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March 29, 2013

Thermonuclear Micro-Bomb Propulsion for Fast Interplanetary Missions by Friedwardt Winterberg

To reduce the radiation hazard for manned missions to Mars and beyond, a high specific impulse-high thrust system is needed, with a nuclear bomb propulsion system the preferred candidate. The propulsion with small fission bombs is excluded because the critical mass requirement leads to extravagant small fission burn up rates. This leaves open the propulsion with non-fission ignited thermonuclear micro-explosions, with a compact fusion micro-explosion igniter (driver), and no large radiator. It should not depend on the rare He3 isotope, and only require a small amount of tritium. This excludes lasers for ignition. With multi-mega-amperegigavolt proton beams and a small amount of tritium, cylindrical deuterium targets can be ignited. The proton beams are generated by discharging the entire spacecraft as a magnetically insulated gigavolt capacitor. To avoid a large radiator, needed to remove the heat from the absorption of the fast neutrons in the spacecraft, the micro-explosion is surrounded by a thick layer of liquid hydrogen, stopping the neutrons and heating the hydrogen to a temperature of ~ 100,000 Kelvin, which as a fully ionized plasma can be repelled from the spacecraft by a magnetic mirror.

This is a new nuclear fusion microbomb propulsion design for a spaceship to Mars that would travel at 200,000 miles per hour. This would enable a travel time of about 2 weeks.

Winterberg is well-respected for his work in the fields of nuclear fusion and plasma physics, and Edward Teller has been quoted as saying that he had "perhaps not received the attention he deserves" for his work on fusion. His thermonuclear microexplosion ignition concept was adopted by the British Interplanetary Society for their Project Daedalus Starship Study. His work was the theoretical basis of the global positioning system.




In DT fusion about 80% (in D fusion less) of the energy is released into neutrons, which cannot be deflected by a magnetic mirror, and still worse would heat the spacecraft, therefore requiring a large radiator. While this poses a problem for deep (interstellar) missions requiring the largest possible specific impulse, for less ambitious missions within the solar system, in particular missions to Mars, a nice solution exists for this problem: surrounding the neutron releasing micro-explosion with a sufficiently thick layer of liquid hydrogen, stopping in it the neutrons and heating it to high temperature to render it a fully ionized plasma which can be deflected by a magnetic mirror. For missions to Mars it has the additional bonus that it increases the thrust by lowering the exhaust velocity of the order 100 km/s., which optimal for this mission.

It is here proposed to place the thermonuclear target in the center of a liquid hydrogen sphere, with the target to be ignited by a GeV ion beam—passing through a pipe. To increase energy output, the hydrogen sphere can be surrounded by a shell made from a neutron absorbing boron. The energy released as energetic α-particles by the absorption of the neutrons in the boron not only increases the overall energy output, but also compresses the hydrogen sphere. Following the ignition and burn of the target, the hydrogen is converted into an expanding hot plasma fire ball used for propulsion.

For this idea to work, the radius of the liquid hydrogen sphere must be large enough to slow down and stop the neutrons, but not be larger than is required to keep its temperature at or above 100,000 K. This condition can be met for liquid hydrogen spheres of reasonable dimensions.

As in fission reactors, the neutron physics is determined by the slowing down and diffusion of the neutrons in the blanket. Assuming that the radius of the target is small compared to the outer radius of the neutron-absorbing blanket, one can approximate the neutron source of the burning target as a point source.

To charge the spacecraft to the required gigavolt potentials we choose for its architecture a large, but hollow cylinder, which at the same time serves to act as a large magnetic field coil. If on the inside of this coil thermionic electron emitters are placed, and if the magnetic field of the coil rises in time, Maxwell’s equation induces inside the coil an azimuthal electric field.

The way propulsion is achieved is straightforward and explained in the first figure in this article. The minifusion bomb F is catapulted into the focus of a parabolic magnetic reflector R, made from steel and placed inside a large magnetic field coil C.

The wall of the magnetic mirror is insulated against the hot plasma of the expanding fire ball by the thermomagnetic Nernst effect. It generates currents in the boundary layer between the cool wall and the hot plasma of the fire ball, producing a magnetic field between the wall and the fire ball just sufficiently strong to repel the fire ball from the wall.

At a temperature of T ≈ 100,000 K the exhaust velocity is about equal to the thermal expansion velocity of a hydrogen plasma which at this temperature is 30km/s, with higher exhaust velocities up to 100 km/s reached with a temperature 10 times higher.

Using the entire space craft as a large, magnetically insulated capacitor, inductively charged up to gigavolt potentials, leads to an electron cloud in the vacuum surrounding the spacecraft. For the injection of the GeV proton beam into the pipe reaching the DT target as shown, the liquid hydrogen sphere must be grounded against the electron cloud surrounding the spacecraft. This can be done by a small plasma jet emitted from the surface of the hydrogen sphere as shown below. The jet can be produced by a laser beam heating the material making up the jet.



For the recharging of the entire space craft to gigavolt potentials by inductive charge injection, the electromagnetic pulse induced in the induction loop surrounding the rocket propulsion chamber is used to radially inject electrons at the top of the spacecraft into the rising magnetic field of an auxiliary coil. Apart from a different design of the rocket propulsion chamber, this is the same kind of inductive charge injection proposed for pure deuterium bomb
propulsion.

Because it uses the entire spacecraft as a gigavolt capacitor to be discharged into an intense ion beam for ignition, the proposed concept is expected to be superior to all other designs intended to reach Mars in the shortest time possible, because all the other designs depend on a massive radiator. To reduce the mass of the radiator, “droplet” radiators have been proposed, but they suffer from the loss of droplet mass by evaporation. This not only applies to the VISTA and ICAN concept, but also to various electric propulsion concepts. None of them can compete with the compact gigavolt capacitor ignition concept ideally suited for the high vacuum of space

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