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Comment by CIPHERSTONE | Hacker News Reader

This is what I don't understand, can someone ELI5 it for me:

"Even nutrition, the report said, could be problematic, because any Mars vehicle will likely be significantly smaller than the International Space Station, and astronauts will be too far away to receive the regular resupply missions they now get on the station."

Why does that have to be the case? Why are we limiting the size of the spaceship to what can be launched from Earth? Wouldn't it be better to construct teh ship in space so it could be a behemoth that had ample supplies, etc. and in effect could be a semi-permanent space station in Mars orbit that could be launched from to the surface with smaller ships?

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The short answer is: launch costs and crap rockets.

Building a big ship, right now, means lifting lots of mass into orbit and assembling it there. That in itself will be stunningly expensive. But then you want to actually send it to Mars.

Earth orbit to Mars orbit costs about 6 km/s (see http://i.imgur.com/SqdzxzF.png), which is comparable to the 9.4 km/s needed to get from the surface of Earth to low earth orbit. Now, we are in space, which means we can use slow, efficient burns rather than the fuel-belching behemoths you need for a launch... but right now we're limited to inefficient chemical rockets, which means a hell of a lot of fuel.

The ISS masses about 400 tonnes. Assuming hydrazine as a fuel, with an isp of about 350, that means that the wet mass of your vehicle has to be (dry mass) * e^(deltav / isp9.8) = 400e3 e^(6000 / 350*9.8) = 2300 tonnes. So you'll need 1900 tonnes of fuel. That's 380 Falcon 9s full of rocket fuel. (Assuming my maths are right.)

So you try to make your spacecraft as small as possible. That way you don't need to carry the fuel needed to move the fuel. The rocket equation is a bummer.

(Of course, nuclear engines, are vastly more efficient, change all this. Shame we don't have any. If you were willing to wait long enough, an ion thruster with an ISP of 20000 would require only 12 tonnes of fuel. But you wouldn't live that long.)

Now factor in all the failures on the pad. Lets round to 400 Falcon-9s. And thats just for the ISS. To go to Mars, you also need to get off Mars. Here are some of the challenges to doing that:

Constraints are placed on the MAV by the Martian environment and the entry, descent and landing (EDL) system used. The MAV will have to endure daily temperature swings of up to about 100C with an annual maximum of about 24C and minimum of 111C. (These values were calculated using the NASA Ames Global Climate model for Holden Crater, which was the MSL candidate site with the greatest variation in temperature and is assumed to be representative of a MSR landing site.)Current MSR architecture places the MAV in a sky-crane type lander along with a fetch rover to re-trieve the samples. The EDL system constrains both geometric and mass limits on the system. The baseline design is a solid rocket. However, the thrust of solid rockets is dependent on their temperature. Additionally, any cracking of the fuel grain could lead to a cata-strophic failure."

http://www.lpi.usra.edu/meetings/marsconcepts2012/pdf/4342.p...

Also, and I will have nerd-sniped away any productivity now from you dear reader, all of NASA's papers are public. Here is the whitpaper on a Mars Ascent Vehicle:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2011001...

(be warned, this is going to kill any work for the day)

That's "only" 36 falcon heavy launches (with an expected mass to LEO of 53 tonnes). With it being mostly reusable (at least the two boosters), this isn't quite so daunting an idea.

I mean, we did build a space station. We've done similar things in scale and complexity. And we did it with the ridiculously inefficient space shuttle.

Yes, but realistically one could mine the Moon poles and develop the fuel there...

Better yet expand it grow food on moon and refuel and load up on food in moon orbit.

Does growing food on the moon actually help? Unless you can mine the nutrients for the food, you still need to bring up all of the mass that would go into the food, at which point you might as well just bring up already processed food (which would also be denser in value/weight). The main benefit of growing food in space is that it lets you recycle nutrients. (I am including the atmospheric gasses as nutrients in this).

Yes, you'd have to mine the nutrients there for this to be effective. As you say, there's no point lifting it all from Earth.

Organic life is largely made out of carbon, hydrogen, oxygen and nitrogen (the so-called CHON). There's not much carbon or nitrogen. This is one reason why people are so interested in hypothetical ice deposits at the poles. There's a reasonable chance that there'll be stocks of this elements there (in the form of ammonia or hydrocarbons).

Fixation on modern implementation of Apollo, go there as a stunt and never come back.

Probably they're specifically talking about some kind of transfer vehicle doing a Hohmann transfer orbit. The idea of sending very small resupply and spare part payloads on a non-Hohmann likely very expensive trajectory is interesting, but it'll never work as a substantial fraction of the mass of the ship, like food will have to be. So you could use the entire delta-V of a major booster to deliver a spare circuitboard to a vehicle 3/4 of the way to Mars but significant masses of food won't work.

Another problem is organizational inertia, if a Hohmann transfer takes 600 days or whatever and a major international mission takes 10 years of administrative planning, then if a super emergency resupply is required on day 400, it won't arrive until 3400, in other words a bit too late to matter. So technically you could waste a normally 20 thousand pound booster on a 10 pound emergency payload of circuit boards and med supplies, but organizationally "we" are too slow to do it.

aka, The Martian

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