What are the lessons learned from nuclear reactor design and analysis?

[Azael]

I have been reading a bit lately about project orion and the NERVA project. Project orion seems like such a crazy project that you can't help to love it I am not suprised it got shut down however.

But is there any research going on anymore in the states or any other country that is similar to NERVA?

As I understand it NASA's recent Prometheus project was oriented towards using reactors for electricity production to power equipment and ion engines. But no nuclear thermal propulsion?

Is nuclear thermal propulsion research more or less dead all over the world right now?

 

[Astronuc]

The Russians have had an active program, at least up through about 2005, in direct nuclear thermal. Jim Tulenko and others at University Florida's Innovative Nuclear Space Power and Propulsion Institute (http://www.inspi.ufl.edu/ ) have had a program on high temperature materials for fuel, e.g. U-Zr-C and derviatives.

There is no full scale NTR program in the US, but there are limited experiments on high temperature materials.

Prometheus is more or less dead. There are very limited activities, and I believe some of the work has moved over to some of the GenVI activities.

I have some contacts in DOE and NASA with whom I can check.

 

[Azael]

That would be great! :)

A related question. If the states where to begin a big project again similar in scale to NERVA. Would it be completely impossible for a foreigner to work on the project?
Next year I am going to have my masters diploma and then follow it up with a phd in reactor physics. I would love to work on nuclear propulsion at some time in the future, but sweden will never start such a program. I doubt ESA will either. So only the states, russia and china left.

 

[Astronuc]

A related question. If the states where to begin a big project again similar in scale to NERVA. Would it be completely impossible for a foreigner to work on the project?

It would not be impossible, and actually there may be collaborative projects between ESA and NASA. Now one complication might be if the research and development work were done by Bettis or Knowles Laboratories. These places do classified work on nuclear propulsion for the military, US DOD. It is effectively impossible for foreigners to work there.

NASA's Marshall Center has the lead on boosters and heavy lift vehicles, so perhaps it would be possible to get a job there. However, it depends on whether or not there is an opening, and whether or not there is a program like Prometheus.

Here is an abstract from a recent conference.
Nuclear Thermal Propulsion
Mars Mission Systems Analysis and Requirements Definition
http://www.inspi.ufl.edu/space07/program/abstracts/2072.pdf

Another conference dedicated to space nuclear is STAIF
http://www.unm.edu/~ISNPS/conferences/conferencec.html#ct203

See also
http://www-rsicc.ornl.gov/ANST_site/nasa10079.pdf

http://anst.ans.org/RelatedLinks.html - lots of archival material on this page, like the link above

Next year I am going to have my masters diploma and then follow it up with a phd in reactor physics. I would love to work on nuclear propulsion at some time in the future, but sweden will never start such a program. I doubt ESA will either. So only the states, russia and china left.

If China has a program, they are not saying much if anything about it.

 

[StuMyers]

That would be great! :)

A related question. If the states where to begin a big project again similar in scale to NERVA. Would it be completely impossible for a foreigner to work on the project?
Next year I am going to have my masters diploma and then follow it up with a phd in reactor physics. I would love to work on nuclear propulsion at some time in the future, but sweden will never start such a program. I doubt ESA will either. So only the states, russia and china left.

Who says you have to be a foreigner? Come be American!

Always liked Orion myself. I thought we abandoned it because of some treaty we signed with the Soviets in the 60's.

 

[Azael]

It would not be impossible, and actually there may be collaborative projects between ESA and NASA. Now one complication might be if the research and development work were done by Bettis or Knowles Laboratories. These places do classified work on nuclear propulsion for the military, US DOD. It is effectively impossible for foreigners to work there.

NASA's Marshall Center has the lead on boosters and heavy lift vehicles, so perhaps it would be possible to get a job there. However, it depends on whether or not there is an opening, and whether or not there is a program like Prometheus.

Here is an abstract from a recent conference.
Nuclear Thermal Propulsion
Mars Mission Systems Analysis and Requirements Definition
http://www.inspi.ufl.edu/space07/program/abstracts/2072.pdf

Another conference dedicated to space nuclear is STAIF
http://www.unm.edu/~ISNPS/conferences/conferencec.html#ct203

See also
http://www-rsicc.ornl.gov/ANST_site/nasa10079.pdf

http://anst.ans.org/RelatedLinks.html - lots of archival material on this page, like the link above

If China has a program, they are not saying much if anything about it.

