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This is a variant on Robert Zubrin's salt water rocket. The salt water rocket itself is unlikely to work (at least with 235U), because the fission cross-section and reproduction factor of 235U rapidly declines as neutron energy increases away from the thermal region. This means the salt water rocket would reach criticality, there would be a low energy pulse and most of the propellant would leave the nozzle without even reaching boiling point. See here:
http://wwwndc.jaea.go.jp/Reproduction_F … 235_0K.jpg
One way of avoiding this is to start criticality with a hard neutron spectrum - say in the KeV range. As temperature increases, the reproduction factor will increase and the reactivity will increase along with it. A good candidate fuel would be Uranium Tetrachloride, which is molten at temperatures above 590C. The high atomic weight of chlorine limits any neutron spectrum softening and the moderate melting point means that the engine chamber and fuel handling apparatus can be made from stainless steels.
An interesting option would be to add small amounts of deuterium and tritium to the engine whilst in operation. This would undergo fusion, releasing superfast neutrons, which would boost reactivity, potentially allowing for reduced critical mass.
Last edited by Antius (2017-09-15 09:09:48)
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It doesn't seem to me that any variant of the Nuclear Saltwater Rocket would have conditions that are favorable for fusion. What makes you think it would occur?
Fusion will occur because the plasma will have temperature in the millions of kelvin. The fusion will contribute no more than about 10% of the total energy of exhaust. Too much fusion fuel will start to add an unacceptable level of moderation to the mix. Most of the energy from deuterium tritium fusion is released as super fast neutrons, which will boost the fission yield and reduce critical mass. So the fusion is there to produce neutrons, rather than energy.
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The optimal temperature for DT fusion is 13 keV, or 150 million K, which is maybe possible. On the other hand fusion rates go way down when you dilute the fuel. I guess this is a bit of a nitpick though. It's an interesting idea!
-Josh
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Sounds to me like this is unknown territory between the end-points of (1) it won't work at all, and (2) it might blow sky-high. Where would you safely test such a thing?
Flight test as initial test is not an option: you must have a stable thrust stand and lots of room for instrumentation, plus a bunker in which to cower.
I suggest a lunar crater. No neighbors to endanger or annoy, no air or water to pollute, a ringwall to capture debris from the inevitable test failure. No need to capture and decontaminate or dispose of exhaust plumes, either.
GW
Last edited by GW Johnson (2017-10-05 11:27:38)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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That would be the more expensive option, surely? Getting to Luna is expensive enough, but at least we can ship it out there using chemical propulsion and build a bunker using in-situ materials...
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
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This proposal envisages a very powerful rocket, I would think. It might have a significant effect on the orbit of a space rock. You would have to have a very big one to make this negligible. Using the moon is much simpler.
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True, asteroids are farther, but the NSWR is a far-outer-system and interstellar kind of engine. By the time we're working on it the belt should already have been settled (admittedly, hopefully lightly settled).
Let's say there's basically no way to scale down, and your engine goes through 100 kg/s of propellant with an exhaust speed of 0.01 c (3,000 km/s). That represents a force of 300 MN. If you fire continuously for one hour, you will have generated an impulse of 1.08e12 N-s. If you want to keep your delta-V to 10 m/s or less, that means your asteroid needs to mass no more than 1.08e11 kg. If the asteroid has the density of water, it could be a sphere with a diameter of 600 m. There's plenty of asteroids that big.
There's another reason you might want to use a smaller object, which is that it actually gives you a pretty good way of measuring the impulse the rocket generates. If you have a precise measurement of the mass of the body you are using as a test stand, by measuring the change in its orbital characteristics before and after the test fire (this is something you can do pretty accurately) and by recording the precise firing time, you can get a really good estimate of its mean exhaust velocity.
-Josh
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Actually, I'd just as soon use a thrust transducer, and not have to worry about changing the orbit of my thrust stand. The moon is big enough not to be affected. It's close enough to reach with the rockets we already have.
Seems almost made-to-order for that very purpose.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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