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Space Technology

NASA Sticking To Imperial Units For Shuttle Replacement 901

JerryQ sends in a story at New Scientist about the criticism NASA is taking for deciding to use Imperial units in the development of the Constellation program, their project to replace the space shuttle. "The sticking point is that Ares is a shuttle-derived design — it uses solid rocket boosters whose dimensions and technology are based on those currently strapped to either side of the shuttle's giant liquid fuel tank. And the shuttle's 30-year-old specifications, design drawings and software are rooted in pounds and feet rather than newtons and meters. ... NASA recently calculated that converting the relevant drawings, software and documentation to the 'International System' of units (SI) would cost a total of $370 million — almost half the cost of a 2009 shuttle launch, which costs a total of $759 million. 'We found the cost of converting to SI would exceed what we can afford,' says [NASA spokesman Grey Hautaluoma]."
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NASA Sticking To Imperial Units For Shuttle Replacement

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  • by eldavojohn ( 898314 ) * <> on Wednesday June 24, 2009 @11:01AM (#28452601) Journal
    How many cwts [] of Mars Orbiters [] must be lost before we learn?!
    • Re: (Score:3, Insightful)

      by fishbowl ( 7759 )

      That *is* the lesson learned, and being followed.

      There are other issues. A machine setup that can make a .5 inch bore to ten-thousandths of an inch precision, cannot necessarily be changed to make a 1.27cm bore with the same precision. Many of the machine tools used in aerospace are calibrated in SAE units, and the machines cannot be replaced economically, if at all -- lathes, milling machines, grinders etc., still in service since the 1960s or even 1940s, refit for CNC, still turning out high-precision w

  • by elrous0 ( 869638 ) * on Wednesday June 24, 2009 @11:05AM (#28452659)

    You know, a lot of Europeans probably think that U.S. reluctance to embrace the metric system is just another example of our arrogance. But a lot of Americans (like me) are genuinely interested in adopting this system. We even passed a law [] in 1975 trying to mandate it.

    The real problem is that it is surprisingly hard to embrace a new system of measurement when you've spent your entire life thinking in different terms. Try as I might, I still can't picture a kilometer without converting it to a mile first, and still can't picture a centimeter without converting it to inches. The meter is a lot easier because it's pretty analogous to the yard. I think maybe your brain gets locked into a certain measurement pattern pretty early in life and it's very difficult to get out of it, even though many of us would happily embrace it. I'm still trying to think more in metric, but it requires a surprising amount mental effort to do so.

    It's not that Americans are really all that arrogant or stubborn about the imperial system. We've actually been trying to embrace the metric system [] for some time.

    • by mrvan ( 973822 ) on Wednesday June 24, 2009 @11:09AM (#28452733)

      I don't buy it

      I lived in guilders all my life, and the first couple years in Eurotime I could only "imagine" a price by converting back to guilders and thinking whether the price sounded right. Now, I can only "imagine" a guilders price by converting it to euros

      I've lived in the UK and US for 1.5 and .5 years, respectively, and I started thinking natively in most units pretty quickly, esp. inches and miles, and of course pints in the UK. Some units are more difficult, either because they have an offset as well as a scale difference (fahrenheit) or because they just don't make any sense (a 22 fluid ounces drink?? gimme a pint, damnit!)

      I think the UK is busy converting mostly to metric system, so maybe some UKians can chime in with their experience?

      • Re: (Score:3, Informative)

        by jonbryce ( 703250 )

        We have some people called the Metric Martyrs who think metric units are some unnecessary EU interference in our affairs.

        It gets confusing at times when for example the distance you drive in a car is measured in miles, fuel for it is sold in litres, and fuel efficiency is either miles per gallon or litres per 100km. We really need a miles per litre measure, but I guess that isn't going to happen.

        The same law that prevents the Metric Martyrs from selling their vegetables in pounds and ounces also prevents p

      • by FTWinston ( 1332785 ) on Wednesday June 24, 2009 @11:27AM (#28453013) Homepage

        I think the UK is busy converting mostly to metric system, so maybe some UKians can chime in with their experience?

