Author Topic: Pu 238  (Read 6052 times)

Offline krytek

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Pu 238
« on: 11/09/2011 07:43 PM »
Remember Heinlein's "The man who sold the moon" ?
The radioactive rocket fuel was made in orbit instead of being made on earth.
Can Pu 238 be made in space, can we make a sort of fuel depot for very deep space missions?

Offline Robotbeat

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Re: Pu 238
« Reply #1 on: 11/09/2011 08:23 PM »
Remember Heinlein's "The man who sold the moon" ?
The radioactive rocket fuel was made in orbit instead of being made on earth.
Can Pu 238 be made in space, can we make a sort of fuel depot for very deep space missions?
1) Yes (though haven't read it).
2) No, at least not until you have ridiculous amounts of industry in space. It really is expensive on the ground and would be a lot more expensive in space. The Moon I believe is relatively uranium poor, or at least contains no more uranium than the Earth. (EDIT: Yes, the Moon is quite uranium-poor: http://www.space.com/8644-moon-map-shows-uranium-short-supply.html )
3) Yes, that makes sense for chemical fuels, but since nuclear fuels are so energy-dense, there's really no need for a fuel depot for them (though a fuel depot for the hydrogen propellant used in a nuclear thermal rocket would likely make sense).
« Last Edit: 11/09/2011 08:26 PM by Robotbeat »
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Offline Proponent

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Re: Pu 238
« Reply #2 on: 11/10/2011 12:03 AM »
The Moon I believe is relatively uranium poor, or at least contains no more uranium than the Earth. (EDIT: Yes, the Moon is quite uranium-poor: http://www.space.com/8644-moon-map-shows-uranium-short-supply.html )

And this post and the ones following it discuss why one would not expect to find deposits of uranium on the moon.  Basically, uranium oxides are soluble in water, so uranium can be concentrated by oxygenated flowing water, which is unlikely to have ever been abundant on the moon.

Offline simonbp

Re: Pu 238
« Reply #3 on: 11/10/2011 03:54 PM »
Right, the prime fissionable substance on the Moon (as Earth, actually!) is thorium, which can be enriched to uranium. Thorium concentrations do exist on the Moon in KREEP deposits, which means nearside volcanism (see the second figure down here).

How economical it would be to extract thorium from KREEPy lunar regolith I have no idea, but I can't imagine it's worth it unless you have a large lunar base to support.

Offline Robotbeat

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Re: Pu 238
« Reply #4 on: 11/10/2011 04:52 PM »
Right, the prime fissionable substance on the Moon (as Earth, actually!) is thorium, which can be enriched to uranium. Thorium concentrations do exist on the Moon in KREEP deposits, which means nearside volcanism (see the second figure down here).

How economical it would be to extract thorium from KREEPy lunar regolith I have no idea, but I can't imagine it's worth it unless you have a large lunar base to support.
Considering the level of technology required to make mining fissionables sensible, you're talking millions of people, minimum. Fissionable fuel is incredibly value-dense, like a hundred thousand dollars per kilogram or more. In order to have millions of people on the Moon, the cost of getting to the Moon would have to be incredibly low, probably $100/kg or less. The only time you'd ever mine thorium on the Moon would be if we run out on Earth (not likely to happen any time soon... hundreds or thousands of years of easily available thorium, probably a billion years if really necessary).
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Offline DarkenedOne

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Re: Pu 238
« Reply #5 on: 11/10/2011 08:00 PM »
Remember Heinlein's "The man who sold the moon" ?
The radioactive rocket fuel was made in orbit instead of being made on earth.
Can Pu 238 be made in space, can we make a sort of fuel depot for very deep space missions?
1) Yes (though haven't read it).
2) No, at least not until you have ridiculous amounts of industry in space. It really is expensive on the ground and would be a lot more expensive in space. The Moon I believe is relatively uranium poor, or at least contains no more uranium than the Earth. (EDIT: Yes, the Moon is quite uranium-poor: http://www.space.com/8644-moon-map-shows-uranium-short-supply.html )
3) Yes, that makes sense for chemical fuels, but since nuclear fuels are so energy-dense, there's really no need for a fuel depot for them (though a fuel depot for the hydrogen propellant used in a nuclear thermal rocket would likely make sense).

