# Why mixed units for motors? 18mm by 2.75 inches

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#### Zeus-cat

##### Well-Known Member
18mm X 2.75 inches.
24mm X 2.75 inches
24mm X 3.75 inches

Mixing measurement systems was a sure way to get points taken away on any homework or test when I was in school. So why is it the standard way to describe an Estes or Quest motor?

One of those accidents of history, as I understand it. And the inconsistency has been recognized by all involved, practically since day one. Once you get used to it, it seems to work, though. Someone more knowledgeable than me about the very earliest days of model rocketry can provide a more detailed explanation. (I was flying rockets during some of the early days, but I don't go THAT far back.)

BTW, NAR S&T motor certification documents always report all of the dimensions and weights (and thrust, of course) in metric units.

MarkII

In its first few catalogs, at least, FSI listed all measurements of its motors and its kits in metric units.

https://www.ninfinger.org/rockets/catalogs/fsi67/67fsicat.html

By 1970, though, even they were starting to mix units (see the component pages).

https://www.ninfinger.org/rockets/catalogs/fsi70/70fsicat.html

Canaroc appears to have listed dimensions in both systems in its catalogs. (I don't remember when Canada officially went all metric, but it was sometime in the 1960's, as I recall.)

https://www.ninfinger.org/rockets/catalogs/canaroc80/canaroc80.html

MarkII

metric units rules...
i can't understand inches, foot, arms, livers

why divide by 12 instead of dividing by 10???
i have 10 fingers in my hand, do you(non metric users) have 12 in yours???

eheheh

about this post, i can't figure out why there is this difference in units, that's a big pain for me!!!

i think units must be revisited, i mean, we don't need more than one unit to measure something, isn't it? so why don't use only the easy and reliable metric units worldwide??

The fractional, or Imperial, or whatever, system is intuitive if you grew up with it; otherwise, I can see your point. And the metric system of measurement, for those of us who did not grow up with it, is non-intuitive. Sure, we understand that everything is in units of 10, but when you are used to the other system, you don't immediately know what distances such as a millimeter, a meter and a kilometer correspond to in your everyday world. If I was to invent a system of measurement from scratch, I doubt that I would come up with anything like the Imperial system. But for me and other people in the US, it is not a matter of whether it makes sense; it is just what is - it's the norm. Words like "feet," "inches" and "miles" are part of our everyday speech, and are commonly used in a metaphorical as well as a literal sense; words like "meter" and "kilometer" are not. The United States, BTW, has officially been on the metric system for awhile now, but the law has no enforcement provisions.

In model rocketry in the US, metric measurements pertain specifically to rocket motors; everything else is expressed in Imperial units. This includes kit dimensions, launch site dimensions in the Safety Code, etc. The reason that there are mixed units is that in the mid 1960's, NAR and the model rocket industry decided to switch to using metric units to describe rocket motor performance. But nothing else was converted over. Yes, it's kind of odd. But you get used to it pretty quickly.

MarkII

metric units rules...

why divide by 12 instead of dividing by 10???
i have 10 fingers in my hand, do you(non metric users) have 12 in yours???

eheheh...
Oh, it's even worse than that. Distances between one inch and one yard are expressed in units of 12, but distances of less than one inch are expressed in units of 8. The distance of one mile, which seems to be so intuitive and obvious to those of us in the US, seems to be rather arbitrarily set at 5280 feet (or 1760 yards). Neither the number 5280 nor the number 1760 have any particular significance otherwise, and neither one is an obvious or convenient multiple of more basic units like the foot or the inch. Most users of the Imperial system do not even think of the distance of one mile in terms of feet or yards. And even though our automobiles measure the distance they are driven in miles, they also break down the miles into fractions of 10. The division of the mile into tenths is done nowhere else but in the automobile. Most Americans cannot even tell you how far one-tenth of a mile is, unless they drive it in their cars.

As for the terms for the different units, Americans do not ever think of the unit of one foot in terms of a literal body part. To us, the fact that the same word can describe two different things is just a coincidence. It is not until we get older, and we study history, do we learn that the unit of one foot was originally based on the actual length of a human foot. But we understand that there is no connection between the two anymore.

I can see how all of this seems to be just crazy, but when you grow up with that system, it is how you see your world, and so it all makes sense.

MarkII

The United States, BTW, has officially been on the metric system for awhile now, but the law has no enforcement provisions.

The Metric Conversion Act of 1976, signed by Gerald Ford.

And then there are the tire companies, that cannot make up there minds ... like the 235/55-17 tires on our minivan ... metric width on an Imperial-sized wheel.

I grew up in the US, so the imperial measurements are what I am used to. However, I also learned all the metric units in science and engineering courses I have taken. I could switch to metric tomorrow with only one problem - temperature.

Fahrenheit temps are just completely ingrained in me and I can't handle Celcius. Meters, kilometers, liters, no problem. Tell me the temp will be 15C and I have to convert it to F to know if it is hot or cold. I know 0 is freezing and 100 is boiling, but everyday Celsius temps are just meaningless numbers to me.

I grew up in the US, so the imperial measurements are what I am used to. However, I also learned all the metric units in science and engineering courses I have taken. I could switch to metric tomorrow with only one problem - temperature.

Fahrenheit temps are just completely ingrained in me and I can't handle Celcius. Meters, kilometers, liters, no problem. Tell me the temp will be 15C and I have to convert it to F to know if it is hot or cold. I know 0 is freezing and 100 is boiling, but everyday Celsius temps are just meaningless numbers to me.
Except that -40° is -40°.

Come up to the Adirondacks in January, and see for yourself.

