Path to Megawatt Direct Drive Fusion Propulsion Has No Scientific Roadblocks

A Masters Thesis analyzed a mission to Saturn’s moon Titan using the Princeton Plasma Physics Laboratory Direct Drive Fusion system. The Direct Fusion Drive (DFD) is based on a D-3He fueled, aneutronic, thermonuclear fusion propulsion system.

He considered a 2-MW-class single DFD module, which provides 8 N of constant thrust and a specific impulse of 10,000 seconds. Two different profile missions have been considered: the first one is a thrust-coast-thrust profile with constant thrust and specific impulse and the second is a continuous and constant thrust profile, with a switch in thrust direction operated in the last phases of the mission. Each mission is divided into four different phases, starting from the initial Low Earth Orbit departure, the interplanetary trajectory, Saturn orbit insertion and the Titan orbit insertion. For all mission phases, maneuver time and propellant consumption have been calculated. The results of calculations and mission analysis offer a complete overview of the advantages in terms of payload mass and travel time. The first scenario analyzed is the thrust-coast thrust profile mission which is based on the assumption that the DFD is capable to turn off and on the thrust generation, though without restart the engine.

The paper is Trajectory design for a Titan mission using the Direct Fusion Drive by Marco Gajeri, July 2020.

Princeton Satellite has had various NASA, SBIR and IR&D grants to develop a multi- megawatt-class nuclear fusion propulsion and space power system. This funding has enabled them to precisely simulate their designs and performed experiments. They have stated they would need $100 million and five years to actually make a full megawatt propulsion system. They have not received the level of funding needed to proceed with the main development. Studies of electron heating in PFRC-2 surpassed theoretical predictions and recently reaching 500 eV with pulse lengths of 300 ms, and experiments to measure ion heating with input power up to 200 kW are ongoing. When scaled up to achieve fusion parameters, PFRC would result in a 4-8 m long, 1.5 m diameter reactor producing 1 to 10 MW. DFD uses an innovative radiofrequency (RF) plasma heating system.

Several previous attempts to heat FRC plasmas with RF only reached near-FRC plasma with “open” field lines. The “open” field lines let the plasma to escape.

The PFRC exploits a rotating magnetic field (RMFo) with odd-parity symmetry, produced by the oscillation of the current in four quadrature-phased radio-frequency (RF) antenna. The magnetic field on one side of each figure-8 is in the opposite direction as the other side and closed field lines in the generated FRC. The closed field lines keep the plasma trapped when it is heated.

Current is generated and conditions heat the plasma ions and electrons. These enable compact devices and excellent stability due to the fact that a small, high-temperature FRC plasma, it is said to be kinetic rather than fluid-like and is stable against the tilt mode.

In an FRC reactor, current-carrying electrons will have very high peak energy, about 5 times greater than in D-T tokamak fusion reactors.

The RMFo method improves energy confinement, current drive, plasma heating, and plasma stability.

The simple geometry of the machine, low radiation, and moderate magnetic field strength all contribute to lowering development and maintenance costs. There are no hazardous fuels or materials required. The DFD has been designed to be safe and affordable.

A conservative value for the specific power was chosen for the Masters thesis study. 0.75 kW/kg results in an engine mass of about 2660 kg, which is the estimated mass used for the calculations. A Brayton cycle was chosen for electric power generation.

Current space-qualified radiators will be too heavy but there are upcoming radiator materials that will make the radiators mass a small fraction
of the engine total. NASA is currently supporting research on carbon-carbon radiators. The goal is to reduce the areal mass of radiators from about 10 kg/m2 currently to 2 kg/m2 or less and an average temperature of 625 K. Other essential topics are the superconducting coils and the shielding design. Available data suggests that research in all these areas has made tremendous progress and no roadblocks have been identified.

The analysis shows that the direct drive fusion mission could bring large payloads to Titan in less than two years.

SOURCES – Princeton Satellite, CUNY & Torino Masters Paper – Marco Gajeri
Written By Brian Wang,