Eternal sunshine of the space mad
Dec. 29th, 2017 01:06 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
glaurungPosting this here so I can find it again.
Space travel is expensive and difficult, so we humans do very little of it. We launch satellites, we send up astronauts and sometimes space probes, but not many. The market for space launches is small and almost entirely government subsidized. Which has crippled attempts to make getting into space less difficult and expensive, which has prevented the development of uses for space that would justify the development of less expensive ways to get there. Egg, meet chicken.
This state of affairs is frustrating for space enthusiasts who feel that humans ought to be colonizing other worlds, living in space, and generally going out beyond Earth in a big way. Space enthusiasts (and I am one) tend to be susceptible to trying to invent a way that "doing space in a big way" could be economically sensible. Far too often, they do so without bothering to do the math or the fact checking required to determine if their latest scheme makes any actual sense or not.
Hence Helium 3 mining on the moon (for a method of fusion that we do not know how to do and if we did know, it would be far cheaper to mine it from supplies on Earth, or manufacture it by making tons of tritium and waiting for it to decay), Solar power satellites (a rube goldberg way of making electricity where just building the solar power installations down here would be only 3x less efficient and be 10x or more cost effective). And so on.
One meme that I keep seeing come up is how there are mountains and crater rims on the north and south poles of the Moon that get eternal sunlight, right next to craters that are eternally dark and therefore contain ancient ice. The idea is that all we need to do is install solar panels on those mountains and we will have oodles of solar power for mining lunar ice, then using that to fuel our vast armada of space ships/keep our space colonies supplied with water/what have you.
The total area of moon that gets 24/7 sunlight is, according to the above link, only "a few football fields" - call it 10 hectares.
On earth, on average, one hectare of solar panels delivers one gigawatt-hour of power per year (after converting to real units from the "only in America" stupidity of acres per gigawatt). Divide a gigawatt by the hours in a year and you get 114 kilowatt hours of power.
In space, thanks to 24/7 sunlight and no atmosphere, you get 3x the power as on Earth. So one hectare of panels gives 350kwh of power, rounded up. Ten hectares gives 3500 kwh of power. At 3.6 megajoules per kw, that's 12,600 megajoules per hour, 302,000 megajoules per day. Sounds like a lot.
An earth based open pit mining operation that processes 10,000 tons of rock and ore per day will consume 4,750 liters of diesel fuel for its mining vehicles. IC diesel engines are usually 40-50% efficient (whereas electric motors are over 90%), so half that, times the energy content of diesel (40 mega joules/liter) is 95,000 megajoules per day (assume the benefit of lower gravity is offset by the handicaps of hard vacuum and -170 degree ambient temperature). SO that's 1/3 of our power budget, and all we've done is dig up the "ore" (ice). We still need to melt it (334 mj per tonne), and (for rocket fuel) use electrolysis to split it into hydrogen and oxygen (20,000 mj per tonne). Nine tons of water yields 1 ton of hydrogen, so 180,000 mj of power to create one ton of hydrogen. Mining and making one ton of hydrogen per day uses up nearly all of our power budget.
A nuclear powered cargo ship using oxygen/hydrogen fuel and carrying a 15 tonne payload from Earth orbit to Lunar orbit would require 40 tonnes of hydrogen and 35 tons of oxygen per 3 day round trip. Our eternally lit solar panels could make enough fuel for one earth-moon cargo ship trip once a month.
And we haven't allotted any power for turning the hydrogen and oxygen we make into liquids (10,000 kw/tonne to produce LH2), providing life support for human staff (if any), lighting up the floor of the perpetually dark crater so we can see what we're doing, running the ore processing machinery, etc, etc, etc.
Those eternally lit mountains at the lunar poles sound like a great place for solar power generation, but even if we use every square meter, they're too small to support any kind of large scale operation.
Space travel is expensive and difficult, so we humans do very little of it. We launch satellites, we send up astronauts and sometimes space probes, but not many. The market for space launches is small and almost entirely government subsidized. Which has crippled attempts to make getting into space less difficult and expensive, which has prevented the development of uses for space that would justify the development of less expensive ways to get there. Egg, meet chicken.
This state of affairs is frustrating for space enthusiasts who feel that humans ought to be colonizing other worlds, living in space, and generally going out beyond Earth in a big way. Space enthusiasts (and I am one) tend to be susceptible to trying to invent a way that "doing space in a big way" could be economically sensible. Far too often, they do so without bothering to do the math or the fact checking required to determine if their latest scheme makes any actual sense or not.
Hence Helium 3 mining on the moon (for a method of fusion that we do not know how to do and if we did know, it would be far cheaper to mine it from supplies on Earth, or manufacture it by making tons of tritium and waiting for it to decay), Solar power satellites (a rube goldberg way of making electricity where just building the solar power installations down here would be only 3x less efficient and be 10x or more cost effective). And so on.
One meme that I keep seeing come up is how there are mountains and crater rims on the north and south poles of the Moon that get eternal sunlight, right next to craters that are eternally dark and therefore contain ancient ice. The idea is that all we need to do is install solar panels on those mountains and we will have oodles of solar power for mining lunar ice, then using that to fuel our vast armada of space ships/keep our space colonies supplied with water/what have you.
The total area of moon that gets 24/7 sunlight is, according to the above link, only "a few football fields" - call it 10 hectares.
On earth, on average, one hectare of solar panels delivers one gigawatt-hour of power per year (after converting to real units from the "only in America" stupidity of acres per gigawatt). Divide a gigawatt by the hours in a year and you get 114 kilowatt hours of power.
In space, thanks to 24/7 sunlight and no atmosphere, you get 3x the power as on Earth. So one hectare of panels gives 350kwh of power, rounded up. Ten hectares gives 3500 kwh of power. At 3.6 megajoules per kw, that's 12,600 megajoules per hour, 302,000 megajoules per day. Sounds like a lot.
An earth based open pit mining operation that processes 10,000 tons of rock and ore per day will consume 4,750 liters of diesel fuel for its mining vehicles. IC diesel engines are usually 40-50% efficient (whereas electric motors are over 90%), so half that, times the energy content of diesel (40 mega joules/liter) is 95,000 megajoules per day (assume the benefit of lower gravity is offset by the handicaps of hard vacuum and -170 degree ambient temperature). SO that's 1/3 of our power budget, and all we've done is dig up the "ore" (ice). We still need to melt it (334 mj per tonne), and (for rocket fuel) use electrolysis to split it into hydrogen and oxygen (20,000 mj per tonne). Nine tons of water yields 1 ton of hydrogen, so 180,000 mj of power to create one ton of hydrogen. Mining and making one ton of hydrogen per day uses up nearly all of our power budget.
A nuclear powered cargo ship using oxygen/hydrogen fuel and carrying a 15 tonne payload from Earth orbit to Lunar orbit would require 40 tonnes of hydrogen and 35 tons of oxygen per 3 day round trip. Our eternally lit solar panels could make enough fuel for one earth-moon cargo ship trip once a month.
And we haven't allotted any power for turning the hydrogen and oxygen we make into liquids (10,000 kw/tonne to produce LH2), providing life support for human staff (if any), lighting up the floor of the perpetually dark crater so we can see what we're doing, running the ore processing machinery, etc, etc, etc.
Those eternally lit mountains at the lunar poles sound like a great place for solar power generation, but even if we use every square meter, they're too small to support any kind of large scale operation.
(no subject)
Date: 2017-12-29 02:38 pm (UTC)