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I debated about putting this here or in the 'Unmanned Probe' forum, but that seems mostly to deal with real, current missions, and this topic has to do directly with Mars Direct and eventual manned missions, so here goes...
Has anyone ever done a cost estimate on an unmanned mission to send a small propellant generation system (like Zubrin developed previously) to land on Mars to prove its viability? How cheaply could this really be done?
It could even be combined with a sample-return mission, so that if the propellant creation works, we even get more bang for the buck.
Think this could be done on a lander about the size of Viking? Would an RTG work for the mission's power requirements?
If it's only a few hundred million, what are the odds we could find funding? Possible ideas:
Getting the Mars Society, Planetary Society, et. al. to join together to find funding
Private investors (get Bill Gates to do some good with his bankroll)
Do the hard-sell with NASA and convince them to use this mission as part of their (near-term) Mars exploration program, now that Bush put manned Mars missions on the table again (down the road).
Thoughts?
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I'm not sure why the US won't consider in-situ with sample return. Zubrin advocates it in *The Case for Mars.* Here are a few facts that help figure out the answer:
1. You can make a kilogram of methane/oxygen fuel for about 5 kilowatt hours. Thus a 100-watt RTG or solar panel, which puts out 2.4 kilowatt hours per day (0.1 kw x 24) could make half a kilo per day.
2. The delta-v from the Martian surface to Earth, with gravity losses and 0.1 km/sec of midcourse correction, is 6.5 km/sec. For methane and oxygen, assuming an exhaust velocity of 3.7 km/sec (Zubrin assumes 3.8 km/sec in his book), the mass ratio is 5.8 (4.8 as much fuel as payload, including tanks and engines).
A small vehicle of this sort would have difficulty achieving a mass ratio of 5.8 because tanks and engines of that small size are relatively heavy. Let us assume the vehicle masses 100 kg and can fly 1 kg of samples to Earth. To achieve a mass ratio of 5.8, you need 480 kg of fuel. To make 480 kg of fuel (which is 5% hydrogen by mass) you need 24 kg of liquid hydrogen. But I don't know whether a vehicle of this size is right. This gives some rough numbers.
To make 480 kg of methane and oxygen you need about 2400 kilowatt-hours, or to use a bigger time period, 100 kilowatt-days. So a continuous 1 kw of power makes the fuel in 100 days, and 100 watts would take 1,000 days. Solar panels to make 24 kw-hrs per day on Mars require about 21 square meters of panels, massing about 100 kilograms (I suppose they could be as little as 50 kg). I think a radioactive power source would have about the same mass range; 50 to 100 kg.
So we are landing on Mars a 100 kg dry mass vehicle, 25 kg of hydrogen (to round up a bit) and a 100 kg power source. Plus an in-situ resource utilization device massing maybe 100 kg more. probably That's 325 kg; Spirit and Opportunity mass 400 kg, if I remember right. Add a light rover massing about 250 kg to get some samples, and you've got your mission. But you have these complications:
1. Getting it to Mars.
2. Making sure the power source works after landing. The radioactive source isn't a problem, but deploying the panels could be tricky. On the other hand, we now know not all of Mars is covered by boulders, so maybe that isn't so difficult.
3. Making sure the ISRU works.
4. Making sure the launch system works.
5. Making sure the rover works. We already have seen that this can be complicated.
Steps 2 through 4 are vastly simplified if one lands a solid-fueled Mars launch vehicle, or a vehicle fueled with UDMH and nitrogen tetroxide (which are easily storable for long times and are hypergolic; they ignite on contact, which means your engine is simple, especially if you use pressurization instead of a pump to feed the propellants to the engine). But the specific impulse is rather low, so the mass quickly gets too big. The solution is to land two vehicles, one with the rover to get the sample. That doubles the mission cost and dangers of failure. Or, you use an ascent vehicle to get the sample to orbit alone (delta-v, 4.1 km/sec only) and a second launch to send an earth return vehicle. Again we have the dual mission problem, plus the problem of robotic orbital redezvous and docking.
The new idea is a single launch with an ion-propelled earth return vehicle from Mars orbit. Even so, you have to throw so much mass to Mars, it's difficult and expensive.
And all these systems are undeveloped, so you have to spend an unknown amount of $ to develop something that works reliably the first time; otherwise you spend a billion or two and look like a fool. Just consider the problem of developing the surface launch system; if that 100 kg vehicle has 1 kg of mass creep in the design and development phase, it has no cargo capacity at all!
So it's understandably complicated.
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