Pulse Unit, an extract from page 23 of GA-5009 Volume 3

From Page 23of GA-5009,
A General Atomic study for NASA


"2. 1. 4. The Pulse Unit
The energy for the propulsion of the nuclear-pulse vehicle is
provided by the pulse unit; it converts the energy released by a nuclear
explosion into a well-focused cloud of high-velocity propellant vapor.
As shown in Fig. Z. 6, the pulse unit consists of the propellant, channel
filler, radiation case, nuclear explosive device, delivery case, and
fusing and firing mechanism.
When the nuclear device is exploded, the channel filler absorbs
the radiation emitted and rises to a high temperature. The radiation
case serves to contain the energy released by the explosion so that
more energy is absorbed by the channel filler than is emitted into the
solid angle it subtends relative to the source. The high pressure
achieved in the heated channel filler then drives a strong shock into
the propellant, which vaporizes the propellant material and accelerates
it toward the pusher.
The specific impulse of the propulsion module is directly pro-portional
to the fraction of the mass of the pulse-unit propellant inter-cepted
by the pusher. In order to maximize this fraction, it is necessary
to control the manner in which the propellant expands and to ensure that
its mass forms a large fraction of the total mass. The degree to which
the expansion is controlled by shaping and the degree to which the
expanding vapor remains focused is proportional to the propellant
diameter-to-thickness ratio. This ratio should be large. But if the
diameter of the pulse unit is to be kept to a reasonable size, the pro-pellant
must be made from a high-density material. Furthermore, the
thickness and opacity of the channel filler should be such that the
radiation does not penetrate the propellant."