We have discussed this in other threads, but the basic premise is that the whole idea that anything in space will cost you more than it does here on Earth is false.  It that was true than there would never be a market for any space product. 

My guess is that nuclear operations in space will be far cheaper than they are here.  There are no real safety concerns other than the threat to the crew. 

Offline Kaputnik

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Re: Pu 238
« Reply #6 on: 11/10/2011 08:15 PM »
the basic premise is that the whole idea that anything in space will cost you more than it does here on Earth is false.  It that was true than there would never be a market for any space product.

And what, exactly, is the current market for space-produced goods?
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Offline DarkenedOne

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Re: Pu 238
« Reply #7 on: 11/11/2011 12:44 PM »
the basic premise is that the whole idea that anything in space will cost you more than it does here on Earth is false.  It that was true than there would never be a market for any space product.

And what, exactly, is the current market for space-produced goods?

Imagery and telecommunications.

Offline kevin-rf

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Re: Pu 238
« Reply #8 on: 11/11/2011 01:49 PM »
the basic premise is that the whole idea that anything in space will cost you more than it does here on Earth is false.  It that was true than there would never be a market for any space product.

And what, exactly, is the current market for space-produced goods?

Imagery and telecommunications.

And the moon makes a great platform for those two products how?
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Offline douglas100

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Re: Pu 238
« Reply #9 on: 11/11/2011 03:21 PM »
Remember Heinlein's "The man who sold the moon" ?
The radioactive rocket fuel was made in orbit instead of being made on earth.
Can Pu 238 be made in space, can we make a sort of fuel depot for very deep space missions?

Yes, I read it when I was a teenager. The kind of "isotope X" fuel is described as being used in some kind of propulsion system. The story was written (late thirties, early forties?) when nuclear reactions were just starting to be understood. The idea in the story is of course completely fictional.

Are you suggesting that plutonium 238 can be used in a nuclear rocket? It can't, it's  a non fissile isotope. Its space use is in power generation.

As far as manufacturing it in space is concerned, that's a non starter for reasons given already by posters on this thread.
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Offline Robotbeat

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Re: Pu 238
« Reply #10 on: 11/11/2011 03:32 PM »
Remember Heinlein's "The man who sold the moon" ?
The radioactive rocket fuel was made in orbit instead of being made on earth.
Can Pu 238 be made in space, can we make a sort of fuel depot for very deep space missions?

Yes, I read it when I was a teenager. The kind of "isotope X" fuel is described as being used in some kind of propulsion system. The story was written (late thirties, early forties?) when nuclear reactions were just starting to be understood. The idea in the story is of course completely fictional.

Are you suggesting that plutonium 238 can be used in a nuclear rocket? It can't, it's  a non fissile isotope. Its space use is in power generation.

As far as manufacturing it in space is concerned, that's a non starter for reasons given already by posters on this thread.
It could definitely be used for electric propulsion, though. It would not be worth it until you got out to Jupiter, though, because RTGs are generally quite heavy for the power they produce compared to solar panels in the inner solar system.

And I think I remember some concepts about using the heat source for a sort of hopper propulsion. Also, I think there was some talk of using radioisotopes in a "fission-fragment" propulsion mode (putting fission in quotes, here).
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Offline douglas100

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Re: Pu 238
« Reply #11 on: 11/11/2011 09:33 PM »


It could definitely be used for electric propulsion, though. It would not be worth it until you got out to Jupiter, though, because RTGs are generally quite heavy for the power they produce compared to solar panels in the inner solar system.

And I think I remember some concepts about using the heat source for a sort of hopper propulsion. Also, I think there was some talk of using radioisotopes in a "fission-fragment" propulsion mode (putting fission in quotes, here).


Yes it could be used for electric propulsion but the power to mass ratio is inferior to solar cells. Beyond Jupiter a nuclear reactor like the JIMO proposal would have a much better performance than a radioisotope generator.

There was a Mars hopper proposal by Zubrin, I think, using Mars's atmosphere as working fluid. I don't remember if he first suggested using a radioisotope source to heat the propellant, but later on he was talking about heating it electrically.

