Consider two hypothetical spacecraft. The Orion vehicle would have worked by setting off low-yield nuclear devices behind a massive pusher plate, driving forward a payload attached at a safe distance from the pusher (and protected by mind-boggling shock absorbers). Even if we had the nuclear devices at our disposal, agreed to use them for such a purpose, and found the political will to construct an Orion craft for deep space exploration, a problem still remains: most of the energy from the nuclear blasts is dissipated into space, and the craft thus requires a huge critical mass of fission explosives.
Orion, in short, is not efficient in using its energies. Now consider Project Daedalus, the hypothetical mission to Barnard’s Star designed by members of the British Interplanetary Society back in the 1970s. Daedalus was designed to use fusion microexplosions instead of fission. One of the reasons the Daedalus craft demanded as much fuel as it did is that the ignition apparatus, whether laser or particle beam, to ignite fusion is massive, adding unwanted heft to the vehicle. Daedalus would have massed an overwhelming 54,000 tons.
But there is a third option, discussed by Friedwardt Winterberg (University of Nevada/Reno) in a new paper in Acta Astronautica. Winterberg describes so-called ‘mini-nukes,’ which are devices in which the mass of the fission explosive is hugely reduced by a reflector in which a deuterium-tritium (DT) fusion reaction takes place — the fusion neutrons thereby increase the fission rate, and the increased fission rate, in turn, increases the fusion production rate. The implosive compression to ignite the process is provided by chemical high explosives.
We wind up with a fission-fusion assembly with serious advantages: 1) we eliminate the weight of a laser or particle beam generator to ignite fusion, and 2) we add to the specific impulse and thrust of the exhaust, which is composed of combustion products both from the nuclear and chemical reactions.
And no longer do we, Orion-style, lose the bulk of our energies into space. “With mini-nukes,” Winterberg writes, “the situation is much better because the explosions can take place there in the focus of a parabolic reflector positioned in close proximity to the spacecraft. Since still smaller mini-nukes appear possible, with their feasibility depending only on the technical perfection to focus the chemical energy for ignition, the mini-nukes can be detonated inside a large combustion chamber…”
The paper is Winterberg, F., “Mini-fission-fusion explosive devices (mini-nukes) for nuclear pulse propulsion,” Acta Astronautica Vol. 57 (2005), pp. 707-712. Some of Winterberg’s earlier work was studied, incidentally, by the Daedalus designers as they set about examining various ways of making an inertial confinement fusion system that could power their starprobe.
And incidentally, check this Fusion News Update at the Star Spangled Cosmos site for news of a possible development in inertial confinement fusion in South Korea.