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PostPosted: Jan 07, 2014 9:11 pm 
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That isn't what I am proposing though.

I am proposing a standard fission fragment rocket based on a dusty bed.

EDIT:

Since my engines are drastically larger than the ones that the barely-critical result was calculated for I think I might be able to get a nonzero conversion ratio, at least on the larger engines.

So now I have to work out if 238U in the fuel would be a good idea or not, since there will be no time for it to decay through Neptunium, and as far as I can tell it is not fissionable (I have no figures for (n,f) for 239Np and a small value for 239U).

232Th has the same problem.
If I can reduce the fissile content of the fuel below weapons grade this is obviously a big help.
Does anyone know any near stable isotopes that produce fissile materials from neutron capture without significant intermediate decays?

I might be able to the power of each individual particle but that will have a weight, and thrust, penalty.
But it might be the only option if I want to have practical fuel storage.

How about 231Pa if it can be obtained in sufficient quantities?
It appears sufficiently refractory (although I can't find figures for PaN), is borderline fissionable itself and 232Pa is fissile - this decays rapidly to 232U and is fissile itself, although that will drastically increase the radiation flux inside a deactivated reactor assembly since the only other major radiation source after shutdown is likely 14C in the moderator.


Last edited by E Ireland on Jan 07, 2014 9:59 pm, edited 1 time in total.

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PostPosted: Jan 07, 2014 9:55 pm 
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Joined: Nov 30, 2006 9:18 pm
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E Ireland wrote:
I am proposing a standard fission fragment rocket based on a dusty bed.
That's a rather vague description.

But if I get the gist of it..... the typical problem with these types of concepts is ship acceleration: As soon as it starts, the fuel gets dumped out the back nozzle, before any more useful heat is generated.
It's a bit like the proverbial motorcycle passenger who gets dropped because she wasn't holding on.....


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PostPosted: Jan 07, 2014 9:58 pm 
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jaro wrote:
E Ireland wrote:
I am proposing a standard fission fragment rocket based on a dusty bed.
That's a rather vague description.

But if I get the gist of it..... the typical problem with these types of concepts is ship acceleration: As soon as it starts, the fuel gets dumped out the back nozzle, before any more useful heat is generated.
It's a bit like the proverbial motorcycle passenger who gets dropped because she wasn't holding on.....


Due to my very low accelerations the forces aren't enormous and the electromagnetic field around the nozzle should be able to keep the relatively low energy dust particles inside the reactor.
After all all the field has to do is allow the relativistic charged particles out, not the enormously massive (compared to the fission fragments) slow moving dust.


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PostPosted: Jan 08, 2014 8:41 pm 
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In hindsight due to the low fission and capture cross sections of 239U and 239Np It hink a certain amount of plutonium production would be viable.

So I am going to set fuel enrichment at roughly 50%.
50% 233U and 50% 238U.

At worst it will marginally reduce my max attainable ISP but it will seriously reduce the cost of a mass unit of fuel since only half of it is fissile obtained at significant cost from reactors or accelerators.

I imagine it also reduces my fuel handling issues.


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