Analysis of a Radioisotope Thermal Rocket Engine

Abstract

The Triton Hopper is a concept for a vehicle to explore the surface of Neptune's moon Triton, which uses a radioisotope heated rocket engine and in-situ propellant acquisition. The initial Triton Hopper conceptual design stores pressurized Nitrogen in a spherical tank to be used as the propellant. The aim of the research was to investigate the benefits of storing propellant at ambient temperature and heating it through a thermal block during engine operation, as opposed to storing gas at a high temperature. Lithium, lithium fluoride and beryllium were considered as possible materials for the thermal block. A heat energy analysis indicated that a lithium thermal mass would provide the highest heat energy for a temperature change from 900⁰C to -100⁰C. A heat transfer analysis was performed for Nitrogen at -100⁰C flowing through 1000 passages inside a 1 kg lithium thermal block at a temperature of 900⁰C. The system was analyzed as turbulent flow through a tube with constant surface temperature. The analysis indicated that the propellant reached a maximum temperature of 877⁰C before entering the nozzle. At this exit temperature, the average specific impulse (Isp) of the engine was determined to be 157 s. Previous studies for the stored heated gas concept suggest that the engine would have an average Isp of approximately 52 s. Thus, the use of a thermal block concept results in a 200% engine performance increase. A tank sizing study was performed to determine if the concept is feasible in terms of mass requirements. The mass for a spherical carbon-fiber pressure vessel storing 35 kg of nitrogen at an initial temperature of -100⁰C and a pressure of 1000 psia, was determined to be 7.2 kg. The specific impulse analysis indicated that the maximum engine performance is obtained for a mass ratio of 5 kg of Nitrogen for every kilogram of lithium thermal mass. Thus, for 35 kg of Nitrogen the required thermal mass would be 7 kg. This brings the total mass of the system to 49.2 kg, which is less than the 56 kg landing payload capacity of the Triton Hopper. Finally, an insulation analysis using 10 mm of Multi-layer insulation indicated that a total of 22 watts of heat are lost to the environment. With the heat loss known, the power required to heat the thermal mass to 900⁰C in 24 days was determined to be 2.15 Watts. The study's results allowed us to conclude that the thermal mass concept is a better option for the Triton hopper propulsion, due to the performance increase provided, the low power requirement and its compliance with the landing mass requirement of the Triton Hopper.

Hopper Missions to Triton and Pluto using a Vehicle with In-Situ Refueling

“…The GPHS produces heat at a nominal temperature of up to 1200°C. This energy is then transferred to a thermal mass, which is used to heat the propellant Nitrogen is passed through the heated thermal mass and then expanded through a rocket nozzle to produce the thrust needed for the hop [7]. A number of different possible materials were possible for the thermal mass; for the baseline case we assumed molten lithium, due to its high heat capacity and thermal conductivity, but several other choices (e.g., phase-change salts) also achieve similar performance.…”