42 feet per gallon | Hacker News Reader

42 feet per gallon

nasa.gov

46 pointsmarkbnine36 comments

You know, 42 feet per gallon sounds pretty terrible, but I'd say moving 5.5 million pounds of equipment 42 feet with one gallon of gas is pretty damn impressive.

Put differently, a 30mpg, 4500lb car gets 713e6 feet-pounds/gallon (30m/g * 5280f/m * 4500lb). A loaded crawler gets 756e6 feet-pounds/gallon (42fpg * 18e6 lb). Not so different after all! And the car looks much less attractive if you look at just the payload.

This is a meaningless calculation. It's not as though the amount of energy required to move something a specific distance is weight related. The amount of force required to accelerate it to a specific velocity is, but once at that velocity, energy required is a function of friction. Of course larger objects have more air friction on average, but really what you want is some measure of the surface area of the object.

Bah, not that it matters in the least.

"It's not as though the amount of energy required to move something a specific distance is weight related. "

" really what you want is some measure of the surface area of the object."

http://en.wikipedia.org/wiki/Rolling_resistance

Interesting comparison.

A container ship does about 24knots = 40feet/second

It uses around 350tons of fuel/day = 4.5 l/s = 1.2USgall/s

So it does a very similar consumption - around 35feet/gallon

While carrying 150,000tons of cargo = 320million pounds.

The low speed (90 rpm) diesels in those ships are by far and away the most thermally efficient internal combustion engines on the planet. The biggest ones approach 50% efficient.

http://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA96...

Why is that? Why don't they use these to generate electri ity?

The slow diesel engine can be connected directly to the propeller shaft, making it mechanically very efficient. I could be wrong, but I don't think a 90 RPM engine with extremely high torque would be appropriate for hooking directly to an alternator.

Because external combustion engines (boiler + turbine) are more efficient and cheaper to build at large scale

So why not use those in ships?

According to this marine engineering book ( http://books.google.com/books?id=PCSpWWuTgkkC&lpg=PA53&#... ) and this website ( http://library.fentu.ru/book/gumi/101/English/section_1_main... ) which seems based on the book, slow diesel engines are more efficient, but turbines are used in ships.

The steam turbine has until recently been the first choice for very large power main propulsion units. Its advantages of little or no vibration, low weight, minimum space requirements and low maintenance costs are considerable. In steam turbines high pressure steam is directed into a series of blades or vanes attached to a shaft, causing it to rotate. This rotary motion is transferred to the propeller shaft by gears. Steam is produced by boiling water in a boiler, which is fired by oil. Recent developments in steam turbines which have reduced fuel consumption and raised power output have made them more attractive as an alternative to diesel power in ships. They are 50 per cent lighter and on very large tankers some of the steam can be used to drive the large cargo oil pumps. Turbines are often used in container ships, which travel at high speeds.

The weight of the crawler is 5.5 million pounds, it carries 12 million pounds on top for the trip. I'm not sure if the FPG calculation includes the cargo

Very cool. I had to compare it with the Airbus A380-800: 1,300,000 lbs max takeoff weight, using 85472 us gal max fuel capacity, and 6400 mi max range

= 395 ft/gal or 513.5e6 ft-lbs/gal

So, 12,8m/3,78541178l or EU standard combined: 29573,53l/100km?

I wish at least sciency articles used SI.

The numbers you just put above mean nothing to me. Feet per Gallon is something humans can visualize.

I'm sure I'll get downmodded to hell for expressing this opinion here, but I really do prefer imperial measurements.

Where by "humans" I assume you mean "Americans"? I've lived in three countries, none of which use gallons, and only one of which used miles...

I'm from the UK where we often use imperial.

> The numbers you just put above mean nothing to me.

And they will continue to mean nothing because not even sciency articles bother to use SI. Save a concerted effort to learn, obviously. A meter is about 3 feet or one yard and a litre is about a quart, if that helps.

> Feet per Gallon is something humans can visualize.

Kind of a broad statement, no? Humans who are used to imperial units, sure. To folks not accustomed to them it seems at times that "imperialists" measure everything in football fields…

> […] but I really do prefer imperial measurements.

Do you prefer them in an objective comparison or out of habit? SI units are, I think, easier to use because of the decimal-basedness and simpler conversions.

It's only something you can visualise if you grew up with imperial units; "42 feet per gallon" is not inherently easier than "3.4 meters per liter", nor "125.7 gallons per mile" than "296 liters per km".

The advantage to the metric system is the ease of converting between units. I can convert from metres per litre to litres per kilometre by dividing the former into 1000.

In this case 1000/3.4 = 294 (Approx)

But all of those are far more accessible than 29573.53L/100km, which most people would just round up to "a hell of a lot". Using conventional units doesn't really help when the magnitude is so high.

30000 litres is the same as 30 cubic metres, i.e. "small swimming pool". Easy.

It consumes about the same as 5500 mid-sized cars?

OK, so that's about 50 feet per gallon, using imperial gallons instead of US gallons (assuming they used dry gallons rather than wet ones).

Hmmm. Maybe it's better to say 1.5 chains to the peck to avoid confusion over the different gallons.

It would have been more meaningful if rue had normalized it to 3.38 m/l. Those are human visualizable units.

Quite true, but I specifically wanted to get the standard combined "fuel efficiency" figure to compare against other vehicles easily. As has been demonstrated, it is quite simple to convert to simpler figures.

Well, he multiplied both numbers by a huge number and included far too many digits, obscuring what it means. Here comes the advantage of SI (notice that only multiplication by 10 is needed):

    29573.53l/100km = 0.29573531 l/m =~ 0.3 l/m

A way to visualize this is that if you put the fuel needed in a pipe along the route the vehicle rides, then the pipe has a cross section of 3 cm^2 (hey those SI units are handy!).

> The numbers you just put above mean nothing to me. Feet per Gallon is something humans can visualize.

Can you tell me quickly, without getting out pen and paper, how many gallons in 42 cubic feet? I suspect I can calculate how many litres in 42 cubic meters a damn sight quicker.

Mike Rowe of Dirty Jobs got to drive the crawler in one episode. It was a cool episode. You appreciate just how big it is when you see him standing next to the treads.

Article doesn't say how far it typically has to travel.

Traveling quarter mile, no big deal. Traveling 10 miles, much more expensive.

Seems to be about 3 miles from most of the launch pads to the VAB, so about 377 gallons for a one-way trip (http://www-pao.ksc.nasa.gov/nasafact/count3teaf.htm)

The crawler can travel about 1 MPH loaded; the tour guide at KSC (at least when I was there several years ago) made a big deal about the blazing 2 MPH it can do when unloaded. :)

The closest observation point is three miles from the launch pads, so yes, the VAB (Vehicle Assembly Building; it's insanely large) is probably around three miles as well.

Why the three mile distance? Back during the Apollo program, engineers determined that if the Saturn V were to explode on the launch pad, the resulting fireball would be around four or file miles in diameter. I think it's a similar radius for the Shuttle as well.

That sounds like an impossibly huge fireball for a rocket. IIRC even nukes don't create fireballs that large. Maybe I'm underestimating the amount of energy in a rocket.

My guess is that the main threat would be an off course rocket, or debris from an explosion, and that the probability of damage is acceptably low 3 miles away.

So the fireball would still engulf the buildings? Doesn't that still seem too close?

Diameter not radius.

So 4-5 miles diameter = 2-2.5 miles radius.

3 miles away produces a 6-mile diameter safety zone.

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