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The Changing Definition Of 'Kilogram' 964

Posted by timothy
from the jenny-craig-kilograms dept.
DrLudicrous writes "The NYTimes is reporting that the platinum-iridium standard mass for the kilogram is shedding at an appreciable rate -- at least compared to other reference masses. The Pt-Ir cylinder is kept in France, and measured annually, and the slight discrepancy is important because the kg is an SI base unit- thus other quantities such as the Volt are based on it. A new standard is being sought- the two frontrunners are counting the number of atoms in a perfectly spherical single crystal of silicon, and another technique uses a device known as the Watt balance."
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The Changing Definition Of 'Kilogram'

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  • Re:yay! (Score:2, Informative)

    by sky_fire (109496) on Tuesday May 27, 2003 @12:37AM (#6044702) Homepage
    actually since the kilo weighs less you weigh more because it takes more kilograms to equal your mass. :p
  • Look here... (Score:4, Informative)

    by switched4OSX (668686) on Tuesday May 27, 2003 @12:40AM (#6044725)
    Here is a site that gives some reasons why the are thinking of replacing the standard: l. No registration necessary
  • Re:Kilogram? (Score:5, Informative)

    by el-spectre (668104) on Tuesday May 27, 2003 @12:45AM (#6044760) Journal
    It's not a matter of dark ages, it's a matter of infrastructure... while not the largest country in the world (the US is probably third or fourth, I'm not sure), we have by far the most technological infrastructure. It is not feasible to change all that in a short period of time.

    A friend is in construction, and guestimates that it will take over 100 years to replace all failing/obsolete tech with the versions in metric equivalents. It just does not make any economic sense to replace a set of, say, water pipes with the metric standard if the current ones will last 20 years. It'll have to be a gradual thing.

    Just to be difficult, though, I'd mention that most construction is done in 'tenths of feet', even the surveying equipment is marked this way. Has nothing to do with the metric system, it just makes the math easier...
  • by Midajo (654520) on Tuesday May 27, 2003 @12:47AM (#6044773)
    Scientists Struggling to Make the Kilogram Right Again

    RAUNSCHWEIG, Germany -- In these girth-conscious times, even weight itself has weight issues. The kilogram is getting lighter, scientists say, sowing potential confusion over a range of scientific endeavor.

    The kilogram is defined by a platinum-iridium cylinder, cast in England in 1889. No one knows why it is shedding weight, at least in comparison with other reference weights, but the change has spurred an international search for a more stable definition.

    "It's certainly not helpful to have a standard that keeps changing," says Peter Becker, a scientist at the Federal Standards Laboratory here, an institution of 1,500 scientists dedicated entirely to improving the ability to measure things precisely.

    Even the apparent change of 50 micrograms in the kilogram -- less than the weight of a grain of salt -- is enough to distort careful scientific calculations.

    Dr. Becker is leading a team of international researchers seeking to redefine the kilogram as a number of atoms of a selected element. Other scientists, including researchers at the National Institute of Standards and Technology in Washington, are developing a competing technology to define the kilogram using a complex mechanism known as the watt balance.

    The final recommendation will be made by the International Committee on Weights and Measures, a body created by international treaty in 1875. The agency guards the international reference kilogram and keeps it in a heavily guarded safe in a château outside Paris. It is visited once a year, under heavy security, by the only three people to have keys to the safe. The weight change has been noted on the occasions it has been removed for measurement.

    "It's part ceremony and part obligation," Dr. Richard Davis, head of the mass section at the research arm of the international committee.

    "You'd have to amend the treaty if you didn't do it this way."

    That ceremony has become a little sorrowful as the guest of honor appears to be, on a microscopic level at least, wasting away.

    The race is already well under way to determine a new standard, although at a measured pace, since creating reliable measurements is such painstaking work.

    The kilogram is the only one of the seven base units of measurement that still retain its 19th-century definition. Over the years, scientists have redefined units like the meter (first based on the earth's circumference) and the second (conceived as a fraction of a day). The meter is now the distance light travels in one-299,792,458th of a second, and a second is the time it takes for a cesium atom to vibrate 9,192,631,770 times. Each can be measured with remarkable precision, and, equally important, can be reproduced anywhere.

