Nuclear propulsion

For a breakdown of main torch specifications, see Main Propulsion. For RCS thruster specifications, see Reaction Control System.

Nuclear propulsion generally converts the heat energy of fission into kinetic energy by flowing propellant through a reactor core. The propellant expands and plasmafies, then can be further accelerated by other devices such as magnetoplasmadynamic accelerators before exiting the spacecraft through a thruster nozzle at high speed.

Counterintuitively, thrusters with high efficiencies and low propellant flow tend to use more reactor heat than less efficient thrusters. This is because heat usage is less dependent on propellant flow and moreso on exhaust velocity; efficient thrusters need to accelerate the propellant to higher speeds before it leaves the nozzle, so they draw more heat from the reactor to do so.

Nuclear Thermal Engines

Nuclear thermal engines directly turn fission's heat energy into kinetic energy by exhausting reactor-heated propellant out of a nozzle. They are the simplest kind of nuclear propulsion, and the cheapest to procure and maintain.

Nuclear thermal engines achieve better thrust per unit mass than other modes of nuclear propulsion, but are much less efficient on account of their low exhaust velocities.

Nuclear-Assisted Magnetoplasmadynamic Engines

Nuclear-assisted magnetoplasmadynamic (MPD) engines have one or more stages after the propellant exits the reactor. These stages use superconducting electromagnets to further excite and accelerate the plasma before it leaves the nozzle.

Engines utilising MPD technology offer good efficiency and thrust at the expense of electrical power, with the one exception being the BWM-T535 and its integrated turbine and generator, and thermal consumption.

The technology is extended by closed-cycle MPD electrical generators commonly used in colony infrastructure and large spacecraft.

Magnetohydrodynamic Engines

Nuclear-assisted magnetohydrodynamic (MHD) engines employ the same fundamental principles as their plasma-focused counterparts, but instead treat the plasma flowing through them as a unified body that is influenced by a moving magnetic field. Higher propellant flow, and thus greater thrust, can therefore be achieved with minimal efficiency losses compared to MPD engines.

Ion Thruster

Ion thrusters use copious amounts of electrical energy to strip the electrons from atoms, creating ions that can then be accelerated to remarkable exhaust velocities. Generally, they are either electrostatic, accelerating propellant through the Coulomb force along the direction of an electric field, or electromagnetic, accelerating propellant through the Lorentz force along the direction of a magnetic field.

Their high efficiency is offset by their low thrust, per-unit mass, and power consumption.

Fusion Engines

Fusion engines are a largely-theoretical type of engine that has only recently seen adoption with the introduction of Elon Interstellar's proprietary "ZAP" torch. By confining fusion inside some means of containment, propellant and fusion products can be accelerated to astronomical velocities with equal efficiency.

Fusion engines offer better efficiency than any other type of nuclear propulsion system, but are markedly fragile and require massive amounts of power to ignite.

Z-Pinch

The Z-Axial Pinch fusion torch works on the basis of Z-pinch inertial confinement fusion. Fusion fuel is held within a glob of plasma, which is then simultaneously imploded and inductively heated by a powerful magnetic field, triggering fusion and creating a long, narrow line of fusing plasma known as a filament.

Z-pinch fusion is notoriously prone to kink instability, where slight peturbations in the shape of the filament result in exponential destabilisation and cessation of the fusion reaction. This issue is resolved in the ZAP through use of sheared-flow stabilisation. Propellant plasma is introduced to the filament, creating rapidly-flowing annular layers that continuously maintain the filament's form.