Apollo 10 contingency plans

[Dalhousie]

I don't doubt for an instant that there are non-scientists and non-engineers who have been exposed to metric units their whole life who would get confused by some uses of SI units.  For instance, a layman probably doesn't know what a Newton is or how it's used.

Because they don't understand that force and mass are two different things...

I learned about Newtons in high school physics.  It's a safe bet that anyone who needs to understand the difference between force and mass will know whata Newton is.

You guys are confirming the point I'm trying to make.  There is nothing instinctive about either system of units, they must be learned.  Most layman can use a tape measure, know how far it is to the next town, can measure out a portion of food, and know when it is time to loose weight because of what their bathroom scale tells them, but other than that they're largely ignorant of units of measure.  To know more than the basic everyday life stuff, it must be studied.  In one system a person learns that there are basic units of length (meter) and mass (kilogram), and there's a derived unit of force (Newton).  In the other  system a person learns that there are basic units of length (foot) and force (pound), and there's a derived unit of mass (slug).  Once those basics are learned, what's the problem?  It is no more difficult to solve a problem using English units than it is to solve a problem in SI units.  I just don't accept the argument that one system is intuitive while the other is some screwed up mess.  People who need to know do know, and those who don't know are going to be confused by some aspects of either system.

I learned both systems and problem solving in SI I find much, much easier.  So to the vast majority of people.

[Bob B.]

I learned both systems and problem solving in SI I find much, much easier.

I've learned and used both systems extensively.  SI is easier to understand in the way the base units are defined, and in the use of decimal multiples. SI just makes more sense.  However, once learned, I find almost no difference in using one system versus another in problem solving.  What's easier about plugging meters into an equation instead of feet?

So to the vast majority of people.

I can't testify to what other people think.

[ka9q]

As a physics student we were studying the subject from a purely esoteric perspective and learning other unit systems was never really critical, but the engineers learned CGS.

I'm an electrical engineer but I don't think I was ever formally taught cgs. MKS (meter-kilogram-second) was already the basis of SI when I was in high school in the early 1970s, so those were the units we learned.

If EEs ever used cgs-based units, it was before my time. Volts, amperes, watts, ohms, farads and henries are all MKS/SI. I can recognize cgs units on the rare occasion I see them in old physics papers, but I usually have to look up their meanings.

When I got to Cornell I was astounded to see some of the mechanical engineering professors use English units. I made so many mistakes with them that I routinely converted to SI, performed the calculations there, and then converted the results back to English units if that's what they wanted.

[Sus_pilot]

My .02:  when I fly, I use knots and nautical miles for speed and distance, record times in tenths of an hour, use feet for altitude, inches of mercury for barometric settings, and Celsius for temperature.  It's all arbitrary.

[VQ]

For which the best response would be an immediate rendezvous and docking with the CSM, not a landing...

This is a fictional scenario, but I handwaved that with the imaginary solar flare so severe that staying in the moon's shadow was the only means of survival available to the LM crew. This assumes that a rendezvous with the CSM would have required about a half orbit and exposure to the sun.

[ka9q]

What's easier about plugging meters into an equation instead of feet?

Well, the fact that so many important derived units are based on meters, not feet, with unity conversion factors.

It's far easier to remember that a newton is the force that accelerates a kilogram at 1 meter per second per second than to remember that 0.03108095 pounds-force accelerates 1 pound-mass at 1 foot/sec/sec, or that 1 pound-force accelerates 32.174049 pounds-mass (1 "slug") at 1 foot/sec/sec.

And then you have the ambiguity of the common word "pound", as opposed to lbm or lbf.

But my biggest daily gripe about the English system is its widespread use of fractions. True, nothing inherently requires this, but that's what everybody does. So, quick, which is bigger: a 17/64" socket or a 1/4" socket? How about a 6 mm socket vs a 7 mm socket?

[VQ]

My .02:  when I fly, I use knots and nautical miles for speed and distance, record times in tenths of an hour, use feet for altitude, inches of mercury for barometric settings, and Celsius for temperature.  It's all arbitrary.

