Thread: What is the origin of Impulse categories?

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What is the origin of Impulse categories?

Hi all,

This is a question for the history buffs out there. What is the origin of the commonly used nomenclature for impulse categories in model rocketry? Why was it decided to use 2^n *10 N-s? Why is each category twice the previous instead of say an arithmatic progression? Why use N-s instead of slugs-fortnight for units?

Kevin

2. To answer the "2^n *10 N-s" not sure, but probably just a handy way to organize in terms that make sense for different motor sizes.

For the second part, it is the SI unit for impulse, so I assume that's why it's used

3. G_T
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The arithmetic progression question is pretty straightforward. If you want such a progression, N-s already provided it. A new method wasn't needed. But N-s starts to be a pretty big number if one is talking about SRBs for instance. A logarithmic scale is usually used in science and engineering to handle these sort of scale problems. But the letter designations are not used for engineering purposes anyway. They just give you a rough idea of how much energy a motor provides.

I wouldn't totally be shocked if the reason for the letter designations was something as simple as an early hobby motor got called version "A". Then a more powerful one came out called "B". The trend stuck... (I'm just making this up; I have no idea of the real reason).

Gerald

4. Originally Posted by G_T

I wouldn't totally be shocked if the reason for the letter designations was something as simple as an early hobby motor got called version "A". Then a more powerful one came out called "B". The trend stuck... (I'm just making this up; I have no idea of the real reason).

Gerald
Once upon a time there was but one model rocket motor - the Model Missiles type "A" originally made buy Brown inds, then Estes.Estes then came out with the "Super B" which filled the casing used at that time, don't know if it was quite double, and also 1/2A and 1/4A motors and the NAR came up with the coding system that doubled each category based on what Estes already was making. Originally we used pounds for average thrust and pound-seconds for impulse, but in the late '60s changed to newton, because if the USA didn't change to the metric system we would be a third world country by the year 2000

EDIT: checking out ninfinger you can see that the original "B" and "C" ranges were not doubled, every other category was - all based on what Estes was already making
http://www.ninfinger.org/rockets/cat...4/64cen20.html

2nd EDIT: Original type A was .7 lb-sec and the type B was 1.1 lb-sec - the NAR code was in place by 1960
http://www.ninfinger.org/rockets/cat...60/60est2.html
Last edited by Bazookadale; 23rd July 2012 at 08:37 PM. Reason: update

Once upon a time there was but one model rocket motor - the Model Missiles type "A" originally made buy Brown inds, then Estes.Estes then came out with the "Super B" which filled the casing used at that time, don't know if it was quite double, and also 1/2A and 1/4A motors and the NAR came up with the coding system that doubled each category based on what Estes already was making. Originally we used pounds for average thrust and pound-seconds for impulse, but in the late '60s changed to newton, because if the USA didn't change to the metric system we would be a third world country by the year 2000

EDIT: checking out ninfinger you can see that the original "B" and "C" ranges were not doubled, every other category was - all based on what Estes was already making
http://www.ninfinger.org/rockets/cat...4/64cen20.html

What countries besides the U.S. have not adopted the metric system?
Many U.S. teachers think the answer is "Liberia and Burma" (make that Myanmar). Let's give Liberia and Myanmar a break! All countries have adopted the metric system, including the U.S., and most countries (but not the U.S.) have taken steps to eliminate most uses of traditional measurements. However, in nearly all countries people still use traditional units sometimes, at least in colloquial expressions. Becoming metric is not a one-time event that has either happened or not. It is a process that happens over time. Every country is somewhere in this process of going metric, some much further along than others.

Since 1875, in fact, the United States has subscribed to the International System of Weights and Measures, the official version of the metric system.

6. If you go back to the early days of rocketry, motors were measured in terms of pound-seconds of impulse. At the time, the US was on the slow track to go completely metric in about 10-15 years (by the early 80's IIRC) and NAR was one of the "early adopters". The pound-second measurements were difficult to work with because MOST motors available at the time actually ended up being a FRACTION (decimal actually) of a pound-second. For instance, IIRC going from memory, the B6-4 was actually a B.8-4 under the old pound second system (or something like that... you get the idea). This is a very clumsy way of dealing with the numbers. Science labs had pretty much all switched to metric, and since rocketry is a "scientific hobby" the decision was made to switch to metric, hence the switch to newton-seconds for impulse, millimeters and centimeters for dimensional measurement, grams for weight, cubic centimeters for volume, etc.

