Use of Metric Units

[jamescobban]

Participants who were educated in science or the 95% of the world's population who live outside of the United States of America have only very limited familiarity with American Customary Units outside of narrow limits.  For example they may have an intuitive understanding of Fahrenheit temperatures over the range of weather reports (-40 to +40), of pounds over the range of shopping purchases (1 to 50), and so on.  Similarly nobody in your audience has an intuitive understanding of what speeds in thousands of statute miles per hour are, because they have never driven a vehicle above even 100 mph.  Your audience are not going to intuitively understand American units outside of those limited ranges. 

For example in the discussion of LOFTID on 20/11/2022 the NASA representatives mentioned that it had to withstand up to 2900F.  I sincerely doubt that anyone in your audience has an intuitive understanding of what that means because they have never observed anything at that temperature.  By contrast SpaceX describes all of its experiments and system in metric, including indicating the temperature its TPS must handle in Celsius, not Fahrenheit.  The mission progress indications on SpaceX launches report altitudes and ranges in km and speeds in m/s.  If SpaceX can do that in the United States, why can NASA not use metric?  Could you please ask your guests to use the standard units of science and engineering, not the popular US environment.  It is my understanding that because of the Mars Climate Observer failure that all internal engineering and operations in NASA missions is performed in metric.  Note that in the Apollo program NASA gave distances not in US statute miles, because US statute miles have never been used in aerospace technology, but rather in Nautical Miles.  For example the air speed indication in an aircraft only has two options: km/h or nautical miles per hour.  But the NASA web site currently reports everything in US statute miles, pounds, feet, and so on with no option to view the information in the units of science and engineering.   Because of this I have requested the Canadian Space Agency (CSA) to work with the European Space Agency (ESA) and Japan Aerospace Exploration Agency (JAXA) to create a copy of the NASA web site which displays everything in the scientific metric system.

[Airlocks]

I'm french and thus from the very country that created and then enforced the metric system. Yet, along the years (and with Google precious help, TBH) I worked my way through Imperial units.

1000 pounds is 454 kg.
2000 pounds, 908 kg.
That's the bare minimum for rocket payload conversions.

I do know that 37°C is 102°F because in high school I LMAO trying to imagine a person with a 102°C fever - that person would boil itself from the inside.

1 miles = 1.609 km, and that one is a huge PITA.

Feet are slightly better: 30.5 cm. Orbital velocity is 30 000 feet (aproximately).

I learned about inch to cm conversion (2.54) thanks to the NRO spysat mirrors. KH-8, KH-10, KH-11: 44 inch, 72-inch, 94 inch, and also rocket diameters.

As far as rocket diameters are concerned, 10 ft is close enough from 3 m (3.05 exactly) and the number in inch is not too tedious: 120 inch, Hubble mirror diameter before 1975 and Congressman Bolland, that idiot.

Worst thing is Machs to km/h to mph conversions - that's complete Hell on Earth.
 

[DanClemmensen]

I'm french and thus from the very country that created and then enforced the metric system. Yet, along the years (and with Google precious help, TBH) I worked my way through Imperial units.

1000 pounds is 454 kg.
2000 pounds, 908 kg.
That's the bare minimum for rocket payload conversions.

I do know that 37°C is 102°F because in high school I LMAO trying to imagine a person with a 102°C fever - that person would boil itself from the inside.

1 miles = 1.609 km, and that one is a huge PITA.

Feet are slightly better: 30.5 cm. Orbital velocity is 30 000 feet (aproximately).

I learned about inch to cm conversion (2.54) thanks to the NRO spysat mirrors. KH-8, KH-10, KH-11: 44 inch, 72-inch, 94 inch, and also rocket diameters.

As far as rocket diameters are concerned, 10 ft is close enough from 3 m (3.05 exactly) and the number in inch is not too tedious: 120 inch, Hubble mirror diameter before 1975 and Congressman Bolland, that idiot.

Worst thing is Machs to km/h to mph conversions - that's complete Hell on Earth.

I always remember that the speed of light is 300,000 km/s and is 186,000 mi/s.  From there you can easily convert miles and kilometers.

[daedalus1]

The problem with Mach conversion is that Mach is a variable number.  It is higher at lower altitudes.

[Proponent]

Odd fact:  At some point centuries ago, the length of the foot was decreased by 10%  Some longer measures of length were increased by 10%, so that measures of land did not need to be changed.  That is why, for example, a rod is a rather odd 5.5 "modern" yards or 16.5 "modern" feet.  The mile would have been a relatively round 4800 old feet rather than the awkward 5280 modern feet (I mean, seriously, what's the point of a unit that is conveniently divisible by 11?).

