Interstellar Travel w/o Magic: Part 5, Pounds

Before I get into the details of propulsion systems and vehicle design I want to make 2 brief post about the units of measurement I’ll use when discussing these things.  No, no… don’t go to sleep.  It is really interesting stuff.  Or at least more so than you may suspect.

I will generally use British Gravitational Units in these posts (a.k.a., the Imperial System) because they are a more useful system of units*.  For some modern technologies (electronics, nuclear power, etc.) I may slip into metric units sometimes when that is appropriate because the fields are so young that there are no long-established Imperial Units in common use and/or familiar to most readers.  Similarly when dealing with certain phenomenon where natural units or dimensionless units or astronomical units are easier to work with (like speed in fraction of light speed a.k.a. C, or acceleration as a fraction of Earth gravitation a.k.a. Gees, or distance in light-years) and are commonly understood by most readers, I will do so.  If I throw in any weird units (like specific structure cost), that are necessary technical jargon I think most readers won’t be familiar with I will try to explain them either here, in the post referencing them, or both.

The first thing I want to clarify is my use of the units of “pound”.  Yes, units plural.  “Pound” is perhaps rivaled only by ton and barrel for the diversity of units that word represents.   There are at over half a dozen common, currently used units known as “pounds” and but I will probably only use 3 of them in my posts.  The first is, of course, as a unit of mass.  For those foreign readers a pound of mass is roughly the amount of mass in one pint of beer.  If you don’t drink pints of beer, I understand… I’m a Baptist myself too.  You can just imagine a pint of water.  The second pound unit is a unit of force.  A pound of force is the force exerted by one pound of mass accelerated at normal earth gravity (about 32.2 ft/s^2 or 1 g).  When I need to differentiate between pound (force) and pound (mass) I will abbreviate the former as lbf. and for the latter as lbm.  Most of the time, however, it is clear from context if the unit is a force or a mass, in which case I’ll just refer to it as a pound and abbreviate it with the obvious shorthand of lb.  (for libra: Latin for what you use to measure both types the aforementioned pounds and also the similar Roman unit).  The reason we have pounds of both force and mass, is because it predates the understanding that the weight of an object (a force) and the mass of an object were not the same thing.  There was an attempt to solve this confusion by inventing an alternate Imperial unit of mass, the slug (equal to about 32.2 lbm.).  The slug never caught on, however, possibly due to the fact that it has such a lazy, depressing name.  The third use of pound is as a measure of energy.  Really, it is the same as lbm. but converted to the energy equivalent (E=mc^2).  One pound of energy is about 3.9*10^13 BTUs (4.1*10^16 Joules) or the about amount of energy the United States consumes every three and a half hours.

Other common uses of pound are: a measure of explosive power (equal to the explosive power of 1 lb. of TNT), a measure of British currency, an alternate unit of mass used only for precious metals, and an alternate unit of weight (force) also only used for precious metals.  The alternate pounds used for precious metals are called Troy pounds to differentiate them from mundane pounds (which are technically Avoirdupois pounds).  So, yes, a pound of feathers does not weigh the same as a pound of gold bullion because they are not the same pounds.  (See, I told you this was interesting… you could use this to win you pound of beer in a bar bet). There is an exception, in that if the precious metal is being measured for mundane use, and not as a valuable or financial instrument, (such as figuring out how much the gold foil insulation on your spaceship masses, or how many rail cars you need to transport a load of silver bars) then it is still measured in the regular Avoirdupois pounds.  Interestingly a Troy pound of precious metals is lighter than a regular pound, but a Troy ounce of it is heavier than a regular ounce, as there are only 12 Troy ounces in a Troy pound.

The other unit of measurement confusions I need to clarify are exhaust velocity vs specific impulse and cost vs specific cost.  I’ll explain that in the next post, however.

*Yes, I know about the Systeme International (which I will refer to as the Metric System, as that is the common American term) and how easy it is to convert by multiplying or dividing by a factor of 10.  Back during the sliver of time when “computer” was a job title instead of a device that was certainly important.  Nowadays, I can type 12 into my calculator as fast as I can type 10 and my computer can multiply by 5280 as fast as 1000.  While that calculation disadvantage of of the Imperial System is gone, the advantages remain.  Remember, there have been innumerable units of measure invented over millennia.  The ones that stuck around through history to become known as “customary units” were invented once also, and they were only kept while all the others were discarded because they were easier or more useful to the specialists who used them than all the other units invented.  It is a particular arrogance of the Metric inventors, matched only by Socialists and Esperanto speakers, that what THEY invented will supplant everything before because THEY are so much smarter and logical than all generations before and the ordinary people with their unthinking habits.  America didn’t fail to adopt Metric because we were backwards hicks, but because we were the one nation of, by, and for the people where the Ivory tower intellectuals and fanatical micro managers that orbit all capitol cities did not have a powerful central gov’t that could force tradesmen and specialists to do what the “Idea Men” and their model train fantasies wanted them to do.

Quick story: When helping a client stand-up a manufacturing facility in Red China, we had to order some metal plates.  I assumed that they would come in even increments of 1 meter, but my local engineering team beamed with pride when they told me that due to Chinese innovative thinking, the plates were sized in increments of 1.2 meters (e.g. 2.4m x 3.6m).  You see, they explained, it was often necessary to subdivide material into a small number of equally spaced lengths (as, in fact, we needed to do).  A 1m based panel could be evenly divided into 2 or 4 units (or 5, but that’s not needed as often) with a tape measure marked in cm or mm… but if you cut it into 3 parts the division would fall always  between marks and the exact point to cut would have to be estimated.  Different workers would estimate differently, and even the same worker would have more error estimating between marks.  By having the panel sizes be based on 1.2m, the size could always be divided into 2, 3, or 4 equal parts (and 6, but that not needed as often) and cut reliably.  “What,” they asked, “Did we do in America when we faced the same problem?”  They were very dejected when I explained that they had just re-invented a base 12 linear measurement system and that Americans never had that problem because we were smart and stubborn enough to not abandon ours in the first place (despite the protestations from the gov’t and intellectuals that we must)… that that was why there were 12 inches to the foot.

If you wanted to take things further with being able to evenly divide things, you could base your measurement system on the smallest number that is evenly divisible by 1,2,3,4,5, and 6 (and also 10) and use 60 as the basis for your system, which is exactly what the Babylonians did.  Before you begin to think that mathematics based on 60 instead of 10 seems like a lot of trouble, I’ll point out that the Babylonian customary system is still used for measuring the most frequently subdivided item we have: time.  So even the most ardent metric supporter reading this uses customary units when they need to measure the most valuable thing they have.

Non-engineers may find this odd, but the obvious superiority of basing measurement on 12 but the fact that we have only 10 fingers was one of my most disconcerting theological mysteries… until I learned that I had just been counting wrong my whole life.