As far as using "fission-fragment" propulsion is concerned, the thrust would be remarkably low, don't you think?

Bottom line: there is no reason to manufacture Pu 238 in space.
« Last Edit: 11/11/2011 09:37 PM by douglas100 »
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Offline Robotbeat

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Re: Pu 238
« Reply #12 on: 11/11/2011 09:41 PM »


It could definitely be used for electric propulsion, though. It would not be worth it until you got out to Jupiter, though, because RTGs are generally quite heavy for the power they produce compared to solar panels in the inner solar system....


Yes it could be used for electric propulsion but the power to mass ratio is inferior to solar cells. ...
Is there an echo in here?
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Offline kevin-rf

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Re: Pu 238
« Reply #13 on: 11/11/2011 10:20 PM »
Is there an echo in here?

If in space no one can hear you scream, where would an echo come from?
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Offline douglas100

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Re: Pu 238
« Reply #14 on: 11/12/2011 08:52 AM »
Quote
Is there an echo in here?

Only because we are in agreement on that point!  :)
« Last Edit: 11/12/2011 08:56 AM by douglas100 »
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Offline Robotbeat

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Re: Pu 238
« Reply #15 on: 11/13/2011 03:54 AM »
Quote
Is there an echo in here?

Only because we are in agreement on that point!  :)
:)
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Offline mlorrey

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Re: Pu 238
« Reply #16 on: 11/13/2011 06:04 AM »
In Heinleins alternative future history, nuclear power plants were put into space and their power beamed to earth to reduce the risk of nuclear accidents poisoning the environment. This didn't exactly work very well, the first one launched into orbit failed. Then 100% efficient solar technology was invented and the fuel for the nuclear plants was surplused out for asteroid and lunar mining explosives and ship propulsion. needless to say, things didnt turn out exactly has he'd predicted...
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Offline DarkenedOne

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Re: Pu 238
« Reply #17 on: 11/13/2011 12:48 PM »
Remember Heinlein's "The man who sold the moon" ?
The radioactive rocket fuel was made in orbit instead of being made on earth.
Can Pu 238 be made in space, can we make a sort of fuel depot for very deep space missions?

Yes, I read it when I was a teenager. The kind of "isotope X" fuel is described as being used in some kind of propulsion system. The story was written (late thirties, early forties?) when nuclear reactions were just starting to be understood. The idea in the story is of course completely fictional.

Are you suggesting that plutonium 238 can be used in a nuclear rocket? It can't, it's  a non fissile isotope. Its space use is in power generation.

As far as manufacturing it in space is concerned, that's a non starter for reasons given already by posters on this thread.
It could definitely be used for electric propulsion, though. It would not be worth it until you got out to Jupiter, though, because RTGs are generally quite heavy for the power they produce compared to solar panels in the inner solar system.

And I think I remember some concepts about using the heat source for a sort of hopper propulsion. Also, I think there was some talk of using radioisotopes in a "fission-fragment" propulsion mode (putting fission in quotes, here).

Electric propulsion using Pu-238 is inefficient.   Radioisotope thrusters are better because they convert the heat directly into heat for a rocket.   

Offline douglas100

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Re: Pu 238
« Reply #18 on: 11/14/2011 09:08 PM »
No, in terms of propellant use, an electric thruster would be much more efficient than a radioisotope thermal thruster. In addition to having much higher specific impulse, an electric thruster is much easier to control than an isotope heat source which cannot be turned off.

Electric thrusters allowed and Hayabusa and Dawn to successfully complete their missions. Do you know of any radioisotope thruster being currently designed for a spacecraft?

There's still no case for manufacturing radioisotopes in space.

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Offline mlorrey

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Re: Pu 238
« Reply #19 on: 11/14/2011 10:22 PM »
No, in terms of propellant use, an electric thruster would be much more efficient than a radioisotope thermal thruster. In addition to having much higher specific impulse, an electric thruster is much easier to control than an isotope heat source which cannot be turned off.

Electric thrusters allowed and Hayabusa and Dawn to successfully complete their missions. Do you know of any radioisotope thruster being currently designed for a spacecraft?