    The kilogram was conceived to be the mass of a liter of water, but accurately measuring a liter of water proved to be very difficult. Instead, an English goldsmith was hired to make a platinum-iridium cylinder that would be used to define the kilogram.

    One reason the kilogram has lagged behind the other units is that there has been no immediate practical benefit to increasing its precision. Nonetheless, the drift in the kilogram's weight carries over to other measurements. The volt, for example, is defined in terms of the kilogram, so a stable kilogram definition will allow the volt to be tied more closely to the base units of measure.

    A total of 80 copies of the reference kilogram have been created and distributed to signatories of the metric treaty. The sometimes colorful history of these small metal cylinders underscores how long the world has used the same definition of the kilogram.

    Some of the metal plugs were issued to countries that later vanished, including Serbia and the Dutch East Indies. The Japanese had to surrender theirs after World War II. Germany has acquired several weights, including the one issued to Bavaria in 1889 and the one that belonge
  • Re:crazy question (Score:2, Informative)

    by Midajo (654520) on Tuesday May 27, 2003 @12:55AM (#6044816)
    how exactly do they derive volts from kilograms?

    A volt is 1 newton-meter per coulomb.
    A newton is the force required to accelerate a 1 kilogram mass 1 meter per second, per second.
    Most (all?) units of (metric) measurement are based on kilograms, meters, and seconds.
  • by BJH (11355) on Tuesday May 27, 2003 @12:56AM (#6044817)
    A meter is the distance light travels in a certain fraction of a second. c is invariant no matter where you are, and a second is defined as a particular number of caesium atom vibrations, giving you a very precise basis for measurement of distance.
  • Re:Annually (Score:5, Informative)

    by sould (301844) on Tuesday May 27, 2003 @12:59AM (#6044839) Homepage
    Why exactly does it have to be measured annually......Anyone care to enlighten me?

    It doesn't exactly have to be measured. They just do that to check it's still right. Go read about the history of the Systeme International the NIST site [] and the definition of a kilogram at the same place []

    But essentially, its part of a way of ensuring that the measuring units Scientists use around the world are the same, not slightly different.

    For instance, anyone around the world can reproduce (in a well equipped lab anyway) the definition for time (The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom).

    There are only 7 base SI units (meter, kilogram, second, ampere, kelvin, and candela) from which many more units [] are derived. Hence, if kilo is out/changing many of these are changing too.

    and why should I care if it detoritates?

    Presuming you're American, you would use feet, pounds, find metric too complicated, etc, etc - so probably wont care if it does.

  • by Yosho (135835) on Tuesday May 27, 2003 @01:00AM (#6044848) Homepage
    The unit of "volt" can also be expressed as m^2kgs^-3A^-1.
  • Re:Look here... (Score:4, Informative)

    by sc00p18 (536811) on Tuesday May 27, 2003 @01:01AM (#6044850)
    Nice info. Here it is Linkified []
  • by Anonymous Coward on Tuesday May 27, 2003 @01:07AM (#6044895)
    Didn't see one of these yet (but of course there will be one posted right before I submit this...): tml?ex=1054612800&en=a5e16828b6cf309b&ei=5062&part ner=GOOGLE []
  • Re:i'm confused (Score:3, Informative)

    by madcow_ucsb (222054) <{ten.sknas} {ta} {2todhsals}> on Tuesday May 27, 2003 @01:21AM (#6044979)
    Apparantly you skipped physics class. E=m*c^2.

    But on your 2nd point, you're right. A joule is defined based on a kg, not the other way around.
  • yeah it's a mess (Score:5, Informative)

    by Trepidity (597) <delirium-slashdot AT hackish DOT org> on Tuesday May 27, 2003 @01:23AM (#6044990)
    The various prefixes -- kilo, Mega, Giga, and so on -- are very precisely defined SI prefixes that have been in common use in the sciences for quite some time now. In computing though, 1024 bytes was originally termed a "kilobyte" because it was very close to an actual "kilo" of bytes (1000 bytes), and so was a convenient term to use. In other computer-related disciplines though, in particular engineering, the correct SI usage prevailed, so your 128 kbps mp3s actually have 128000 bits per second, not 128 * 1024.