Thanks ALOT for the links! Just the kind of links I have been looking for. Perfect for the 22 hour trainride I will embark on tomorrow.

It would seem like the best thing to do is to keep fingers crossed that nuclear thermal propulsion will get a new life now that nuclear power is becoming more and more accepted. Il be damned if I don't end up working on it at some point in my life.

Who says you have to be a foreigner? Come be American!

Always liked Orion myself. I thought we abandoned it because of some treaty we signed with the Soviets in the 60's.

Well maby someday I will come and be a american if I see a need for it Lower taxes and higher salaries compared to Sweden sure is tempting

It would have been nice to se if Orion could fly. If there was some way to limit the fallout.

 

[sanman]

Burning Plasma?

It seems like most of the obstacles/liabilities with the nuclear rocket are
a) fear of Chernobyl disaster due to launch failure
b) fear of irradiating the crew

Well, B could be addressed if the rocket is purely intended to lift heavy unmanned payloads, allowing the crew to get to orbit by separate alternate means.

As for A, I'm wondering if the rocket could be designed to eject the core to have it parachute to the ground, in the event of a launch failure.

Or else, what about the idea of a fusion process -- like a burning plasma?
Fine, a burning plasma is just a temporary high-energy state which doesn't last, but likewise so too are the H2-burning space shuttle main engines only producing output for about 8 minutes.

The longest duration burning plasma was made to last 28 seconds by Japan's JT-60 experiment.
http://www.jaea.go.jp/english/news/p06052303/index.shtml
http://www.jaea.go.jp/english/news/p06052303/all.jpg

IIRC, that was a hundred-fold improvement over previous trials.
If that could be further improved by a factor of 15, then it could be enough to get a rocket into orbit.

When the ITER experiment is conducted, it is expected/targeted to achieve a burning plasma duration of 500s, at 500megawatts of fusion power:
http://www.eurekalert.org/features/doe/2003-05/dnal-bpt051203.php

So let's suppose that goal is achieved. Couldn't it then serve as the basis for a powerplant in an Earth-to-Orbit heavy launch vehicle?

Comments? Analysis? Speculations?

 

[Astronuc]

Nuclear Thermal Rockets would likely be lifted into space for use in moving from LEO to Martian orbit. Radiation is certainly a critical issue, not only for the crew, but for anyone in the vicinity of the rocket motor.

The greatest exposure from NTR's comes from the nozzle end, since there is effectively not shielding there - it's basics the exit (lower/bottom) plenum of the core.

On the sides and top, shielding is provided by the structure and the propellant. Hydrogenous propellants (e.g. H₂, NH₃, CH₄ or B₂H₆) provided shielding from neutrons, which 'leak' from the core. Metal hydrides like LiH or ZrH₂ also can provide shielding for neutrons. However, the goal in rocket design is to minimize mass in order to minimize energy requirements.

Fine, a burning plasma is just a temporary high-energy state which doesn't last, but likewise so too are the H2-burning space shuttle main engines only producing output for about 8 minutes.

Fusion will not work for propulsion from the Earth's surface. Then plasma densities are way too low, which means thrust is too low, AND the magnets of a confinement system are too heavy.

In space, fusion propulsion MIGHT work - but I have strong reservations about that. We still need to perfect controlled fusion on ground. Propulsion systems are an entirely and much great challenge.

 

[sanman]

Fusion will not work for propulsion from the Earth's surface. Then plasma densities are way too low, which means thrust is too low, AND the magnets of a confinement system are too heavy.

In space, fusion propulsion MIGHT work - but I have strong reservations about that. We still need to perfect controlled fusion on ground. Propulsion systems are an entirely and much great challenge.

Suppose the plasma is used to heat a propellant, even mixing into its exhaust stream for max heat transfer, with the plasma's energy supplementing the usual H2 + O2 heat of combustion. Suppose you also use the cold temperature of the liquid H2 & O2 to cool some magnets made out of some lightweight superconductor? Again, the purpose is to enable/provide heavy lift capacity from Earth-to-Orbit. Instead of lifting several tons or even 100 tons of payload to orbit, you want to lift hundreds of tons to make things more economical.

