The operating principle of the Nuclear Lightbulb is simple: a self-sustaining uranium plasma would be contained within a glass vessel; radiant energy would pass through the “bulb” and heat hydrogen gas surrounding it, which in turn would be contained within a metal chamber. The superheated gas would then pass through a nozzle and generate thrust in the normal fashion.
While simple to describe, almost nothin about this was simple to design. The engineering involved would have been monumental. And while the Nuclear Lightbulb is typically described in simple terms and illustrated with simple sketches, it turns out that a lot of work was done on this engine concept, (seemingly) mostly by the United Aircraft Corporation. The United Tech Research Lab produced a fairly detailed design for a reference engine, and did much more work – including many physical experiments – on the componant designs than is generally known. The reference engine had seven separate “bulbs,” each a cylinder 6 feet long by 2.3 feet in diameter. The engine operated at 500 atmospheres pressure.
First off, the uranium plasma. Generating the plasma would be fairly straightforward… simply get enough of the right fissile material into an enclosed volume of the right size, and nuclear chain reactions will do the job. In this case, a critical mass of 34.7 pounds of Uranium 233 spread between the seven chambers would cause the uranium to melt, vaporize and finally become a plasma.
Step one in the process would be to actually gather that much U233 within the chambers. Obviously it could not be stored as a solid block, but instead scattered and diffuse so that nuclear reactions would not begin until it was in the reactor. In order to accomplish this, three methods were proposed:
1) Store the uranium in the form of uranium hexafluoride (UF6). The UF6 gas would be simply pumped or injected into the reactors like any other gas. Storage of the UF6 was not described, but it probably would have involved very large tanks of very low pressure (and thus low density) filled with a neutron absorbing foam. As the UF6 began to gather under increasing pressure and density, nuclear reactions would begin to take hold and the temperature would increase. At a pressure of 200 psi, the UF6 would totaly dissociate by 13,000 degrees Rankine, allowing the fluorine to be drawn off from the Uranium. The problem, of course, is that the byproduct would be fluorine gas at about 13,000 degrees Rankine. Fluorine is trouble enough at room temperature. While the gas would be cooled prior to contact with any solid structural material, fluorine that can be described in any way as “hot” is a terrifying notion.
2) Inject molten uranium. This, however, would require some means to melt the uranium, as well as inject it. The melting temperature of uranium is 1403 degrees Kelvin; while this is by no means impossibly hot to work with (metals such as tungsten and many ceramics have melting points far higher), it would still be a complication. While the plan was that the molten uranium would be injected into the reactor in the form of an aerosol suspended within a high temperature carrier gas such as neon, it was expected that the uranium would plate on the mechanical portions of the injector system.
3) A third option was similar to the molten uranium aerosol, but with the temperature lowered so that the uranium was still a solid. Here the uranium would be divided into an extremely fine dust… pumpable, injectable, would not plate out onto the structural surfaces. As the dust begins to build up within the reactor, nuclear chain reactions would cause it to heat, eventually becoming a plasma. Once the full critical mass was injected into the reactor and the system reached equilibrium, the plasma would reach an average temperature of 42,000 degrees Rankine (23,333 Kelvin). This superheated plasma would glow fiercely, providing the radiant energy needed to superheat the hydrogen propellant. But there is no material known, certainly no transparent material, that can withstand anything remotely like the temperature of the uranium plasma. A solution to containment, however, was found.
To be continued…