        Unfortunately, not really. All street signs still measure distance in miles, and eighths of miles, and the like, and half the population think that the metric system is (like the euro) just another damn frenchie scheme to undermine our sovereignty. We have a long history (this [], for instance) of coming up with crazy conspiricies to demonstrate why the imperial system is our God-given right, and why the French would like nothing better than to force their evil organised system of measurement upon us.

        Meanwhile, for at least a couple of decades now, kids grow up being taught nothing but metric, and wonder why the grown ups still insist on using imperial, and what on earth a fluid ounce actually is. Cos everyone seems to use it, but I don't think anyone under 25 has actually been taught it.

      • by kazade84 ( 1078337 ) on Wednesday June 24, 2009 @11:39AM (#28453201)

        I'm from the UK, and my mental image of measurements is fucked.

        I know how much a pint is. I know how much 1kg is, but I don't know how much is 1 pound in weight. I know how tall I am in feet and inches, but not in meters.

        All because we use metric for some reasons, and we are still stuck in imperial for others. My milk comes in bottles that are labelled 568ml although *everyone* refers to it as a pint, obviously our alcoholic drinks come in pints and half pints. Our speed limits are measured in miles per hour, yet we used to run the 100 meters at school. My height has always been given to me in feet and inches (while growing up by my parents) and if you speak to pretty much anyone they will also give their height in feet and inches, yet if I go to the doctor, they want me to know how high in meters. If you go under a low bridge, the height is given in feet.

        When I go swimming the pool is in meters, when referring to medium distances anyone aged over 40 refers to yards, everyone below that refers to meters, at larger distances it's rare for anyone to use kilometers. Anyone over 40ish only understands Fahrenheit, everyone below uses degrees centigrade.

        Generally speaking things are moving to metric (thankfully) but it will take many many years for imperial to die here currently we are in one big measurement mess and we will be for some time, especially as every traffic sign is in imperial.

      • Re: (Score:3, Interesting)

        by dkf ( 304284 )

        I think the UK is busy converting mostly to metric system, so maybe some UKians can chime in with their experience?

        Almost everything now metric. Exceptions are for beer and milk (pints, though milk is also sold in metric units; total muddle), spirits (fractions of a gill) and road distances (miles). Next to nobody uses imperial weight measures any more.

        Beer and spirits are imperial because it would take a major piece of legislation to change. (English law is very very strict there, and pints and gills do have precise metric definitions these days...)

        • Re: (Score:3, Insightful)

          by xaxa ( 988988 )

          Beer is imperial because it would take a major piece of legislation to change.

          It would take a tiny piece of legislation to change it (and the rest of them). But the Daily Mail wouldn't like it, so it hasn't happened yet.

    • by Arthur B. ( 806360 ) on Wednesday June 24, 2009 @11:10AM (#28452747)

      No it's not. I've was born and raised in France, moved in the US at 23, 4 years ago. The only unit I'm still uncomfortable with is F (also one of the stupidest) I have no problem thinking in inches, miles, gallons, ounces without converting.

      • Re: (Score:3, Informative)

        by ivan256 ( 17499 )

        Fahrenheit is one of the few units I prefer over the metric counterpart. At least when talking about weather or indoor climate.

        When expressed as an integer (temperature frequently is when talking about weather), Fahrenheit is a more precise unit.

    • by Snowblindeye ( 1085701 ) on Wednesday June 24, 2009 @11:17AM (#28452849)

      The real problem is that it is surprisingly hard to embrace a new system of measurement when you've spent your entire life thinking in different terms.

      Yes. Thats why the Canadians haven't been able to do it either. Or the Irish. Not Australia and New Zealand either. Or India.

      Oh wait, they *have* all done it. So how come they can, but for the US it's just too hard?

      • by Sperbels ( 1008585 ) on Wednesday June 24, 2009 @11:28AM (#28453031)

        Oh wait, they *have* all done it. So how come they can, but for the US it's just too hard?

        No, I think it's because too few people care, so politicians don't care...and it never gets done. Simple as that.

