Using Amercium RTGs for a reusable Manned Lunar Lander-Could it work?

[fredinno]

55 minutes ago, SomeGuy123 said:

I don't know if there are any "space rated" CH4 cells, but that's barely relevant.  The Dragon wasn't "space rated" or any of the pumps or tanks or engines or computers or any of that equipment until it was (almost all of SpaceX stuff is newly designed).  Mr. Musk's team of engineers could space rate one in a matter of months, it's conventional technology.  

In the far future when you can actually mine for water, sure, great.  The reason to use methane until then is this chart.  Also, methane is liquid at 111 kelvin, versus 20 kelvin for hydrogen, and it's also far, far, far easier to make it stay in it's tank.  

As for Am-241 being cheaper : there's also orders of magnitude difference in energy output per gram.  So...

Only problem is that Pu costs an arm and a leg, and there isn't enough for both manned and unmanned missions.

 

Also, I know why you want methane, but you can't easily get carbon on the moon- unless you bring it, which negates the purpose of ISRU. Thus, the only option would be energy-hungry regenerative scrubbers. https://en.m.wikipedia.org/wiki/Carbon_dioxide_scrubber#Regenerative_carbon_dioxide_removal_system And I'm not even sure it releases pure CO2, or something else.

 

What's the molecular weight of pure carbon, oxygen and hydrogen? We might be able to get away with reacting carbon dust with hydrogen.

[SomeGuy123]

3 hours ago, fredinno said:

Also, I know why you want methane, but you can't easily get carbon on the moon- unless you bring it, which negates the purpose of ISRU. Thus, the only option would be energy-hungry regenerative scrubbers. https://en.m.wikipedia.org/wiki/Carbon_dioxide_scrubber#Regenerative_carbon_dioxide_removal_system And I'm not even sure it releases pure CO2, or something else.

Remember, my proposal is just using the rocket fuel and oxidizer you have to bring anyway as an energy buffer for Lunar nights.  The real power source is solar.

As for getting water on the Moon - that's not a near term thing, is it?  What exactly do you think the water gathering equipment would look like?  It's all frozen, so it's not a well.  You can't just slap a tiny drill on the outside of your lander and drill into the surface. 

I don't know myself.  If I had to guess, I'd say you would be forced to strip mine the lunar surface, same way we do on Earth, for massive quantities of rock.  The rock would be fed into ovens that extract the water, then there would have to be distillation and filters.

The current equipment on earth that does this is some of the largest industrial equipment known to man.  Some of it is even super sized.  So you can scale things down, and you can make equipment lighter to take advantage of the reduced gravity, but however you slice it, I suspect you would need to land dozens of lunar landers, if not hundreds, worth of pieces for your mining equipment.

You then have to reassemble it - robots and self assembly could help, but it wouldn't be simple.  

Then the equipment, if it works, starts making fuel.

The point is, it's an investment.  Unless you're making a certain volume of lunar landings and ascents, it isn't a positive return, and it's cheaper to just bring your fuel instead.  Mars ISRU is easier because the only mining equipment you need is an air compressor - most ISRU proposals only use the atmosphere.

[Kaos]

7 hours ago, fredinno said:

What's the molecular weight of pure carbon, oxygen and hydrogen? We might be able to get away with reacting carbon dust with hydrogen.

You mean the weight ratio in methane combustion? Well, you have 4 H, 1 C and 3 O as combustion result per methane molecule. That is 4 u atomic mass for hydrogen, 12 u carbon and 48 u for oxygen. So if you just bring the methane to the moon and collect the oxygen locally, you already get a fuel reduction by 75%, which is an acceptable amount to begin with.

I would try to make an aluminum/oxygen engine. You can get both stuff on the moon and it is efficient enough to construct a single stage moon surface -> moon orbit -> moon surface vehicle. That would be a great vehicle to transport anything from anywhere to anywhere on and around moon, if it is flown automatically.

And to get back closer to the topic: Burn the aluminum with oxygen in the night to get heat and electricity. The energy density is quite good.

[fredinno]

6 hours ago, Kaos said:

You mean the weight ratio in methane combustion? Well, you have 4 H, 1 C and 3 O as combustion result per methane molecule. That is 4 u atomic mass for hydrogen, 12 u carbon and 48 u for oxygen. So if you just bring the methane to the moon and collect the oxygen locally, you already get a fuel reduction by 75%, which is an acceptable amount to begin with.

I would try to make an aluminum/oxygen engine. You can get both stuff on the moon and it is efficient enough to construct a single stage moon surface -> moon orbit -> moon surface vehicle. That would be a great vehicle to transport anything from anywhere to anywhere on and around moon, if it is flown automatically.

And to get back closer to the topic: Burn the aluminum with oxygen in the night to get heat and electricity. The energy density is quite good.

Only problem is that Aluminium is a solid. A powdered version might form clumps and combustion instabilities, or be abrasive to the tank. And a Monopropellant version with Al and Lox in the same tank might be dangerous, even with AlO3 covering the Al. Even if it didn't, the hybrid rocket motor for it needs a LOT of development. And rocket engines are expensive to develop- that's why people avoid it. It's also less efficient.

 

But the CH4 Lox sounds pretty good- 3/4 of the propellant mass not having to be carried sounds worth it. O2, though, does require thigh temperatures to dissassociate it from the lunar soil, but a base woul likely have enough provisions for that anyways.

[Kaos]

I know the problems with aluminum. Nevertheless, I think it might be worth the effort. At least if we build several or bigger moon bases. Less efficient is not that a problem, as it is efficient enough for the proposed usage.

Perhaps there is also a combination with other stuff from moon, which works better? Thermite like stuff, perhaps; thermite seems to be producable from moon stuff.

[fredinno]

On 1/18/2016 at 1:05 PM, Kaos said:

I know the problems with aluminum. Nevertheless, I think it might be worth the effort. At least if we build several or bigger moon bases. Less efficient is not that a problem, as it is efficient enough for the proposed usage.

Perhaps there is also a combination with other stuff from moon, which works better? Thermite like stuff, perhaps; thermite seems to be producable from moon stuff.

Iron and silicon are also common on the surface- iron which you can combine with Aluminum to make Thermite. I doubt Thermite could be made into a decent fuel, though, and in any case, it would only be able to be used in non-reusable SRBs.

Edited January 19, 2016 by fredinno

[batman78781]

Back to Americium, I am just curious where it is produced? I know ESA is working with it, but if the USA has a handful NASA might as well use it, I don't think there are military uses to eat it up. Also, how much is actually available? The whole reason we have no plutonium is b/c Russia probably ran out and they aren't making more, so we basically have cold war leftovers now, I think I read there is something like 22 kg for NASA.

EDIT: Apparently NASA can make 4 PU generators, one is for 2020 rover.

http://www.scientificamerican.com/article/within-nasa-a-plutonium-power-struggle/

Edited January 20, 2016 by batman78781

[fredinno]

54 minutes ago, batman78781 said:

Back to Americium, I am just curious where it is produced? I know ESA is working with it, but if the USA has a handful NASA might as well use it, I don't think there are military uses to eat it up. Also, how much is actually available? The whole reason we have no plutonium is b/c Russia probably ran out and they aren't making more, so we basically have cold war leftovers now, I think I read there is something like 22 kg for NASA.

EDIT: Apparently NASA can make 4 PU generators, one is for 2020 rover.

http://www.scientificamerican.com/article/within-nasa-a-plutonium-power-struggle/

I would assume worldwide, since the stuf is also used in smoke detectors, and it's comparitvely easy to make.