Energy conversion options that are usually considered are heat engines (Rankine cycle, Brayton cycle and Magnetohydrodynamic – MHD) and direct conversion engine (the so-called Traveling Wave Direct Energy Convertor – TWDEC).
Low specific mass (less than 3 kg/kW) in-space electric power and propulsion can drastically alter the paradigm for exploration of the Solar System, changing human Mars exploration from a 3-year epic event to an annual expedition. A specific mass of ~1 kg/kW can enable 1-year round-trips to Mars, regardless of alignment, with the same launch mass to low Earth orbit (350 mT) estimated by the Mars Design Reference Architecture 5.0 study for a 3-year conjunction mission. Key to achieving such a propulsion capability is the ability to convert, at high efficiency and with only minimal losses rejected as heat via radiators, the energy of charged particle reaction products originating from an advanced fission or aneutronic fusion source directly into electricity conditioned as required to power an electric thruster. The TWDEC concept accomplishes this by converting particle
beam energy into radio frequency (RF) alternating current electrical power, such as can be used to heat the propellant in a plasma thruster.
The TWDEC project is core to the development of multi-MW power for electric propulsion. The technology developed will enable high power systems which have specific mass in the low single-digits and which are sun-independent, require no neutron shielding, and produce no radioactive waste.
There has been NASA studies like the Direct Energy Conversion for Nuclear Propulsion at Low Specific Mass Project in 2012-2014 at the Johnson Space Center
Thin core nuclear fission with Traveling wave direct energy conversion would be superior to Vapor core nuclear fission. Thin core nuclear fission or nuclear fusion systems if developed would enable annual round trip missions to Mars.
Vapor Core Nuclear Fission
Another fission concept that has received attention for space applications is the vapor core fission reactor. In this concept, fission fuel in the form of uranium tetrafluoride (UF4) gas is made critical by a shock wave, and the resulting hot (~2500 K) partially ionized gas is passed through a magnetohydrodynamic (MHD) power convertor. While the physics of such a reactor are understood, none has ever been made to go critical experimentally. The total of an EPP system based on vapor core technology has been estimated in published studies [Knight 2004]. Based on the Knight 2004 estimate, the power to weight ratio of a vapor core reactor is estimated to be as low as 0.04 kg/kWout.
“Thin” Core Nuclear Fission
Yet another fission concept to consider is known as “thin” core fission. In this long known concept, uranium fission fuel is deposited in a very thin layer (e.g., 6 x 10-3 g/cm2 [Safanov 1954]), allowing fission product fragments to fly free and be collimated through a direct energy conversion device, thus avoiding the Carnot limits of heat engine conversion. As with vapor core fission, the physics of such a reactor are understood, but none has ever been made to go critical experimentally. Analytical research continues [Slutz 2000]. The present analysis assumes power to weight of 0.15 kg/kWout. It is also important to note that cooling the magnets necessary to collimate fission product ions into a direct conversion subsystem, as well as cooling the core itself may take a substantial amount of power. Of the cited references only Safonov mentions the escaped or absorbed neutron flux associated with such a reactor and then only speculates that only a small fraction of the source neutrons would support fission. Thus, for conservatism, the present study conjectures that 10% of the reactor thermal output power would be required to run heat pumps for cooling the superconducting, collimating magnets and that neutron shielding requirements would be the same as for a solid core.
In traditional nuclear thermal rocket and related designs, the nuclear energy is generated in some form of reactor and used to heat a working fluid to generate thrust. This limits the designs to temperatures that allow the reactor to remain whole, although clever design can increase this critical temperature into the tens of thousands of degrees. A rocket engine’s efficiency is strongly related to the temperature of the exhausted working fluid, and in the case of the most advanced gas-core engines, it corresponds to a specific impulse of about 7000 s Isp.
The temperature of a conventional reactor design is the average temperature of the fuel, the vast majority of which is not reacting at any given instant. The atoms undergoing fission are at a temperature of millions of degrees, which is then spread out into the surrounding fuel, resulting in an overall temperature of a few thousand.
By physically arranging the fuel into very thin layers or particles, the fragments of a nuclear reaction, can boil off the surface. Since they will be ionized due to the high temperatures of the reaction, they can then be handled magnetically and channeled to produce thrust.
Rotating fuel reactor design by the Idaho National Engineering Laboratory and Lawrence Livermore National Laboratory uses fuel placed on the surface of a number of very thin carbon fibres, arranged radially in wheels. The wheels are normally sub-critical. Several such wheels were stacked on a common shaft to produce a single large cylinder. The entire cylinder was rotated so that some fibres were always in a reactor core where surrounding moderator made fibres go critical. The fission fragments at the surface of the fibers would break free and be channeled for thrust. The fibre then rotates out of the reaction zone, to cool, to avoid melting.
The efficiency of the system is surprising; specific impulses of greater than 100,000s are possible using existing materials. This is high performance, although not that which the technically daunting antimatter rocket could achieve, and the weight of the reactor core and other elements would make the overall performance of the fission-fragment system lower. Nonetheless, the system provides the sort of performance levels that would make an interstellar precursor mission possible.
A newer design proposal is the Dusty Plasma design by Rodney L. Clark and Robert B. Sheldon theoretically increases efficiency and decreases complexity of a fission fragment rocket at the same time over the rotating fibre wheel proposal. In their design, nanoparticles of fissionable fuel (or even fuel that will naturally radioactively decay) are kept in a vacuum chamber subject to an axial magnetic field (acting as a magnetic mirror) and an external electric field. As the nanoparticles ionize as fission occurs, the dust becomes suspended within the chamber. The incredibly high surface area of the particles makes radiative cooling simple. The axial magnetic field is too weak to affect the motions of the dust particles but strong enough to channel the fragments into a beam which can be decelerated for power, allowed to be emitted for thrust, or a combination of the two. With exhaust velocities of 3% – 5% the speed of light and efficiencies up to 90%, the rocket should be able to achieve over 1,000,000 seconds.
The Kilopower and Kilopower 2 concepts and other nuclear electric propulsion concepts are reviewed from material from Lee Mason at the Future In-Space Operations (FISO) Working Group.