    The big problem is that 2^(10x) and 10^(3x) diverge as x increases: 1024 is 2.4% more than 1000, 1048576 is 4.9% more than 1000000, 1073741824 is 7.4% more than 1000000000, and so on. So obviously the "close enough" thing is getting less and less true -- when there's a 10% difference between the two measurements they're not even close enough for everyday colloquial speech.

    So the solution of both the SI and the IEEE is to reassert the original meanings of the SI prefixes (kilo = 1000, Mega = 1000000, etc.), but to add new base-2 prefixes in recognition of their usefulness in computing. These are kibi, Mebi, Gibi, etc. (basically the same as the SI prefixes but with the last two letters replaced by "bi"). Their standard abbreviations are the same as for the SI prefixes, but with a lowercase 'i' appended (so ki, Mi, Gi, etc.).

    The conversion is obviously nowhere near complete, and irritates some computer people who don't want to change the terms we've been using for decades, but this seems to be the only really reasonable way of doing things. The only other two options are to either force the rest of the sciences to change to use the base-2 definitions (which is obviously not going to happen, and they got there first anyway), or to maintain the current ambiguity, which is also obviously undesirable.
  • by mishac (75996) <slashdot AT mishac DOT com> on Tuesday May 27, 2003 @01:23AM (#6044996)
    I believe bhutan is a buddhist/hindu country.
  • by Dhrakar (32366) on Tuesday May 27, 2003 @01:27AM (#6045016)
    The posted article, while interesting, is wrong about the volt being based on the Kilogram. Since about 1990, the volt is defined to be the voltage applied to a Josephson junction that produces a frequency of 483,597.9 GHz. This new standard was implemented in order to get away from relying on 'artifact' standards (such as the Kg cylinder). One quick source page on Josephson junctions (which completely revolutionized the field of Metrology back when I was a calibration tech in the AF) is: squid.html
    If I recall correctly, the eventual goal of the international standards organization was to find ways to define everything in terms of frequency/time since we can measure time so accurately/precisely.
  • Re:Counting Si (Score:5, Informative)

    by dhovis (303725) * on Tuesday May 27, 2003 @01:30AM (#6045033)

    Vacancies are not necessarily a problem. As you say, vacancies are entropically favored, but there is also a formation energy associated with a vacancy. So thermodynamics tells us there will be a balance between the energy required to create a vacancy with the entropy gained by creating one.

    Thus, there is an equilibrium number of vacancies in any crystal. As long as you know what the equilibrium value is for a given temperature and you maintain that temperature, then you will also know how many vacant sites you will have on the crystal lattice. I don't have any of my texts handy, but I'm sure someone can chime in with the numbers for silicon.

    To sum up. All crystals will have vacancies because vacancies are thermodynamically favored. However, the number of vacancies will tend towards an equilibrium value which allows them to be accounted for.

  • by LastToKnow (449735) on Tuesday May 27, 2003 @01:47AM (#6045116) Homepage
    a kilogram is, however, a measure of mass, not weight. it is therefore unaffected (if properly measured) by the force of gravity.
  • by Graff (532189) on Tuesday May 27, 2003 @02:07AM (#6045207)
    A leap year is just because somebody screwed up with time (which I personally think that if we could accept the 1.25 year year, then we'd be a lot better for it)

    Hmm, so instead of a year being 365 days long you would want it to be 456 days long? (365 days * 1.25 = 465 days)

    A leap year has nothing to do with anyone screwing up. The problem is that a year does not have an integral number of days. A year is 365 days, 5 hours, 49 minutes (365.2424 Universal days)*. That means that it takes about 526,297 minutes for the Earth to make a full trip around the sun. After the Earth has rotated about its axis 365 times it will still take about 350 minutes until it reaches the same spot it started from.

    That means that if you tried to have the year be an even number of days, say 365, you would fall behind almost 1 full day every 4 years. It's not much but if you let it go for a while you will start having winter during the hottest times of the year. There are a few other rules that adjust the calendar besides the "extra day every 4 years" rule and because of these rules we are able to keep the seasons approximately where they should be.

    To learn more about how the calandars were changed visit this web site [].