More to the point, the old foot was one-third of a meter to an accuracy of about 1%.  A factor of three between feet and meters is less convenient that a factor of ten, but still better than 3.28.

In China, some unique imperialized metric units are sometimes used.  For example, if you have clothing made, your tailor may measure you in "market inches," a market inch being one-tenth of a market foot, which, in turn is one-third of a meter.  If you're accustomed to being measured in imperial inches, you suddenly feel very slim!

While I am all for SI units, if I could go back in time, I would encourage the adoption of 6 or 12 rather than 10 as the base of the number system.  Ten is a really poor choice.

EDIT: Punctuation

[Proponent]

The problem with Mach conversion is that Mach is a variable number.  It is higher at lower altitudes.

Strictly speaking, the speed of sound is an increasing function of temperature (and a decreasing function of molecular weight), with the result that it decreases up to about 10 km and starts to increase again above about 20 km.

[whitelancer64]

*snip*

why can NASA not use metric?
*snip*

NASA does use metric. NASA 100% uses metric with few exceptions for old legacy equipment. NASA switched to 100% use of metric for all engineering and design work ever since the unfortunate units mix up that caused the demise of the Mars Climate Orbiter.

NASA only uses US standard / imperial units for interfacing with the US public, which largely does not understand metric. 

NASA's broadcasts and public announcements, educational information, etc. are specifically for the US public, they are not really intended for an international audience.

[nicp]

Metric is fine, but if used I think a further step is necessary - namely S.I units, so none of this km/h nonsense. Metres per second PLEASE.

I'm English and just young enough to have never been officially taught Imperial units, but of course I've been surrounded by them forever so I'm more or less happy - except volumes. British and American measure of say, a gallon, is different for some reason. A 9:7 ratio sounds about right...

Feet per second is an odd one, but dividing by 3 and a bit ain't so hard.

[Airlocks]

so none of this km/h nonsense. Metres per second PLEASE.

It is NOT nonsense. My Fiant Grande Punto max speed (when it was much younger) is 175 km/h, and certainly not... (Google convertor) 48.615 m/s.

[Comga]

*snip*

why can NASA not use metric?
*snip*

NASA does use metric. NASA 100% uses metric with few exceptions for old legacy equipment. NASA switched to 100% use of metric for all engineering and design work ever since the unfortunate units mix up that caused the demise of the Mars Climate Orbiter.

NASA only uses US standard / imperial units for interfacing with the US public, which largely does not understand metric. 

NASA's broadcasts and public announcements, educational information, etc. are specifically for the US public, they are not really intended for an international audience.

This may be incorrect.
It was said that all SLS engineering is done in "Imperial" units.
(I will try to verify this with my friend who worked on Orion.)
Look at the display inside Orion.  The distance to the earth is stated in miles. (six digits, no comma separator, one decimal place. )
The Metric-Imperial debate was lost in the 1980's, and some of us still groan about it.
When I advise student I insist on metric, even while the readily available bolts, bores, and taps are all funky Imperial, even in machine shops dedicated to aerospace hardware.

On the other hand, does it matter what kind of units NASA uses when the data displayed in incorrect to start with, like the altitude above the moon during this morning's fly-by and LOI burn?  First things first.

And yes, nicp, the SI unit "m/s" would be preferable to the non-SI "km/hr", even for SpaceX, but that's so minor compared to ft, lbs, "tons", statute miles, gallons, etc.  (Is NASA still using nautical miles for orbit heights like they did for Apollo?  If it was good enough for Neil and Buzz, it should be good enough.....)

But I "feel your pain", jamescobban.  And on behalf of an extreme minority of your neighbors to the south, I appologize.
 

[laszlo]

Participants who were educated in science or the 95% of the world's population who live outside of the United States of America have only very limited familiarity with American Customary Units outside of narrow limits.  For example they may have an intuitive understanding of Fahrenheit temperatures over the range of weather reports (-40 to +40), of pounds over the range of shopping purchases (1 to 50), and so on.  Similarly nobody in your audience has an intuitive understanding of what speeds in thousands of statute miles per hour are, because they have never driven a vehicle above even 100 mph.  Your audience are not going to intuitively understand American units outside of those limited ranges. 