As it turned out, the general population never wanted to switch over to metric, and the gov'ts attempts at FORCING the switch didn't go well. SO, the US finally quit pretending and dropped the "official" metric switchover, for the most part. (Metric is indeed much handier for many things, especially scientific endeavors where the easy conversion of measurements is critical, since it's all based on powers of ten, rather than abstract fractions like halves, quarters, eighths, sixteens, thirty-seconds, sixty-fourths, etc. like imperial measurements are...) Lots of stuff in the industrial world is pretty much all metric now, or the vast majority is, like hydraulic fittings, bolts, nuts, even driveline shafts have mostly gone metric now... but there's still just enough standard stuff around to complicate things.

Interestingly enough, most of the engineering at NASA has been done in imperial measurement, whereas the science side of NASA switched to metric basically from the get-go. This resulted in the loss of at least one spacecraft when the unit conversion wasn't done and the imperial units were entered into the guidance system operating on metric without being converted over in the units, so only a fraction of the propulsive power needed was actually produced, causing the vehicle to crash. I think that this drove the change to all-metric within NASA IIRC... (can't recall exactly).

Why newton-seconds?? Simple... newtons is the measure of force, and seconds is the measure of time. Total impulse is thrust multiplied by time. Hence newton-seconds. In imperial measure, pounds-force is the measure of force (to differentiate it from pounds-mass, to describe what's actually being measured) and seconds is the measure of time, so hence pounds-seconds.

Later! OL JR

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Originally Posted by kruland
This is a question for the history buffs out there. What is the origin of the commonly used nomenclature for impulse categories in model rocketry? Why was it decided to use 2^n *10 N-s? Why is each category twice the previous instead of say an arithmatic progression? Why use N-s instead of slugs-fortnight for units?
The answer is not historical, it is scientific.

http://en.wikipedia.org/wiki/Logarithmic_scale

Originally Posted by wikipedia
A logarithmic scale is a scale of measurement that displays the value of a physical quantity using intervals corresponding to orders of magnitude, rather than a standard linear scale.

A simple example is a chart whose vertical axis has equally spaced increments that are labeled 1, 10, 100, 1000, instead of 1, 2, 3, 4. Each unit increase on the logarithmic scale thus represents an exponential increase in the underlying quantity for the given base (10, in this case).

Presentation of data on a logarithmic scale can be helpful when the data covers a large range of values. The use of the logarithms of the values rather than the actual values reduces a wide range to a more manageable size. Some of our senses operate in a logarithmic fashion (Weber–Fechner law), which makes logarithmic scales for these input quantities especially appropriate. In particular our sense of hearing perceives equal ratios of frequencies as equal differences in pitch. In addition, studies of young children in an isolated tribe have shown logarithmic scales to be the most natural display of numbers by humans.

8. Originally Posted by luke strawwalker
As it turned out, the general population never wanted to switch over to metric, and the gov'ts attempts at FORCING the switch didn't go well. SO, the US finally quit pretending and dropped the "official" metric switchover, for the most part. (Metric is indeed much handier for many things, especially scientific endeavors where the easy conversion of measurements is critical, since it's all based on powers of ten, rather than abstract fractions like halves, quarters, eighths, sixteens, thirty-seconds, sixty-fourths, etc. like imperial measurements are...)

Later! OL JR
I don't consider fractions to be abstract, they are very obvious and useful to me. I do like metric for lab work but don't like it forced on me for everyday usage. My Subaru gets 152 rods to the hogshead of gas and I like it that way!

I don't consider fractions to be abstract, they are very obvious and useful to me. I do like metric for lab work but don't like it forced on me for everyday usage. My Subaru gets 152 rods to the hogshead of gas and I like it that way!
Maybe "abstract" was a poor word choice... perhaps "confusing in the conversion" or "difficult to convert" would have been better choices...