There's still no case for manufacturing radioisotopes in space.



Isotope heat sources can be very easily "turned off" with neutron moderating control rods that shut down supercritical reactions.
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Offline 93143

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Re: Pu 238
« Reply #20 on: 11/15/2011 02:20 AM »
Isotope heat sources don't undergo fission.  It's just radioactive decay heat.

And the achievable temperature is very low - substantially lower, most likely, than that of a fission NTR, but certainly no higher.  You'd be restricted to hydrogen if you wanted an Isp above 500.  Hydrogen is not easy to store.

In contrast, a electric thruster running xenon can pull 3000 seconds easily.

Offline mlorrey

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Re: Pu 238
« Reply #21 on: 11/15/2011 07:05 PM »
Isotope heat sources don't undergo fission.  It's just radioactive decay heat.

And the achievable temperature is very low - substantially lower, most likely, than that of a fission NTR, but certainly no higher.  You'd be restricted to hydrogen if you wanted an Isp above 500.  Hydrogen is not easy to store.

In contrast, a electric thruster running xenon can pull 3000 seconds easily.

And a bank of electric thrusters powered by a fission reactor can pull high impulse and generate a respectable acceleration for interplanetary travel.
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Offline douglas100

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Re: Pu 238
« Reply #22 on: 11/15/2011 07:41 PM »

And a bank of electric thrusters powered by a fission reactor can pull high impulse and generate a respectable acceleration for interplanetary travel.

Indeed they can, but this thread is about isotopic propulsion and the possibility of producing such isotopes in space, not about fission reactors. Let's not confuse the two.
« Last Edit: 11/15/2011 07:42 PM by douglas100 »
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Offline DarkenedOne

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Re: Pu 238
« Reply #23 on: 11/15/2011 07:59 PM »
Isotope heat sources don't undergo fission.  It's just radioactive decay heat.

And the achievable temperature is very low - substantially lower, most likely, than that of a fission NTR, but certainly no higher.  You'd be restricted to hydrogen if you wanted an Isp above 500.  Hydrogen is not easy to store.

In contrast, a electric thruster running xenon can pull 3000 seconds easily.

The decay of plutonium 238 takes the form of alpha radiation.  The alpha radiation gets absorbed within the material thus generating thermal energy.  The temperature of the isotope is directly related to the thermal energy contain in it.

Like any heat source its temperature will rise until it loses as much heat as it generates.  If an thermal insulator is used to prevent heat from escaping than the temperature will rise unabated.

Therefore the achievable temperature of plutonium is as high as the system you build can handle. 

Offline DarkenedOne

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Re: Pu 238
« Reply #24 on: 11/15/2011 08:11 PM »
No, in terms of propellant use, an electric thruster would be much more efficient than a radioisotope thermal thruster. In addition to having much higher specific impulse, an electric thruster is much easier to control than an isotope heat source which cannot be turned off.

Electric thrusters allowed and Hayabusa and Dawn to successfully complete their missions. Do you know of any radioisotope thruster being currently designed for a spacecraft?

There's still no case for manufacturing radioisotopes in space.

We are talking about too different types of efficient.  I was talking about power efficiency.  You cannot get much better efficiency than direct heat to thrust conversion.  With electric power systems you will always have loses.  With radioisotope generators you have only 30% efficiency of heat to electric conversion.

Electric propulsion has better ISP, but much lower power due to conversion loses.

Which one is better depends on your mission. 

Offline douglas100

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Re: Pu 238
« Reply #25 on: 11/15/2011 11:23 PM »
[quote author=douglas100 link=topic=27264.msg829231#msg829231

We are talking about too different types of efficient.  I was talking about power efficiency. 

Electric propulsion has better ISP, but much lower power due to conversion loses.

Which one is better depends on your mission. 

Yes, I understand the difference. But if your mission is propulsion (and I thought it was, for this thread) then my point is that specific impulse is the measure of efficiency.

If you are using Pu238 to directly heat the propellant, then you're right, a much larger percentage of the heat generated would be transfered to the exhaust. But the specific impulse would so low that I doubt the thruster would have any advantage of a conventional chemical engine.
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Offline 93143

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Re: Pu 238
« Reply #26 on: 11/16/2011 03:11 AM »
Isotope heat sources don't undergo fission.  It's just radioactive decay heat.

