What is the origin of Impulse categories?

[MarkII]

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.

Basing a system on an arbitrary unit with a precise definition, rather than on a natural one that can be interpreted in a variety of ways, has a lot to recommend it.

The Romans had some distance units based on the finger, thumb, width of palm, length of palm, foot, step and pace, among others. Volume was based on fractions of a standard Roman amphora (storage vessel or jar). The Romans originated the concepts of ounce and pound for weight.

[JPVegh]

I'd wager 100 quatloos that an alien species isn't going to give a centons thought as to how many miles or kilometers are in a kellicam. By the way, when the aliens show up and explain that their unit of measurement is based upon the average distance between the flonk and flork of their 4th sex, I wouldn't call them scientifically incompetent. They might take it the wrong way and force you to narftle the garthok.

You really have to pardon me for being human centric, I was born that way.

[CarVac]

Someone should make an altimeter that reads out apogee in Smoots.

[Nerull]

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.

The average male human foot is about 9 inches long, the average female 7 inches. There are people with foot long feet, but they sure as hell aren't average.

A 'foot' is a distance most people can deal with because they were raised with it. That's really all there is to it. It's no more closely related to any "human terms" than a meter is.

Most people have zero "human" connection to any of the imperial/us units. What's the human connection to a mile? A furlong? A chain? An acre? How about a pound? Why is a mile different lengths depending on if you are on land, in the air, or on the water? Why is a pint different depending on who you ask? Just what is human about cups, quarts, and gallons? Nothing.

Why is temperature based on one arbitrary point to a completely different arbitrary point more "human" than the freezing and boiling temperatures of water, which we see everyday?

[iter]

Why is a mile different lengths depending on if you are on land, in the air, or on the water?

I know Statue miles (originally, one thousand paces of a Roman soldier, useful for measuring marching distances) and Nautical miles (originally, one minute arc of earth circumference, useful for maritime navigation). Please tell me about air miles.

Why is temperature based on one arbitrary point to a completely different arbitrary point more "human" than the freezing and boiling temperatures of water, which we see everyday?

Daniel Gabriel Fahrenheit originally defines the top of his scale (100 deg F) by measuring the temperature of his own body.

Ari.

[JPVegh]

Sure the average human male foot is nine inches if you include all of the little asian feet in the world. But the average greco-roman foot was over 11 inches when the measurement originated.

A yard was based on the average mans girth. For a lot of years I was a 36 waist, now I'm a 38. I guess I'm creeping up on a meter, going metric and didn't even know it.

People still measure horses in hands.

An in inch was width of a mans thumb.

An acre was established as the amount of land that a person could plow in one day.

This seems arbitrary when considering the variations in the size of people but from the origins came standardized measurements.

[Nerull]

I know Statue miles (originally, one thousand paces of a Roman soldier, useful for measuring marching distances) and Nautical miles (originally, one minute arc of earth circumference, useful for maritime navigation). Please tell me about air miles.



Daniel Gabriel Fahrenheit originally defines the top of his scale (100 deg F) by measuring the temperature of his own body.

Ari.

Aircraft also use nautical miles.

[Nerull]

Sure the average human male foot is nine inches if you include all of the little asian feet in the world. But the average greco-roman foot was over 11 inches when the measurement originated.

A yard was based on the average mans girth. For a lot of years I was a 36 waist, now I'm a 38. I guess I'm creeping up on a meter, going metric and didn't even know it.

People still measure horses in hands.

An in inch was width of a mans thumb.

An acre was established as the amount of land that a person could plow in one day.

This seems arbitrary when considering the variations in the size of people but from the origins came standardized measurements.

The average american foot size is also around 9 inches. The average English/European size is around 9 and a half. I don't think there are enough Asians to throw the numbers off that much.

And the length of feet in a particular part of the world 2000 years ago is a piss poor basis for a system of measurement. And still doesn't relate to 99% of the world any better.

[JPVegh]

The average american foot size is also around 9 inches. The average English/European size is around 9 and a half. I don't think there are enough Asians to throw the numbers off that much.

And the length of feet in a particular part of the world 2000 years ago is a piss poor basis for a system of measurement. And still doesn't relate to 99% of the world any better.

