A while back, I read Anthony Tate's article Opening the Next Frontier dealing with a Gas Core Light Bulb SSTO. I was impressed by the potential performance that an GCLB SSTO could bring; however there's just one thing I need some clarification on. Most of the literature on the subject indicated that the Hydrogen propellant needed to be "seeded" with tungsten or some other material in order to make the propellant opaque, thus better absorb the heat from the fused silica core. The engine described in Mr. Tate's article made no mention of this; Is there a way around this problem?
The nuclear lightbulb reactor uses uranium or uranium tetraflouride gas as the fuel carrier--in the former, just uranium, the gas will be extremely hot--perhaps as hot as 50,000K (120,000 degrees Fahrenheit) so it will actually be a fissioning plasma...
If one looks at a hot, dense gas at such high temperature, it will behave somewhat like a Blackbody radiator--so at 50,000K most of electromagnetic radiation will be emitted as ultraviolet radiation and not as 'heat.' Hydrogen gas is mostly transparent to this radiation, so you have to use something to intercept it: carbon granules, or tungsten can work--tungsten is most often spoken of because it is practically inert to hydrogen, and since it is so much denser than hydrogen, it can be effectively seperated from the gas stream by cyclonic action...
The trouble I personally have with the idea, is that a fissioning plasma will also emit a heck of a lot of fast neutrons which will smack into the UV transparent quartz window--the energy deposited in the window alone from the neutron flux will be enough to damage the window, let alone the accumulated radiation damage will cause the window to quickly opaque because of the formation of free silicon (when oxygens are disslocated out of latice cites by energetic bombardment with gamma-rays and fast neutrons...) The result is that in a very short time, the window will cease to function...and the engine will too...
It seems to me that if have to go to the trouble of mixing a heavy metal seed with your hydrogen working fluid, and use a cyclonic seperator to seperate the reaction mass from your energy 'surrogate' material anyways, then why not just mix the reaction mass with the fuel (the uranium gas) and use the seperator to get the very hot reaction mass out again? If done right, the cyclonic seperator could also seperate out the lighter fission products too, so the engine would naturally 'clean' its own fuel supply as well...however, since it is really a partially open cycle the engine would have to be carefully tuned to avoid excessive loss of fissile fuels...and of course this also means that the engine could only be used in space...
It is my suspicion that a vapor core reactor using UF4 gas is probably a bit more practical--and with a lower total reactor power, would do nicely for powering something like a VASIMR array...
Interestingly enough, there is another proposal that is being slowly worked on by Andrews Aerospace of a concept called MiniMag Orion, which uses electromagnetic forces in a Zeta pinch arrangement to implode tiny spheres of fissile materials such as Curium-244--the tiny nuclear explosions create plasmas which are directed by a magnetic nozzle...Theoretically thrusts in the hundreds of Kilo Newtons, to low Mega Newtons could be produced with Isps of thousands of seconds...power levels in the hundreds of Gigawatts may be possible...it looks like an exciting future propulsion technology, and I keep an eye on it from time to time...
First, thank you for responding to my post. I found it very helpful, however I would like to know what kind of performance to expect from an GCLB engine if " seeding" of the hydrogen was NOT done.
That's hard to say--without building one and trying it out, the next best thing would be to create a mathematical model of it and run a simulation: a full up, thermohydraulic simulation is no trivial feat in of itself.
I know that trying to couple electromagnetic radiation to hydrogen is tricky: hydrogen is naturally very transparent to electromagnetic radiation at nearly all wavelengths. It is however a very efficiently heated by conduction or convection, which is why a 'seed' particle is introduced: the tungsten dust intercepts the ultraviolet and gets very hot: the hydrogen gas then convects this heat away from the seed particles, and itself gets hot, so its a two step process.
How efficient is energy transport without the seeds? I don't know. I do know that in conventional solid-state Nuclear Thermal Reactors, the Isp generated from hydrogen gas is approximately in the 800 to 960 seconds "neighborhood", so if I were to guess about nuclear lightbulb reactor's performance I would use those numbers as a low end: high end probably no more than 1200 seconds or so.
Years ago (almost 20...?) I messed around with an idea for a gas core fission reactor that used direct contact with hydrogen and fissioning fuel to heat the reaction mass. I came to the conclusion that hydrogen at thermal equilibrium with the plasma temperature of around 250,000 Kelvin, the hydrogen plasma exhaust ought to have a specific Impulse of around 5,000 seconds...However, the degree of ionization of the uranium plasma at that temperature (somewhere around +2 or +3) would radiate more power than the reactor could produce...so the plasma would immediately quench and cool. So the conclusion I came to then was that it was not possible to operate a gas core reactor at those temperature levels...i.e., 5000 seconds of Isp was out of the question. Maybe 2000 seconds, but not 5000.
Once again, thank you for responding to my post. I found it very helpful. Since they, I've been busy reading up on this subject and come accross a few wrinkles that may or may not allow Anthony Tate's Gas Core Light Bulb engine to work without seeded hydrogen. The only person that can settle this once and for all is Anthony Tate himself; that's if I can get a hold of him. Any help you, Bruce Behrhorst or Terry McNeeley can provide would be greatly apreciated.
I know that Anthony Tate was once an active member of the board, but I don't know if he's still active. However, having said that, I did a little digging and found a plethora of technical information on the NTRS (NASA Technical Reports Server) dating from the late 1960's and early 1970's.
And you should find 16 records: these are technical papers published by NASA at the time of Project Apollo. If there is interest, we may be able to upload these to our sites permanent technical paper library, but copyright permissions must be obtained for us to post them. However, they should be free for you and or anyone else following that link--each paper will indicate whether or not it is available for free online, or wether it may have te be purchased. Most are available free for anyone to use.
It has been my personal experience that the NTRS is a wonderful resource provided by NASA and is a very cost-effective way to obtain detailed information in doing initial research for a particilar topic.
