Interstellar Ramjets by Dr Andrew Collin, continued...

The problem is, then; how to build a spacecraft that will arrive at its destination in a reasonable time for the occupants (if not for the society that launched it, due to relativistic constraints); and how to overcome the mass ratio problem? The answer (and the exception to the "slowboat" approaches above) is to use a ramjet.

Bussard (1960) is usually credited with the first published mention of a ramjet, although he credits the idea to Ackeret (1946) in an earlier paper. In concept the idea is elegantly simple. A ramjet carries no fuel at all, but uses the interstellar medium itself, both as a fluid to provide reaction mass, and as an energy source via thermonuclear fusion. In the direction of travel a large area of the interstellar medium is swept out, like a whale trawling for krill, and the material channelled into a fusion reactor attached to the payload. Exothermic fusion reactions then occur between individual hydrogen atoms, which form the overwhelming bulk of interstellar material, and the end products are expelled from the reaction with more momentum than they had initially, and so the ship accelerates.

Most subsequent work on ramjets has assumed that the scoop will be magnetic in nature, for reasons given below.
 

2 PLASMA CONTAINMENT AND FUSION

The theory underlying nuclear fusion is well understood by physicists, and most people working in the fusion programme today would probably agree that the major barriers to building an economic fusion reactor are technological (and political!). Uncontrolled fusion has been demonstrated by the testing of the "hydrogen bomb"; unfortunately, controlled fusion in a terrestrial environment is much more difficult, and fusion in a ramjet throws up further problems still.

Nuclear fission involves the release of some of the nuclear binding energy of a heavy nucleus, when the nucleus splits into two 'daughter' products. Fusion occurs in the joining together of two light nuclei to form a third; energy can also be released in this case if the stability of the resulting nucleus is greater than that of the two parent nuclei, and if the relative energy of the two parents is high enough to overcome electrostatic repulsion. The degree of this repulsion is referred to as the reaction cross section. For instance, the two isotopes of hydrogen, deuterium and tritium can fuse together to give a helium-4 nucleus and a neutron, with both products carrying off large amounts of kinetic energy. We note here that fusion reactions between deuterium nuclei are much easier to reach than between single protons, or hydrogen nuclei; although the latter reaction is energetically favourable the reaction cross section is vastly smaller than that for deuterium nuclei. Unfortunately, deuterium appears to be much rarer in the Universe than hydrogen; the best estimate seems to suggest an abundance of one deuterium atom for every ten thousand in the interstellar medium. Thus, for a given acceleration and mass, a deuterium powered ramjet would need a scoop cross section of about 10⁶ times larger than a proton burning ramjet.

Fusion is notorious for requiring extremely high temperatures and careful confinement techniques. To run a deuterium fusion reactor the reaction vessel must attain the Lawson criterion

which relates the number density of the reaction plasma and the confinement time. The criterion requires temperatures of around 100 million degrees Kelvin - comparable to that at the centre of our Sun - and only at these temperatures can the deuterium nuclei overcome their mutual repulsion and allow fusion into stable helium nuclei to take place. At these temperatures the electrons around the nucleus are stripped away and both nuclei and electrons form a 'soup', called a plasma. For proton fusion - which reaction our ramjet will depend on - even higher temperatures will probably be required.

(1) Also shows that careful confinement of a reaction plasma to allow high densities is needed. Containment of a plasma at these temperatures presents particular problems; the plasma must be in a vacuum and cannot be allowed to contact any material object, or temperatures will fall drastically to unusable levels. The generally accepted technique for plasma confinement is to use powerful, carefully directed magnetic fields in various topologies (although others, such as inertial containment, are also being worked on). We need only note at present that the same magnetic field may suffice for both reaction confinement and for the ram scoop.

Bussard's 1960 analysis discovered some of the main constraints in the construction of a ramjet. Bussard considered an idealized vehicle that moves in one dimension; he purposely ignored the conceptual engineering problems, and restricted his analysis to the acceleration capabilities. Even so, some important design constraints came out.

The ramjet engine is considered to swallow, burn and expel a  certain mass of interstellar hydrogen every second. A small fraction of this mass is converted to energy, following Einstein's relation E = MC² , and the rest is expelled. Some of the energy generated is converted to kinetic energy with efficiency x, fraction (1 - x) being lost as thermal radiation transverse to the direction of flight. Bussard notes that in principle this parameter is under the control of the reactor designer, whereas 'a' is controlled by the laws of nature.

Bussard then solves the relativistic energy and momentum equations to determine the system motion.
 

Feel free to contact our president direct at: andy-nimmo@ntlworld.com
 Back