      • Re: (Score:3, Insightful)

        by Malc ( 1751 )

        Some things are deeply ingrained. In Canada, the building industry is still imperial, and people generally talk about their weight in pounds (not stones and pounds like the UK). Australia seems to have converted more thoroughly, although I could talk to older people in imperial.

        Inches and feet are units of a nice sized. Most things can be expressed as a whole unit, and when working precisely, they're easy to sub-divide (1/2, 1/4, 1/8, 1/16, etc). Try quartering a cm - you end up with fractions of mm. C

        • Re: (Score:3, Insightful)

          by Dr. Hok ( 702268 )

          Try quartering a cm - you end up with fractions of mm. Cm and m seem to be constantly odd numbers or funny fractions.

          A question of POV. I have a couple of wrenches whose sizes are written in SI and imperial units, and I really find 5/16'' more odd than 8 mm.

          Americans seem particularly resistant to change. It will take a government with a lot of will to make such a change. A good starting place would be if the government mandated everything it does is metric.

          I think it worked in Germany by outlawing old units. You were forced to sell your stuff in kg instead of pounds etc.

          It also helped a lot that Germany was a conglomerate of various kingdoms, each of which had different units. On markets near the border between, say, Prussia and Hanover people were sick of converting the Prussian ell (pound, mile etc) into their Hanoveri

    • Re: (Score:3, Interesting)

      by Lumpy ( 12016 )

      Most Americans cant picture a mile. picturing a kilometer is easy, it's very close to 7 city blocks.

      Or for you suburbians the distance from abercrombie to starbucks.

      It's easy if you simply use it. Problem is ask any of your co-workers how big an inch is and most will be very wrong.

    • Re: (Score:3, Informative)

      Countries have changed, with varying success. You can do it if you want to.

      Canada sort of changed, but has slipped back a bit. However, our road signs are in kilometers, our weather forecasts are in degrees Celsius, we sell liquids by the litre, and few people under the age of 50 have any issue with this. I was in elementary school when we changed our weather forecasts (I'm 47), and I find U.S. weather forecasts and road signs and such meaningless unless I translate them to proper units.

      While the price

  • by oneirophrenos ( 1500619 ) on Wednesday June 24, 2009 @11:06AM (#28452669)

    1 foot = 0.3048 meters

    There you go, NASA. That one's for free.

  • The sticking point is that Ares is a shuttle-derived design â" it uses solid rocket boosters whose dimensions and technology are based on those currently strapped to either side of the shuttle's giant liquid fuel tank. And the shuttle's 30-year-old specifications, design drawings and software are rooted in pounds and feet rather than newtons and meters.

    And in 20 years, that'll be the same excuse given for building Ares's replacement with imperial units.

  • Horses Asses (Score:5, Interesting)

    by the phantom ( 107624 ) on Wednesday June 24, 2009 @11:09AM (#28452729) Homepage
    This email goes around archaeological circles every once in a while (I'm sure it goes around other circles, too), and I just got a new copy of it from my uncle yesterday, so it seems as good a time as any to share:

    People are always asking why we do things the way we do. Well, here is the reason: railroad tracks.

    The US standard railroad gauge (distance between the rails) is 4 feet, 8.5 inches. That's an exceedingly odd number.

    Why was that gauge used? Because that's the way they built them in England, and English expatriates designed the US railroads.

    Why did the English build them like that? Because the first rail lines were built by the same people who built the pre-railroad tramways, and that's the gauge they used.

    Why did 'they' use that gauge? Because the people who built the tramways used the same jigs and tools that they had used for building wagons, which used that wheel spacing.

    Why did the wagons have that particular odd wheel spacing? Well, if they tried to use any other spacing, the wagon wheels would break on some of the old, long distance roads in England because that's the spacing of the wheel ruts.

    So, who built those old rutted roads? Imperial Rome built the first long distance roads in Europe (including England) for the legions. Those roads have been used ever since.

    And the ruts in the roads? Roman war chariots formed the initial ruts, which everyone else had to match for fear of destroying their wagon wheels. Since the chariots were made for Imperial Rome, they all had the same wheel spacing. Therefore, the United States' standard railroad gauge of 4 feet, 8.5 inches is derived from the original specifications for an Imperial Roman war chariot. Bureaucracies live forever.