A recent report on auxiology combined with common anthropometrics states that the avarage American male has a height of 178.2 cm(5'10.2") with a foot length of 26.73cm(10.52").
The same report states that the average chinese man has a height of 164.8 cm(5'4.9") with a foot length of 24.72cm(9.73")
The range varies wildly in Europe with Spanish men averaging at 170cm (5'7") high to 184.8 cm(6' 0.8") high for men in the Netherlands and foot lengths of 25.5 cm(10.3") and 27.72cm(10.91") respectively. Interestingly it seems the Guatemalans(mayans) are among the smallest people with an average height for men of 157.5cm(5'2") with a foot length of 23.63cm(9.3").

We can deduce from the numbers above that the average foot length for the average American male is 10.52" and the average foot length for the average European male falls somewhere between 10.3" and 10.91". Further more since the average foot length for the average Guatemalan male is 9.3" we can safely deduce that the average human male foot is longer than 9 inches and is likely to be in the 9.75 to 10.25 inch range.

What does any of this have to do with the foot as a unit of measure? Not a whole lot unless you don't have access to a rule or tape measure. In which case you'll find that a Guatemalan foot is about an inch shorter than the rest of the world. It's easy to say in this world of precision instrumentation that one unit of measurement is archaic compared to another but we really aren't that far removed from the technology of our ancestors. One good solar flare and a cubit becomes a relevant unit of measurement again(perhaps a bit of an exaggeration). Given that most of the structures we build revolve around human habitation it is intuitive that our units of measurement be humancentric. Just be prepared to duck your head when entering the doorway of a mayan home. Of course if you're designing computer chips then a foot, hand or cubit might not be that useful to you.

Yes, I have nothing better to do than look up seemingly useless data all day long.

[iter]

Of course if you're designing computer chips then a foot, hand or cubit might not be that useful to you.

In contrast with a meter, which--deriving as it does form the size of the Earth--makes it easier to compute the size of silicone transistors.

Ari.

[cwbullet]

We are slowly going to the metric system. Nthe rerstnofnthe world makes funifnus for not having done it yet.

Look at the bottles of soda. We have used 2 liters for a long time. Did you notice that the small bottles changes recently in the grocery store? They went from 16, 20, and 24 Ounces to 16.9 ounces. Hmm what is that? ...... 500 ml?

The is no excuse. We need to the more scientifice measurement system.

[karlbaum]

The average american foot size is also around 9 inches. The average English/European size is around 9 and a half. I don't think there are enough Asians to throw the numbers off that much.

And the length of feet in a particular part of the world 2000 years ago is a piss poor basis for a system of measurement. And still doesn't relate to 99% of the world any better.

My foot is a about a foot long and my thumb is about an inch wide so it works well for me.

[JPVegh]

My foot is a about a foot long and my thumb is about an inch wide so it works well for me.

You'll be in high demand after the zombie apocalypse.

[CarVac]

My hand span is almost exactly ten inches.

[Bazookadale]

And the length of feet in a particular part of the world 2000 years ago is a piss poor basis for a system of measurement. And still doesn't relate to 99% of the world any better.

It's as good a starting point as any thing else

[jadebox]

My hand span is almost exactly ten inches.

I suggest that we not take the "size of body parts" discussion any further ......

-- Roger

[JPVegh]

It's as good a starting point as any thing else

Exactly my point in this discussion.

I suggest that we not take the "size of body parts" discussion any further ......

-- Roger

Hah! You weren't the only one thinking that we were on dangerous ground here.

[bill_s]

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

Saying the reason is to be logarithmic is true, but may or may not really explain it. The log scale is useful anytime differences are figured as multipliers rather than addition. For example, 10% larger than 10 N-s is 11 N-s. 10% larger than 100 is 110. Adding 1 to 10 is 10%, but adding 1 to 100 is only 1% larger.

Other examples are in music, an octave is a doubling of frequency, and the difference between 2 high notes next to one another is a lot more Hz than between 2 low notes. Sound pressure is also logarithmic and also is often notated in decibels, which converts it to numbers that are added and subtracted instead of multiplied -- and has caused plenty of confusion in the process. Approx. 6 dB is a doubling of sound pressure -- it gets more steps than rocket engines. 10 decibels are 1 Bel which is a somewhat large quantity to work with.

