Nuclear rocket propulsion

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  • #26
Astronuc
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Regarding the 3He, I'm saying that there are still some small supplies of the material here on Earth, which could be used for a proof-of-concept experiment.
Once you know it works, then it could justify going to the moon or elsewhere to acquire more.
He-3 is an indirect by-product of the weapons program in the sense that tritium T-3 decays by beta emission to He-3.

He-3 is pretty expensive stuff since there is not a lot of it. It's a great neutron absorber.

I don't see why a tokamak with a low density of burning plasma inherently has to be heavier than a large chemical combustion rocket.
Because tokamaks don't have lot of mass flow.

Chemical rockets have very low Isp (specific impulse), so the thrust comes from high mass flow rate. That's not the case with tokamaks.
 
  • #27
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Astronuc, I'm not saying that the tokamak's plasma current has to be directly vented as an exhaust flow. It could be used to power RF heating of hydrogen propellant, for example. All you need for that is adequate power-to-weight ratio. Yes, the 3He is a good neutron absorber, which any nearby astronauts would appreciate. Its fusion reaction is also a low in neutron emission, which the aforementioned astronauts would appreciate even more. Since it's expensive to produce, it would be cheaper to mine.

Greg, since 3He+D emits most of its energy output as thermal protons and alpha particles(helium nuclei), then that would be advantageous for a spacecraft, since less of a blanket would be required for energy capture. Then you don't need the cathedral.
 
  • #29
Astronuc
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Astronuc, I'm not saying that the tokamak's plasma current has to be directly vented as an exhaust flow. It could be used to power RF heating of hydrogen propellant, for example. All you need for that is adequate power-to-weight ratio.
And that is the problem - the power to mass ratio of a tokamak.

The tokamak's current is simply used to heat (ohmic heating) the hydrgoen plasma to temperatures to enable fusion reations. Ideally the fusion reaction gets going and some of that heat maintains the temperature of the plasmsa. Then fresh D, He3 (or whatever fuel) is fed into the system, and the products e.g. He4 are removed. Then the extra energy not needed to sustain the plasma temperature is extracted for more useful purposes such as electricity or thermal energy or propulsion.

If one builds an RF heating system for the H-propellant, that will another massive system, which has to be coupled to the power conversion system, which is another massive system, which has to be coupled to the tokamak. A mirror machine would make more sense, but that is also massive.

I don't want to discourage creativity, but people who throw out ideas for nuclear propulsion really need to do proper research, do the math/engineering and get a feel for what can and cannot be done with respect to plasmas and magnetic confinement systems. One needs to know what are reasonable numbers in terms of plasma density and pressure, and what limits there are on magnetic fields and materials limitations.

Controlled thermonuclear fusion is not a simple proposition, otherwise we would have perfected it long ago.

My last comments apply to NTR and anti-matter systems.
 
  • #30
Astronuc
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For more than a decade, the Particle Bed Reactor (PBR) has been a capability in search of a mission. The nascent PBR technology promises higher operating temperatures than those of conventional solid core reactors such as were developed in the 1960s under the NERVA program, which can translate into a more efficient power generator, or a more capable propulsion system.
I disagree with the promise of higher operating temperatures. And to what operating temperatures do they refer - fuel or coolant. It's the coolant temperature that counts, and that is constrained by the structural materials avaiable in the coolant transport system and power conversion system.

Such claims without supporting technical information are essentially meaningless. Of course raising the temperature allows better performance, but is it feasible to raise the temperature, which is limited by 'real' technical/physical constraints, namely material strength and endurance.
 
  • #31
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Here's another Wisconsin article for you:

fti.neep.wisc.edu/presentations/jfs_jannaf_1205.pdf

Lots of Helium-3 goodness
And if you look at slide8, you'll see why neutron emissions are bad
 
  • #32
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Lightweight Proton Accelerator

Here's an article about a lightweight proton accelerator:

http://www.technologyreview.com/Biotech/19084/

Perhaps this could be useful for a lightweight accelerator-driven reactor that operates at subcritical conditions.
 
  • #34
what was the propellant used in NERVA? hydrogen?
 
  • #35
Astronuc
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Yes - hydrogen. That offers the highest specific impulse, and doesn't introduce decomposition as would NH3 or CH4. Thermal conductivity is reasonably good compared to other gases.

The turbo pumps developed for this program essentially evolved into those use for the Shuttle SSME's.
 
  • #36
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The problem is that it just doesn't like staying inside a small, manageable volume

What about that Laser Thruster propulsion that I mentioned in another thread?

Some fellow from BAE Insitute says laser propulsion would be far better.

Well, even though photons have miniscule momentum/thrust, they would seem to be a propellant of boundless supply, since you can generate as many photons as you want as long as you have the energy, without suffering from any limitation of onboard supply.

Therefore a nuclear-powered laser thruster could generate as much photonic propellant as it had energy available for.

That guy from BAE Institute said that while his demo was only generating 35-micronewtons, it could be scaled up to kilonewtons of thrust by using nuclear power.

I'm not quite sure how it works, though. Can anyone elaborate?
It seems that he's using a stationary laser source to hit against a vehicle equipped with some sort of resonant cavity. This resonant cavity then bounces the photons around and extracts more energy out of them than would otherwise be the case. By doing this, you get more thrust from your laser beam.

But so would this be suitable for earth-to-orbit launch? If you laser source is independent of the vehicle, then it could be as heavy as you liked. But you'd need the laser emitter to be poking through the top of the atmosphere to avoid being blocked by it. Could this then point towards some kind of buoyant floating launchpad, perhaps mounted on a dirigible/blimp? Your blimp could either be carrying the nuclear power supply, or else it could dangle a wire down to the ground where the nuclear reactor would be sitting.
 
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  • #37
Yes - hydrogen. That offers the highest specific impulse, and doesn't introduce decomposition as would NH3 or CH4. Thermal conductivity is reasonably good compared to other gases.

The turbo pumps developed for this program essentially evolved into those use for the Shuttle SSME's.
is the energy transfer mechanism completely thermal, or does the hydrogen ionize like in VASMR? (sorry, i just can't seem to find many details about NERVA online). if it's thermal, why would decomposition be a problem (for instance, why not use water or something more dense than hydrogen, like Hg) if it is all going out the back anyway? and wouldn't you get decomposition of the dihydrogen into protons (reactive) anyway?

i can only guess that (a) cryogenic storage of H2 isn't a problem for the <20 K environment of outer space or (b) the light mass of hydrogen offers the prospect of it's leaving the reactor fast enough that the reactive protons are less of a problem. (i.e., the material of the reactor can't be easily protonated?)

that is cool that the turbopumps for the SS had their origins in NERVA, it's amazing that they have never failed.
 
  • #38
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