Manned Spacecraft Propulsion through Direct Conversion of Nuclear Energy

Fission-based power sources have been used in space as early as the Voyager missions. However, the use of nuclear power for manned space exploration has yet to be accomplished. Reactor-based (thermal or electric) propulsion efforts have been stymied by enormous launch costs and extremely long mission times, which endanger the health of the astronauts, and increase the risk of failure due to the need for complex space rendezvous and assembly. All of this is avoided with a compact, high efficiency, single-launch spacecraft such as that projected for the Fission-Powered Pulsed Plasma Propulsion concept (F4P) proposed here. In the F4P scheme, the immense energy of the nuclear reaction is to be harnessed in a more direct and powerful way to propel the spacecraft with the optimal range of jet power (30–100 MW), and at the most efficacious exhaust velocity (30–50 km/sec). It is accomplished in the following manner: (1) Large, thin hoops of Lithium (Li) metal are driven at the proper angle and speed for convergence onto fissile target injected from the F4P thruster throat. (2) The converging Li shells form a thick blanket compressing the fissile material thereby producing a large increase in reactivity and fission energy release. (3) Virtually all of the energy released (plasma thermal, radiant, shell-ohmic, and fission-fragment) is absorbed by the Lithium shell which is subsequently vaporized and ionized. This Li plasma plume expands out against the thruster's divergent magnetic field which acts as a barrier and directs the plasma flow out of the thruster resulting in minimal interaction with the spacecraft. In this manner, both high exhaust velocity and thrust are achieved with only minor interaction with the spacecraft. To understand the issues involved with this approach, as well as design an experimental test, a 1D analytical model was developed and 3D ANSYS Explicit Dynamics® of the liner implosion were carried out. The inductive coupling and drive efficiency of the compression for a single liner was evaluated with the one-dimensional model which included the key circuit, magnetic field and liner parameters. The model was then used to characterize the liner motion as a function of liner mass, resistivity, stored energy, coil voltage and initial magnetic field. The stability and structural behavior of multiple liners was studied with the 3D ANSYS code. Techniques for controlling liner bucking as well as liner rotation for Rayleigh-Taylor stability were examined with these codes Experimental testing at the relevant scale have validated the metal hoop compression process. The major unknown to be resolved is the functional dependence of the fission energy yield on the several key parameters of the compression process. This must be fully understood and tested to prove the viability of the F4P concept. The current effort is focused on (1) a detailed physics model of the heating and compression of the fissile fuel from a solid to an ionized gas. (2) Calculations to determine the fission amplification and energy yield throughout the compression cycle as a function of fissile mass and density.