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accooper

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OK, here is a question I have been wanting to ask for years. Why are model rocket motor thrust in Newtons instead of pound?

I have never understood this.

Andrew From Texas
 

Bazookadale

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In the '60s America was challenged to "Think Metric". There was a nationwide crusade to change to the metric system like the rest of the industrial world, or by the magical year 2000 the USA would be unable to compete internationaly and would be a third world country. The NAR and the model rocket industry converted to metric, most of the rest of the USA didn't.

The metric system does have advantages over the English system (or the American version of Imperial units) particularly in science. My 7th grade science class in 1966 spent about 4 weeks covering the metric system unfortunately I'm forgot much of that.

Edit - I remember my 11th grade Physics teacher explaining that one problem with the English system was that it used the same unit (pounds) to define both weight and force. A newton is force, not weight.
 
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MarkII

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I don't know. I prefer Pecan Sandies, myself. :p

Because SI measurements are easier to state and to calculate, I guess.

Stated in Newtons, the progression in maximum impulse from the "B" motor class to the "G" motor class, for example, is: 5-10-20-40-80-160.

State in Pounds, the same progression is: 1.10-2.2-4.4-8.8-17.62-35.24.

http://www.tripoli.org/tmt/motor_classes.shtml

And as Dale said, the Newton is a unique measurement of force; a Newton only means one thing. The same cannot be said for the Pound measure. Although here in the US we are quite accustomed to our system of measurements and it seems natural to us, our system is in fact the oddball one. Even though the Imperial (or "English") system evolved in England many centuries ago, we can call it ours now because we are the only industrialized nation that still uses it. Our continued use of it demonstrates that it is still a workable system, but it makes for awkward communication when we need to deal with vendors and manufacturers from outside of our borders. And converting from one system to the other can lead to some catastrophic errors.

MarkII
 

MarkII

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Perhaps a better question would be "Why do we continue to use Imperial measurements for everything else?"

MarkII
 

cjl

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Because metric is better?

Honestly, because metric in general makes more sense, and is the standard unit pretty much everywhere but here.
 

adrian

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Edit - I remember my 11th grade Physics teacher explaining that one problem with the English system was that it used the same unit (pounds) to define both weight and force. A newton is force, not weight.
[pedant]
Weight is a force. The kilogram is the SI unit for mass, not weight. Weight is the force exerted by gravity upon an object, calculated as mass x gravitational acceleration, and is measured in newtons.
[/pedant]

What this means is that going into space is a good way to lose weight. :)
 

TWRackers

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[pedant]
Weight is a force. The kilogram is the SI unit for mass, not weight. Weight is the force exerted by gravity upon an object, calculated as mass x gravitational acceleration, and is measured in newtons.
[/pedant]

What this means is that going into space is a good way to lose weight. :)
Weight, yeah. Mass, no. Unless you lose your lunch.
 

Bazookadale

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Perhaps a better question would be "Why do we continue to use Imperial measurements for everything else?"

MarkII
Because this is the United States of America and we do things the way WE want! My Subaru gets 564480 rods to the hogshead and that's the way I like it!:neener:
 

GregGleason

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It is interesting that the NASA PAO (or whatever he was) who was giving the liftoff play-by-play yesterday still uses good old imperial units.

" ... kilometers downrange at a velocity of 890 meters per second ..." just doesn't sound quite right.

Greg
 

tibadoe

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Just my 2 cents but I think metric is easier to work with.
 

clreynolds

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It all depends on what you are used to.

We tend to be a bit schizophrenic. Almost everyone knows motor diameters in SI units, 18mm, 29mm, 54mm, etc. As well as the tubes they fit in. But talk about the length, and most will use english units 2.75", 3.75", etc. And if we are talking about airframe tubes instead of motor tubes, we will use english units 3", 7.51". Or maybe just an old random number used as a size designator BT-60.

It doesn't really make sense, but that is the way we like it.;)
 

MarkII

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It all depends on what you are used to.

We tend to be a bit schizophrenic.
Sorry, but I believe you mean "schizoid," not schizophrenic. There's a difference.

If we were schizophrenic, we would believe that the whole "going metric" thing was part of a larger conspiracy to impose a one-world government. And to mandate that the US adopt French as its official language. No one here in America believes that, right?

