Is There Any Logic To Body Tube Designations

[DaHabes]

This is why I love TRF; in so many other places on the internet, being challenged is a way to get into a fight. I hope it was clear that "I read it somewhere, and I can't find the source right now" is tantamount to "I'm not too sure of this, but..."

I, too, would rather go metric on this (and many other things, if not everything). But I must point out that the OD is not 25 mm. If 0.976 in. is correct to three significant digits then it comes to 24.8 mm. Apogee reports the same, though that may be converted from 0.976 just as my number is.

I have a spreadsheet. Common name, dimensions in inches, dimensions in metric, by vendor (Estes, Madcow, Mach1, LOC, Always Ready Rocketry, PML, Quest, Centuri (eRockets)). It covers body tubes, tube couplers, nose cones, Aeropak retainers, etc. Impossible to build scratch without a full set of dimensions to put part together.

 

[MetricRocketeer]

I, too, would rather go metric on this (and many other things, if not everything). But I must point out that the OD is not 25 mm. If 0.976 in. is correct to three significant digits then it comes to 24.8 mm. Apogee reports the same, though that may be converted from 0.976 just as my number is.

Hi @jqavins and everyone else,

Thank you for your reply.

That's fine. Exactly 0.976 inches = exactly 24.7904 mm. I will be glad to use as many significant digits as anyone wishes. Let's go with one significant digit. Than we are at 24.8 mm. Let's assume that we could measure the outer diameter of the tube to the nearest tenth of a millimeter. I would still prefer to use that figure than to use 0.976 inches.

In fact, the typical ruler would not have any of those numbers on it in inches, right? It would not show 0.976 inches, nor would it show 0.98 inches, nor would it even show 0.9 inches. To get good precision, we would have to start using fractions, and that is way harder than using decimals.

Stanley

 

[OzHybrid]

As a child of the late 50's I was brought up initially on imperial measurement and switched to metric. I CAN work in either. I prefer metric for most things but given a choice I use both. For measuring rockets, I use mm, for being hit over the back of the knuckles I prefer an Imperial measurement as the standard rulers are slightly shorter...........
Sometimes there is no logic to be found, it just is what it is.

The Serenity Prayer encompasses body tube logic.

God, grant me the serenity to accept the things I cannot change,
The courage to change the things I can,
And the wisdom to know the difference.

 

[MetricRocketeer]

Hi @RocketScientistAustralia and everyone else,

Thank you for your message.

I congratulate you on your maximum-altitude flight measuring 26,240 feet. How many miles was that? Let's get out the calculator to divide 26,240 feet by 5280.

And I wish you good success on your flight in which you expect to achieve about 70,000 feet. So how many miles will that be? Out comes the calculator, and we get about 13 miles.

Let's do the last one using the metric system. You expect to achieve an altitude of about 21336 metres. Hmm ... I wonder how many kilometres that will be. Please wait while I retrieve my calculator ... no wait, 21336 metres equals 21.336 kilometres. Done and dusted.

Stanley

 

[lakeroadster]

canons? you mean cannons?

You write cannons, ThirstyBarbarian writes canons... let's call the whole thing off

 

[neil_w]

I congratulate you on your maximum-altitude flight measuring 26,240 feet. How many miles was that? Let's get out the calculator to divide 26,240 feet by 5280.

… etc …

I hope you are not laboring under the assumption that any of us are unaware or in denial that these sorts of calculations are simpler in metric.

 

[MetricRocketeer]

I hope you are not laboring under the assumption that any of us are unaware or in denial that these sorts of calculations are simpler in metric.

Hi @neil_w and everyone else,

Then let's act on our knowledge and start using the metric system.

Rocket kits should be labeled in metric units. Altitudes should be recorded in metres and kilometres, and weights should be listed in grams and kilograms.

