The fusion torch engine
The invention of the fusion torch engine in the late 20th century was perhaps the most important in human history: in 2093, the first fusion torch spacecraft, the Prometheus, flew to Mars in twenty-five days. In the thirty years since, the fusion torch engine has engendered the formation of the World Union for Space Policy, put the resources of the entire Inner Solar System within a month’s travel from Earth, and prompted unprecedent prosperity and peace.
The fusion torch engine works on the principle of magnetically-insulated inertial confinement fusion (MICF). The Prometheus made use of a deuterium/tritium reaction, which still sees use in older fusion reactors. The deuterium/tritium reaction, however, produces excessive neutron radiation, which embrittles components and limits engine life. Tritium is also prone to radioactive decay, restricting its useful lifetime as a fuel. These were critical limiting factors on the early colonisation of space. Breakthroughs in 2102 led to the adoption of the cleaner and more powerful deuterium/helium-3 reaction, which opened up the Asteroid Belt to humanity. Today, the deuterium/helium-3 reaction is the most common fusion process in commercial and government applications, as it is largely aneutronic and its two components can be produced from water, typically obtained from Lunar or asteroid ices. Indeed, human expansion throughout the Solar System has largely been dictated by the accessibility of fusion fuels.
Fusion torch fuel pellets are composed of deuterium and helium-3 and surrounded by a shell of lead and lithium. These fuel pellets are injected into the engine nozzle and ignited by a fibre laser at a distance of 300 to 500 metres from the ship. The deuterium/helium-3 reaction produces roughly 75% thermal radiation and 20% charged particles, the remainder being neutron radiation produced by “stray” deuterium-deuterium reactions. These neutrons are absorbed by the pellet’s metallic shell. A magnetic field generated by the engine nozzle captures the fusion reaction to provide thrust. The distance at which the fusion reaction occurs means that the ship is largely protected from thermal radiation, and remaining waste heat from the reaction is absorbed by each engine’s tantalum hafnium carbide heat shield and expelled by the ship’s radiators.
The base deuterium-helium-3 fusion reaction is highly efficient. The merit of the fusion torch engine, however, is that it significantly improves thrust power by using water as reaction mass: water is injected into the engine nozzle, where it is ionised by the fusion reaction and the plasma captured by the magnetic nozzle. This combination of high thrust and high fuel efficiency allows fusion torch ships to perform Brachistochrone transfers, which entail constantly accelerating until the halfway point to the target, then decelerating for the remaining distance to come to rest with respect to the target. Brachistochrone transfers reduce journey times between planets from months or years to weeks or even days: most civilian ships can sustain an acceleration of between 0.01 and 0.02 g, while ships of the Space Patrol Service can sustain an acceleration of up to 0.03 g. A journey from Earth to Ceres in the Asteroid Belt by fusion torch ship lasts no more than fifty-five days. By contrast, the same journey relying on an impulse transfer would take at least five months.
Fusion torch engines’ structural components are typically composed of nanocrystalline tungsten-tantalum-vanadium-chromium alloys, which exhibit excellent resistance to both thermal radiation and neutron embrittlement. Civilian torch engines typically resemble open cages to minimise both weight and heat retention. The engines of Space Patrol Service ships, by contrast, are totally enclosed to protect their components from weapons fire. This has unavoidable consequences for efficiency and thermal management: Patrol engines consume considerably more fuel and propellant than their civilian counterparts, and are fitted as standard with regenerative cooling systems and treated with a pyrolytic carbon external finish to maximise their thermal emissivity.
Controlling thirty per cent of the market share, Solar Dynamics is the leading provider of fusion torch engines today, and is the sole supplier of engines to World Union Government ships. Solar Dynamics perfected the deuterium/helium-3 reaction in 2102, stealing a march over its rival, the Bennett Fusion Corporation. Bennett Fusion developed the original deuterium/tritium fusion torch engine design, but today focuses largely on planetside power generation. It is, however, renowned for its bespoke engine designs for custom spacecraft.