At first glance it almost seems to have more weapons applications then propulsion.  But, I guess that it could be used for either.  I wonder how that would compare with state of the art nuclear thermal systems.

The problem with the direct pumped nuclear laser is the power conversion system--converting the energy of the fission reaction into some means of exciting a population inversion in some medium. On the ground, a Direct Pumped Nuclear Laser--such as a large gas dynamic laser I have mentioned several times, in which the heat from a nuclear reactor is used to increase the enthalpy of a high pressure (nearly supercritical) mixture of carbon monoxide, helium, and usually a tad bit of xenon, and then passing the superheated mixture through a system of parallel de Levaal nozzles to cause the population inversion: an external infrared laser will then stimulate the emission of more radiation at the characteristic wavelength of the stimulator. Thus the whole thing will act as a laser amplifier--and it should have outputs ranging from a few hundred kilowatts right up to a few hundred megawatts. The upper end should be powerful enough for a ground launch of a laser powered launch vehicle nearly the mass of a Volkswagon. However, the power generation and conversion systems will mass several thousands of tons--way too much to create a 'self powered' direct nuclear pumped laser propulsion system.

To create a steady stream of laser pulses or even continuous power--powerful enough to be sufficient for laser-thermal propulsion--will require Gigawatts of nuclear power. You might as well just use a smaller 1 Gigawatt-thermal reactor to heat the hydrogen directly...it won't be quite as high performance in some circumstances, the weight sivaings will more than anything else, increase the performance sufficiently that the added Isp (doubtful it would be any better than 900 seconds!) from DNPL/Laser Thermal propulsion will not matter in the least!

And this is different from the "X-ray stimulated Hafnium Isomer Transition Reactor" mentioned in that article, Bruce. The Hafnium Isomer Reactor was supposed to be a compact gamma-ray source that could be used to provide the energy for an anolog of a nuclear-thermal jet engine. A more refined version of the Project: Pluto nuclear thermal ramjet engine originally developed in the late 1950's just before ballistic missile development really took off. The Hafnium-Isomer Reactor has been since proven to be totally bogus as the transitions cannot be stimulated by bombarding the Isomers with low power x-ray emissions. However, there are some interesting studies looking at Th-229 isomers:

http://blog.wired.com/defense/2007/04/livermore_claim.html

and of Tantalum-180:

http://cc.ysu.edu/~jjcarrol/abstracts.html

The jurry is still out on isomer 'reactors.' There is some interesting hints, but if there is anything really juicy out there it is being kept quiet. Any new compact, high -energy density source using nucleonics with energy densities approaching 800 MW*hr/Kg is going to be a dead ringer for new weapons systems: propulsion, directed energy weaponry, and of course very compact nuclear wepoans. So if it is real--and real big--it is likely "black-ops" hush, hush at Lawrence Livermore...!

This is a very interesting concept but the problem I see for propulsion is that at 600 deg K you aren't going to have very much in the way of specific impluse from the cooling hydrogen flow and the photon beam probably (I haven't done any calculations yet) won't contribute that much thrust. I see that is the reason for adding the carbon. I really doubt that will be superior to what we can do with state of the art nuclear thermal systems. The old NERVA type systems did have some errosion that released radioactivity but I thought that the newer concepts has solved some of that. Also, if we did have an accident would UF6 be the best form of a release to clean up afterward?

It is an interesting technology and thanks for bring it to up.

Zarimus wrote:

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I'm not sure what that would translate to in the form of heat energy in the hydrogen propellant, I suspect it would take an immense amount of computer modeling to find out.

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My guess is that it would depend on the amount of laser beam focusing.
Theoetically, extremely high temperatures could be achieved -- but the volume heated to such high temperatures would be quite small.
So not a design suitable for ground launch, but perhaps a simple high-Isp device for in-space propulsion ?

Zarimus wrote:

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As for accidents, the UF6 tank would probably be built fairly sturdily. The gas in the laser would be pumped back into the storage tank for containment (or if released in a catastrophic accident would be minor). It could be made reasonably safe, and that's all anyone could expect.

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The risk would not be from the hex -- rather, it would be from fission products which, if the reactor is operated at high power for a long time, could be very substancial.....