MarkII
 

GregGleason

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... part of a larger conspiracy to impose a one-world government. And to mandate that the US adopt French as its official language. No one here in America believes that, right?

MarkII
You know, this seems like deja vu. Hey, wait a minute ...

... I think I feel the need to watch a Jerry Lewis movie.

:eek:

Greg
 

luke strawwalker

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Depends on what you're doing....

I like regular imperial units because I can 'relate' how far or long or deep or tall or heavy something is easily because I'm used to dealing with those units for a lifetime. If I see a cotton bale, I instantly "know" that it's about 500 pounds, that my tractor is 8,000 pounds, or 4 tons, that my other farm is 90 miles away, my rocket flew to 1,000 feet, etc. But, if someone tells me that the cotton bale is 250 kilos, or my tractor is 4 metric tons, or the other farm is 180 kilometers away, my rocket flew 300 meters, etc. I have to actually CONVERT that measurement into something I can 'relate' to-- IE FEET, MILES, POUNDS, TONS before I can get a 'grasp' of what's really being said. Even my little examples above are pretty far off the mark because a kilo isn't REALLY 2 pounds, a meter isn't REALLY a yard or 3 feet (that one I know-- 1 meter is 39.4 inches) etc. and it gets even MORE 'out there' when you start talking about measuring things like liters vs. gallons, cubic centimeters vs. cubic inches, centigrade vs. fahrenheit etc. because you REALLY have to go look those up and do the math to get any meaningful information out of the 'measurement'. How many liters does my 200 gallon spray tank hold?? I don't have a clue. How cold if 5 centigrade?? how hot is 40 centigrade?? Is it hot or just warm?? How many cc's is a Ford 302 V-8?? (Ok, that one I know, because I'm a mechanic-- 302 CI= 5.0 liters, or 5,000 cc, since there's 1,000 cc per liter) but otherwise it gets REALLY confusing.

Now, being a mechanic, I usually like to think in terms of decimal parts of an inch (thousandths of an inch) rather than the fractional parts of an inch, like you do with woodworking and carpentry. I end up having to use both. It's MUCH easier to convert saying 1.250 inches rather than 1 1/4 inch, etc. but they don't make tape measures that way. Steel machinists rules, yes, but 30 foot tape measures, no. Shame really. I can do either one but usually I keep a fractional/decimal conversion chart handy. Anytime you're scaling anything, the decimal stuff is MUCH easier to use! Of course it helps to have everything in the same units, say inches, rather than using different units like some stuff does... feet for some things, inches for others, etc.

I REALLY like metric for working with rockets, and I can see why it makes SO much better sense for engineering... scaling and switching units is as easy as moving decimal points over-- eminently practical. The main thing is to make sure everything is in the same units (not some measurements being meters and others centimeters or millimeters) so the decimal gets moved the right number of spots. And, of course, make sure it's not some hodge-podge mixture of imperial and metric units! NASA got burned by that one awhile back!

SO, I'd say that the PAO, in directing it's comments to the 'uninformed' general public, chooses imperial units because that's what people can 'envision' easier and get a grasp of what's being said... like in the movie Apollo 13 when Gary Sinise as Ken Mattingly says, "on my mark, your velocity will be 35,302 feet per second-- MARK! Stand by for entry interface..." Well, being a rocket guy, and knowing there's 5280 feet per mile, that's roughly 7 miles a second. If they'd given that in meters/second, I'd have to look it up and convert, so the number has no 'impact' without translation.

As to the OP-- well, ORIGINALLY the imperial units of pounds-force were used in engine designations, and it's more confusing IMHO... when I read about engines from the late 50's/early 60's being a B1.3-4 is a lot less meaningful than saying B4-4 or something like that. Introducing decimals into the mix is kinda silly, because really using pounds to measure our rocket engines, until you get into the HPR realm, is really using a unit that is too big and so it must be broken down to decimals or fractions. It would be like a cartoon I once saw-- a General is standing on one of those big scales with the readout at eye level, and the readout says, "0.0000000000000000194 MEGATONS"... kinda silly to use such mismatched units... about like using miles to measure distances much beyond Mars distances... the numbers are so huge they become meaningless...

SO, anyway, it depends on what you're doing and what you're comfortable with. I remember until a few years ago, ALL the vehicles had both MPH and KPH on the speedometers, and a few years ago I noticed that the KPH had all gone away-- now I see some that have it and some don't depending on the market the car is being built for and if it's exported or sold internationally, etc.