I'm like you... I don't like stuff "forced" on me. Dealing with measurements on a regular basis (in agriculture) you DO get used to working in a particular system and it's easy to visualize and estimate once you do... and some measurements just work better than others... for instance, I MUCH prefer "acres" of area measure than hectares... hectares is VERY difficult to visualize unless you've worked in metric your whole life... a hectare is a rather larger measure than an acre and unless you're talking about fields that are literally hundreds of acres in size, the acre is easier to deal with than a hectare...

later! OLJR

10. Meh. Spend a single year using nothing but metric and you'll have every bit as intuitive a feel. Kg per hectare is within 10% of lbs per acre if you're talking spread. If you prefer tons, then it's 2:1, give or take that 10%. A newton is a quarter of a pound. If you have to deal in gravity, you'll really like that it's so close to 10m/s2 as to be inconsequential to all but the most detailed calculations.

The only thing that gets substantially more difficult is finding errors. Off by 16 - you forgot to convert ounces/pounds; off by 12 or 144 - it's feet and inches that screwed you up; off by 32.2 - yup, gravity; missed your vibration calc by 20 - gravity...in inch units (12*32.2 = 386.4, or inch-slugs, as I like to call it; sqrt(386.4)=19.6). Screw up in the metric system and you're off by a factor of 10, or 3 (square roots) and who knows where you slipped a digit.

11. It cracks me up every time people bring up how backwards inches and pounds are. I lived in metric countries form birth to 21 years, so I have no "traditional" bias for English units. When I first came to the States, I had to adjust, and the experience very much resembles the experience of learning to drive on the left when I moved to Scotland a few years later (I used to travel a lot). So now that I can use both sets of units, I can wholeheartedly say that I see no particular benefit to one set or another, except that for certain narrow ranges, people seem to prefer units where whole numbers smaller than 10 or 12 are meaningful (I notice a lot of people talk about grams and inches in the same sentence: as luke strawwalker is saying, 3.5N is more convenient than .8lbs). So people use grams until about 20g and then switch to oz, and then to pounds. I find it a lot more convenient to say "a pound" than "half a kilogram." "Half a kilogram" is two levels of indirection: 1/2 -> 1000 -> g and it gets awkward. In America, many trades define their own units of measure that are convenient for the narrow range they operate in, instead of trying to use a centralized set that is equally awkward for everyone. As an example, an American electrician might refer to 24 ga wire where his European colleague might call it 0.5mm wire. Electricians care about current-carrying capacity rather than diameter, and their (logarithmic wrt diameter) scale reflects that. The impulse letter system we use for motors is essentially a private measuring system that is relevant to the quantities we care about.

Now the supposed ease with which you can perform arithmetic on metric units is a total red herring. Sure, grams to kilograms is easy. But try to convert m/s to km/h, and it's exactly as easy (or hard) as fps to mph. The original metric system--the product of French revolutionary rationality--defines a minute as 100 seconds and a right angle as 100 decimal degrees (gradians). It tries to ground everything in rational reality--literally. Rejecting old-fashioned arbitrariness of dark ages, the French turn to something as Universally central as the Earth. Meter--the basic unit in SI--is 1/40,000,000 of Earth's circumference (since the right angle now is 100 degrees, and each degree is 100 minutes and each minute is 100 seconds; the original definition of kilometer is the length of a decimal-minute-arc on the equator, much as a Nautical Mile is one normal arc-minute). A liter is the volume of one cubic decimeter and a kilogram is the weight of one liter of water. So when you go grocery shopping, you can weigh a kilogram of milk and know that you're paying for about a liter. Yay! Where the whole easy-to-convert-between-units narrative breaks down is at metric units that are not decimal. Since everyone but the French rejects decimal gradians and everyone--including the French--rejects decimal time, anything to do with time or angles is as non-metric in SI as it is in "traditional" systems. The other problem is that the rational world around us doesn't divide itself into rational fractions, much less into decimal ones. The French revolutionary calendar coerces the year into 10 months, but the number of days in a year is still irrational. Free-fall acceleration is an inconvenient 9.8 m/s^2 so one Kilogram of mass exerts a force of 9.8 Newtons. The square root of 2 is irrational, so a square with an area of 10 m^2 is ~3.1622776602 m on the side...