There are psychological factors that may make one engine class have more choices within than another, or differences seem more significant, but each class is still the exact same range.

[kruland]

Saying the reason is to be logarithmic is true, but may or may not really explain it. The log scale is useful anytime differences are figured as multipliers rather than addition. For example, 10% larger than 10 N-s is 11 N-s. 10% larger than 100 is 110. Adding 1 to 10 is 10%, but adding 1 to 100 is only 1% larger.

Other examples are in music, an octave is a doubling of frequency, and the difference between 2 high notes next to one another is a lot more Hz than between 2 low notes. Sound pressure is also logarithmic and also is often notated in decibels, which converts it to numbers that are added and subtracted instead of multiplied -- and has caused plenty of confusion in the process. Approx. 6 dB is a doubling of sound pressure -- it gets more steps than rocket engines. 10 decibels are 1 Bel which is a somewhat large quantity to work with.

There are psychological factors that may make one engine class have more choices within than another, or differences seem more significant, but each class is still the exact same range.

Bill,

Thanks for getting this thread back on track. Sometimes its kinda fun to see where threads end up, and this one went where I didn't expect.

Sound pressure has a tremendous range so compressing it with a logarithm allows for easier manipulation of both the high and low end. However, in the end, it was a decision to measure the log of sound pressure with the Bel. I wouldn't be surprised if when Bel introduced this idea, there was quite a bit of discussion about the convention that he chose.

The doubling in the octave is more to do with harmonics than anything else. The sensation induced in the mind by two notes one octave apart is somehow pleasing. I do not think the same arbitrary decision was made here. Frequently, the idea of "octave" is applied to other waves (such as light) which is more arbitrary.

Back to impulse classes. I'm mostly curious why somebody decided that 10N-s would be (the top end) of C (kinda like when Fahrenheit decided where 0 should be). It is very convenient to say two C's is a D. So the doubling does again make some sense. But I have to say there is quite a bit of difference between a little J and a big J.

Another question, when somebody says "it's 2% L" for example, what does this mean mathematically?

Kevin

[iter]

Another question, when somebody says "it's 2% L" for example, what does this mean mathematically?

It means 2% of the difference between K and L: 2560+2560*0.02=2611.2N

Ari.

[bandman444]

It means 2% of the difference between K and L: 2560+2560+0.02=2611.2N

Ari.

What he meant was to multiply the low range by the percent plus one.

2%L is 2560(full K) times (1+0.02) you get 2611.2

A 44%L would be 2560x1.44=3,686.4

Hope that helps.

[bill_s]

The doubling in the octave is more to do with harmonics than anything else. The sensation induced in the mind by two notes one octave apart is somehow pleasing. I do not think the same arbitrary decision was made here. Frequently, the idea of "octave" is applied to other waves (such as light) which is more arbitrary.

True, it wasn't just a case of asking "how big a step do we want?", the octave is totally nonarbitrary the largest interval not heard, essentially, as a smaller interval+octaves. Even with other types of waves, it is relevant because of the way waves combine, such as forming beat frequencies.

Back to impulse classes. I'm mostly curious why somebody decided that 10N-s would be (the top end) of C (kinda like when Fahrenheit decided where 0 should be). It is very convenient to say two C's is a D. So the doubling does again make some sense. But I have to say there is quite a bit of difference between a little J and a big J.

Another question, when somebody says "it's 2% L" for example, what does this mean mathematically?

Kevin

The case of sound pressure may be the best analogy here, sometimes when looking at the "macro" side of things some may say a few dBs isn't that much, and it isn't when comparing a mid-volume sound to the range from extremely quiet to extremely loud. On the other hand, when putting the final tweaks on a mix, a dB or even a fraction of one can be huge. If you're flying Alphas (or fullsize Saturn 5s) the difference between Js is insignificant to you, but if you're flying J rockets it becomes very important. I've never even seen a J in person yet, so make very little difference to me, but different Es make a big difference to me right now (admittedly, magnified by differences in thrust rate not just total impulse, as I'm doing minimal flights of a heavy-ish rocket).

EDIT: I could be wrong as J rockets (I'm guessing) are often transonic. There are some thresholds here, just as with sound.

