How accurate are motor diameters?

Not sure how to ask this without sounding too stupid, but....
How accurate are motor diameters?
38mm is just a tad under 1.5 inches, will a 38mm motor actually fit in a 1.5" ID tube or is there some play in the measurements?
54mm comes out to 2.12598..... inches or just a smidge more than 2 1/8, but not much; is a 54mm motor actually a 2 1/8 motor rounded to the nearest mm?
Hope I don't come off as too stupid here, just wondering.

Greg

Originally posted by 11Bravo
Not sure how to ask this without sounding too stupid, but....
How accurate are motor diameters?
38mm is just a tad under 1.5 inches, will a 38mm motor actually fit in a 1.5" ID tube or is there some play in the measurements?
54mm comes out to 2.12598..... inches or just a smidge more than 2 1/8, but not much; is a 54mm motor actually a 2 1/8 motor rounded to the nearest mm?
Hope I don't come off as too stupid here, just wondering.

Greg

Click to expand...

Our casings are designed to an inches standard, their millimeter designation is the closest equivalent.

Motor diameters are english inch sizes. Most are expressed in millimeters as a convenience in product respecification, such as in kits.

13mm 0.49
18mm 0.69
24mm 0.93
29mm 1.12
38mm 1.50
54mm 2.12
66mm 2.50
75mm 3.0
95mm 3.75
98mm 3.875
152mm 6

Jerry

Thanks guys, that's kind of what I figured.
Have a spankinly good one,

Greg

Shouldn't that be:

76mm = 3" ??



JD

Originally posted by Jerry Irvine
Motor diameters are english inch sizes. Most are expressed in millimeters as a convenience in product respecification, such as in kits.

13mm 0.49
18mm 0.69
24mm 0.93
29mm 1.12
38mm 1.50
54mm 2.12
66mm 2.50
75mm 3.0
95mm 3.75
98mm 3.875
152mm 6

Jerry

Click to expand...

Originally posted by JDcluster
Shouldn't that be:
76mm = 3" ??
JD

Click to expand...

In theory but the term 75mm has become a "term of art" and the actual case OD is sometimes actually 2.98" (75.692mm).

Jerry

in rocketry, 75 mm = 76 mm = 3 inch

Scott McNeely

Why wasn't 3" actually used - it seems like 3" OD aluminum would be easy to source than having to turn it down slightly.

Edward

Originally posted by edwardw
Why wasn't 3" actually used - it seems like 3" OD aluminum would be easy to source than having to turn it down slightly.

Edward

Click to expand...

The real answer is the kit makers all ordered 3" ID tubes not 3.02" to fit unground cases.

Inertia.

Jerry

The real reason that I was asking is that I had just toyed around and came up with the design I posted here-
https://www.rocketryforum.com/showthread.php?s=&threadid=15117
and was wondering what size tube to get as the engine tube for a 54mm motor.
Specifically if 2 1/8 would work or if I would need to get something else and see if I could get the inside bored out a bit.
I just now have found some that I'm pretty sure would work at wicks aircraft.
Thanks for the info,

Greg

Originally posted by 11Bravo
The real reason that I was asking is that I had just toyed around and came up with the design I posted here-
https://www.rocketryforum.com/showthread.php?s=&threadid=15117
and was wondering what size tube to get as the engine tube for a 54mm motor.
Specifically if 2 1/8 would work or if I would need to get something else and see if I could get the inside bored out a bit.
I just now have found some that I'm pretty sure would work at wicks aircraft.
Thanks for the info,

Greg

Click to expand...

You want a LOC BT-2.15 or a USR BT-22

Jerry-
Thanks for the suggestions, but if you would, could you reread the original post and have a look at the .rkt file; the rocket in question is make entirely out of 6061 aircraft aluminum.
Not so much to be a rocket to fly, although that would be great too, but a project to better my TIG welding abilities.

Greg

The OD of a 54mm case is 2.125 - but the ID is much smaller than thinwall (.067, .058, .049) - many people are turning down Schedule 80 2" aluminum pipe to 2.125" and that makes a '54 mm' motor. You can also leave a thrust ring in it by not turning down the last 1/2" or so.

Edward