OK, I best get off my apple crate now.

Ari.

12. Originally Posted by luke strawwalker
I'm like you... I don't like stuff "forced" on me. Dealing with measurements on a regular basis (in agriculture) you DO get used to working in a particular system and it's easy to visualize and estimate once you do... and some measurements just work better than others... for instance, I MUCH prefer "acres" of area measure than hectares... hectares is VERY difficult to visualize unless you've worked in metric your whole life... a hectare is a rather larger measure than an acre and unless you're talking about fields that are literally hundreds of acres in size, the acre is easier to deal with than a hectare...
I was born in Soviet Russia. There, the government was very particular about forcing things on people, as about collectivizing agriculture and achieving communal economies of scale. On their huge collective farms, hectares are more meaningful than in our fragmented, private-ownership world. Interestingly, "hect-" in "hectare" means "one hundred of" and is very uncommon prefix in SI. The base unit, "are" is 100m^2, or about 1,000 sq.ft. The French word "are" is unknown in Russia, but for measuring private property the Russians use an informal unit, "sotka" which translates as "one hundredth," implicitly one hundredth of a hectare.

Ari.

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Originally Posted by luke strawwalker
I MUCH prefer "acres" of area measure than hectares... hectares is VERY difficult to visualize unless you've worked in metric your whole life... a hectare is a rather larger measure than an acre and unless you're talking about fields that are literally hundreds of acres in size, the acre is easier to deal with than a hectare...

later! OLJR
OLJR, the interesting thing is I have no way to visualize any measure of area. I have no particular problem with mass, or length (in either sets of units) but, area - forget it. I have no feeling for the measure.

Kevin

14. The fractional A to O system of impulse categories was adopted for use with model rocket motors only. A particular motor with a given letter designation does not necessarily contain 2x the impulse of a motor with the next lower letter designation. The letter designations specify ranges, and a motor with total impulse falling anywhere within that range is classified with that letter. Hence a reload like the Pro29 G125, with a total impulse of 159 N-s, is properly classified as a G motor, while the Pro29 H133 is properly designated as an H with 163 N-s of total impulse, just 4 more N-s than the G. The G reload falls just inside the top end of the G designation, while the H measures out to be just on the other side of the dividing line. Any adult over 18 years old can buy the G125, but one will need to possess Level 1 certification (at a minimum) to purchase the H, even though it is just a hair more powerful. Sounds rather arbitrary, but they had to draw the lines somewhere, and those categories long predated the development of those two reloads. There are many more examples of this in other letter categories; it isn't at all uncommon.

Once you get above around a "P" designation, the letter categories start to cease making sense. They aren't used by anyone outside of the hobby rocketry community.
Last edited by MarkII; 24th July 2012 at 04:21 AM.

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Originally Posted by MarkII
Once you get above around a "P" designation, the letter categories start to cease making sense. They aren't used by anyone outside of the hobby rocketry community.
Mark,

That's kinda where I was going. The big motors - I'd even say K and higher start having such a large range of total impulse that the categories no longer make sense. I like the way CTI puts total impulse in front of the letter designator for this reason.

I also get a kick out of Alway's ROTW where he computed the category for real rocket motors. I'd love to see a ZZ fly!

Kevin

16. Originally Posted by kruland
Mark,

I like the way CTI puts total impulse in front of the letter designator for this reason.

