Since the NERVA project, hydrogen has been the most proposed propellant for nuclear propulsion systems thanks to its overwhelming performance. However, its low density and the need to store it in cryogenic conditions, combined with the long duration of the foreseen interplanetary missions, led to the design of propulsion systems with enormous envelopes and the introduction of complex systems for propellant management. On the other hand, ammonia presents a clear loss of performance compared to hydrogen due to its higher molecular weight, limiting the spectrum of missions made accessible by the use of this propellant in a nuclear thermal propulsion system. However, the greater density of ammonia and its high vapor pressure at around 373 K allow for much more compact and less complex propellant management system configurations. This work proposes a new propellant management configuration for an ammonia-fueled nuclear thermal propulsion system for a class of missions involving cargo displacements between LEO and the lunar orbits of interest for future space programs. The suggested configuration maximizes the advantage deriving from the self-pressurization of ammonia by exploiting the thermal power lost by the nuclear reactor towards the vacuum space due to the escaping particles. In this layout a tank containing ammonia in saturated conditions is placed near the nuclear reactor and receives an input thermal power proportional to the dose of gamma rays and neutrons absorbed by the ammonia and the tank walls. This thermal power accelerates the vaporization process of the saturated ammonia, thus increasing the pressure in the tank. A pressure regulator valve exploits this overpressure to pressurize the ammonia propellant contained in a run tank to the level required by the mission by connecting the two ammonia volumes. The pressure achieved inside the run tank pushes the propellant with an adequate mass flow rate inside the nuclear reactor. The Homogeneous Equilibrium Model and the Two-Temperature Model describe the coupled dynamics of these two tanks, while a study of neutron and photon transport performed with the Monte Carlo code OpenMC provides the thermal power input received from the pressurizing ammonia tank. The developed analysis shows how this propellant management system can provide a constant mass flow to the nuclear reactor without using a turbopump assembly. This capacity depends on the relative distance between the tank and the nuclear fission reactor. Another advantage of the proposed concept concerns the reduction of the mass of the radiation shield: it does not have to protect the tanks from nuclear radiation, as happens for hydrogen-based projects, and therefore, its dimensions are governed exclusively by the radial size of the payload. For some missions, this equates to having a smaller shield size and, consequently, a lower mass.
Analysis of an Autogenous Propellant Pressurization System for Nuclear Thermal Rocket
Puccinelli E.;Giusti V.;Pasini A.
2024-01-01
Abstract
Since the NERVA project, hydrogen has been the most proposed propellant for nuclear propulsion systems thanks to its overwhelming performance. However, its low density and the need to store it in cryogenic conditions, combined with the long duration of the foreseen interplanetary missions, led to the design of propulsion systems with enormous envelopes and the introduction of complex systems for propellant management. On the other hand, ammonia presents a clear loss of performance compared to hydrogen due to its higher molecular weight, limiting the spectrum of missions made accessible by the use of this propellant in a nuclear thermal propulsion system. However, the greater density of ammonia and its high vapor pressure at around 373 K allow for much more compact and less complex propellant management system configurations. This work proposes a new propellant management configuration for an ammonia-fueled nuclear thermal propulsion system for a class of missions involving cargo displacements between LEO and the lunar orbits of interest for future space programs. The suggested configuration maximizes the advantage deriving from the self-pressurization of ammonia by exploiting the thermal power lost by the nuclear reactor towards the vacuum space due to the escaping particles. In this layout a tank containing ammonia in saturated conditions is placed near the nuclear reactor and receives an input thermal power proportional to the dose of gamma rays and neutrons absorbed by the ammonia and the tank walls. This thermal power accelerates the vaporization process of the saturated ammonia, thus increasing the pressure in the tank. A pressure regulator valve exploits this overpressure to pressurize the ammonia propellant contained in a run tank to the level required by the mission by connecting the two ammonia volumes. The pressure achieved inside the run tank pushes the propellant with an adequate mass flow rate inside the nuclear reactor. The Homogeneous Equilibrium Model and the Two-Temperature Model describe the coupled dynamics of these two tanks, while a study of neutron and photon transport performed with the Monte Carlo code OpenMC provides the thermal power input received from the pressurizing ammonia tank. The developed analysis shows how this propellant management system can provide a constant mass flow to the nuclear reactor without using a turbopump assembly. This capacity depends on the relative distance between the tank and the nuclear fission reactor. Another advantage of the proposed concept concerns the reduction of the mass of the radiation shield: it does not have to protect the tanks from nuclear radiation, as happens for hydrogen-based projects, and therefore, its dimensions are governed exclusively by the radial size of the payload. For some missions, this equates to having a smaller shield size and, consequently, a lower mass.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