    So, the next time you are handed a specification/procedure/process and wonder 'What horse's ass came up with this?' you may be exactly right. Imperial Roman army chariots were made just wide enough to accommodate the rear ends of two war horses (two horses' asses). Now, the twist to the story.

    When you see a Space Shuttle sitting on its launch pad, there are two big booster rockets attached to the sides of the main fuel tank. These are solid rocket boosters, or SRBs. The SRBs are made by Thiokol at their factory in Utah. The engineers who designed the SRBs would have preferred to make them a bit fatter, but the SRBs had to be shipped by train from the factory to the launch site. The railroad line from the factory happens to run through a tunnel in the mountains, and the SRBs had to fit through that tunnel. The tunnel is slightly wider than the railroad track and the railroad track, as you now know, is about as wide as two horses' behinds.

    So, a major Space Shuttle design feature of what is arguably the world's most advanced transportation system was determined over two thousand years ago by the width of a horse's ass. And you thought being a horse's ass wasn't important? Ancient horse's asses control lots of things...


    • by sakdoctor ( 1087155 ) on Wednesday June 24, 2009 @11:18AM (#28452871) Homepage

      Why is my mailbox full of unfunny spam?

      Because a bunch of horses' asses keep hitting the Forward button.

    • Re:Horses Asses (Score:5, Informative)

      by H0p313ss ( 811249 ) on Wednesday June 24, 2009 @11:32AM (#28453087) []

      Claim: The United States standard railroad gauge derives from the original specification for an Imperial Roman war chariot.

      Status: False

      • Re:Horses Asses (Score:5, Interesting)

        by Kjella ( 173770 ) on Wednesday June 24, 2009 @11:59AM (#28453555) Homepage

        That must be one of the weakest "false" results I've seen on Snopes. As it says itself:

        Origins: This is one of those items that although wrong in many of its details isn't exactly false in an overall sense and is perhaps more fairly labeled as "True, but for trivial and unremarkable reasons."

        In fact, it collaborates that the English railway was made in the same size as double-horse carriages, that the US share that width because they shared tools and that it's the dominant standard today since the northern US won the civil war. It's a bit of a stretch that double horse carriages were popular only because the romans did it, but they certainly did do it first and built a massive network of them.

        Finally, on the space shuttle thing snopes is just being silly. The largest carriage in the table listed by snopes is 9-10 feet. According to wikipedia the shuttle boosters are a little over 12 feet. So while the part about being "slightly wider than the track" is a liberal description, it's certainly possible they couldn't be built bigger because the tunnels aren't bigger.

  • $370 million? (Score:3, Interesting)

    by sanosuke001 ( 640243 ) on Wednesday June 24, 2009 @11:10AM (#28452749)
    What the hell are they spending this money on? If they paid their engineers $150,000/year, they could hire almost 2500 engineers for a year-long project. It's not like they're building anything new or buying raw materials; they just need someone to re-draw plans with new measurements in a different system. The fitting/testing for the Ares should already be budgeted for so it shouldn't fit in with this cost. No wonder we're in debt...
    • Re:$370 million? (Score:4, Insightful)

      by Volante3192 ( 953645 ) on Wednesday June 24, 2009 @11:22AM (#28452911)

      You make it sound so simple...when, in fact, this is quite literally rocket science here.

      One of the common stories here is people needing to rewrite an entire project because of a new language fad. The old project worked. Rewriting it first means you have to replicate the old project and then deal with new bugs while the old project had all the bugs mostly ironed out.

      Why do we insist NASA to reinvent the wheel when we're so against it in our own profession?

      • Re: (Score:3, Insightful)

        by atamido ( 1020905 )

        I'm sorry, it really sounded like you just called the metric system a "new language fad". Seriously?

        Look, it's not about saving money now, it's about the future. They've already lost a multi-million dollar project because they haven't gone metric yet, and a similar mistake in the future is not unlikely.