Kevin
I'd like to see that become the standard for all motors

17. Originally Posted by iter
It cracks me up every time people bring up how backwards inches and pounds are. I lived in metric countries form birth to 21 years, so I have no "traditional" bias for English units. When I first came to the States, I had to adjust, and the experience very much resembles the experience of learning to drive on the left when I moved to Scotland a few years later (I used to travel a lot). So now that I can use both sets of units, I can wholeheartedly say that I see no particular benefit to one set or another, except that for certain narrow ranges, people seem to prefer units where whole numbers smaller than 10 or 12 are meaningful (I notice a lot of people talk about grams and inches in the same sentence: as luke strawwalker is saying, 3.5N is more convenient than .8lbs). So people use grams until about 20g and then switch to oz, and then to pounds. I find it a lot more convenient to say "a pound" than "half a kilogram." "Half a kilogram" is two levels of indirection: 1/2 -> 1000 -> g and it gets awkward. In America, many trades define their own units of measure that are convenient for the narrow range they operate in, instead of trying to use a centralized set that is equally awkward for everyone. As an example, an American electrician might refer to 24 ga wire where his European colleague might call it 0.5mm wire. Electricians care about current-carrying capacity rather than diameter, and their (logarithmic wrt diameter) scale reflects that. The impulse letter system we use for motors is essentially a private measuring system that is relevant to the quantities we care about.

Now the supposed ease with which you can perform arithmetic on metric units is a total red herring. Sure, grams to kilograms is easy. But try to convert m/s to km/h, and it's exactly as easy (or hard) as fps to mph. The original metric system--the product of French revolutionary rationality--defines a minute as 100 seconds and a right angle as 100 decimal degrees (gradians). It tries to ground everything in rational reality--literally. Rejecting old-fashioned arbitrariness of dark ages, the French turn to something as Universally central as the Earth. Meter--the basic unit in SI--is 1/40,000,000 of Earth's circumference (since the right angle now is 100 degrees, and each degree is 100 minutes and each minute is 100 seconds; the original definition of kilometer is the length of a decimal-minute-arc on the equator, much as a Nautical Mile is one normal arc-minute). A liter is the volume of one cubic decimeter and a kilogram is the weight of one liter of water. So when you go grocery shopping, you can weigh a kilogram of milk and know that you're paying for about a liter. Yay! Where the whole easy-to-convert-between-units narrative breaks down is at metric units that are not decimal. Since everyone but the French rejects decimal gradians and everyone--including the French--rejects decimal time, anything to do with time or angles is as non-metric in SI as it is in "traditional" systems. The other problem is that the rational world around us doesn't divide itself into rational fractions, much less into decimal ones. The French revolutionary calendar coerces the year into 10 months, but the number of days in a year is still irrational. Free-fall acceleration is an inconvenient 9.8 m/s^2 so one Kilogram of mass exerts a force of 9.8 Newtons. The square root of 2 is irrational, so a square with an area of 10 m^2 is ~3.1622776602 m on the side...

OK, I best get off my apple crate now.

Ari.
Well Ari, I appreciated the time you spent on your apple crate to explain that!

For rockets, I prefer to use decimal inches for linear measure, and grams for weight. That's on the smaller scale. When things are bigger I prefer feet and inches (not decimal feet) and pounds (in decimals, not ounces). Strange, but it works for me.

Greg

18. I remember reading somewhere that a space shuttle SRB is an AC class motor.

19. Originally Posted by kruland
OLJR, the interesting thing is I have no way to visualize any measure of area. I have no particular problem with mass, or length (in either sets of units) but, area - forget it. I have no feeling for the measure.

Kevin
Yeah, it's difficult... farmers can generally estimate the size of a field within about 10-20% error (depending on how good they are at it) after years of experience... and of course on how big the field is and the shape of it...

For rancher's it's easier... a square mile (1 mile by 1 mile) is 640 acres. My Shiner farm is 1/4 this size, or 1/2 mile by 1/2 mile, or 160 acres. The Needville farm is 1/2 mile by 1/4 mile (roughly) in size, or 80 acres (actually 87). A field 1/4 mile by 1/4 mile is 40 acres (or 1/2 mile by 1/8 mile).

later! OL JR
Later! OL JR

20. Originally Posted by MarkII
The fractional A to O system of impulse categories was adopted for use with model rocket motors only. A particular motor with a given letter designation does not necessarily contain 2x the impulse of a motor with the next lower letter designation. The letter designations specify ranges, and a motor with total impulse falling anywhere within that range is classified with that letter. Hence a reload like the Pro29 G125, with a total impulse of 159 N-s, is properly classified as a G motor, while the Pro29 H133 is properly designated as an H with 163 N-s of total impulse, just 4 more N-s than the G. The G reload falls just inside the top end of the G designation, while the H measures out to be just on the other side of the dividing line. Any adult over 18 years old can buy the G125, but one will need to possess Level 1 certification (at a minimum) to purchase the H, even though it is just a hair more powerful. Sounds rather arbitrary, but they had to draw the lines somewhere, and those categories long predated the development of those two reloads. There are many more examples of this in other letter categories; it isn't at all uncommon.

