**1. Introduction**

Nuclear thermal propulsion (NTP) is a technology that uses a nuclear reactor to provide the necessary energy to power a spacecraft for extraterrestrial operations [1]. At the most basic level, nuclear thermal propulsion is simply the use of nuclear fission to heat a gas to a high exit velocity. In this sense, it is very similar to a chemical rocket, in which the exothermic reaction of hydrogen and oxygen provides the energy used to heat the reaction product—gaseous H2O—to generate thrust. However, in an NTP engine, molecular hydrogen (H2) is used as the propellant. The H2 is used to remove heat from a reactor core by convection; the added energy provides a high speed exit velocity to generate thrust.

For an NTP engine using an H2 propellant, the engine is two to three times as efficient as an H2/O2-fueled rocket engine. Here, efficiency is measured in terms of *specific impulse* (*Isp*). The Isp is the amount of time (in seconds) that a rocket engine can generate thrust with a fixed Earth weight (mass *g*o) of propellant, when the weight is equal to the engine's thrust [2]. Here *g*<sup>o</sup> is the gravitational constant on Earth, about 9.81 m/s<sup>2</sup> , and relates mass to weight. For an H2/O2 engine, the *Isp* is around 450 s. For nuclear thermal propulsion with H2, the *Isp* is approximately 900 s [3]. Hence, the United States (U.S.) National Aeronautics and Space Administration (NASA) has had a long interest in use of NTP for propulsion, with recent interest in missions to Mars between 2030 and 2050 [4], and for cislunar operations, with a plan to demonstrate an NTP system above low Earth orbit (LEO) by 2025 [5].

This chapter is organized as follows. First, background will be provided on historical NTP work and current needs for operation—specifically, the functionality of an NTP engine. Next, we will detail key components of core physics design, focusing on the nuclear subsystem of the larger plant. We will briefly discuss the balance of plant as it relates to the nuclear subsystem, then conclude with a presentation of simulation results for a conceptual nuclear thermal propulsion system.