This is similar to my experience as a mechanical engineer in the United States. I encounter and use a large number of units of measurement, some of them less organized than the foot-pound system. For example, I routinely use or encounter all these units of energy or heat transfer rate: ton (of refrigeration), Btu/hr, kBtu/hr (frequently abbreviated MBH), W, kW, HP, therm/y, kWh/y. I do not think this is ideal and certainly is more prone to errors than SI, but to do my job I need to be comfortable using units from whatever system of measurement I encounter.

[ka9q]

Yeah, heat transfer is probably the very worst for unit proliferation. Energy supply is sometimes almost as bad. My pet peeve are those who give the output of some new power plant in "kilowatts per year". This phrase would be valid only if you were talking about a plant that produces solar panels or wind turbines.

Another peeve is the (more correct) citation of plant output in gigawatt-hours/year. Why not just give the average power?

[Luther]

Yeah, heat transfer is probably the very worst for unit proliferation. Energy supply is sometimes almost as bad. My pet peeve are those who give the output of some new power plant in "kilowatts per year". This phrase would be valid only if you were talking about a plant that produces solar panels or wind turbines.

So the derivative of power output with respect to time?  Maybe they're phasing the plant in slowly?

Another peeve is the (more correct) citation of plant output in gigawatt-hours/year. Why not just give the average power?

Works a lot better in SI, because there are ten hours per day, ten days per month, and ten months per year 

[Luther]

But my biggest daily gripe about the English system is its widespread use of fractions. True, nothing inherently requires this, but that's what everybody does. So, quick, which is bigger: a 17/64" socket or a 1/4" socket? How about a 6 mm socket vs a 7 mm socket?

I could answer them both pretty quickly, and I think my answers were correct.

I don't mind this particular aspect of the English system, but I did rather resent having to have two complete sets of tools.  And often having to use both of them.  On the same device 

[Echnaton]

Another peeve is the (more correct) citation of plant output in gigawatt-hours/year. Why not just give the average power?

I suppose it depends on the audience and what what "gigawatt-hours/year" actually means.  As a financial analyst looking at a bond issue of a merchant power plant, I would be very interested in the proposed generation output per year.  More so than in the potential capacity of say, running full time and wide open.  Because I'd want to examine the regional spark spread to determine the potential annual cash flow generation.  "Gigawatt-hours/year" could also mean the maximum designed capacity assuming normal downtime, which is also of interest to an investor, who could look for an independent judgement on the need for electricity on the regional grid and match that to the type of plant in making a credit assessment.

[Bob B.]

Well, the fact that so many important derived units are based on meters, not feet, with unity conversion factors.

That get's back to the point I made early that the vast majority of error and confusion comes from the fact that there are two systems of units.  No matter what system you're most accustomed to, there are going to be times when you encounter sources that give other units.  I think incorrect conversions is where most errors are made.  The problem is having to keep straight two systems of units in your head.  It's not because there is something inherently evil about English units.  If the world worked exclusively in one system or the other, I doubt there'd be many problems either way. 

It's far easier to remember that a newton is the force that accelerates a kilogram at 1 meter per second per second than to remember that 0.03108095 pounds-force accelerates 1 pound-mass at 1 foot/sec/sec, or that 1 pound-force accelerates 32.174049 pounds-mass (1 "slug") at 1 foot/sec/sec.

When I worked extensively in English units, the only number I had to keep in my head was 32.174 ft/s².  That was no harder than remembering 9.80665 m/s².  The numbers that one needs to remember becomes so engrained through constant use that I don't see how anyone can consider it difficult.

And then you have the ambiguity of the common word "pound", as opposed to lbm or lbf.

I went through all my years of schooling, got my engineering degree, and I don't remember ever encountering the term "pound-mass".  It wasn't until 20 years ago when I started reading old NASA and rocketry literature that I came across extensive use of the term.  When I was in school and solving problems in English units, the correct unit of mass was the slug, no exception.  We would typically be given a problem in which the weight of an object was given in pounds.  We would instinctively divide by 32.174 to convert to slugs and away we went.  Pound-mass is just another unit is mass (i.e. the mass that has a weight of 1 pound in standard gravity) that has to be converted into slugs.  It's no different than having to convert tonnes into kilograms, except in that case you can make the conversion in your head because you're simply dividing by 1000 instead of 32.174.