Once you get above around a "P" designation, the letter categories start to cease making sense. They aren't used by anyone outside of the hobby rocketry community.
A good point... for the smaller "model rocket" motors up to about E impulse, basically each motor is double the impulse of the previous motor letter class. By the time you get to "D" motors, since the ranges are double the width of the previous letter classes range, there's a LOT more room in there for "less than full" size motors in that impulse range (for instance, an Estes "D" isn't a "FULL D" since it's only 17 n-s, whereas a "full D" would be 20 N-s. With the E's, F's, and G's, it's even worse since their scales are not only double the previous letter, but the width of the range is double as well. So there's a LOT more room in there for different size motors that fall from the low end of the range to the upper end of the range... but are still in the same impulse letter category...

Later! OL JR

I'd like to see that become the standard for all motors
For all motors larger than about a "D" or "E" it really starts making a lot of sense! OL JR

22. Originally Posted by luke strawwalker
A good point... for the smaller "model rocket" motors up to about E impulse, basically each motor is double the impulse of the previous motor letter class.
It wasn't always that way and it isn't uniformly that way even now. Many, but far from all, black powder and composite motors double in impulse from A (and fractional A) through D because they are deliberately designed that way; there is nothing inevitable about it and it has nothing to do with the "tightness" of the smaller impulse categories. That progression didn't always happen in the early days. In the present day there is variation in total impulse among motors in most of the low power classes. Compare:

1/8A class (up to 0.3125 N-S)
Quest 1/8A.2 MicroMaxx I = 0.135 N-s (43% 1/8A)
Quest 1/8A.5 MicroMaxx II = 0.22 N-s (70% 1/8A) (this is a significant difference at this impulse level)

There is only one motor in the 1/4A class (0.313 to 0.625 N-s). The two available motors in the 1/2A class (0.626 to 1.25 N-s) do have identical total impulses.

A class (1.251 to 2.50 N-s)
Quest A3T = 1.71 N-s (36% A)
Estes A3T = 2.5 N-s
Quest A8 = 1.86 N-s (49% A)
Estes A8-3,5 = 2.50 N-s
Estes A8-0 = 2.15 N-s (72% A)
Estes A10-0T = 1.88 N-s (50% A)

B class (2.51 to 5.0 N-s)
Estes B4 = 5.0 N-S
Quest B4 = 3.84 N-s (53% B)
Estes B6-0 = 4.9 N-s (96% B)
Quest B6-4 (C)* = 4.85 N-s (94% B)
Estes B6-x = 5.0 N-s

C class (5.01 to 10.0 N-s)
Estes C6 = 9.0 N-s (80% C)
Quest C6 (C)* = 8.76 N-s (75% C)
Estes C11 = 9.0 N-s (80% C)

D class (10.01 to 20.0 N-s)
Quest D5-4,6 = 17.61 N-s (76% D)
Quest D5-P = 19.6 N-s (96% D)
Quest D8 = 18.59 N-s (86% D)
Estes D11 = 18.0 N-s (80% D)
Estes D12 = 17.0 N-s (70% D)
AeroTech D9 = 20.0 N-s
Apogee, AeroTech D10 = 18.3 N-s (83% D)
AeroTech D21 = 20.0 N-s
AeroTech D24 = 18.5 N-s (85% D)

All statistics are from NAR S&T (http://www.nar.org/SandT/NARenglist.shtml) except for "(x% Y)" which I calculated.
Last edited by MarkII; 25th July 2012 at 07:31 AM. Reason: Corrected "% X" figures

