click to expand...

This is on my to-do list at some point, one of many reasons behind my quest for tiny DD altis. Come to think of it, a Goblin would work pretty much out of the box....

dhbarr said:

...one of many reasons behind my quest for tiny DD altis.

Click to expand...

How small do you need? A Quark will fit in a 24mm coupler. Battery is under the quark. There is also a switch in there but not visible in picture.

I have a Quark, an AltiDuo SMT, and a Telemini V3 so far on the DD side. Great stuff.