MarkII

Except that -40° is -40°.

Come up to the Adirondacks in January, and see for yourself.

MarkII

No need, I grew up in Minnesota. Our state motto is: "Minnesota, great for winter sports 10 months of the year - the other two months are to cold."

particularly since they are not 18mm but 17.526mm - sorry, just saw the chance to be geeky!

Things get really goofy when you measure things with engineering tools like calipers and micrometers. Now the inch is divided by factors of 10. Bazooka Dale's number, 17.526mm = .69 inches

I just built a racing engine with a piston to cylinder wall clearance of .002, 2 thousanths of an inch. No idea of that distance in mm, now I think that's in micrometer distances.

edit: .002 = 50.8 micrometers

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Things get really goofy when you measure things with engineering tools like calipers and micrometers. Now the inch is divided by factors of 10. Bazooka Dale's number, 17.526mm = .69 inches

Actually I didn't measure anything - I worked backwards. Estes catalogs throughout the '60's and early '70s listed the diameter of standard motors as.690" Somewhere online I found a really early Estes catalog that listed them as 11/16" OD and 13/32"ID (for the original thick wall casings)

...
why divide by 12 instead of dividing by 10???
i have 10 fingers in my hand, do you(non metric users) have 12 in yours???...

eheheh

One thing about the foot unit of length: one foot is divisible by 2, 3, 4 and 6.

One other thing: in the USA we also measure land area in Rhode Islands.

MarkII

As a Canuck kid I was in grade school when we switched to metric. Naturally we resented having to change as we had already learned the Imperial system. Many Canadians saw it as more of Trudeau's frenchification of Canada.
A lot of the work in this country is still done in Imperial units, fabrication, construction etc. but it's inconsistent so were forced to know both systems. I need two socket sets to work on my old safari van.
However, when converting to scale the multiplication factor will give decimal imperial units. I find it much simpler to multiply decimal inch values by 25.4 to give the value in millimeters than to determine how many 32nds of an inch it is. Millimeters being smaller than 16ths also allow for more precise measurement.
I really appreciate the vendors who provide metric equivalents in their instructions. Ted

All of this debate overlooks the fact that use the metric (SI) system is much more widespread in the United States than people often assume. In the medical and pharmaceutic fields, for instance, it is used exclusively. A growing number of everyday items are now packaged or designated in SI units, and Americans have accepted them without any problems or objections. (Just try to buy bottled water by the pint or quart anywhere in this country; it can't be done. You can only find it in liters.) Weather forecasts frequently report the temperature in Celsius, sometimes exclusively in that scale. If you mention a millimeter, a meter or a kilometer, most people in the US have an immediate intuitive grasp of what those distances look like. The gram unit of mass is becoming more and more widely recognized. In our own hobby, we long ago converted our force measurements to Newtons without any difficulty, and I am beginning to hear that term mentioned on occasion in popular media and in news reports. These are all just a few examples. But as Zeus-Cat illustrated in his original post, the process of conversion is not yet complete here. But it is happening, bit by bit, and at an ever-increasing rate, I think. Our children are taught it in school, and I, too, was taught it in grade school some 45 years ago. But back then, I never heard metric units used for anything in my everyday world. Now, I encounter it all the time. So the question, "why doesn't the United States use the SI system" is a false one. The plain truth is, we already do! It is just not used exclusively yet, but it will be, eventually, and probably sooner than many critics think. Our government isn't forcing it down our throats, though; instead, we Americans are adopting it on our own without requiring any mandate from a central authority to do so. Has any other country ever done that?

MarkII

I remember during the oil crisis in the 1970's, that many gas pumps had to be set up to dispense in liters, because they could not set the price per gallon high enough. Ahh, the days of mechanical gauges! :roll:

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The distance of one mile, which seems to be so intuitive and obvious to those of us in the US, seems to be rather arbitrarily set at 5280 feet (or 1760 yards). Neither the number 5280 nor the number 1760MarkII

The mile is indeed a very obvious unit being 80 chains. A chain being 22 yards or the length of a cricket pitch. A furlong for horse racing is 10 chains. 10 square chains is one acre. An acre is historically the area of land that can be ploughed by a team of 8 oxen in the morning.

All totally logical - mind you for engineering use I would rather work in watts rather than foot poundals / second or Newton metres rather than ounce feet :shock:

The speed of light in vacuum is 1.8 Terafurlongs per fortnight.

Bob

slugs aren't bad, what gets confusing is when one class is mostly in SI, one is mostly English Slugs/pounds/feet/ect/ and one uses both systems and pound-force / pound-mass.

And then there is the nameless wounder
ounce force * s^2
. inch
or a variety of similar unit types derived from swapping ounces for pounds and inches for feet from the original slug. One professor gave us a list of about 20 different names for this unit and it all depended on what company you worked for. That is one way to protect trade secrets work in your own unit system.

The speed of light in vacuum is 1.8 Terafurlongs per fortnight.

Bob

Ha!

The speed of light in vacuum is 1.8 Terafurlongs per fortnight.

Bah! Light can't travel in a vacuum. There's nothing for it to push against.

-- Roger

But seriously ...

I don't think mixing of units is a bad thing in rocketry. It's easier to refer to a "quarter-inch" launch lug than a "6.35 mm" launch lug. And "18 mm motor" is easier to remember than ".71 inch motor."

As long as there's no need to, for example, multiply launch lug diameter by motor diameter, the mixed units don't cause problems.

For calculations, metric units are much easier to use, of course. But, for measuring, especially when you need to divide something into segments, using feet or inches is often much more practical.

-- Roger

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