Stanley

 

[OzHybrid]

Moving back to the question asked, I've never been able to find a logical reason for the Body Tube sizing numbers. Other than they are numbers with gaps between the numbers for other tubes bigger than the next smaller number tube but smaller than the next bigger number tube. It also depends on said manufacturer of the BT, as we then get into manufacturing tolerances. So I either buy a kit where the manufacturer of the kit has taken all this into account or buy body tube, or fibreglass tube or phenolic and work from the tube manufacturers spec before it arrives and measure it once I've got it in my possession.
The units you measure things in are up to you. I'm easy with both as long as they are accurate. That would be inches to 3 decimal places =mm to 2 and a bit decimal places. So 2dp is not enough and 3 is too many decimal places...... so specifying an imperial size to 3 dp in metric to 3 dp is specifying it too accurately. @MetricRocketeer is right.....ish. In a Utopian world, metric would be a better system. But we're not in Utopia. Which is why I've just made them the first person to get put on my ignore list.

 

[BEC]

The explanation for Estes body tube size designations has been posted in this thread....straight from an interview with Bill Simon who created the "system". Centuri used something slightly more rational in that ST-7 was about 0.7 inches in diameter, ST-10 was about 1 inch in diameter, ST-16 was about 1.6 inches in diameter, and so forth (sizes corresponding roughly to Estes BT-20, BT-50 and BT-60, though ST-10 is enough bigger than things don't interchange well — or 18mm, 25mm and 40mm). The real mystery to me in the Estes "system" is BT-10 and BT-20, which both have the same ID, but BT-10, which was mylar, had much thinner walls, so the OD was less....!

And yes, the standard 18mm diameter model rocket motor diameter was set by the company who made the first mass-produced ones for Model Missiles Inc in the late 1950s because they made a firework (Buzz Bomb) that used the same tube. The handmade Carlisle motors were closer to 1/2 inch in diameter.

I don't know what other LPR companies did "back in the day" as I was only familiar with Estes and Centuri in my first rocketry period (late 1960s to early 1970s). I still think in Estes BT tube sizes even though the system is pretty much nonsense, as Mr. Simon has admitted, as that is what I used back when I would wear out a catalog looking at it over and over and over. Quest, which didn't exist when I got out of rockets the first time, and now has been acquired by AeroTech, uses millimeter sizes for its tubes.

As for MPR and HPR, it seems they mostly work in inch sizes it seems, save for motor mounts. I have no idea why motor mounts are generally referred to in millimeters when everything else is in the King's units..... . But this is not my area of experience or expertise, so I could be all wrong about that.

 

[bjphoenix]

I have a spreadsheet...

Do you share?

 

[MetricRocketeer]

As for MPR and HPR, it seems they mostly work in inch sizes it seems, save for motor mounts. I have no idea why motor mounts are generally referred to in millimeters when everything else is in the King's units..... . But this is not my area of experience or expertise, so I could be all wrong about that.

Hi @BEC and everyone else,

I have a theory about why motor mounts are referred to in millimetres. It is because motors are specified exclusively in wall-to-wall metric terms -- their diameter and length are stated solely as millimetres, their thrust is identified in newtons (a metric unit), their total impulse is indicated in newton seconds (a metric unit), and their propellant mass and total mass are given in grams (a metric unit).

Stanley

 

[Blast it Tom!]

Hi @neil_w and everyone else,

Then let's act on our knowledge and start using the metric system.

Rocket kits should be labeled in metric units. Altitudes should be recorded in metres and kilometres, and weights should be listed in grams and kilograms.

Stanley

Aw, you take all the fun out of it!

Do you share?

It's pretty easy to look it up on erockets. They have lots of data on body tubes, couplers and so on.

 

[OzHybrid]

There's more on the subject in this old thread. And there's some more information here.

Using data from this page, I condensed out the table attached. (The page gives dimensions in inches. The sheet of millimeter values has conversions therefrom.)

I've reviewed the source and your spreadsheet data. Both are the victim of rounding errors. In the original source the dims are imperial. Adding 2x wall thickness to the inner or subtracting from the outer does not result in the other dimension. Additionally when they have been converted to metric and rounded again a further size creep has gone in due to metric rounding. Having said that, I plan on using it with that knowledge that its almost correct..... and close enough for me. Thanks for taking the time to put it together.