For example in the discussion of LOFTID on 20/11/2022 the NASA representatives mentioned that it had to withstand up to 2900F.  I sincerely doubt that anyone in your audience has an intuitive understanding of what that means because they have never observed anything at that temperature.  By contrast SpaceX describes all of its experiments and system in metric, including indicating the temperature its TPS must handle in Celsius, not Fahrenheit.  The mission progress indications on SpaceX launches report altitudes and ranges in km and speeds in m/s.  If SpaceX can do that in the United States, why can NASA not use metric?  Could you please ask your guests to use the standard units of science and engineering, not the popular US environment.  It is my understanding that because of the Mars Climate Observer failure that all internal engineering and operations in NASA missions is performed in metric.  Note that in the Apollo program NASA gave distances not in US statute miles, because US statute miles have never been used in aerospace technology, but rather in Nautical Miles.  For example the air speed indication in an aircraft only has two options: km/h or nautical miles per hour.  But the NASA web site currently reports everything in US statute miles, pounds, feet, and so on with no option to view the information in the units of science and engineering.   Because of this I have requested the Canadian Space Agency (CSA) to work with the European Space Agency (ESA) and Japan Aerospace Exploration Agency (JAXA) to create a copy of the NASA web site which displays everything in the scientific metric system.

First, there are 2 types of countries - those who have landed men on the Moon and those who use metric.   Sorry, couldn't resist. I'll be good from now on.

As I understand it, in the Apollo days, the AGC did all its calculations in metric but, due to the astronauts' explicit preferences, the displays were in imperial units.

BTW, having dealt with casting and welding aluminum and with molten glass and with lava (long story), I actually do have an intuitive understanding of what 2900°F looks and feels like. I've also personally operated multiple vehicles well in excess of 100 mph. And if you count people who have looked up at jets and the people inside who were looking back down at them, there are a lot of us who are quite familiar with speeds much greater than 100 mph.

Airspeed is not reported in nautical miles per hour. It's in knots, where 1 knot equals 1 nautical mile per hour. I know, I know, but  since we're being pedantic about units here, let's be consistent.

Miles per hour has a very useful attribute in that the speed limits on US highways are approximately 60 mph (1 mile per minute) so that it's very easy to make a first-order approximation of how long a cross-country trip will take. Jets tend to cruise near 10 miles per minute, so same deal. 100°F is also a nice round number that corresponds closely to human body temperature. It's a bit more memorable than 37°C. So for those of us used to thinking in imperial units, there are some good calculation shortcuts that metric thinkers don't have. Those shortcuts will help keep imperial entrenched in the US.

NASA faces a sticky situation in that it doesn't want to alienate the public which pays for it. The US is not that cosmopolitan, so I think that we'll keep seeing imperial units from NASA for quite a while.

[nicp]

so none of this km/h nonsense. Metres per second PLEASE.

It is NOT nonsense. My Fiant Grande Punto max speed (when it was much younger) is 175 km/h, and certainly not... (Google convertor) 48.615 m/s.

We could compromise on angstroms per annum?

[Comga]

Yup! 
A direct quote from my friend on Orion:

Imperial [units]. LM [Lockheed Martin] has an edict that all engineering be done in imperial. (Except for optics cuz that entire industry is metric as you know)
And horror, we had to use Fahrenheit for temperature ugh

 

It seems clear that Lockheed did this at the direction of NASA.

[TrevorMonty]

If you think UK government couldn't get any worst think again. What idiot thought this was a good idea.

https://www.euronews.com/culture/2022/05/30/uk-to-revive-imperial-measurements-to-bring-back-british-culture-and-heritage-says-mp

[edkyle99]

When Boeing builds the Core Stage, is it designed to be 8.4 meters or 27.6 feet in diameter (as described by NASA)?  My guess is that neither of those values are accurate.  They're either doing things in millimeters or in inches.  330 inches, maybe, or 331, or 8400 millimeters.  Etc.  (STS ET is sometimes listed at 331 inches.)
 Good luck finding that answer!

If it is built using U.S. customary units, then the most accurate way to describe it is using those units.  If metric, same.  Otherwise, conversion and rounding errors are introduced.

I prefer metric, but I don't mind feet pounds.  It isn't that hard to do the conversion, or to use Google for the conversion.

 - Ed Kyle

[1]

....nobody in your audience has an intuitive understanding of what speeds in thousands of statute miles per hour are, because they have never driven a vehicle above even 100 mph.  Your audience are not going to intuitively understand American units outside of those limited ranges. 

Although I agree with this, it's ultimately a self-defeating argument. If the target audience (which, in this case, is the US general public) does not have any intuitive feel of high values of imperial units, they won't have an intuitive feel in any other system of units either. And, you could probably successfully argue that the rest of the general world public has no intuitive feel for metric values that high, so what difference would it make? Ultimately, this is why content intended for the general public often ends up containing some kind of analog that might be better intuitively understood. For example, I've lost count how often Starship's properties (payload, thrust, interior volume, CO2 emissions, etc) have been benchmarked against a Boeing 747. I'm chin-deep in aerospace, and have intuitive feel for neither 2.3MN nor 500Klbs-f.