The only grip I have with the old NASA and rocketry literature is that they would often put the lbm to slug conversion factor right in the equation.  This allows lbm to be entered directly into the equation without having to first convert into slugs.  This went against my prior training and I had to adjust.  Whenever I used any of these equations in my web page, I stripped the conversion factor out of them.  This makes the equations equally useable in either English or SI units; however, when using English units, mass must be expressed as slugs, as it should be.

A good example of the above is the specific impulse equation, which is often expressed in old literature as I_sp = F/ṁ.  This is a case in which the lbm to slug conversion has already been included.  However, if you use the proper unit of slugs, the equation is I_sp = F/(ṁ*g_o).  In this case, if we are given mass in units of lbm, we covert to slugs by diving by g_o and we see that g_o cancels out.

I_sp = F/((ṁ/g_o)*g_o) = F/ṁ,  where ṁ is in lbm.

I admit this can cause a brief moment of confusion, but once it's explained, everything is good.  Once one becomes familiar with this way of doing things, there shouldn't be any difficulty going forward.  Of course somebody accustomed to SI units might get confused, but that comes back to the point about most problems arising from having two system of units.

But my biggest daily gripe about the English system is its widespread use of fractions. True, nothing inherently requires this, but that's what everybody does. So, quick, which is bigger: a 17/64" socket or a 1/4" socket? How about a 6 mm socket vs a 7 mm socket?

That can sometimes be a problem, though I'm pretty use to it.  Through frequent use I have most decimal equivalents memorized.  For instance, with barely having to think about it I know that 2 5/16" is 2.3125".  And I know that 1' 5" is 1.41667'.  The difficulty comes in having the convert something like 1'-2 5/16" to feet.  Of course it is no more difficult than having to work with time or degrees.  I find that having to convert something like 26^o 23' 51" into decimal degrees to be a far more annoying problem than working with feet and inches.

[Bob B.]

This reminds me of the untis in electostatics, I studied with SI units. My lecturer kept waffling on about other units but said we did not need to know them. As a physics student we were studying the subject from a purely esoteric perspective and learning other unit systems was never really critical, but the engineers learned CGS.

My college years were 1976-81.  I don't remember if I did anything in CGS, if I did it was likely chemistry.  I only had one class in electrical networks I don't remember what we used.  Thermodynamics, as I recall, was all MKS.  My mechanics and engineering courses were mostly FPS, though with a good bit of MKS thrown it.  I remember when I took mechanics it was very common to switch back and forth between systems of units.  For instance, problem #1 on a test might be in FPS and problem #2 was in MKS.  We were forced to become very proficient at both.  When switching back and forth I found no difference in one versus the other in terms of difficulty.

SI units make more sense than English units, and I don't doubt that SI is probably easier to learn for a person is starting from scratch.  However, if one it truly proficient at both, I just don't understand how one can describe SI as "much, much" or "far" easier.  In my experience that is not the case.  Conversions are easier within SI, but other than that, it's just different names for the same thing.

[gwiz]

As I may have mentioned before, I spent most of my working life using both SI and Imperial, basically because we had two major projects on the go, one dating to before and the other after the UK switch to SI.  Once I'd switched from aerospace to F1 I could forget the Imperial units.  As Bob says, you can work happily in either system.

[Allan F]

How much mass had to be lost before the LM could get to any orbit with half fuel? 1000 kg?

About half its mass.  Δv is a function of the ratio of the fully fuelled mass to the empty mass.  If you cut the fuel mass in half, then you have to cut the empty mass in half in order the maintain the same ratio.

I just realized something important when I re-read this thread. The RATIO of descent fuel to the mass of the complete LM and the ratio of ascent fuel to the mass of the ascent stage is  . . . . .

the same (close, anyway).