I prefer to stick with imperial for stuff you 'need to visualize easily' and use metric for more 'scientific' measurements, but that's just me. It's hard to visualize a 'pound-force' so I prefer the 'newtons' since it's an easier number to deal with since no decimal is used.

Later! OL JR :)
 

JoeLaunchman

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America will never fully convert to metric for one reason: football. Phrases like,"And he's tackled at the 1 meter line!", "This will be a 45 meter field goal attempt", and "That was a spectacular 60 meter kick return" just don't sound as good.

The Imperial system is so ingrained in our culture that the present hybrid metric/Imperial system will continue indefinitely.
 

bradycros

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Because this is the United States of America and we do things the way WE want! My Subaru gets 564480 rods to the hogshead and that's the way I like it!:neener:
So in othe words, your getting 28 mpg.
 

daveyfire

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And, of course, make sure it's not some hodge-podge mixture of imperial and metric units!
Unfortunately, most of the aerospace propulsion industry is just this way -- speeds in ft/sec, altitudes in m, temperatures in K, pressures in psi. And don't even get started trying to justify Imperial units of Isp in seconds :cyclops:

One of my professors once put it best: "we in the propulsion field tend to bastardize units frequently."
 

hardinlw

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I have to deal with thrust specific fuel consumption (TSFC) and in English units that is "pounds thrust per pound mass per second of fuel" which is an odd combination of units that comes out to seconds only if you ignore the fact that a pound mass and pound force really are not the same thing. Metric is even worse because they tried to make the number come out almost the same as the English units and metric TSFC is deca-Newtons of thrust per kilogram of fuel per second. Why deca-Newtons? Because a kilogram under the acceleration of gravity "weighs" about 10 Newtons. So it seems the European propulsion people bastardize units just as much as those of us in the US.
 

Rocketjunkie

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America will never fully convert to metric for one reason: football. Phrases like,"And he's tackled at the 1 meter line!", "This will be a 45 meter field goal attempt", and "That was a spectacular 60 meter kick return" just don't sound as good.
Then there's Canadian football. The field is 100 meters long but marked in yards. The centerline is the 55 yard line :confused::y:
 

MarkII

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Depends on what you're doing....

I like regular imperial units because I can 'relate' how far or long or deep or tall or heavy something is easily because I'm used to dealing with those units for a lifetime. If I see a cotton bale, I instantly "know" that it's about 500 pounds, that my tractor is 8,000 pounds, or 4 tons, that my other farm is 90 miles away, my rocket flew to 1,000 feet, etc. But, if someone tells me that the cotton bale is 250 kilos, or my tractor is 4 metric tons, or the other farm is 180 kilometers away, my rocket flew 300 meters, etc. I have to actually CONVERT that measurement into something I can 'relate' to-- IE FEET, MILES, POUNDS, TONS before I can get a 'grasp' of what's really being said.
Exactly. It makes sense to us in the USA because we are used to it. Whichever system you use, Imperial or SI, becomes part of how you think of the world, part of how you recognize things. But it is not impossible for Americans to learn the metric system. What this requires is that we learn what the various units "look" like and "feel" like. You have to walk and drive 1,000 meters several times, for example, until you start to intuitively recognize that you have gone one kilometer. You have to make a point of measuring the lengths of a number of things using centimeters and millimeters. When you buy a 5-lb. sack of flour, measure it out into a canister until you have put 2 kilos in there. (You'll have a bit left over in the sack.) To learn liquid measures and kitchen measures, get a cookbook that has been printed in Canada and make a few recipes. Instead of putting 2 teaspoons of sugar in your morning coffee, put in 10 ml. (It will taste VERY SLIGHTLY sweeter. But if you can taste the difference, then perhaps you can have a career as a wine or coffee taster.)

Have you ever noticed that bottled water and sports drinks in the US are sold in 16.9 oz. containers? What is so special about 16.9 fluid ounces? Well, it happens to be one half liter.

In other words, to learn SI, you don't have to install a conversion calculator in your brain. You just have to learn to use it the same way that you use Imperial units now, but just recognizing the units intuitively. All it takes is a little practice.

