**4. Space nuclear propulsion**

Space nuclear propulsion refers to using nuclear fission reactors or radioisotope decay to provide power for spacecraft propulsion and other space missions. Unlike conventional chemical propulsion systems, nuclear propulsion systems provide a much higher energy density, allowing faster, more efficient travel through space. There are two main types of space nuclear propulsion: nuclear thermal propulsion (NTP) and nuclear electric propulsion (NEP).

In NTP, a nuclear reactor is used to heat a propellant, such as hydrogen, to high temperatures, creating a high-speed exhaust that provides thrust. NTP offers higher thrust levels and specific impulses (a measure of propulsion efficiency) than conventional chemical propulsion systems. Although operational lifetime requirements are typically low (a few hours), the need for very high temperature (>2500 K) fuel and very high power density (>1 MW/L) for operational systems add design complexity.

On the other hand, NEP uses the electricity generated by a nuclear reactor or RTG to power an electric thruster (e.g., ion, Hall, MPD, VASIMR), which uses charged particles to produce thrust. NEP systems produce much lower thrust than NTP systems but are very efficient and well-suited for certain missions. Early NEP systems in the 10 kWe range may be well suited for deep space science missions, and advanced NEP systems in the 10 MWe range may enable fast (< 1 year) round-trip human Mars missions. NEP is less complex than NTP and offers high specific impulses but lower thrust levels. A primary challenge for NEP systems is ensuring that the specific mass (mass of integrated power/propulsion system divided by power into the propellant) is sufficiently low, typically <50 kg/kW, to enable deep space science missions and < 6 kg/kW to enable very fast human Mars missions.

Space nuclear propulsion has been used in a limited number of missions, including the Mars rovers, which relied on RTGs for power to turn the drive wheels. However, nuclear propulsion systems in space are still primarily limited due to technical and regulatory challenges. As of 2023, a multiagency program is in place to demonstrate a nuclear thermal propulsion system in space.

### **4.1 Differences between chemical and nuclear thermal rocket propulsion**

Chemical rocket propulsion and nuclear thermal propulsion are two types of rocket propulsion systems that differ in how they generate the necessary energy to produce thrust.

Chemical rocket propulsion involves chemical reactions between fuels and oxidizers to generate hot gases that are expelled through a nozzle to produce thrust. Two commonly used propellants in chemical rocket propulsion are liquid hydrogen and liquid oxygen, which produce high-speed exhaust gases with a high specific impulse

(a measure of propellant efficiency). The mathematical formula for the rocket equation, which describes the change in velocity of a rocket, is given by:

$$
\Delta \nu = \nu\_\epsilon L n \left(\frac{m\_0}{m\_f}\right),
\tag{2}
$$

where *Δv* is the change in velocity, *ve* is the exhaust velocity, *m*<sup>0</sup> is the initial mass of the rocket, and *mf* is the final mass of the rocket.

On the other hand, nuclear thermal propulsion involves using a nuclear reactor to heat a propellant, which is then expelled through a nozzle to produce thrust. The main advantage of nuclear thermal propulsion over chemical rocket propulsion is its much higher specific impulse (a measure of propellant efficiency), which allows for much higher exhaust velocities and, thus, a much greater *Δv*. The mathematical formula for the specific impulse of a nuclear thermal rocket is given by:

$$I\_{sp} = \frac{v\_{\varepsilon}}{\mathcal{g}}\,, \tag{3}$$

where *Isp* is the specific impulse *ve* is the exhaust velocity, and *g* is the acceleration due to gravity.

Nuclear thermal propulsion systems have the potential to be much more efficient than chemical rocket propulsion systems, but they also come with several challenges and limitations. For example, nuclear thermal propulsion systems require a reliable, safe, and efficient method of cooling the nuclear reactor and a means of containing and controlling the radioactive materials involved. Additionally, significant technical and regulatory challenges are associated with developing and operating nuclear thermal propulsion systems.

Chemical rocket propulsion and nuclear thermal propulsion are two different types of rocket propulsion systems that differ in how they generate the necessary energy to produce thrust. Chemical rocket propulsion relies on chemical reactions between fuels and oxidizers, while nuclear thermal propulsion relies on the heat generated by a nuclear reactor. While nuclear thermal propulsion has the potential to be much more efficient than chemical rocket propulsion, it also comes with several technical and regulatory challenges that must be overcome.

Typical values for the specific impulse of chemical propulsion rockets for Mars missions is �400 s, whereas the tested nuclear thermal propulsion rockets are in the 850 s range. Solid-fuel nuclear thermal propulsion rockets may be able to achieve 900 s *Isp* (with hydrogen), and liquid-fuel nuclear thermal propulsion rockets may be able to achieve 1800 s *Isp* with hydrogen and 1000 s *Isp* with methane [4].