And the achievable temperature is very low - substantially lower, most likely, than that of a fission NTR, but certainly no higher.  You'd be restricted to hydrogen if you wanted an Isp above 500.  Hydrogen is not easy to store.

In contrast, a electric thruster running xenon can pull 3000 seconds easily.

The decay of plutonium 238 takes the form of alpha radiation.  The alpha radiation gets absorbed within the material thus generating thermal energy.  The temperature of the isotope is directly related to the thermal energy contain in it.

Like any heat source its temperature will rise until it loses as much heat as it generates.  If an thermal insulator is used to prevent heat from escaping than the temperature will rise unabated.

Therefore the achievable temperature of plutonium is as high as the system you build can handle. 

You're talking to someone with two degrees in mechanical engineering, and most of a third in aerospace.  In other words, I know all that.  (Except the "unabated" part, which is an oversimplification - there's no such thing as a perfect thermal insulator.)

I'm not willing to allow that a radioisotope thermal rocket would definitely be able to hit NTR temperatures, partly becaue you can't turn it off (which creates practical issues), and partly because the power density is so low - radiative cooling, for instance, might limit the achievable peak temperature.  I haven't done any detailed analysis, but I believe my remark was more or less fair considering the time I had available to put into it.  Perhaps you read it wrong...?
« Last Edit: 11/16/2011 07:37 AM by 93143 »

Offline Robotbeat

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Re: Pu 238
« Reply #27 on: 11/17/2011 06:17 PM »
Strictly speaking, the lower your Isp, the better your thrust-to-power ratio is, thus you could say the lower your Isp the better your efficiency. But obviously you'll run out of propellant very quickly and very little of your potential energy from the decaying isotope will end up in providing propulsion.


Isotope heat sources don't undergo fission.  It's just radioactive decay heat.

And the achievable temperature is very low - substantially lower, most likely, than that of a fission NTR, but certainly no higher.  You'd be restricted to hydrogen if you wanted an Isp above 500.  Hydrogen is not easy to store.

In contrast, a electric thruster running xenon can pull 3000 seconds easily.
Very true.
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Online mmeijeri

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Re: Pu 238
« Reply #28 on: 11/17/2011 06:39 PM »
What about Polonium 210 with its incredibly high power density and correspondingly short half-life? I bet it's incredibly expensive too, but does anyone have ball park figures for it? What kind of incremental costs would there be for producing hundreds of kg a year and how much investment would it take to get there? Billions, hundreds of billions, or even more than that?
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Offline DarkenedOne

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Re: Pu 238
« Reply #29 on: 11/18/2011 02:14 PM »
Isotope heat sources don't undergo fission.  It's just radioactive decay heat.

And the achievable temperature is very low - substantially lower, most likely, than that of a fission NTR, but certainly no higher.  You'd be restricted to hydrogen if you wanted an Isp above 500.  Hydrogen is not easy to store.

In contrast, a electric thruster running xenon can pull 3000 seconds easily.

The decay of plutonium 238 takes the form of alpha radiation.  The alpha radiation gets absorbed within the material thus generating thermal energy.  The temperature of the isotope is directly related to the thermal energy contain in it.

Like any heat source its temperature will rise until it loses as much heat as it generates.  If an thermal insulator is used to prevent heat from escaping than the temperature will rise unabated.

Therefore the achievable temperature of plutonium is as high as the system you build can handle. 

You're talking to someone with two degrees in mechanical engineering, and most of a third in aerospace.  In other words, I know all that.  (Except the "unabated" part, which is an oversimplification - there's no such thing as a perfect thermal insulator.)

I'm not willing to allow that a radioisotope thermal rocket would definitely be able to hit NTR temperatures, partly becaue you can't turn it off (which creates practical issues), and partly because the power density is so low - radiative cooling, for instance, might limit the achievable peak temperature.  I haven't done any detailed analysis, but I believe my remark was more or less fair considering the time I had available to put into it.  Perhaps you read it wrong...?