    *source: Timeline of interesting calendar facts []
  • Re:Kilogram? (Score:1, Informative)

    by Anonymous Coward on Tuesday May 27, 2003 @02:08AM (#6045209)
    FYI, the pipes in Europe are in inches, or zolls as we call them here. Been that way since who knows when, and it doesn't like that'll be changing any time soon. Standard water and gas pipes here are 1/4" and 1/2".

    Everything else is metric though.

    US is NOT the most technologically advanced country any more, and actually it hasn't been that way for at least five years or more (I know this because I lived in the US for the past seven years, and now I live in Europe again). Previous statement also refers to the infrastructure. Countries like Germany, Switzerland or Japan are ahead of the US now, both in tech and infrastructure.
  • by The Cydonian (603441) on Tuesday May 27, 2003 @02:11AM (#6045226) Homepage Journal

    Fundamentalist Islamic country [] without any telephones []?

    Can I have some of whatever your smoking please?

  • by addaon (41825) <> on Tuesday May 27, 2003 @02:23AM (#6045269)
    Diamond spontaneously decays into graphite... no mass change, I suppose, but it has different absorption of gasses from the air, and different density (matters if they measure mass through weight in an atmosphere). Gold is much more long-lasting.
  • by Anonymous Coward on Tuesday May 27, 2003 @02:23AM (#6045271)
    You don't get it. He's asking why the kilogram isn't just called a gram. He's not asking why the gram isn't the current base unit.
  • by MasterShake (617668) on Tuesday May 27, 2003 @02:25AM (#6045280)
    Just to be the pedantic bastard, pounds are a measure of weight and kilo's are a measure of mass. The imperial measure of mass is the slug which ways ~32lbs under standard conditions See this [] A newton is the SI unit of weight... apples to apples people.
  • by Dr. Spork (142693) on Tuesday May 27, 2003 @02:37AM (#6045330)
    We actually have a metric definition of mass that doesn't depend on anything in Paris. By convention, Avogadro's number of Carbon 12 atoms has a mass of 12 grams. This will remain true no matter what happens in Paris.

    However, maybe I'm implicitly assuming that we have settled exactly what Avogadro's number is. But if we haven't, if we are still holding out for more and more accurate measurements of Avogadro's number, then yeah, we need to really nail down what a kilogram is. But that seems weird to me, because Avogadro's number has no units. It's just a count of atoms, playing the same grammatical role as the word "dozen".

  • by Dr. Spork (142693) on Tuesday May 27, 2003 @02:45AM (#6045365)
    What? We don't define the speed of light. Definitions are up to us, and the speed of light is fortunately not. We define how long a second is, and we define what a meter is in terms of seconds and the speed of light. But we don't define the speed of light. That's just given by nature, and that's why it's so useful.

    The same deal with Plack's constat. It's value is not up to us, but up to nature. "Defining" it would be like defining pi as 3.

  • by Jeremy Gray (223298) on Tuesday May 27, 2003 @02:57AM (#6045417)

    The same problems are still there, regardless of material. Changing the composition doesn't change the fact that there is only one standard in only one laboratory, that stray particles and cleaning will affect its mass upon measurement, and that the standard may be damaged in some way.

    The other solutions presented as candidates to replace the standard rely on invariant physical constants, i.e. Avogadro's number. Distance and time standards are already defined in this way, from the speed of light and the frequency of a two-state cesium transition in the microwave region.

    This shifts the accuracy of the standard from it's care and maintenance to the measurement of constants, with the added benefit of any appropriately equipped laboratory being able to measure the standard.

  • by Proteus (1926) on Tuesday May 27, 2003 @02:58AM (#6045422) Homepage Journal
    Um... the kg isn't a measure of weight, it's a measure of mass. Your weight may change based on where on earth you are, but your mass is not dependent on location -- even in near-zero gravity, your mass is the same.
  • by radish (98371) on Tuesday May 27, 2003 @03:07AM (#6045457) Homepage
    Other than:

    Distances/speeds (miles/mph), beer (pint) and (in some cases only) milk (pint), we are fully metric. Personally I'd welcome a full switch but we have to wait for the old people to die first :)
  • Re:Aaargh. (Score:5, Informative)

    by 2nd Post! (213333) <> on Tuesday May 27, 2003 @03:12AM (#6045474) Homepage
    Except that no one has actually counted/created a structure composed of a mol of any particular element.

