The topic is: If it were decided to go to Mars, what is the fastest, most cost-effective way, within a flat NASA budget? Since the vaunted 50-odd unmanned spacecraft doing all that exciting research suck up fully half the budget, and we need $2B USD per year to keep ISS flying into perpetuity, that means we have about $7B USD per year to work with--maybe $8B if we're lucky, and Congress is in a generous mood.
Despite what Robert Zubrin of Mars Direct and offspring say, there is an alternative paradigm: the one recently sketched out over last few years in the series of ULA AIAA white papers, and the writings of Paul Spudis, Dennis Wingo, and others. The main idea of these papers is a general approach: (1) use depots to stockpile chemical energy in space; (2) emphasize commonality of the BLEO spacecraft inorder to minimize design and manufacturing costs, and to maximize reliability; (3) emphasize diversity of design for Earth-launch vehicles in order to drive competition to serve the large propellant market that a robust, ongoing BLEO program would represent; (4) emphasize reusable lunar SSTO's to get lunar-derived propellant into the game, and (5) the main design criterion is not minimization of mass and delta v--rather the goal is to minimize cost while maximizing capability. In other words, it's the original VSE as envisioned by President Bush: get up to the Moon, make me some propellant, and use that to get to Mars!
It is a myth that the ULA affordable architecture paper is an argument against heavy lift: although it shows that an aggressive BLEO program can be done it 30 ton chunks, it also says that reasonably sized HLV's would "amplify" their proposed architecture (e.g., they just put out a 2010 paper arguing that ULA could develop a 70 ton HLV for about $3B USD). For a number of reasons, the current plan is to have Marshall Space Flight Center design and build a 70-ton SLS. Thus presumably, if this craft gets off the ground, and gets the flight rate high enough, in theory it could get launch costs to LEO down to about $3000 USD/kg.
With launch costs down to $3000/kg, the "aggressive" option detailed in the ULA paper (3 crew flights and 2 20-ton cargo flights to the Moon per year) becomes feasible. (The "conservative" option they also reviewed--2 crew flights and 1 20-ton cargo flight per year--assumed about $10,000/kg). We'll take the $3000/kg figure as a starting point, but will also work out the implications of the conservative scenario.
Now, a centerpiece of the new paradigm is the use of lunar-derived propellant to leverage our spaceflight capabilities; this is something that Dr. Spudis has been arguing for years, but it's something I and a few others here have only grokked in just the last few months.
Let's start by thinking about what's basically required for a base to produce a few thousand tons of propellant annually. We'll work backwards:
1) We'll start with 4,000 tons of propellant. Since propellant has a mixture mass ratio of 5 (i.e., 5 kg LO2 per 1 kg LH2), and water has a natural mixture ratio of 8 (16 protons and neutrons per 2 protons), then 8/5 * 4,000 = 6400 tons of water that must be cracked per year.
2) Let's say the density of the ice is 0.9 gm/cc. That leaves the purity of the ice in the northern anomalous craters. I've seen at least one estimate as 90% pure, but the LCROSS results show that lunar volatiles are a veritable witches brew of various chemical species. I'm going to use my geologist's perogative and say that the polar deposits contain about 80% water. I say this because I suspect that at the LCROSS site, water preferentially sublimates sooner than most of the other observed species, and so the other species get concentrated relative to water. On the other hand, regolith density is going to be on the order of 2 gm/cc, so let's say the average density of the material is 1.1 gm/cc, which equals 1100 kg/m3. Thus we can expect to extract water at approximately the rate of 1100 * 80% = 880 kg/m3. 6400 mt/880 kg/m3 = 7272 m3 of material that must be excavated.
3) The circular polarization radar that discovered the water deposits have a wavelength of about 14cm. This sets a minimum depth of the water deposits at about 2 meters, but it's probably a lot thicker. But for our purposes, lets assume it's 2m thick. Since we need 7272 m3 of material, then we need to excavate a pit that's 3636 m2. To put this into perspective, a 50m X 100m football field has an area of 5000 m2. In the USA, when bidding out excavation jobs, 150 m3 per hour is typical. Let's just say the excavation rate on the Moon with our underpowered equipment will be more like 72 m3 per hour. So to excavate 7272 m3, then it would take about 100 hours to get 'r done. 2 guys working 4 hour shifts would have to spend 25 days out of a year to do the required excavation. Don't like my 80% water estimate? OK fine: call it 40%: that means the guys will have to do 50 EVA's in one year to do the excavation.
4) Thus the next question is how much earth moving equipment do we need to get 'r done. I would say 20 tons worth of earth moving equipment (i.e., one cargo flight) is enough; but if you think we need at least 40 tons, because we want a backhoe, a loader, a bulldozer, a dump truck, and probably a rock crusher with a conveyer, then OK fine. (Don't worry about the cold--we'll use nickel alloys that retain their strength and ductility down to cryogenic temperature, and we'll use sealed bearings filled with cryogenically rated grease to get around the dust--power will come from fuel cells. Caterpillar is already on it.) So we need 1 or 2 cargo flights to provide the requisite earth-moving equipment.
That's enough for now. More to follow. Main point for now is to lay to rest the oft expressed intuition that "OMG! Thousands of tons of propellant! You must be crazy if you think that can be done at a 1st generation Moon base!" Simple numerical calculations show that it ain't an Anaconda copper mine on the Moon that we need. The required "mine" is more like the local mom 'n' pop gravel pit that they work on the weekends for extra money.
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