 

[BEC]

Hi @BEC and everyone else,

I have a theory about why motor mounts are referred to in millimetres. It is because motors are specified exclusively in wall-to-wall metric terms -- their diameter and length are stated solely as millimetres, their thrust is identified in newtons (a metric unit), their total impulse is indicated in newton seconds (a metric unit), and their propellant mass and total mass are given in grams (a metric unit).

Stanley

Well, actually, model rocket motors' thrust and total impulse were expressed in Imperial units until Estes went to metric units some time in 1967, as the conversion, including a chart to show old and new designations appeared in the 1968 catalog. Here's a screen shot of those pages from the 1968 Estes catalog as posted on Estes' site:


Also, to me, Estes motors have always been 2.75 inches long or 1.75 inches long as shown here, and later 3.75 inches long (E9s/E12s) and 4.5 inches long (E16s/F15s), so no, they're not exclusively and universally referred to only in metric units. The later motors did not exist when this catalog page was made. All 24mm and 29mm diameter Estes motors, and even the 13mm diameter ones (which are 1.75 inches long also) came after this 1968 catalog page, in some cases several decades later.

Here are the equivalent charts from the 2021 catalog:



Interestingly, here the diameters are given only in millimeters and the lengths are not listed at all.

 

[OzHybrid]

Well, actually, model rocket motors' thrust and total impulse were expressed in Imperial units until Estes went to metric units some time in 1967, as the conversion, including a chart to show old and new designations appeared in the 1968 catalog. Here's a screen shot of those pages from the 1968 Estes catalog as posted on Estes' site:
View attachment 491411

Also, to me, Estes motors have always been 2.75 inches long or 1.75 inches long as shown here, and later 3.75 inches long (E9s/E12s) and 4.5 inches long (E16s/F15s), so no, they're not exclusively and universally referred to only in metric units. The later motors did not exist when this catalog page was made. All 24mm and 29mm diameter Estes motors, and even the 13mm diameter ones (which are 1.75 inches long also) came after this 1968 catalog page, in some cases several decades later.

I like the prices..

 

[BEC]

Yeah.....though as a junior high school kid in 1968, even those prices were, to me, a bit of a barrier.

 

[Initiator001]

I recall reading (no time to locate now) that the original 18 mm engine diameter came from off the shelf available parallel wound tubes from the fireworks industry. And probably the same for the 24 mm engines

Thanks to G. Harry Stine the history of how the first mass-produced model rocket motors came to be .69" in diameter by 2.75" long has been preserved (Yes, it was decided by the easy availability of a tube used by a fireworks company).

However, how was the size of the D13/D12/D11 motor casing determined?

During NARAM-52 in Colorado I had the opportunity to discuss this with Ed Brown.
Ed had been hired by Vern Estes in the mid-1960s to operated the motor making machines at Estes and retired from there after 40 years of employment.

Ed had a wealth of knowledge about making black powder motors.

According to Ed, Vern asked him to develop a new, larger rocket motor.
The criteria Ed was give by Vern was:
1) The motor had to fit in a BT-50 body tube.
2) The motor had to be able to use the same metal motor hooks as the 18mm motors.

Ed told me it wasn't easy and even after the D13 had been developed, tested and released the motor had to be detuned to a D12 as there were too many reports of problems from users.

So, sometimes the determining factor in producing and releasing a new product has more to do with making it compatible with existing materials than anything else.

 

[BEC]

Thanks for that, Bob. I wondered if the 24mm Ds (and later C11s) weren't sized to use BT-50 tubes as motor mounts. So that has lived on in black powder and composite motors ever since...

And that constraint of using the standard motor hooks rings true. I had a chance, at one time a few years ago, to spend an afternoon and evening with Bill Simon and his wife and one of the things that came up in our conversation was that even at the beginning, the creation of new parts was discouraged.

 

[Initiator001]

Quest, which didn't exist when I got out of rockets the first time, and now has been acquired by AeroTech, uses millimeter sizes for its tubes.

Quest tubing diameters are in millimeters because the molded plastic parts come from the molds made by MPC in 1969-1970.

