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the asinine cretin, on 23 May 2012 - 09:13 AM, said:

The main driver behind that push is the fact that domestic production plutonium 238 has been shut down since 1993. Funding to get that domestic production restarted is barely there, and it's only on the NASA side. Producing ²³⁸Pu the job of the Department of Energy, and Congress for some reason refuses to fund the DOE side. Logjam.

If there was an adequate supply of plutonium 238 NASA would happily continue using RTGs. They are a very simple and very trusted technology. No moving parts. They don't fail. While much more efficient, those Stirling generators are new technology and are significantly more complex than RTGs. Increased complexity means a significantly increased likelihood of failure. That it is new also increases the risk of failure. A lot.

The ASRGs will be used for low power consumption devices such as sensors and communications. Using them for a high power consumption device such as a VASIMR engine is not going to get a vehicle to Mars. At least not very fast. Do the math. A VX-200 sucks 200 kW of electrical power, or the output of 1400+ ASRGs. At 32 kg per ASRG, the power source needed to supply a VX-200 masses over 45,700 kg. That's just for the power source, not the housing for those 1400+ units or the electrical cable needed to connect them to the engine. All for 5 newtons of thrust.

the asinine cretin, on 23 May 2012 - 04:58 AM, said:

No news is most often bad news when in comes to companies that delve into new technologies. Or it might mean they are quietly working on things where progress is measured in years. Many, many years in this case.

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There's a lot more to RTGs than thermocouples, particularly an RTG that contains 100 billion lethal doses of ²¹⁰Po. The fuel is encapsulated in ceramic, surrounded by iridium, which in turn is surrounded by high-strength carbon, which in turn are placed in housings, which in turn is surrounded by an aeroshell. There is a whole lot more to an RTG than just the fuel and a couple of thermocouples.

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Those higher efficiency systems are not RTGs. They are heat engines. Now you have a working fluid, moving parts, a heat exchanger, a generator, and you still need massive amounts shielding to keep those 100 billion lethal doses from escaping in case of some kind of accident. There's a good reason that none of the world's space agencies have gone to these speculative devices. The four fold boost in efficiency is not worth the added mass.

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Obviously not. Where does it come from? That in situ manufacturing would require building on Mars an automated bismuth mine, an automated bismuth refining plant, an automated nuclear reactor, an automated neutron bombardment facility to bombard the mined and refined bismuth with neutrons, an automated purification system to isolate the polonium, and an automated storage system. It's ludicrous.

the asinine cretin, on 22 May 2012 - 05:00 PM, said:

No news is no news. Or in some cases, it's bad news. Lots and lots of proposed technologies start with a flourish but then just fade away.

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What you are doing here is accounting for the mass of the fuel but failing to account for the mass of all of the infrastructure needed to make that fuel useful. The decay of polonium 210 generates heat. VASMIR runs on electricity. You are ignoring the fairly massive infrastructure needed to convert that heat into electricity plus the extremely massive infrastructure needed to ensure that the PO-210 remains safely encapsulated even in the case of an explosion. That alone changes your many kilowatts per kilogram into many kilograms per kilowatt. Then there's the problem of the efficiency of that conversion of heat into electricity. RTGs are simple devices with no moving parts. The downside: They are extremely inefficient. Typically efficiency is about 5%. Your two inch cube of polonium-210 would generate about 7 kW of electricity.

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Swap it out? With what? Another PO-210 based RTG? Won't work. Polonium 210 is highly radioactive. The polonium doesn't care if the generated heat is put to productive use or is just radiated out into space.

Yes, there is at least one habitable planet or moon in our solar system.

TheLivingMartyr, on 26 April 2012 - 03:17 PM, said:

The problem is not that this integral doesn't exist. It most certainly does exist. The problem is that this integral can't be expressed in terms of the elementary functions. All the neat tricks that you have learned are aimed at solving those integrals than can be expressed in terms of elementary functions.

Most integrals cannot be expressed that way. For example, students regularly ask how to integrate or . Play around with u-substitutions, integration by parts, etc., and no matter what you do you will either get an intractable mess or just get right back to square one.

That doesn't mean the integral doesn't exist. It does exist. It turns out that is a very important function. It appears so often, and in so many different settings, that it has been given a special name (the error function) and a special symbol . The error function is just one of many integrals that can't be expressed in terms of elementary functions and that rear their ugly heads over and over again. These "special functions" will be the subject of multiple advanced math classes should you go that far in math.

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Amongst other things, the arc length along a segment of an ellipse. Elliptical integrals are one of those things that keep coming up again, and hence they are given a special name.