Even my little examples above are pretty far off the mark because a kilo isn't REALLY 2 pounds, a meter isn't REALLY a yard or 3 feet (that one I know-- 1 meter is 39.4 inches) etc. and it gets even MORE 'out there' when you start talking about measuring things like liters vs. gallons, cubic centimeters vs. cubic inches, centigrade vs. fahrenheit etc. because you REALLY have to go look those up and do the math to get any meaningful information out of the 'measurement'. How many liters does my 200 gallon spray tank hold?? I don't have a clue. How cold if 5 centigrade?? how hot is 40 centigrade?? Is it hot or just warm?? How many cc's is a Ford 302 V-8?? (Ok, that one I know, because I'm a mechanic-- 302 CI= 5.0 liters, or 5,000 cc, since there's 1,000 cc per liter) but otherwise it gets REALLY confusing.
If the only way that you can recognize metric units is by mentally converting them into Imperial units, then it will always be burdensome and a headache. It is like learning a second language; you will never be fluent, or even conversant, in it if you have to constantly translate everything into your first language. You just can't do it fast enough to keep up. You have to learn to "think" in the new language.

5°C is a typical spring day in the Adirondacks. 40°C is an average summer day in Texas. To get a reference point for the centigrade scale, remember that 0° C is the temperature at which water freezes (equiv. to 32° F) and that 100° C is the temperature at which water boils (equiv. to 212° F). So 5°C is a little bit warmer than freezing (equiv. to 41°F) and 40°C is almost halfway to boiling (and is equiv. to 104°F). And a typical overnight low temp in mid-January in the Adirondacks reads the same in Fahrenheit and Celsius. (-40° on both scales.) :y:

I REALLY like metric for working with rockets, and I can see why it makes SO much better sense for engineering... scaling and switching units is as easy as moving decimal points over-- eminently practical. The main thing is to make sure everything is in the same units (not some measurements being meters and others centimeters or millimeters) so the decimal gets moved the right number of spots. And, of course, make sure it's not some hodge-podge mixture of imperial and metric units! NASA got burned by that one awhile back!

As to the OP-- well, ORIGINALLY the imperial units of pounds-force were used in engine designations, and it's more confusing IMHO... when I read about engines from the late 50's/early 60's being a B1.3-4 is a lot less meaningful than saying B4-4 or something like that. Introducing decimals into the mix is kinda silly, because really using pounds to measure our rocket engines, until you get into the HPR realm, is really using a unit that is too big and so it must be broken down to decimals or fractions.

SO, anyway, it depends on what you're doing and what you're comfortable with. I remember until a few years ago, ALL the vehicles had both MPH and KPH on the speedometers, and a few years ago I noticed that the KPH had all gone away-- now I see some that have it and some don't depending on the market the car is being built for and if it's exported or sold internationally, etc.
If you buy a car from an independent dealer (i. e., one that isn't tied to any particular brand) in my area, you will sometimes get one with the speedometer marked off in kph. (Also, the odometer will have spaces for more digits, because it will be measuring the distance driven in kilometers.) Independent auto dealers obtain much of their stock at auction, and around here, they frequently travel to Canada for some auctions.

I prefer to stick with imperial for stuff you 'need to visualize easily' and use metric for more 'scientific' measurements, but that's just me. It's hard to visualize a 'pound-force' so I prefer the 'newtons' since it's an easier number to deal with since no decimal is used.

Later! OL JR :)
If you can learn to recognize Newtons, then you can learn to recognize the other units, too. The United States has been "officially" on the metric system for the past couple of decades, but the enabling legislation contained no enforcement provision. Still, we are SLOWLY changing over. The United States will be the only industrialized nation that switched to SI because the citizens just started choosing to use it all on their own, rather than having their government cram it down their throats.