    The minute you can do that, then you can reliably and predictably create a fixed metric by which any one in any place can measure mass.
  • Re:Counting Si (Score:5, Informative)

    by neodymium (411811) on Tuesday May 27, 2003 @03:30AM (#6045523) Homepage
    If you really would try to build such a crystal, vacanies could very well be the problem. As you said, there is an equilibrium value of defects in any crystal. This equilibrium value is temperature dependant with a exp(-Eform/kT) law, where Eform is the formation enthalpy. High temperature means high rate of defects.

    Si single crystals are usually prepared at very high temperatures out of molten Silicon (1414C, Czochralsky method). Essentially, this will lead to a freezing of the defect structure at temperatures close to the melting point, because the lattice reorientation kinetics (point diffusion) also are thermally activated.

    You would have to temper the crystal for _very_ long times at temperatures of i.e. 300C to get a thermal equilibrium of defects at this temperature. These times could be >>years !

  • by BJH (11355) on Tuesday May 27, 2003 @03:32AM (#6045526)
    What you're saying is that it's impossible to define a gram as the mass of a particular number of atoms, because we measure mass in grams.

    Using grams to express the mass of an atom, or m/s to express c, is fine, but remember that both of these are actual physical constants, not arbitrary measures. Thus, you can measure c to be of a particular, definite and universal value, independent of the units in which it is expressed.

    The units do not define the constant - the constant is just expressible in those units.
  • Mass not weight guy (Score:4, Informative)

    by MrYotsuya (27522) on Tuesday May 27, 2003 @03:38AM (#6045549)
    A kilogram is the measurement of a certain amount of matter, not it's weight. Sure the two are related, but not the same.

    A kilogram is the same on the surface of the earth, in outer space, or one the moon. Weight however, varies with gravitational pull or acdeleration.

    In other words, weight is basically the mass of on object multiplied by whatever gravitational field you happen to be in.

  • So is the US (Score:5, Informative)

    by pommiekiwifruit (570416) on Tuesday May 27, 2003 @04:36AM (#6045786)
    IIRC the pound (or most versions of it anyway) is defined in terms of the kilogram these days, rather than the average weight of a grain of barley. Otherwise they would have problems if the US genetically modified barley to include jellyfish genes or whatever the seed companies reckon you should be forced to eat (and unless you grow your own food, you are eating it).

    And the metre is defined properly these days (as is the second) in terms of wavelengths of radiation.

  • by mattrix2k (632351) * on Tuesday May 27, 2003 @04:44AM (#6045825)
    No, weight is measured in newtons (N), so somebody with a mass of 80kg weighs about 784N (assuming gravity is 9.80m/s^2).
  • by spiny (87740) on Tuesday May 27, 2003 @05:17AM (#6045922) Homepage Journal
    i've often wondered this, so i checked:

    " First of all bhp and ps are slightly different animals. Bhp or brake horsepower is a calculation where the engine horsepower is measured with no load from a chassis or any accessories attached to the engine whatsoever. It's also called gross horsepower. PS, on the other hand, is a measurement of net horsepower. It's horsepower calculated with the engine installed in the vehicle, complete with accessories and ductwork. So you can't really convert one measurement to the other with a simple formula.

    However, just to give you a better idea on what "ps" stands for, we can talk a little more about horsepower. Just as there are different ways of measuring temperature, as in Centigrade or Fahrenheit, you can measure the power of your engine in several ways, too. Horsepower is simply your engine's ability to move mass over a certain amount of time. If you want to be technical about it: one horsepower can lift 33,000 pounds up one foot in one minute. And that's the measure of horsepower you're familiar with, the one used in the U.S., and it's the standard set by the Society of Automobile Engineers, which is why it's also called SAE horsepower. The ps you're asking about is short for the German word Pferdestarke, the term for metric horsepower, also known as DIN horsepower. DIN is short for Deutsche Industrie Normen which simply translates to "German industrial standard".

    So, in summary, hp is the U.S. standard for horsepower, while ps is the standard in continental Europe. If you want to dazzle your friends, you can say that one horsepower is equal to 1.0139 ps, making SAE horsepower roughly 98.6% of the metric DIN measurement."

    so, as far as i can work out, 'x BHP at the wheels' should be the same as 'x PS' put very simply.

