Relativistic spaceflight and the catalytic nuclear ramjet

Abstract

The interstellar ramjet concept allows us, at least hypothetically, to get around the mass-ratio limit inherent with relativistic fuel-carrying rockets. The required ramjet energy must be extracted from the nuclear reserves of the interstellar hydrogen (interstellar deuterium is too scarce). Unfortunately, the usual PPI chain for converting hydrogen to helium is hopelessly slow due to the minute weak interaction cross section of the reaction p + p → 2 D + e + + v at essentially any energy. It is shown here that this problem can be avoided, in principle, by exploiting a proton burning catalytic cycle. The best known catalytic cycle is the CNO Bi-Cycle occurring in sufficiently hot main sequence stars. The catalyst “fuel” may be carried on board the ship since it is not depleted, and the ultimate source of energy is the interstellar hydrogen. It is shown that with potentially realistic parameters the energy requirements for a ramjet accelerating at 1 g to near luminal velocities can be met with either the “hot” CNO Bi-Cycle or the Ne-Na cycle. Some of the formidable technological problems associated with a conventional heavy ion fusion reactor are briefly discussed. It is not absolutely necessary that a conventional plasma reactor model be used; for example a laser fusion reactor or a large scale Migma-type reactor (if technologically possible) might be employed. The problem of interstellar drag is considered and a model is suggested in which the incident proton's kinetic energy is stored in the electric field between charged grids and returned to the exhaust particles after the relevant nuclear reactions are completed. Such a combination of electric and magnetic fields would make less severe other suggested limitations to the ramjet's performance. The problem of ionizing the interstellar medium directly in front of the ramjet when operating in non-HII regions of space is also discussed. It is shown that laser ionization is energetically possible for interstellar number densities ≳10 cm −3; however, since the laser also tends to sweep the interstellar matter out of the ramjet's path, more quantitative study is necessary before the method can be considered feasible at any density. The possibility of using a thin stripper foil, placed over the intake cross section, to ionize the interstellar medium is suggested. This method has the advantage of efficiently ionizing the interstellar matter independent of density.