        Much of the production for space parts occurs outside of the US, where they use what? Metric. What kind of overhead do you think they charge to supply imperial equipment as well as the metric that they su

    • Re:$370 million? (Score:5, Insightful)

      by Anonymous Coward on Wednesday June 24, 2009 @11:37AM (#28453171)

      It's not like they're building anything new or buying raw materials; they just need someone to re-draw plans with new measurements in a different system.

      Frankly, and without trying to be insulting, you're so ignorant of what the issue is that it's laughable that you even have an opinion on it.

      This isn't a matter of trivia, where we are worried if plans are marked in inches or mm. Change to metric, now every bolt must be metric pitch thread, every nut must be changed to accomodate. Every calculation of mass and structural integrity has to be reexamined and recalculated for new components. You don't just magically say "ok, our 3/8" bolts are now to be called 9.525mm bolts" and call it a day.

    • Re:$370 million? (Score:4, Insightful)

      by DerekLyons ( 302214 ) <> on Wednesday June 24, 2009 @12:24PM (#28453975) Homepage

      What the hell are they spending this money on? It's not like they're building anything new or buying raw materials; they just need someone to re-draw plans with new measurements in a different system.

      And once the drawings are re-drawn, you have to verify the individual drawings. Then you have to verify the interfaces to make sure that vendor 'A' didn't round his tolerances in a direction that means his part will no longer properly mate with a part from vendor 'B'. Then you have to withdraw the old drawings from service and replace them with the new in an orderly fashion. Somewhere along the way you also have to not only update the references between drawings, but also the hundreds of thousands of pages of documentation, specifications, etc... that reference these drawings.
      The individual steps are bone simple - but there are a lot of individual steps and they interact in various complicated ways.
      An additional problem is that all this has to be done while those drawings, specifications, etc... etc... are in daily use at facilities scattered across the country, which means you have a fairly difficult problem not only in making these changes - but in ensuring everybody is 'on the same page'...

    • Re:$370 million? (Score:5, Insightful)

      by Just Some Guy ( 3352 ) <> on Wednesday June 24, 2009 @12:37PM (#28454221) Homepage Journal

      If they paid their engineers $150,000/year, they could hire almost 2500 engineers for a year-long project.

      Or, pay 10 engineers to make sure that the adapter between the (imperial) boosters and (metric) Ares is properly sized and be done with it. If you're pulling a boat behind a truck, you don't care if the truck engine's bolts are metric and the boat's are imperial because they don't have anything to do with each other. As long as the hitch pieces are compatible, you're golden.

    • Re:$370 million? (Score:5, Insightful)

      by Buelldozer ( 713671 ) on Wednesday June 24, 2009 @01:31PM (#28455183)

      Please advise us on how you're going to train every machinist, and QC agent, just to name two job categories on how to measure 30.22mm with calipers that are intended to measure in Imperial? If they can't do this, do it reliably, and do it accurately then you're going to have some funny fitting parts on those Ares.

      That is just one very simple example in two very limited job categories where changing from SI to Metric would introduce horrible, and potentially disastrous, difficulties.

  • by gregg ( 42218 ) on Wednesday June 24, 2009 @11:12AM (#28452763)

    Abe Simpson: The metric system is the tool of the devil! My car gets forty rods to the hogshead and that's the way I likes it.

  • by Audiophyle ( 593650 ) on Wednesday June 24, 2009 @11:29AM (#28453045)
    Most systems engineers in the space industry know that it's difficult to completely use metric for space missions. There are usually many components and subsystems that are designed by different vendors that have their own paradigms set up. These paradigms are usually kept do a legacy of proven use, and engineers will agree with me that if a product works well on-orbit, why on earth would you want to change a product simply due to unit conversions. You simply take note of the units and move on. I never thought I'd have to deal with microinches, to be honest, but it's no big deal since everyone knows 1 uin = 0.0254 microns.
  • by jcochran ( 309950 ) on Wednesday June 24, 2009 @11:30AM (#28453057)

    the issue isn't just one of redoing the drawings along with the various checks and cross checks to make certain the units were converted properly. I'm sure they could that, but the resulting set of new drawings would be extremely prone to encouraging mistakes. As a minor example. Let's assume that on one piece they currently have a dimension of 12 inches +/- 0.01 inches. So they convert this dimension to metric giving a new value of 30.48 cm +/- 0.025 cm. Excuse me?!?!? That's a rather odd and strange dimensional target to hand off to the machinest. And you'll be getting these rather strange dimensions for everything on the original design. Frankly using the metric measurements would make that rocket utterly hell to construct. So the "proper" solution would be to use the original design and then stretch/shrink various dimensions in order to make the dimensions "rounder" and easier to manufacture. But upon doing that, they have effectively come up with a new design that has to be recertified.