Why wouldn't this then work? What further obstacles would there then be?
Has anyone ever proposed anything like this approach, which I could try to read up on?

 

[Morbius]

Why wouldn't this then work? What further obstacles would there then be?
Has anyone ever proposed anything like this approach, which I could try to read up on?

sanman,

Astronuc is correct - the plasma densities are too low. If the plasma
density is low - the amount of energy you are adding is TRIVIAL!

You have to work the numbers to see this is a shovel with a rope handle.

Dr. Gregory Greenman
Physicist

 

[mheslep]

I thought the idea for nuclear heavy lift was you carry your own thrust mass and heat it/accelerate it w/ nuclear sourced energies. And who said anything about magnets! Tokamaks are not the beginning and end of fusion.

mheslep

 

[mheslep]

...
The longest duration burning plasma was made to last 28 seconds by Japan's JT-60 experiment.
http://www.jaea.go.jp/english/news/p06052303/index.shtml
http://www.jaea.go.jp/english/news/p06052303/all.jpg

IIRC, that was a hundred-fold improvement over previous trials.
If that could be further improved by a factor of 15, then it could be enough to get a rocket into orbit.

I find it frustrating that all of these press releases fail to release numbers in pure Lawson terms - density*time any maybe energy. A benchmark of time confinement alone is meaningless except for hinting at an increase in $ budgets. 'Normalized' plasma pressure? 'High' pressure? Come on.

 

[Danger]

Always liked Orion myself. I thought we abandoned it because of some treaty we signed with the Soviets in the 60's.

I believe that it was the original SALT, which banned the deployment of nuclear weapons in space. Since the Orion 'engine' was essentially a sizeable supply of H-bombs, they had to pull the plug. (I don't know, however, whether the story is true or a myth.)

 

[Astronuc]

I thought the idea for nuclear heavy lift was you carry your own thrust mass and heat it/accelerate it w/ nuclear sourced energies. And who said anything about magnets! Tokamaks are not the beginning and end of fusion.

IEC's are also too heavy.

I believe that it was the original SALT, which banned the deployment of nuclear weapons in space. Since the Orion 'engine' was essentially a sizeable supply of H-bombs, they had to pull the plug. (I don't know, however, whether the story is true or a myth.)

That's part of the it. The other part is the number of nuclear warheads? Do the math, and one finds metric tons of nukes. It's just not feasible.

Suppose the plasma is used to heat a propellant, even mixing into its exhaust stream for max heat transfer, with the plasma's energy supplementing the usual H2 + O2 heat of combustion.

The plasma denisty and mass flow rate are too low! The mass of the plasma is too small compared to mass of H2 + O2. And one still needs to get the plasma heated, which takes a massive system.

I thought the idea for nuclear heavy lift was you carry your own thrust mass and heat it/accelerate it w/ nuclear sourced energies.

It is. The NTR was initiated to carry the prototype thermonuclear weapons which where too heavy for chemical rockets of the time. Then the thermonukes got lighter, and chemical rockets more powerfu. Then the nukes were supposed to used for missions to Mars - and then the program died in the 70's, resurrected under SDI in 80's and died again, resurrected in the 90's and died again, resurrected by Bush II only to die again.

A good book on NTR history is James Dewar's "https://www.amazon.com/dp/0813122678/?tag=pfamazon01-20"

 

[sanman]

Power Demand Profile, Zero-to-Escape

Firstly, a thermonuclear reactor is not a weapon -- no more so than the RTGs which have gone up on various space probes. Actually it's even less so, since there's no potential for the radioactive fallout hazards you could get with fission reactors.

Nextly, even if a tokamak/spheromak's plasma density is low, the power density is still high, and that's ultimately what you want to transfer to your propellant, to make thrust. 500megawatts continuously for 8 minutes (projection for ITER) is nothing to sneeze at. The low plasma density simply means that we have to find a different way to achieve the power coupling with the propellant. Perhaps some kind of magnetic induction heating process? Or else RF heating like VASIMR?

Again, if we use a cryogenic propellant, we can get double benefit -- cryogenics keeps the propellant at a manageable volume, and also the low temp of that propellant can be used to supercool some superconductive magnets on our tokamak/spheromak.

