Development of a Bi-modal Nuclear Propulsion System for Space Applications

Since the 1950s, nuclear propulsion has been considered a game-changing technology to complete humanity’s most ambitious missions in the solar system. Nuclear thermal propulsion allows heating, thanks to the energy released by nuclear reactions inside a reactor on board the spacecraft, a low molecular weight fluid up to 3000 K, obtaining propulsive performances that outclass current chemical propulsion systems; nuclear electric propulsion uses the nuclear reactor to generate electrical power used to power electric thrusters independently of the distance from the Sun, a constraint that instead limits the use of solar electric propulsion systems. Numerous projects on both NTP and NEP have been carried out in the past, producing a large amount of data on the use of different types of propellant, nuclear fuel, and power generation systems for various mission scenarios. The work carried out in this thesis starts from an analysis of past projects to identify the most promising configurations of nuclear thermal propulsion systems for the missions of interest in the next decade. NTP systems are characterized by large dimensions and masses; the first step of the propulsion system design consisted of the optimization of the propellant management system and the nuclear reactor to reduce the dimensions and make it possible to insert the system into Earth orbit with a single launch; to make single-launch to orbit possible, the most proposed propellant for the NTP, namely hydrogen, is set aside, preferring ammonia. Ammonia, although halving the specific impulse compared to hydrogen, has several advantages: it can be stored non-cryogenically, it allows self-pressurization, it has higher density than hydrogen, and it is present in the lunar regolith paving the way for In-Situ resource utilization. The continuation of the work focuses on improving the performance of the ammonia-fueled NTP. The main focus is on the study of a new nuclear reactor configuration that encourages the ammonia decomposition phenomenon thanks to which the specific impulse can grow up to 500 s; importance is also given to the propellant management system that implements a self-pressurizing configuration exploiting the energy lost by the reactor in the form of radiation emitted to pressurize the propellant. The mission analysis for the proposed innovative NTP system highlights how the Cislunar environment is the best for exploiting all the advantages provided by using ammonia as a propellant. The final part concerns the analysis for upgrading the NTP system to a bimodal system in which ammonia is used as the only working fluid in the system, both as a propellant for NTP, as a propellant for NEP, and as a circulating fluid in the electric power generation system. A preliminary reliability analysis identifies the most critical aspects of this bimodal configuration on which to focus subsequent efforts. A final chapter is dedicated to the additional activities carried out; in particular, the CHIPS project is presented, concerning the design, development, and testing of a chemical propulsion system for CubeSat, which allowed participation in an experimental campaign during the PhD and inspired part of the final design for the nuclear propulsion engine