That all said, what I find particularly amusing is that most Americans actually do have an intuitive understanding of m/s, and simply don't realize it. The reason is good-ole' American Football, and the fact that yards are similar in length to meters. So really, the solution is subterfuge. Rather than ask NASA to display metric units, have them instead display 'football' units; and you've won 90% of the battle.

While I am all for SI units, if I could go back in time, I would encourage the adoption of 6 or 12 rather than 10 as the base of the number system.  Ten is a really poor choice.

Couldn't agree more. I've had similar musings with my old university roommate, and settled on 12 as a better base number because we both find need to divide by 3 or 4 much more often than 5.

If it is built using U.S. customary units, then the most accurate way to describe it is using those units.

The fun thing here, for those unfamiliar, is that many US customary units become semi-metricized at small values. It's very common to see mils and microinches (thousandths and millionths of an inch, respectively) in use. The main impediment in aerospace is that a lot of commanding documentation still uses older imperial units in some way; everything from MIL docs to test and screening methods. Revising all of these documents would be a gigantic task, but would need to be done before a complete conversion to metric could be done. It's a straightforward task, but has a huge amount of institutional inertia to overcome. As a result, most people I know have a 'meh' attitude towards it, because we're comfortable in either system. Sure, we lose a Mars probe or two every now and then, but ultimately we can blame that on program management. :-)

[mkent]

OK, as an practicing American engineer, I want to clear a few things up on this thread.

For example they may have an intuitive understanding of Fahrenheit temperatures over the range of weather reports (-40 to +40)

Maybe if you live in Barrow, Alaska.  :-)  Temperatures in most U. S. weather reports range from 0 to 100 degrees F, which is one reason why that scale has remained popular with the public.

Could you please ask your guests to use the standard units of science and engineering, not the popular US environment.

Science, yes, but not engineering.  Most engineering in the United States is done in the British Engineering system.  That's particularly true in aerospace.

NASA only uses US standard / imperial units for interfacing with the US public, which largely does not understand metric. 

NASA's broadcasts and public announcements, educational information, etc. are specifically for the US public, they are not really intended for an international audience.

This.  NASA's broadcasts are for their customer: the U. S. taxpaying public.

It was said that all SLS engineering is done in "Imperial" units.

Correct.  All of the CAD models for the Core Stage and EUS are in inches, and I'd bet a hefty sum that the models for the engines, boosters, and ICPS are too.  That's the way American aerospace is done.

When I advise student I insist on metric, even while the readily available bolts, bores, and taps are all funky Imperial, even in machine shops dedicated to aerospace hardware.

If your students are American engineering students, you are really doing them a disservice.  Their careers are going to be based in British Engineering units.  That's what the machine shops, foundries, fabrication houses, standard parts, tools, tooling, and process specs all use.  Thus, their products will use that too.

A direct quote from my friend on Orion:

Imperial [units]. LM [Lockheed Martin] has an edict that all engineering be done in imperial. (Except for optics cuz that entire industry is metric as you know)
And horror, we had to use Fahrenheit for temperature ugh

 

It seems clear that Lockheed did this at the direction of NASA.

No.  I guarantee you that most engineering at Lockheed is done in British Engineering units, as is most American aerospace.

[spacenut]

I grew up using the English system.  We learned about the metric system, but that is about it in high school.  I still used English in physics in high school.  It was still used when I went to college. 

Fast forward.  I understand metric.  My biggest stumbling block is going from lbs of thrust to whatever metric is used.  I understand that 1 bar is atmospheric pressure.  I understand tons of pressure metric.  It is the Pascals, Newtons, and kilopascals that confuse me in translation to an English equivalent. 

For instance 7.5 million lbs thrust of Saturn V can be translated into would be about 3,402 tons of pressure in Metric.  What should that read in metric?  33,362.9 kilonewtons?  Or, a 30,000 lb thrust engine to what in metric? About 13.6 metric tons?  133.4 kN?  Both seem off when roughly translating. 

[Steve G]

In Canada we started to switch over in the early seventies, and life was great until someone in their infinite wisdom decided that the insanely simple rocket thrust listed in pounds should be changed to Newtons. What the heck is a Newton? Seriously? Fine for rocket scientists and physicists, but for laymen? I'm constantly using online converters to figure out rocket thrust these days.

[John Santos]

so none of this km/h nonsense. Metres per second PLEASE.

It is NOT nonsense. My Fiant Grande Punto max speed (when it was much younger) is 175 km/h, and certainly not... (Google convertor) 48.615 m/s.

We could compromise on angstroms per annum?

Furlongs per fortnight.