  • Re:Planck units (Score:2, Informative)

    by M_Carling (109238) on Tuesday May 27, 2003 @06:32AM (#6046146)
    The Planck mass is the threshold mass of a point particle (assuming String Theory is wrong and there are point particles) above which it would be a Black Hole. In theory, defining the kilogram as a multiple of the Planck mass is a great idea. In practice, we cannot measure the Planck mass (and probably never will be able to) and the only theory we have (Loop Quantum Gravity) which might perhaps be able to calculate the Planck mass to sufficient accuracy is controversial. There are several ways of calculating the Planck mass to three or four significant figures using theories that we know are wrong, but they agree with each other only to one significant figure.
  • by misterpies (632880) on Tuesday May 27, 2003 @07:38AM (#6046358)

    No. In SI units, c is not measured but defined. Physically, c is just a man-made constant of proportionality deriving from the fact that, for historical reasons, we measure time differently from space. In reality, both time and space are physical dimensions and so it makes perfect sense to express both in terms of the same units, be they seconds or metres.

    That's why most theoretical physicists like to do their calculations in "natural units" -- i.e. you set c=1 and h/2pi=1 -- since in reality the values of the fundamental constants are artefacts of your measurement system. Scientifically speaking, it makes sense to set all independent constants to 1 since it brings out the fact that the "equivalence" of eg mass and energy, or distance and time, is really an identicality.
  • by pete-classic (75983) <> on Tuesday May 27, 2003 @08:04AM (#6046468) Homepage Journal
    Uh, could it have something to do with it being (partially) made of iridium?

  • by Bruce Hollebone (22155) on Tuesday May 27, 2003 @09:21AM (#6046972) Homepage
    That's exactly what the Watt Balance method favoured by the americain team does. The method also requires knowing the local gravity though. That's much harder to get correct.
  • Changing to metric (Score:1, Informative)

    by Anonymous Coward on Tuesday May 27, 2003 @11:01AM (#6047966)
    Changing to the metric system has nothing to do with the war. A lot of European countries were on it before WW2, and the British started the change long after postwar rebuilding.

    Canada also started switching in the seventies, and the switch is pretty much complete, as you will know if you come and visit.
  • by Overzeetop (214511) on Tuesday May 27, 2003 @02:35PM (#6050084) Journal
    Things to remember:

    "Pipe" is described by it's nominal diameter and strength ("schedule"). Nominal diameter is neither internal nor external. ex: 4" schedule 40

    "Tube" is defined by the external dimension (not necessarily diameter) and wall thinkness. ex: 4x4x1/4 (a square tube, 4" on a side, with a 1/4" thickness.)
  • The Volt (Score:3, Informative)

    by BluesGeek (160887) on Tuesday May 27, 2003 @03:11PM (#6050372)
    Just as a minor correction ... the volt is no longer defined in terms of the kg. The international definition of 1 Volt is now defined in terms of the "Josephson Effect" and is an effect observed in superconducting materials that are interupted by a normal metal.

    It turns out, that even without an applied voltage, there is still a current in the system, and after a voltage is applied, the current oscillates at a very predicable rate. Thus, the volt is now defined as the potential required to give a specific number of current osciallations in a Josephson Junction.

    Nit-pickey I know, but maybe of interest.
  • by Chris Y Taylor (455585) on Wednesday May 28, 2003 @05:58PM (#6061463) Homepage
    "Specific impulse is measured in seconds no matter what system you use, imperial or metric."

    Not true! In metric the unit of specific impulse (Isp) of a rocket is Newton*second/kilogram.

    The real unit of specific impulse in Imperial is lb*s/lb where the pounds on top are pounds of force and the pounds on the bottom are pounds of mass. Pounds of force and pounds of mass are NOT the same thing and cannot really be canceled out because they are different units (if it helps think of it as lb*s/slug), but everyone cheats and cancels them out anyway because they sound similar. It may seem shocking that rocket scientists would do something so wrong, but I have only found one person (some anal retentive guy at JPL) in all of rocket science who regularly uses the real Imperial units of Isp.