    • Re: (Score:3, Insightful)

      by Sleepy ( 4551 )

      >"Let's assume that on one piece they currently have a dimension of 12 inches +/- 0.01 inches. So they convert this dimension to metric giving a new value of 30.48 cm +/- 0.025 cm.... using the metric measurements would make that rocket utterly hell to construct."

      If the part needs to be that certain length and tolerance, it will be, end of story.
      There's no inaccuracy and it will be machined like so.
      I doubt every part on the Japanese rockets is EXACTLY in 1mm increments.

      The biggest payoff is in all the NO

  • by RabidMoose ( 746680 ) on Wednesday June 24, 2009 @11:40AM (#28453215) Homepage
    $370 million to do undergrad-level (at most) grunt work? Isn't that what unpaid interns are for?
  • I'll do it! (Score:3, Funny)

    by ultraexactzz ( 546422 ) on Wednesday June 24, 2009 @11:40AM (#28453223) Journal
    I'll do the job for only $170 Million, and I'll get it done on time and within budget, something that NASA is not used to.

    For an extra $30 Million, I'll even make sure it's accurate!
  • by jcouvret ( 531809 ) on Wednesday June 24, 2009 @11:47AM (#28453327)
    We're just talking about units of measure. If it is easier to use imperial units because previous design and drawings were done in imperial, then that's the smart choice. I would be upset if NASA was wasting taxpayer money just so that the design could be done in metric. I actually applaud NASA for making a smart, cost/benefit engineering decision.
  • Metric (Score:4, Insightful)

    by kenp2002 ( 545495 ) on Wednesday June 24, 2009 @12:23PM (#28453959) Homepage Journal

    It never ceases to amaze me the resistance to going to metric here in the states for measurements but no one bats an eyelash at the fact our money is basically metric (base 10).

    It is in fact soo damn easy that we can instinctively give somone a $5 and a penny for something that costs $4.01 so we can get back a dollar rather then 3 quarters 2 dimes and 4 pennies....

    Boo metric it's too damn easy to use! Forget cutting a board 1.46 meters in half. it's too damn hard to cut it .73 meters! Better yet that 3 5/8th inch board needs to be cut in half so we need umm... err... need some scratch paper here....

  • by fm6 ( 162816 ) on Wednesday June 24, 2009 @12:56PM (#28454561) Homepage Journal

    Somehow the idea that U.S. units are called "Imperial" units has taken root. That term only applies to a system used in the British Empire/Commonwealth (hence the name) before they went metric. The U.S system is "English units" (because it's based on units that were widely used in England at the time of American independence) or "U.S. Customary Units." The two systems are very close (length and weight are the same) but not identical (volume units are quite different, even thought the names are the same).

    In most other contexts, I'd just say, "OK, sloppy usage eventually becomes the standard, like 'broadband' instead of 'high-bitrate'. Been happening since language was invented, not going to change."

    But in this case you have terms that are defined in standards. And miscommunication can cause much wackiness. For example, suppose I need 10 gallons of something. The nearest store is just across the border in Canada, and they're metric, so I use Google to convert units [] and come up with 45.5 liters. Nice and simple, right?

    Wrong. I only needed a little less than 38 liters []. The U.S. gallon is 20% smaller!

    OK, this particular example is kind of artificial, because most people would just say "gallon" and Google assumes that "gallon" means "U.S. gallon". Still, you need to be careful with this stuff. Like, suppose you're putting fuel in an airplane []!

    Of course, all this extra confusion is yet another reason for the U.S. to go metric. I work for for a computer manufacturer that not only sells widely in metric countries, our actual production is outsourced to companies that are mostly in metric countries. Does this cause headaches? You bet!

