Typical nuclear pulse propulsion has the downside that the minimal size of the engine is defined by the minimal size of the
nuclear bombs used to create thrust, which is a function of the amount of critical mass required to initiate the reaction. A conventional
thermonuclear bomb design consists of two parts: the
primary, which is almost always based on
plutonium, and a
secondary using fusion fuel, which is normally deuterium in the form of
lithium deuteride, and tritium (which is created during the reaction as lithium is transmuted to tritium). There is a minimal size for the primary (about 10 kilograms for plutonium-239) to achieve critical mass. More powerful devices scale up in size primarily through the addition of fusion fuel for the secondary. Of the two, the fusion fuel is much less expensive and gives off far fewer radioactive products, so from a cost and efficiency standpoint, larger bombs are much more efficient. However, using such large bombs for spacecraft propulsion demands much larger structures able to handle the stress. There is a tradeoff between the two demands. By injecting a small amount of
antimatter into a
subcritical mass of fuel (typically
plutonium or
uranium)
fission of the fuel can be forced. An anti-proton has a negative
electric charge, just like an
electron, and can be captured in a similar way by a positively charged
atomic nucleus. The initial configuration, however, is not stable and radiates energy as
gamma rays. As a consequence, the anti-proton moves closer and closer to the nucleus until their quarks can
interact, at which point the anti-proton and a
proton are both
annihilated. This reaction releases a tremendous amount of energy, of which some is released as gamma rays and some is transferred as kinetic energy to the nucleus, causing it to split (the fission reaction). The resulting shower of
neutrons can cause the surrounding fuel to undergo rapid fission or even
nuclear fusion. The lower limit of the device size is determined by
anti-proton handling issues and fission reaction requirements, such as the structure used to contain and direct the blast. As such, unlike either the
Project Orion-type propulsion system, which requires large numbers of nuclear explosive charges, or the various antimatter drives, which require impossibly expensive amounts of antimatter, antimatter-catalyzed nuclear pulse propulsion has intrinsic advantages. A conceptual design of an antimatter-catalyzed thermonuclear explosive
physics package is one in which the primary mass of plutonium usually necessary for the ignition in a conventional
Teller–Ulam thermonuclear explosion, is replaced by one
microgram of antihydrogen. In this theoretical design, the antimatter is helium-cooled and magnetically levitated in the center of the device, in the form of a pellet a tenth of a millimeter in diameter, a position analogous to the primary fission core in the layer cake/
Sloika design. As the antimatter must remain away from ordinary matter until the desired moment of the explosion, the central pellet must be isolated from the surrounding hollow sphere of 100 grams of thermonuclear fuel. During and after the
implosive compression by the
high-explosive lenses, the fusion fuel comes into contact with the antihydrogen. Annihilation reactions, which would start soon after the
Penning trap is destroyed, is to provide the energy to begin the nuclear fusion in the thermonuclear fuel. If the chosen degree of compression is high, a device with increased explosive/propulsive effects is obtained, and if it is low, that is, the fuel is not at high density, a considerable number of neutrons will escape the device, and a
neutron bomb forms. In both cases the
electromagnetic pulse effect and the
radioactive fallout are substantially lower than that of a conventional fission or
Teller–Ulam device of the same yield, approximately 1 kt. == Amount needed for thermonuclear device ==