The issue of whether to diffuse the beam or concentrate is entirely a function of the volumetric optical power conversion density in the reactor. Whatever mechanism that results in the stimulation of the laser beam must deposit sufficient power into the beam to result in significant propulsion. Remember also that if hydrogen is used as a reaction mass, hydrogen is notoriously transparent to radiation except at wavelengths close to the molecular excitation frequencies (vibrations along its covalent bond to make a single H2 molecule.) It is unlikely that the anything less than direct excitation of a UV laser will result in siginificant propulsive activity with hydrogen--that's my "Engineer Wannabe" guess.

The idea of using an "Optocoupler" method seperating the fission reactor from the propulsion system is interesting. However, generally as a rule of thumb, the primary reason why high performance rocket engines work so well at all is that they are mechanically designed to directly convert thermal potential energy (heat of combustion) into mechanical motion (jet power) by capitalizing on a pressure drop across a carefully shaped surface. This may seem trivial to state, but the implication is sound: power conversion is one step from heat to motion (thrust) and thus occurs with very high thermal efficiency (approaching 90-95%.) A DNPL system will involve four steps atleast: (1) reactor energy converted to (2) light (laser) which is then converted to (3) heat in a working fluid which causes conversion of that heat into (4) mechanical motion of the jet. Such a complex power conversion system will suffer because of weight and efficiency issues. Even if the first step occurs with 100% efficiency, the other steps do not, so the whole "cycle" performance will suffer...

My gut feeling on this is that Direct Nuclear Pumped Lasers cannot be effective as a whole flight weight system. Perhaps as a laser launch system from the ground, but not as an integrated flight weight vehicle...

Actually I suspect that you do want to focuss the beam, because diffuse radiation will go through the propellant flow, while a focussed beam will create a small pocket of plasma. Allowing the plasma to interact and quench in the propellant flow will transfer heat kinetically to the molecules of the gas working fluid--this will transfer heat from the plasma to the molecular gas.

To get significant thrust at high Isp is going to need gigwatt power levels. The proof is in doing a little calculus:

The kinetic energy equation is:

K=1/2*m*v^2, where the kinetic energy in Joules, of a mass m in kilograms, moving with velocity v meters per second. Holding v constant, allowing mass to change, and differentiating both sides with respect to time we get:

Pjet=1/2*mdot*v^2 which can be applied to rockets as follows:

Pjet is the jet power, or the actual mechanic power carried away by the reaction mass; mdot is the reaction mass flow rate in kg/s; and v is the exhaust velocity of the jet in meters per second.

For a rocket like the RL-10A2 Centaur upper stage engine, burning liquid hydrogen and liquid oxygen with a specific impulse of 435 seconds (which corresponds to an exhaust velocity of 4266 m/s,) a thrust of about 15,000 lbf and a propellant consumption rate of about 16 kg/s, this corresponds to a jet power of:

Pjet=0.5*(16 kg/s) *(4266 m/s)^2

Pjet=145.6 MW

And this is a 'small' chemical engine!

If we assumed 900 sec of specific impulse (8826 m/s) and a thrust of 15,000 lbf (corresponding to a propellant consumption rate of 15,000 lbf/900 sec = 16.7 lb/s or 7.6 kg/s,) this engine will thus develop a jet power of:

Pjet=0.5*(7.6 kg/s)*(8826 m/s)^2

Pjet=296 MW. Now assuming a laser power to jet power conversion efficiency of 50% (which is probably unrealistically good) this leads us to a laser power of perhaps 600 MW. Now if the reactor can pump the laser with 25% efficiency (very good) then we will need 2400 MW of fission power. This is now a major reactor system!

Meanwhile an NTR system with 900 sec of specific impulse and a 15000 lbf thrust (as before) will only need perhaps Preactor = Pjet/0.7
Preactor = 425 MWthermal of reactor power, which is almost 1/6 of the power required by a the laser system all other things being equal.

Unfortunately entropy kills you: every time there is a power conversion step there is a loss. The most efficient (weight efficient here) engines will use direct conversion: since the mechanism used to accelerate the working fluid uses heat, the it is more efficient thermodynamically speaking to generate the thrust using the heat of the reactor directly, instead of trying for a more complicated power conversion step...

Where such a system may work better though is on the ground as a power station for a nuclear pumped laser launch facility. I suspect that such a setup may infact work well...



-- Edited by GoogleNaut at 00:01, 2007-07-30