  • by CherniyVolk ( 513591 ) on Wednesday June 24, 2009 @01:27PM (#28455083)

    France first adopted the Metric System in 1791 (according to Wikipedia). Let me repeat that... 1791.

    The first public, commercial, industrial use of the Metric System in America was Coca-Cola; Coca-Cola bottles have always displayed their volume in metrics, and they have been around since 1886. Let me repeat that... 1886.

    First shuttle flight was in 1977.

    Now here's the surprise on my part. For as long as I have been alive, all science and math text always focused on the metric system. Aside from off-tasks in grade school of converting Celsius to Farhenheit(sp?) or inches to centimeter... gallons to liters... everything has always been in metrics. Growing up, the total icon of science and math has been primarily NASA. It is very hard to for me to conceive, that given the adoption of the metric system in acadamia and almost exclusive to intellectuals and professionals... that NASA has for so long, and so widespread throughout any of their projects, adopted anything other than the metric system. Had this article not been published, I would have refuted any claim that NASA didn't use the metric system. All I can say in 2009 is "wow".

  • by rssrss ( 686344 ) on Wednesday June 24, 2009 @02:20PM (#28456073)

    The Imperial System of measurements is not the same as the customary measurements used in the United States. The legal arbiter of measurements in the United States is the National Institute of Standards and Technology. Apendixes B [PDF] [] and C [PDF] [] to their Handbook 44 provide a good overview of the structure of the respective standards and their relationship to SI (the science based International System, which was based on the Metric System).

    The word system seems misleading when applied to US customary measures. For example:

    Appendix B. Section 2.2.5. From 1893 until 1959, the yard was defined as equal exactly to 3600/3937 meter. In 1959, a small change was made in the definition of the yard to resolve discrepancies both in this country and abroad. Since 1959, we define the yard as equal exactly to 0.9144 meter; the new yard is shorter than the old yard by exactly two parts in a million. At the same time, it was decided that any data expressed in feet derived from geodetic surveys within the United States would continue to bear the relationship as defined in 1893 (one foot equals 1200/3937 meter). We call this foot the U. S. Survey Foot, while the foot defined in 1959 is called the International Foot. Measurements expressed in U. S. statute miles, survey feet, rods, chains, links, or the squares thereof, and acres should be converted to the corresponding metric values by using pre-1959 conversion factors if more than five significant figure accuracy is required.

    Does this make a difference? From one viewpoint, no, when do you ever need to keep something accurate within 2 mm over a mile? From another, yes, repeated iterations of computations based on incorrect conversions can produce just plain gibberish. Another bit of measurement chaos to keep in mind:

    Appendix B. Section 2.3. British and United States Systems of Measurement. ... In the customary British system, the units of dry measure are the same as those of liquid measure. In the United States these two are not the same; the gallon and its subdivisions are used in the measurement of liquids and the bushel, with its subdivisions, is used in the measurement of certain dry commodities. The U. S. gallon is divided into four liquid quarts and the U. S. bushel into 32 dry quarts. All the units of capacity or volume mentioned thus far are larger in the customary British system than in the U. S. system. But the British fluid ounce is smaller than the U. S. fluid ounce, because the British quart is divided into 40 fluid ounces whereas the U. S. quart is divided into 32 fluid ounces. ...
    1 U. S. fluid ounce = 1.041 British fluid ounces
    1 British fluid ounce = 0.961 U. S. fluid ounce
    1 U. S. gallon = 0.833 British Imperial gallon
    1 British Imperial gallon = 1.201 U. S. gallons

    We also must remember that NASA has proven itself incapable of managing the different systems of measurement before. Ten years ago NASA crashed a Mars bound probe [] because of botched conversions from customary to SI units. You would think that having paid $125 million for that lesson, they would want to avoid a recurrence. But, I suppose that they are from the government and they do not have to care.

  • by whoisisis ( 1225718 ) on Wednesday June 24, 2009 @06:42PM (#28459995)

    Seeing the discussion here, I wonder why nobody has brought this up yet: []

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