Solar Panels

[ikrase]

Also, an RTG cannot be turned off. If you don't use the power, it is wasted as heat. For that reason, really high power RTGs are impractical.


The shuttle is a pretty short duration ship. They can overfuel the fuel cells with a cargo bay module.

 

[Zachstar]

Solar cells become very ineffective past mars and even mars is difficult. However recent advances in solar cell efficiency make possible for a VERY power efficient probe to be sent to Jupiter without RTGs. But after Jupiter you would need a doubling of its efficiency and on top of that even better efficient use of power for Saturn. By then fusion is just better as far as future probes are concerned (VERY few RTGs left for use)

 

[Andy44]

There is no such thing as a self-sustaining fusion reactor, unfortunately.

The plutonium used in RTGs is Plutonium 238, and it's very scarce in the US because the US doesn't make it anymore; it's all bought from Russia. There is currently a push to restart making it in the US, and if that happens it will take 5 years to make enough to be useful.

That leaves fission power for deep space, which will have to happen eventually, but even if a mission were approved today it would likely take longer to develop then it would to make more Pu-238.

 

[jedidia]

Also, an RTG cannot be turned off.

neither can a nuclear reactor, really.

There is no such thing as a self-sustaining fusion reactor, unfortunately.

not yet... I certainly hope they'll come up with one during the next fifty years!

 

[orb]

The plutonium used in RTGs is Plutonium 238, and it's very scarce in the US because the US doesn't make it anymore; it's all bought from Russia. There is currently a push to restart making it in the US, and if that happens it will take 5 years to make enough to be useful.

That leaves fission power for deep space, which will have to happen eventually, but even if a mission were approved today it would likely take longer to develop then it would to make more Pu-238.

How about Am-241 with 432 years of half-life?

 

[Eagle]

neither can a nuclear reactor, really.

You turn a nuclear reactor "off" when you lower the neutron levels below a self-sustaining level, usually this is by inserting cadmium rods which absorb enough neutrons.

 

[Andy44]

I think the nuclear reactors used in most space experiments were controlled by reflectors, weren't they?

How about Am-241 with 432 years of half-life?

Read the wiki article on RTGs.

It gives a pretty good run-down of what isotopes make a good RTG heat source, and why Plutonium 238 is the best overall, especially for space vehicles. Basically, you get lots of power and don't need lots of massive shielding.

 

[orb]

Read the wiki article on RTGs.

I have read, and that's why I asked.

Americium-241 is a potential candidate isotope with a longer half-life than 238Pu: 241Am has a half-life of 432 years and could hypothetically power a device for centuries. However, the energy density of 241Am is only 1/4 that of 238Pu, and 241Am produces more penetrating radiation through decay chain products than 238Pu and needs about 18 mm worth of lead shielding. Even so, its shielding requirements in an RTG are the second lowest of all possible isotopes: only 238Pu requires less.

 

[N_Molson]

Concerning Soyuz spacecrafts, notice that the Soyuz-LOK (designed for lunar missions, extended propulsion module) did not have solar panels, and relied on fuel cells, like Apollo.

I guess that the shadow cone of the Earth could have been a real problem for lunar missions.

In general, it's obvious that a solar power plant build on Mercury will have a much higher output that the same one built on Pluto !

 

[Zachstar]

Actually the reason they would use fuel cells is the same as Apollo. The craft has to be able to "Barbecue Roll" And solar power is not reliable when the sun is not directly aimed at the cells.