Perhaps carbon nanotubes could be used for lightweight wiring, in conjunction with other warm-temp superconductor materials, to make a sufficiently lightweight tokamak/spheromak.

Check out China's EAST tokamak, which was built in just 7 months and used superconducting magnets, even though it didn't attempt any burning plasma experiments:
http://en.wikipedia.org/wiki/EAST

If our burning plasma was made of the nifty Helium-3, then we could efficiently capture all its thermal proton output, while neglecting any trivial neutron emissions.
With the density and mass of a burning plasma being so low, as you've pointed out to me, perhaps the available supply of 3He right here on Earth (byproduct of nuclear weapons) might be sufficient for a large-scale tokamak/spheromak experiment, to achieve/demonstrate a burning 3He plasma with high energy harvest.The reason why I selected the burning plasma is because of the power demand profile of a space launch's flight envelope. Spacecraft expend the most energy up front, while going from zero to escape velocity. After they achieve orbit, the power demand requirements drop considerably. So even if you hypothetically had some ready-to-go nuclear reactor for your space launch, the fact is that you don't need the full mighty nuclear power output except for achieving escape velocity. After you achieve escape velocity, you've got more time to do whatever further acceleration you want for going elsewhere.

If you can come up with a power source that will supply hundreds of megawatts (gigawatts?) continuously for several minutes before fizzling out, then that power source should be suitable for Earth-to-Orbit purposes, providing it's not too bulky/heavy. That's why I'm thinking that a burning plasma in suitably lightweight tokamak/spheromak would be the best fit for the power demand requirements.

Why can't we pick which power source to develop based on the power demand profile of the application? If we don't need the full mighty nuclear power except for those first several minutes when we're trying to achieve escape velocity, then why can't a burning plasma do the trick?

Again, the goal here is to lift payloads of at least a few hundred tons or more to orbit. What better way can you think of to lift that kind of mass without using nuclear power?

 

[sanman]

Read this for an overview of various proposals:

http://www.strangehorizons.com/2004/20040112/nuclear.shtml

 

[sanman]

AIMstar:

http://www.engr.psu.edu/antimatter/Papers/AIMStar_99.pdf

 

[Astronuc]

Nextly, even if a tokamak/spheromak's plasma density is low, the power density is still high, and that's ultimately what you want to transfer to your propellant, to make thrust. 500megawatts continuously for 8 minutes (projection for ITER) is nothing to sneeze at. The low plasma density simply means that we have to find a different way to achieve the power coupling with the propellant. Perhaps some kind of magnetic induction heating process? Or else RF heating like VASIMR?

One has to look at the kW/kg of the system.

Also, the Space Shuttle system produces ~ 11.7 GW of power for several minutes according to Power of a Space Shuttle
http://hypertextbook.com/facts/2001/StaverieBoundouris.shtml

From NASA - At full throttle, the three SSMEs produce 37 million horsepower = 27.6 GW
http://www.nasa.gov/returntoflight/system/system_SSME.html

 

[Morbius]

perhaps the available supply of 3He right here on Earth (byproduct of nuclear weapons)

sanman,

He-3 is NOT a byproduct of nuclear weapons production!

The reaction by which Trititum is made for nuclear weapons gives you He-4; not He-3.

Dr. Gregory Greenman
Physicist

 

[mheslep]

IEC's are also too heavy.

Based on what? Its all 'E' - electrostatic, no superconducting magnets.* The lab units are typically < 1e2 Kg, and scaling up to 1M diameter units is probably still 1e3 Kg. Then I suppose you are assuming a massive blanket to manage neutron 1st wall damage? Perhaps, but at least for a rocket we'd carry all are own tritium at launch, no need for a Li breeder blanket.

*Rostoker's Colliding Beam reactor uses FRC but I don't recall if its superconducting.

mheslep

 

[Astronuc]

Based on what? Its all 'E' - electrostatic, no superconducting magnets.* The lab units are typically < 1e2 Kg, and scaling up to 1M diameter units is probably still 1e3 Kg. Then I suppose you are assuming a massive blanket to manage neutron 1st wall damage?

Or shielding against a 14.1 MeV n from d+t. What is the fusion power level of a 100 kg IEC - 100W, 100 kW, 1 MW?

Then scale it to 10 GW.

