My name is Pavit Hooda, and I was an intern at the Princeton Plasma Physics Laboratory during the summer of 2022. In my time there, I took on the start-up problem of the Direct Fusion Drive (DFD) and developed a compelling solution. A system to power on or re-start the DFD in space is essential for its use, especially in long-duration missions. Therefore, my work has helped us get closer to a space-faring future where the DFD is the means of propulsion for humanity’s missions to the Moon, Mars, and beyond.
The problem at hand was to create an auxiliary power unit that can generate a sufficient amount of power with the use of the Deuterium fuel and liquid Oxygen oxidizer that were on board. The Deuterium is one of the fuels of the fusion within the DFD, and the Oxygen can be recycled from the cabin of the crew. After the power is generated, the objective is to eventually split the deuterium-oxide product back into its constituents for use in their respective areas of the spacecraft. This electrolysis can be done after the fusion core is started and there is a sufficient amount of surplus energy from the DFDs.
The design of the heat engine first begins with the electric pumps that feed the fuel and the oxidizer into the combustion chamber. A turbopump-based feeding system was decided against due to the low mass flow rates that are required to power the DFD. Additionally, the accurate throttle control granted by the use of electric pumps, and the ability to use the batteries on board to spin the pumps, make electric pumps the more viable option. Before the deuterium fuel is fed into the coaxial swirl injector, it is ran across cooling channels surrounding the combustion chamber. This regenerative cooling is performed to heat the deuterium to increase its reactivity and lengthen the lifespan of the combustion chamber by minimizing the effect of the high temperature it is operating at. Additionally, the cooling system provides a healthy temperature gradient for the thermoelectric generation layer that is also wrapped around the combustion chamber. The oxidizer is directly injected into the combustion from its propellant tank.
After passing through the injector and combusting in a successful ignition, the deuterium-oxide steam exhaust is directed towards a turbine system. The turbine system and the combustion chamber are attached with a flange. The turbine system consists of two sets of blades that are separated by a disk that acts like a stator in a steam turbine. The exhaust is first directed towards a doughnut-shaped casing that allows for the heavy water steam to hit the blades in a direction that is parallel to the blade disk’s central normal axis. The two turbine disks are attached to a common axis that extends outside the turbine system’s casing. The rotation of this axle is then used to generate power with an electric generator. Finally, the steam then exits through a large exhaust manifold tube that directs it to a temporary storage container. This design of a heat engine would result in producing 3 MJ, the sufficient amount of power to start up a PFRC, in about 10 minutes. An illustration of the entire design of this system can be seen below.
In the pursuit to study the feasibility of this engine, various parts were selected. A 600 W electric generator that matches both the power and mass specifications of the heat engine was found and is shown below.
Additionally, the turbine casing in the heat engine matches the geometry and function of a turbocharger that is found as a component in some car engines. The part is displayed below.
A significant amount of extensive work still needs to be put into the creation of this heat engine. However, I truly believe that this work presents itself as a good first step in the right direction towards this engine’s small but significant role in humanity’s journey to the Moon, Mars, and beyond.