Gentlefolk, I'm afraid I've been kicking around the advanced propulsion community too long and assume everyone's heard of everything, sorry. The gas core nuclear concept was essentially a radially symmetric drum. A uranium-bearing gas (Uranium hexafluoride, in my memory, but perhaps Henry has better data...) is kept in place near the inside rim of the drum by centrifugal force while much less massive propellant molecules go down the center and are heated to near plasma temperatures by linearly focussed radiation of all kinds. One is moved to say but, but, but... however a lot of calculations were done that indicated it would work and produce high Isp (10's of km/s) and high thrust with respect to similarly high Isp concepts (such as ion engines). With everything working correctly, it might not even have been too dangerous, but my feeling was that just one little thing going wrong and you got a real mess. Start up and shut down are issues, too.
Nerva was a "solid core" design, with hydrogen flowing through an otherwise fairly conventional reactor in tubes. There are also somewhat lighter, more efficient solid core designs using 'microchannels' to produce laminar flow and higher heat transfer efficiency through the core. Timberwind (during SDI) was a particle bed design that eliminates fuel rods. The hydrogen propellant would flow through directly through a porous bed of fuel particles coated with a refractory ceramic. This design was almost as light weight as a conventional rocket engine, and was expected to produce very high thrust at somewhat higher temperatures (and thus exhaust velocities) than conventional solid core designs. The main (real, technical) problem for any nuclear Earth to Orbit crew vehicle concept is neutron backscatter. Neutrons leak out in the exhaust, bounce off air molecules, and back into the cabin. This is what grounded the long duration nuclear aircraft study--crew shielding got just too heavy. There are a hybrid chemical/nuclear designs that take off as LOx/H2 rockets and switch to the nuclear mode 30 km up or so. That's an elegant solution that yields variable Isp benefits, and allows a smaller reactor as well as taking care of the backscatter problem. If memory serves, the original Disney moon rocket was based on such a design. On variable Isp, I mentioned on this list earlier that Goddard patented a rocket turbine device to provide more thrust and efficiency during lift off. One could achieve somewhat the same effect by varying propellants on the way up (never mind the complexity for the moment). Start out burning H202/hydrocarbon, switch to O2/hydrocarbon, then finally to O2/H2. A single engine that could handle all three propellant combinations would be an interesting project. --Best, Gerald _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
