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Going To Mars Via the Moon (mit.edu)
An anonymous reader writes: Getting anywhere in space is a difficult proposition — at least, if you want to get there in a timely manner. Rocket propulsion requires combustion mass. The more mass you take, the more you need. A team at MIT has found that establishing fuel-generating infrastructure on the Moon could reduce launch mass for missions to Mars by up to 68%. "They found the most mass-efficient path involves launching a crew from Earth with just enough fuel to get into orbit around the Earth. A fuel-producing plant on the surface of the moon would then launch tankers of fuel into space, where they would enter gravitational orbit. The tankers would eventually be picked up by the Mars-bound crew (PDF), which would then head to a nearby fueling station to gas up before ultimately heading to Mars." The technology to make this happen is not difficult to build; it just requires a lot of money. Once it's in place, it'll cut down on expensive launch costs. As the commercial space industry gets going and launches happen more often, such an investment starts to make more and more sense.
At some point in the past the Moon must have had lots of fuel. Oil most likely. Look at all the bomb craters on its surface visible even today. If didn't have oil why would have anyone bombed it? QED, the Moon had oil. It still might, but till unless we get the Moonstone XXL pipeline approved, it will remain unexploited.
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the rest is just commentary.
I would rather NASA goes somewhere, even the Moon, than plans to go somewhere even better, such as Mars, but never gets off the ground. The Mars discussions are like the Wright Brothers complaining it's not worth building the Wright Flyer until they solve how to cross the Atlantic, because who really wants to fly 259.7 meters on a sandy beach.
I think the idea is to launch the fuel into orbit FROM the moon.
1.) Goto Moon
2.) Mine/Generate fuel
3.) Launch fuel into orbit - this step is key
4.) Launch smaller payload from Earth
5.) Payload meets fuel in orbit around the Moon
6.) Payload continues to Mars
If you only have to launch the payload with limited fuel, you don't need to use fuel to get other fuel into orbit. Also, the Moon has a much smaller gravity well and NO air resistance so launching anything into orbit around the moon is much more efficient than launching something into Earth orbit - this step 3 and I think where the 68% savings is supposed to come from. You only need rockets big enough to get a payload off the Earth and fuel into Lunar orbit - there is no need for the one big rocket launching everything at once.
TFA quickly says it should be possible to make fuel locally, without many details.
There is water in the moon's polar craters. It can be separated into H2 and O2 using electricity from PV solar panels.
Another good thing about that way of doing it is that by the time you've built the fuel-making plant, you've had to learn how to live there without constant resupply of air and food. Once you've done that, you have the beginnings of a colony there, and you can use what you've learned once you reach Mars. And, building a self-sustaining base on Mars will be easier than on the Moon because Mars has an atmosphere, making pressure issues simpler and giving you some protection against the smaller micrometeorites.
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There's always dull people on stories like these. You boring bastards are always there holding the doers back with your pathetic can't do attitude.
An asteroid mining tug can bring back about 200 times it's starting mass over a reasonable operating life, making multiple trips. The right kind of asteroid is 20% carbon compounds and water, which can be reformed to hydrocarbons + oxygen, i.e. high thrust rocket fuel. So the fuel return ratio is 40:1. Extracting the carbon compounds and water requires an oven, which is pretty easy to do with sunlight and mirrors. You also need an electrolyzer, to split the water, refrigeration to liquefy the oxygen, and hydro-cracking unit to add the Hydrogen to the carbon compounds (they are typically polycyclic aromatics).
If you do the processing in high orbit near the Moon, like the L2 point, you can skip the launch step and just dock and tank up.
Most people also don't know you can "scoop mine" the Earth's upper atmosphere from orbit. Skimming air at 200 km altitude requires adding 7.5 km/s of velocity to bring it to orbit, but electric thrusters have exhaust velocity of ~30-50 km/s. Therefore a fraction of the air you scoop up can make up the drag you create. You need lots of solar arrays to power the thrusters, but they can power bringing multiple times their own mass in air to orbit. The part you keep can be used as additional propellant for other missions, or as air for breathing, or as 8/9ths of the mass of water (you still need to bring the Hydrogen somehow).
Electromagnetic catapults are overkill for small amounts of mass launch from the Moon. If you need a million tons a year, they are great.
For small amounts, a centrifugal catapult works fine. Rotor arm of high strength material, electric motor, and solar arrays to power it. The Moon is small enough that you can reach orbit velocity with ordinary materials. If you have two rotors, you can regeneratively slow down one to reload while spinning up the other, with little energy wasted.
Math on rotor arm:
Lunar orbit velocity + a bit so it misses mountains and can be collected = 1700 m/s.
Assume 1000 g's at the rotor tip. You are launching rock, it doesn't care. Acceleration = v^2/r. Solving for r we get 290 meter radius. Acceleration varies linearly from center to tip, so is 500 g's average x 290 meters = 144.5 g-km.
High strength carbon fiber has a characteristic strength of 361 g-km, but you don't design to ultimate strength. A reasonable value is 150 g-km, giving a rotor taper of about 3, and mass ratio of 6 because it has two arms. You want the rotor to be balanced so it doesn't jerk the axle around, which means you also throw a rock backwards into a hill. That's inefficient, but there is no lack of rocks.
A modern solar array can supply the 1.44 MJ/kg to launch it's own mass of rock in 2.25 hours. Since we throw an equal mass into a hill, we get 4.5 hours, and allowing for inefficiencies, let's assume 6 hours. The Sun is shining half the time, and a solar array lasts ~15 years in space. So a solar array can power launching 11,000 times it's own weight before it wears out. Add whatever the rotors, motors, and other infrastructure you need (rock loaders and gatherers) and you are till way ahead.
Remote controlled from orbit, it's the only way to be sure :-).