Guys read this article.

http://en.wikipedia.org/wiki/Radioisotope_rocket

As you can see they have tested versions that operate at temps similar to NTR.

Offline RanulfC

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Re: Pu 238
« Reply #30 on: 11/18/2011 03:40 PM »
Remember Heinlein's "The man who sold the moon" ?
The radioactive rocket fuel was made in orbit instead of being made on earth.
Can Pu 238 be made in space, can we make a sort of fuel depot for very deep space missions?
Corrections:
In "The Man Who Sold the Moon" Harriman ended up having to use rather conventional chemical fuel, (a three (3) stage rocket with catapult assist from Pikes Peak) because he was unable to get anyone interested in RE-BUILDING the "Atomic-Fuel" depot in orbit which had exploded and was destroyed.

The Atomic-Fuel "depot/production-facility" was actually from an earlier story titled "Blow-Ups Happen" which detailed the rather extrodinary and complex way in which the enriched atomic fuel was being made. The premise of the story was the "assumption" that atomic power plants would TEND to explode pretty much at the drop of a hat, (a rather common theme from that time period I note) and in the story two atomic engineers not ONLY create a new "fuel" complex that allows atomic-reaction motors of unprecedented size and power, they also figure out how to put the entire fuel production process in orbit where it will be "safe" and the atomic powered shuttle craft that services the satellite.

In MWStM, Harriman argues that he can easily re-produce the atomic shuttle to use to get to the Moon and then informed that the government wouldn't allow it because they have evidence that points to the volitile atomic fuel of the shuttle exploding first that set off the depot.

In the end Harriman is "forced" to use more conventional rockets to reach the moon and even though he "eventually" even manages to send a small "colony" effort directly from the Earth to the Moon using the same system the catapult/Shuttle is soon turned into a "simpler-and-cheaper" 1.5-Stage-To-Orbit with launch assist system and is used to build "Supra-New-York" a satillite "way-station" and fuelng depot for space-to-space dedicated Lunar and interplanetary ships.

Of course his later writings go back to using "atomic" space ships but they all use atomic power-plants and "reaction-mass" propulsion (NTRs) rather than actual "atomic-fuel" as in the first story.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline RanulfC

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Re: Pu 238
« Reply #31 on: 11/18/2011 03:51 PM »
Guys read this article.

http://en.wikipedia.org/wiki/Radioisotope_rocket

As you can see they have tested versions that operate at temps similar to NTR.
The article also points out that the POWER levels generated by and RTG are far, far below that of the simpliest NTR. An NTR can generate over a gigawatt of energy while and RTG may only get as much as 5kw. Tested thruster designs barely got an ISP of 700 or less and the design was never considered vialbe for a "main-propulsion" application.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline 93143

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Re: Pu 238
« Reply #32 on: 11/18/2011 06:30 PM »
http://en.wikipedia.org/wiki/Radioisotope_rocket

As you can see they have tested versions that operate at temps similar to NTR.

Depends on your definition of "similar".  A proper fission NTR operates in the range of 2100-2800C.  The RTT folks seem to have managed about 1500-2000C using polonium-210, which has about 300 times the power density of plutonium-238.  I'd say this completely confirms what I said.

I wasn't actually comparing the idea unfavourably with fission NTRs.  I was comparing it unfavourably with electric thrusters, since the T/W of a plutonium RTT would be horrible anyway.  However, given that the test RTTs used polonium, and still ended up with less performance than a fission NTR, it seems quite probable that a plutonium RTT would have very uninteresting performance.

Consider that you have to insulate an RTG pellet for several minutes to get it to glow red...
« Last Edit: 11/18/2011 08:34 PM by 93143 »

Offline douglas100

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Re: Pu 238
« Reply #33 on: 11/18/2011 08:43 PM »
The article was interesting. I'd never heard of "Poodle" before and the reported Isp was higher than I would have guessed.

Nevertheless, the power output of such thrusters is low and the energy source cannot be controlled. I still can't think of an application for such a thruster which could not be better served by other systems.

The article says that the Poodle program ended in 1965. You would think, more than forty years later, if there were a use for such a thruster it would have been developed by now.

Apart from anything else, radioisotopes in kilogram quantities are very expensive.
Douglas Clark

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