Wisconsin IEC gets up to 10⁸ n/sec so one can estimate the power output from the reactions rate.

http://fti.neep.wisc.edu/iec/inertial_electrostatic_confineme.htm
http://fti.neep.wisc.edu/iec/AmericanScientist.htm
http://fti.neep.wisc.edu/iec/operation.htm

Robert W. Bussard, a physicist who founded the Energy/Matter Conversion Corporation of San Diego (which goes by the clever acronym EMC²) is also experimenting with inertial electrostatic confinement fusion. The U.S. Navy has supported him with about $4 million since 1995, in hopes that this technique will someday provide a compact fusion power source. Although practical realization may be far off, Alan Roberts, the official in charge of this Navy research program notes, “You can’t put a tokamak on a ship.”

from Am Sci article.

 

[sanman]

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. Likewise, I don't see why a tokamak can't be scaled up to provide gigawatts of energy. If anything, the expectation is that the larger the tokamak, the better its performance, which scales up cubically with the radius.

South Korea's KSTAR tokamak expects to go live sometime in 2008, and it only has a diameter less than a couple of meters, but is expected to have pulse durations of 1000s or more, due to its efficient superconducting design.

So if a tokamak could be made with a 100m radius, why wouldn't it be able to produce the power necessary to lift hundreds of tons to orbit? Just to reiterate, all it needs to do is produce the high power for several minutes, rather than hours or days or weeks.

 

[sanman]

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.

 

[Morbius]

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. Likewise, I don't see why a tokamak can't be scaled up to provide gigawatts of energy. If anything, the expectation is that the larger the tokamak, the better its performance, which scales up cubically with the radius.

sanman,

The low plasma density means you are going to have a large plasma volume for any\
meaningful amount of energy generation.

If we are talking D-T fusion; most of the energy is coming out in the form of a 14.1 MeV
neutron. Such a high energy neutron has a fairly large mean free path - so if you are
going to retrieve this energy - the thickness of the region just outside the plasma has
to be a few feet thick.

As one of my professors used to say, "Any animal with a skin that is a few feet thick;
is going to be a BIG animal. He used to refer to an operation magnetic fusion reactor
as "the superconducting cathedral".

The only thing that scales with the cube of the scale factor is the volume.

Dr. Gregory Greenman
Physicist

 

[mheslep]

Or shielding against a 14.1 MeV n from d+t.

Ok yes I think we'd have to be talking about some kind of aneutronic reactor to get off the ground.

What is the fusion power level of a 100 kg IEC - 100W, 100 kW, 1 MW?

Then scale it to 10 GW.

Wisconsin IEC gets up to 10⁸ n/sec so one can estimate the power output from the reactions rate.

Hirsch-Meeks IEC ala Wisconsin is limited by grid losses, and would be limited by thermal collisions and Bremsstrahlung at higher power levels. So 1st one needs a virtual cathode (http://link.aip.org/link/?PFLDAS/2/239/1" might be it, in PRL no less. These are some tall hurdles I've just hand waived across, but the nice thing about a spherical E based accelerator is that if it does get that far it can theoretically still be small and lightweight - tabletop even at high powers. It needs no superconductors, no TW picosecond lasers; in sum it needs no 'ignition' minimum size - its an accelerator.

 

[Astronuc]

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.

 

[sanman]

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.

 

[sanman]

Particle Bed Reactor:

http://www.fas.org/nuke/space/c08tw_1.htm

 

[Astronuc]

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.

 

[Astronuc]

Particle Bed Reactor:

http://www.fas.org/nuke/space/c08tw_1.htm

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.

 

[sanman]

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

 

[sanman]

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.

 

[Astronuc]

Reactor Design & Analysis - Lessons Learned

http://www-rsicc.ornl.gov/ANST_site/answinter_hth.pdf

 

[quetzalcoatl9]

what was the propellant used in NERVA? hydrogen?

 

[Astronuc]

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.

 

[sanman]

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.

 

[quetzalcoatl9]

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.

 

[Azael]

The turbo pumps developed for this program essentially evolved into those use for the Shuttle SSME's.

Thats a interesting fact. Has the NERVA program had any other off branches?

Reactor Design & Analysis - Lessons Learned

http://www-rsicc.ornl.gov/ANST_site/answinter_hth.pdf

Nice link