    You can only measure Isp in seconds if you use Imperial and cheat on the pound/pound cancellation but even otherwise metric loving European rocket scientists use the Imperial "seconds of Isp" convention. Metric rocket scientists prefer to use exhaust velocity (which basically measures the same thing) rather than Isp to characterize their rockets so they don't have to ruin the purity of their metricness.

    I admit that exhaust velocity would SEEM to be the better way to go, but for some strange reason rockets seem to blow up more when you use it. The reliability of specific impulse measured rockets compared with exhaust velocity measured rockets may be due to the fact that exhaust velocity is an elegant term to use theoretically, but specific impulse its easier to measure experimentally on an engine test stand (divide the thrust by propellant mass flow to get Isp). Preference to specific impulse would suggest that the rocket calculations had been done by someone who has spent a lot of time experimenting. All other things being equal an engine that has been tested more will be more reliable. Whatever the reason, don't buy a rocket from a guy that insists on using Ve rather than Isp. I would make an exception for V2s.
  • by Chris Y Taylor (455585) on Thursday May 29, 2003 @12:37AM (#6064894) Homepage
    To answer your last question first. I am not really a rocket scientist. I am doing some research on space propulsion, but it is beamed propulsion not rocket propulsion. Saying that you are a rocketless scientist just makes people look at you funny, though. Besides I am an engineer not a scientst. But I do have a fairly good understanding of rockets even though I am trying to make them obsolete.

    If you have done a lot of classical physics, then you should know what impulse is. In engineering, whenever you see "specific" in front of an engineering term it means that value has been normalized by dividing it by some other important system variable. This technique allows easy comparison between otherwise different things. For example looking at the Fuel Consumption of an unknown engine doesn't tell you how efficient that engine is. What might be great for a drag racer would be terrible for a lawn mower. By dividing the fuel flow by the power of the engine, though, you get Specific Fuel Consumption; that tells you how much fuel the mystery engine is burning per horsepower produced. So you now have a variable that can give meaningful efficiency information regardless of how big the engine is; it works just as well for lawn mowers as for automobiles. The "specific" in Specific Impulse tells you that the impulse imparted to the rocket is normalized to the fuel consumed. So, as you would expect, Specific Impulse has units of Impulse divided by Mass and can be calculated from the impulse the rocket receives divided by the fuel burned in producing that impulse. In reality, it is usually determined by looking at the thrust of the engine (Force) divided by the rate of fuel consumption (Mass per unit time); it gives you the same answer and is easier to measure. Impulse has metric units of Newton*seconds, which should reduce to kg*m/s. Mass has units of kg. So specific impulse (impulse delivered / mass of fuel) should reduce to m/s... not s. And that makes sense because I previously mentioned that Metric loving European rocket scientists often prefer the use Exhaust Velocity rather than Specific Impulse as their figure of merit and that that was OK because they were basically doing the same thing. It is not surprising then that Isp and Ve would have the same units (if you really did Isp units correctly, which is not done).

    I am certainly not saying G Dyson is stupid, but perhaps he is giving the "babies are brought by the stork" level of explanation. That is done surprisingly often in aerospace because sometimes the real explanation is both unimportant and very difficult to understand (it IS rocket science). Most school textbooks just tell the kids that the shape of the airfoil forces the air to faster over the top of the wing than the bottom and that makes it generate lift. A few will explain Bernoulli's equation. And most kids go home happy. Very few kids think to ask what is special about an airfoil that forces the air to go faster on the top so why bore the 9,999 others by trying to explain the Kutta condition. Instead, let the one curious one go get a degree in aeronautical engineering.

    As for references, I went looking for them. At first I found NOTHING to support my derivation and I thought that perhaps it had been too long since I did any rocket calculations and that my memory was going bad. Finally I found the following two references that support my claim (and my perhaps not completely faulty memory): 4/ app-b8.htm
    (look down to the section on Specific Impulse)

    and e. html

    Fortunately one of them is from NASA, and despite some of their screw ups that I like to give them a hard time about they still have some people who know rocket science. I did find enough webpages that throw that darn gee-sub-cee in there that I am going to have to double check some things. I suspect that the people that are arguing for usin

If you're not part of the solution, you're part of the precipitate.