MarkII
 

MarkII

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But I still need a calculator to figure out my weight in stones. And my height in hands. :D

(It's actually just about the same number in both measures. More than 14 and less than 18. :eek: )

MarkII
 
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bradycros

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Because this is the United States of America and we do things the way WE want! My Subaru gets 564480 rods to the hogshead and that's the way I like it!:neener:
How many rods do you get to the butt (the word comes from the Romen "buttis")?
 

luke strawwalker

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Exactly. It makes sense to us in the USA because we are used to it. Whichever system you use, Imperial or SI, becomes part of how you think of the world, part of how you recognize things. But it is not impossible for Americans to learn the metric system. What this requires is that we learn what the various units "look" like and "feel" like. You have to walk and drive 1,000 meters several times, for example, until you start to intuitively recognize that you have gone one kilometer. You have to make a point of measuring the lengths of a number of things using centimeters and millimeters. When you buy a 5-lb. sack of flour, measure it out into a canister until you have put 2 kilos in there. (You'll have a bit left over in the sack.) To learn liquid measures and kitchen measures, get a cookbook that has been printed in Canada and make a few recipes. Instead of putting 2 teaspoons of sugar in your morning coffee, put in 10 ml. (It will taste VERY SLIGHTLY sweeter. But if you can taste the difference, then perhaps you can have a career as a wine or coffee taster.)

Have you ever noticed that bottled water and sports drinks in the US are sold in 16.9 oz. containers? What is so special about 16.9 fluid ounces? Well, it happens to be one half liter.

In other words, to learn SI, you don't have to install a conversion calculator in your brain. You just have to learn to use it the same way that you use Imperial units now, but just recognizing the units intuitively. All it takes is a little practice.

If the only way that you can recognize metric units is by mentally converting them into Imperial units, then it will always be burdensome and a headache. It is like learning a second language; you will never be fluent, or even conversant, in it if you have to constantly translate everything into your first language. You just can't do it fast enough to keep up. You have to learn to "think" in the new language.

5°C is a typical spring day in the Adirondacks. 40°C is an average summer day in Texas. To get a reference point for the centigrade scale, remember that 0° C is the temperature at which water freezes (equiv. to 32° F) and that 100° C is the temperature at which water boils (equiv. to 212° F). So 5°C is a little bit warmer than freezing (equiv. to 41°F) and 40°C is almost halfway to boiling (and is equiv. to 104°F). And a typical overnight low temp in mid-January in the Adirondacks reads the same in Fahrenheit and Celsius. (-40° on both scales.) :y:

If you buy a car from an independent dealer (i. e., one that isn't tied to any particular brand) in my area, you will sometimes get one with the speedometer marked off in kph. (Also, the odometer will have spaces for more digits, because it will be measuring the distance driven in kilometers.) Independent auto dealers obtain much of their stock at auction, and around here, they frequently travel to Canada for some auctions.

If you can learn to recognize Newtons, then you can learn to recognize the other units, too. The United States has been "officially" on the metric system for the past couple of decades, but the enabling legislation contained no enforcement provision. Still, we are SLOWLY changing over. The United States will be the only industrialized nation that switched to SI because the citizens just started choosing to use it all on their own, rather than having their government cram it down their throats.

MarkII

I hear ya Mark, and you're right... but it's teaching all us old dogs new tricks that is the REAL trick... :)

As you said with the second language thing, if you don't learn to think in it you won't be fluent in it. That's why I don't bother with a second language, even though here in Texas they're pushing more and more for SPANISH to become the main language than English... more than half the roadside advertisements are in Spanish now so it seems... Still, I'm older now and I'm obstinate, so if ya wanna talk to me you talk in English, or you can junp in a lake, I don't particularly care which... :D

I personally like bars as a measure of pressure, but for most things mechanically related it's always PSI. Don't even get me started on some of the rediculous holdover Imperial units used in surveying and land measuring... chains, rods, and all that baloney... acres I understand because as a farmer I have had to deal with rates of chemicals and seeding rates all these years and that's all in pounds or gallons per acre, and when you have to figure in row widths into the equation (40 inch rows= 13,068 row/feet/acre or 156,816 row/inches/acre used to calculate seed spacings-- ask me how I know THAT off the top of my head!) Every once in a while you'll hear about something in hectares... but it's pretty rare, because hectares is basically anathema to farmers-- NOBODY seems to be able to get a grasp of the size of a hectare in their heads. I can look at a field and guess how many acres is in it by size, but hectares?? Not a chance! I know that 640 acres is a section, or 1 square mile. Then for the horsey-types there's that obscure measurement known as hands... pffft... but then I'm not a horsey-type. Oh well...

Later! OL JR :)
 
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follr

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All I have to say is thank god for the Internet! Its the only way I can make sense of some of this stuff. Then again, the Internet is where I run into all this jargon.... :roll:
 

MarkII

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This appears to be a "D":






MarkII
 
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