I'd like to see that become the standard for all motors
That information isn't hard to find. It is stated on the motor's or reload's packaging, on the manufacturer's web site and on motor certification lists. The motor databases for simulation software like RockSim and OpenRocket also contain this information. With all that being said, I do agree that it is great the CTI places the total impulse right in the reload's name. It puts that information right up front, but on the downside, it can make the motor designation unwieldy. Most often when CTI reloads are mentioned, shortened names are used, with the total impulse part dropped. One purpose of the letter classes was no doubt to make it easier to talk about motors of roughly similar levels of power (as measured by total energy output). Another potential issue with the CTI naming protocol is that it can cause two motors to appear to be more similar to each other than they really are. Compare a 636I216CL with a 634I540WT, for instance. The key difference between them is signaled by the average thrusts, "216" and "540" but these numbers are all but buried in the name. In convention terms these two Pro38 reloads would be referred to as an I216CL and an I540WT, respectively, or even more simply, as an I216 and an I540. You already get a sense of the overall power of the two (they are both "I" reloads), while at the same time the key difference (average thrust) is easy to spot. While knowing the exact total impulse is handy, you already know that neither of them is really going to be comparable to, say, a 650J270G³ (i.e., a J270) for example, or a 654J316P.

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Originally Posted by luke strawwalker
If you go back to the early days of rocketry, motors were measured in terms of pound-seconds of impulse. At the time, the US was on the slow track to go completely metric in about 10-15 years (by the early 80's IIRC) and NAR was one of the "early adopters". The pound-second measurements were difficult to work with because MOST motors available at the time actually ended up being a FRACTION (decimal actually) of a pound-second. For instance, IIRC going from memory, the B6-4 was actually a B.8-4 under the old pound second system (or something like that... you get the idea). This is a very clumsy way of dealing with the numbers. Science labs had pretty much all switched to metric, and since rocketry is a "scientific hobby" the decision was made to switch to metric, hence the switch to newton-seconds for impulse, millimeters and centimeters for dimensional measurement, grams for weight, cubic centimeters for volume, etc.

As it turned out, the general population never wanted to switch over to metric, and the gov'ts attempts at FORCING the switch didn't go well. SO, the US finally quit pretending and dropped the "official" metric switchover, for the most part. (Metric is indeed much handier for many things, especially scientific endeavors where the easy conversion of measurements is critical, since it's all based on powers of ten, rather than abstract fractions like halves, quarters, eighths, sixteens, thirty-seconds, sixty-fourths, etc. like imperial measurements are...) Lots of stuff in the industrial world is pretty much all metric now, or the vast majority is, like hydraulic fittings, bolts, nuts, even driveline shafts have mostly gone metric now... but there's still just enough standard stuff around to complicate things.

Interestingly enough, most of the engineering at NASA has been done in imperial measurement, whereas the science side of NASA switched to metric basically from the get-go. This resulted in the loss of at least one spacecraft when the unit conversion wasn't done and the imperial units were entered into the guidance system operating on metric without being converted over in the units, so only a fraction of the propulsive power needed was actually produced, causing the vehicle to crash. I think that this drove the change to all-metric within NASA IIRC... (can't recall exactly).

Why newton-seconds?? Simple... newtons is the measure of force, and seconds is the measure of time. Total impulse is thrust multiplied by time. Hence newton-seconds. In imperial measure, pounds-force is the measure of force (to differentiate it from pounds-mass, to describe what's actually being measured) and seconds is the measure of time, so hence pounds-seconds.

Later! OL JR
It wasn't so much engineering vs science within NASA. NASA had been metric for 10 years, but Lockheed Martin, the contractor for Mars Climate Orbiter, had not. Lockheed sent over their burn data in pounds-force, and the JPL flight team entered the data into the control software, which expected newtons.

25. Rocketeer Extraordinaire
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Regarding the discussion of metric vs. "imperial" measurements...2 things that favor the imperial side are the following:

1. Imperial measurements are based on human terms, while metric is based on earth terms. A foot, for instance, is the average length of a human foot, whereas a meter is the length of the path travelled by light in vacuum during a time interval of 1⁄299,792,458 of a second. What distance do you a think a human would more easily grasp? How many people have stepped off distances with their feet? How easy would it be it to measure meters with a flashight?
2. Imperial measurments are easier to say. Consider the following:
inch vs. centimeter
foot vs. decimeter
yard vs. meter
mile vs. kilometer
pound vs. kilogram
quart vs. liter

All the imperial measurements above are single-syllable words. At best, liter and meter are two syllables. Some of the metric "equivalents" (in relative terms) are 4+ syllables.

