Author Topic: Pu 238  (Read 6047 times)

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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Offline 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
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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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