|Fig. 1: Concept image of ramjet. (NASA/courtesy of nasaimages.org)|
Popular culture frequently deals with the notion of humankind leaving earth for destinations beyond the local solar system. Reasons given range from practical reasons such as the potential catastrophic destruction of the earth to pure curiosity.  The expansion of humankind beyond the current solar system would fit with the trend of humans expanding ever outward: from Africa to Eurasia and then later to the Americas. In the last century, the first excursions off-planet, first just escaping the atmosphere and then all the way to the moon.
The rate of this expansion has been governed by technology. The development of warm clothing allowed expansion from the warmth of Africa to the cooler climes of Europe. The development of long-range boats allowed post-land bridge expansion from Europe to the Americas. Spaceflight and trips to the moon followed the development of rockets and vessels that could survive the trip. Similarly, any human excursions beyond the solar systems will be entirely dependent on the development of technology that allows such trips.
For trips beyond the solar system, the limiting factor is likely to be the energy available for trip. A staggering amount of energy is required to accelerate and then decelerate a ship to speeds that would reduce duration of an interstellar flight to a human timescale. A trip for a small probe to the nearest non-sun star would require 23 EJ (2.3 x 1019 J).  Current global energy use totals 500 EJ per year.  Since humanity is unlikely to devote 5% of its yearly energy budget to an interstellar mission, a new energy conversion technology would greatly facilitate such an effort. Harnessing of nuclear fusion is one technology that might meet these energy needs.
Nuclear fusion power technology is based on the harvesting of the energy released when atomic nuclei, typically those of isotopes of hydrogen collide and release energy. Several possible fusion reactions are possible, including, but not limited to:
The most visible and familiar examples of fusion reactions found in everyday life are those of the stars, especially the sun. The main fusion reaction pathway that powers the sun is the following :
Fusion reactions require high temperatures and special materials to occur. Currently, two major approaches are taken to manage these high temperatures: inertial and magnetic confinement.  In inertial confinement, the fusion fuel is compressed to high densities and then heated. In magnetic confinement, the fusion fuel is a plasma (ionic fluid) that is contained within an electromagnetic field.
The concept of a nuclear fusion ramjet is in essence a magnetic confinement nuclear fusion reactor that gathers hydrogen from its surroundings as it travels.  The reactor would be placed aboard a spaceship bound for a destination outside the solar system.
The main constituents of the fusion ramjet would be the fusion reactor itself and a magnetic scoop that would gather the fuel. One component that would be lacking would be an extensive fuel storage system. Unlike a traditionally propelled spaceship that would need to carry its fuel out of the earth's gravity well, a nuclear fusion ramjet would offer a tremendous saving in mass and initial launch energy by lacking a fuel storage system. Instead of being carried with the ship, the fuel would be hydrogen gathered from the interstellar medium.
Since the continued propulsion of a nuclear fusion ramjet spaceship is dependent on interstellar hydrogen, the nature of interstellar hydrogen is the main issue of concern when designing such a spaceship. Two aspects of particular interest are the overall density and the isotopic composition of the interstellar hydrogen. The overall density controls the rate at which fusion reactions can take place relative to the craft's speed and the size of the scoop's area. The isotopic composition determines which fusion reaction pathway can be used.
The interstellar density of hydrogen is 0.86 atoms/cm3.  Updating the equations found in  with this value give a scoop area of 105 km2. This, combined with the slow reaction rates of the p + p reaction, severely limits the usefulness of a pure hydrogen fusion ramjet. Instead, consider a fusion reactor using a D + D reaction shown above. However, the number density ratio of D/H of 1.4 × 10 -5 yields any such reactor to need a large scoop area as well. Thus, such reactors using either the p + p pathway or the D + D pathway will likely not be possible without significant developments in magnet and material technology.
Given the scarcity of interstellar hydrogen, particularly the fusion-friendly heavier isotopes, even a nuclear fusion ramjet is not likely to provide the motive power for an interstellar journey. However, the concept might still be useful as a basis for comparison for other methods of propulsion.
© Thomas Parise. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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