The UK officially has used the metric system as the standard, but people there still continue to use imperial terms in conversation because they are easier to grasp...and easier to say.

26. I'd like to note that while metric units are based on grander distances, it is because the scientific community uses metric units, and as such, requires extremely precise definitions of what each measurement is.

Meter was an arbitrary value based on the length of a pendulum that had a half period of one second, second was just an amount of time that could be tracked, kilogram was a mass in france, joule is energy required to raise one gram of H2O one degree centigrade, centigrade is 1/100 the temperature increase needed to change H2O from ice to steam.

They were redefined as follows:

Meter: distance light travels in 1⁄299,792,458 second.
Second: 9,192,631,770 vibrations of a cesium atom at 273.15 kelvin
kilogram: Still a mass in France, but being converted to a sphere of silicon with a near exact number of atoms.
Joule: Energy required to raise one gram of pure water by one degree centigrade
Centigrade: 1/100 the temperature change required to turn ice to steam through liquid phase.

They are all based on atomic principles, as they should be. Other intelligent species, if we ever encounter them, will understand cesium vibrations or change in water temperature better than they'll understand, "Our species feet were around abouts an average of this long on the day we made the measurement."

Ease of use is no excuse for scientific incompetence in schools.

well thats my rant for the day

27. Originally Posted by edwinshap1
Ease of use is no excuse for scientific incompetence in schools.
I wholeheartedly agree.

I also agree that SI now defines many units in terms of atomic measurements. Of course, we now define most corresponding Imperial and American units in the same terms (one yard is, by law, 0.9144m, and consequently derives from the same cesium measurement). So in terms of scientific accuracy, Metric and English units are strictly equivalent. Even a nautical mile is now an exact meter value (1852m) and so derives from the same cesium measurement.

This leaves the original derivation of these units. The English measure their foot. The French measure Earth, Water and Fire: meter is originally one decimal arc-second (1/1,000,000) of the Earth's quarter-circumference (the distance from the North Pole to the Equator on the meridian through Paris, no less); gram is the mass of one cubic centimeter (1/100 of an arc-second of Earth circumference ^3) of water, etc.

The irony that you're apparently missing is that any other intelligent species is likely to find the size of the Earth and human-centric as the size of a human foot.

Ari.

28. It is true that another species would question our motives for having a meter as the standard of length measurement, but it is also defined as ~1/(3*10^8) light seconds, which some may call less humancentric.

My favorite is that the "foot" was defined differently all over the world for a long time.

29. The US isn't as SI-phobic as you think

SI units are the norm in medical (including pharmaceutical) and scientific settings in the United States. You won't find fractional (Standard American) units employed anywhere in these places. Both SAE and metric units are used in automotive and other engineering applications here, and both are widely understood. Fractional, or Standard American (which is not the same as the Imperial System) units of measure, as well as the Fahrenheit temperature scale are still widely used in everyday situations. SI units are actually quite ubiquitous here; they just aren't as prominently displayed as the Standard Am. units. Product sizes tend to be rounded into easily expressed standard units, but metric units aren't as rare or unfamiliar here as is often alleged. The metric system of measurement is taught in all of our schools; I learned it as a schoolboy in the 1960s. Gauges and scales almost always display the results in both systems, or the display can be switched from one to the other. My digital bathroom scale can be switched to display the result in either pounds, kilograms or stone (another non-SI unit). (I'd like to lose about 5 st. to get my weight back to where it ought to be. I'm working on it. ) Perhaps because I leave near the border with Canada, road signs here usually display distances in both miles and kilometers. It has been a very long time since I owned a car that didn't possess a dual mph/kph speedometer. Whenever I drive into Canada, I have no trouble adjusting my driving to conform to posted kph limits because the scale is already on my dash.
Last edited by MarkII; 28th July 2012 at 04:05 AM.

30. That is a good point Mark. We do have km/h and liters and grams on a lot of everyday items.

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