G. Harry Stine was a consultant to General Mills which owned MPC and promoted that the products use metric sizes.
So the body tubes were 20mm, 25mm, 30mm, etc.

Having previously manufactured molds to use saved time and cost to get Quest up and running.

 

[BEC]

Quest tubing diameters are in millimeters because the molded plastic parts come from the molds made by MPC in 1969-1970.

G. Harry Stine was a consultant to General Mills which owned MPC and promoted that the products use metric sizes.
So the body tubes were 20mm, 25mm, 30mm, etc.

Having previously manufactured molds to use saved time and cost to get Quest up and running.

Thanks again. I was vaguely aware of the GHS and MPC connections, but didn't know the story well enough to tell it, so I wimped out and just said they used millimeter sizes, but not how that happened .

 

[Initiator001]

As for MPR and HPR, it seems they mostly work in inch sizes it seems, save for motor mounts. I have no idea why motor mounts are generally referred to in millimeters when everything else is in the King's units..... . But this is not my area of experience or expertise, so I could be all wrong about that.

Coaster/Atlas/Hercules/Mini-Max motors were originally listed as being 1.125" or 1 1/8" in diameter.
When motors went metric they became 29mm.

I vaguely recall that the Coaster motors were 29mm diameter because of some standard tube size Coaster discovered that would work for motor casing (Fireworks?).

 

[rocketlabdelta]

The criteria Ed was give by Vern was:
1) The motor had to fit in a BT-50 body tube.
2) The motor had to be able to use the same metal motor hooks as the 18mm motors.

And what determined the size of BT-50?

The size of early Estes airframes was determined by the largest diameter nose cone that could be turned from standard dimensional balsa stock:

Tube	Balsa Stock	Outside Diameter	Inside Diameter
BT-50	1"	.976"	.95" (24mm)

Granted, this is my recollection based on something I read 20 years ago—and I was wrong about everything else I said earlier—but I feel slightly vindicated.

Coaster/Atlas/Hercules/Mini-Max motors were originally listed as being 1.125" or 1 1/8" in diameter. When motors went metric they became 29mm.

Keep the history coming!

Does anyone know the backstory of 38mm and 54mm motors?

 

[Cape Byron]

Of course the Metric system is more logical. Every child knows that a metre is the distance travelled by light in vacuum in 1/299 792 458 second. Or that it used to be 1/10 000 000 of the meridian through Paris between the North Pole and the Equator.

 

[rocket_troy]

Whenever I see “BT-50” I automatically think of a CNC mill with that metaphorical “Ohhhh… YES I CAN!!!” plastered all over it

TP

 

[6inchmonster]

Is there any logic to the sizing? Does the designation mean anything?

No. The sooner we do away with Estes numbers as some kind of "standard", the better.

Same for "engine" letters for that matter but one dream at a time.

 

[OzHybrid]

No. The sooner we do away with Estes numbers as some kind of "standard", the better.

Same for "engine" letters for that matter but one dream at a time.

Yes. We should number the motors instead in some sort of loose sequence leaving space so we can add in other ones when they get made……

 

[6inchmonster]

Yes. We should number the motors instead in some sort of loose sequence leaving space so we can add in other ones when they get made……

I would have said name them with numbers that actually represent the measured performance characteristics but the first time around I thought that was obvious.

 

[bschultz32]

Seems like there are just some controversies that will never be resolved: PC's vs Macs, metric versus English units, peace in the Middle East, abortion, body tube designations . . . . it's a long list the children of our childrens' children will still be debating many years from now .. . .

 

[6inchmonster]

The eventual heat death of the universe will end BT sizes; probably the rest of those debates too.

 

[MetricRocketeer]

Well, actually, model rocket motors' thrust and total impulse were expressed in Imperial units until Estes went to metric units some time in 1967, as the conversion, including a chart to show old and new designations appeared in the 1968 catalog.

Hi @BEC and everyone else,

I will admit when I am wrong. You are completely correct on the specifications of model-rocket motors. And I was completely wrong.

I stand corrected.

Stanley