As to the % thing, I've only seen it in wRASP, each engine size ranges from 50% (min.) to 100% (max. -- actually sometimes it will call a max. H "50% I" for example). If you wanted to be funny, you could call a 51.2 N-s F a "2% L", but I can't exclude that someone might mean different.

[CarVac]

Even tiny differences in impulse can matter a surprising amount. I have been running simulations of an altitude record rocket and found that a 3.0% difference in impulse yielded a 3.2% difference in altitude, which can (not in this case) be the deciding factor in whether or not you get the record.

[kruland]

What he meant was to multiply the low range by the percent plus one.

2%L is 2560(full K) times (1+0.02) you get 2611.2

A 44%L would be 2560x1.44=3,686.4

Hope that helps.

So it's just like I think it would be - linear scaling in the appropriate impulse range. Funny, the impulse categories are geometric but the convention here is linear.

[jadebox]

If you wanted to be funny, you could call a 51.2 N-s F a "2% L", but I can't exclude that someone might mean different.

A "2% L" would be an L motor that's very close to the bottom of the total impulse range for L motors. A "98% L" would be close to the top of the range.

The "Flight Stats" features at RocketReviews.com adds up to the total impulse of all flights in a time range then computes an "equivalent" motor for the total of the total impulse which is reported as a "% X" motor. We also do this with the club's statistics at http://www.r-o-c-k.com/. It's kind of amusing, for example, to see that dozens of model rocket flights adding up to just the equivalent of a single "50% H" motor or whatever.

-- Roger

[jadebox]

BTW, I think that the "percent letter" designation is easier to interpret than the style used by CTI. I wish it would catch on for labelling and describing larger motors.

Hmm ... i just thought of something to add at RocketReviews.com ......

-- Roger

[bobkrech]

Even tiny differences in impulse can matter a surprising amount. I have been running simulations of an altitude record rocket and found that a 3.0% difference in impulse yielded a 3.2% difference in altitude, which can (not in this case) be the deciding factor in whether or not you get the record.

All else being equal, total impulse is related to potential energy which is the product of mgh where m is the mass, g is the acceleration of gravity, and h is the altitude or height.

If you double the total impulse, you approximately double the apogee altitude of your rocket if the thrust is not greatly changed. If you start out with the same mass, the final weight of the rocket is less with the higher impulse so it may go slightly higher than a rocket with a motor that is half the impulse.

The letter impulse system allow the launcher to estimate how high his rocket will go without doing a sim if he knows the performance of his rocket with another motor of similar thrust. When Estes started making kits, personal computers did not exist, so you were happy to be able to guesstimate the projected altitude to +/- 50%. If your rocket went to about 400' on a B, you could guesstimate it would go to about 800' on a C, and 200' on an A, and 100' on a 1/2 A. Using this simple system and the known field dimensions, you could select a motor so that you won't loose the rocket. It rarely worked when we were young because it was hard to resist the urge to put the biggest motor that would fit into your rocket and inevitably you would loose it.

Bob

[AlphaHybrids]

BTW, I think that the "percent letter" designation is easier to interpret than the style used by CTI. I wish it would catch on for labelling and describing larger motors.

Hmm ... i just thought of something to add at RocketReviews.com ......

-- Roger

I actually prefer CTI's designation. At a quick glance you can get the total impulse of the motor, where it is within the letter class (I,J,K, etc.) and also get an approximate burntime becuase you know the average impulse and total impulse. Take the Pro38 419I800-15A. You know that it is just over an I motor and that it burns roughly 1/2 a second (419/800). If you had the same designation of I800 30.9% I couldn't do the math easily to see how long the burntime is w/o taking ((.309 *320)+320)/800. (419/800) is much clearer.

Edward

[CarVac]

See, that doesn't work with several AeroTech motors. The K550 has nowhere near 550 average newtons of thrust, and the rated impulse of the K1100T and G138 among others exceed the stated impulse. CTI actually names their motors with the average thrust so your method works for them.

[kruland]

See, that doesn't work with several AeroTech motors. The K550 has nowhere near 550 average newtons of thrust, and the rated impulse of the K1100T and G138 among others exceed the stated impulse. CTI actually names their motors with the average thrust so your method works for them.

Shouldn't the manufacturer's designation be more accurate? I guess according to NFPC1127 it only has to be accurate to 20%.