Atmospheric Fusion Drives

While in space thrusts of only a few Newtons sustained over a long period of time are quite satisfactory, this kind of performance is not helpful in VTOL maneuvers in atmosphere, this has led to the re-design of space based fusion flame drives, to produce far greater amounts of thrust than just fusion flame alone, which although extremely hot, does not offer in its self a great degree of thrust.

Drives within planetary atmospheres boil down to Newton’s third law, as exotic space bending techniques do not work well in proximity to large masses, and so all atmospheric engines rely on propelling air or other exhaust fluid in the opposite direction of the desired thrust, and the force gained is from reaction of the moved fluid. The largest linear (as these are most efficient for producing drive flame) fusion reactors used in space flight produce very hot and fast moving exhaust ions, however this in itself only generates a few hundred Newtons, therefore special adaptations have had to be made firstly to increase the available thrust, and secondarily to produce a relatively safe drive (more for the environment than the craft).

Most of the thrust from atmospheric fusion drives comes from the heating and expansion of air. It is a principle law of gases that when a volume of air is heated it desires to expand to occupy a greater volume at the same pressure, or when confined in volume to create a greater pressure inside this volume, this behaviour stems from an increase in the speed of the molecules, which is directly related to heating. Atmospheric fusion drives therefore consist mostly of a means of heating air.

Technologies originally used for hydrocarbon combustion, such as combustion jet engines can be simply modified to accommodate fusion flame as a heat source, and variations on these existing technologies provide much in the way of all atmospheric fusion drives.

Fusion ramjets are a common and easily inbuilt design, the principle of the drive basically relies on an open tube. While the vehicle is moving through the atmosphere, and the ramjet tube aligned to its movement, air forces its way into the forward part of the pipe, a fusion flame then heats and expands this air, and this heated air escapes out of the back end of the tube, and is prevented from escaping from the front by the inrushing air. As the escaping air occupies a greater volume (because of its heating), than the intake air this generates a thrust in the direction of travel, and continues the flight, and sustains the powering process. The advantage to this design is that the ramjet design does not interfere with the fusion flame in space, so ships can readily switch between environments without having to modify their drive geometry or design. The disadvantage is that these drives require high speed to operate, as the engine has no way of drawing air, except at high speed, this means aerodynamic surfaces, and runways are required, and VTOL is impossible.

The next design mimics the jet engine, with compressor blades at the front to draw in and compress air, and turbine blades after the expansion of the gas which take some of the expansion energy and use it to drive the compressor blades, these systems typically need only one moving part. The problem with these devices is that the fusion flame is extremely hot and energetic, and without proper dilution in atmospheric gases the flame can destroy the turbine blades. Designs for fusion turbine systems are much different to their hydrocarbon counterparts, they are generally much longer and often more slender, as plasma drive flames can be readily adjusted, many different variations on engine design can also be seen, from equivalent low temperature turbofan engines, to compressor ramjet/turbojet designs where the turbine blade wheels have been reduced to barreled shafts because of extreme thrust.

This family of engines varies much more greatly in design than the ramjet engines, offering high performance drives which need only be powered by small fusion devices, the systems are often much more nimble in the air than ramjets, and with a motor driven compressor system are able to fire up whilst stationary allowing for VTOL. The disadvantage is that fusion flame has to be mixed with air, otherwise the temperatures within the engine rise unacceptably, leading to failure, this means such engines are exclusively used in atmosphere and can not be used in space.

Apart from drive design, safety is of crucial importance, not only in the respect of drive reliability, and the safety of the piloting crew, but also because of the nuclear nature of the drive. The main concern is pollution of the atmospheric gases, especially the introduction of unstable nuclear isotopes, which could then lead to an increase in surface radioactivity. Most modern fusion drives run on very ‘clean’ reactions, which favour low emission of neutrons, not because of undesirable radiations (which is easily halted by massless neutronium), but because of more energy efficient reactions, this has of course been reversed in atmospheric drives, where low emissions are the greatest concern. The secondary concern is the high energy electromagnetic radiation, which can stimulate nuclear reactions in other particles (as they are extremely high energy). Therefore most fusion drives require plasma pre-processing which deals with these potentially harmful radiations before mixing with atmosphere.

Pre-processing is most successfully integrated in system where drive plasma is carried in conduits before being mixed with atmosphere. Neutrons are separated from other ions by magnetic deflection, as only neutrons will travel unaffected in straight lines through ‘drive elbows’. The electromagnetic radiation is mostly from fusion reactions and the high temperature, generally plasma temperature drop below fusion temperature before they combine with drive ducts, this cuts out hard gamma, Still the drive plasma is extremely hot, and emits X-rays, which are cut out by neutronium ‘swirlers’ which not only creates a fusion plasma vortex for atmospheric combination, but prevents X-ray escape by blocking line of site with atmosphere.

The major remaining problem is that the high temperatures formed within the drive often drive chemical reactions, which could change the composition of exhaust gases. For this reason particular caution has to be observed in peculiar atmospheres such as methane/oxygen mixtures. For the most part, this type of drive causes few problems in Earth like atmospheres, generally only producing small amounts of nitrogen/oxygen compounds, as well as destroying any methane or ammonia.

Atmosphere fusion drives are the principle in atmosphere drives used by atmospheric capable space craft, often requiring only limited additional hardware for such an enhanced ability in operation.