Fuel grain for spherical boost-sustain rocket motor

DETAILED DESCRIPTION OF THE INVENTION Reference is now made to FIG.
1 of the drawing which shows a substantially spherical shaped rocket motor 10, having a casing including an outer shell 11 and an insulation liner 12, a fuel grain 14, an igniter 16, an initiator 17, a nozzle 18 and a throat insert 19.
Typical desired performance pressure-time curve 20 and thrust-time curve 21 for rocket motors, delivering boost-sustain performance, are shown in FIG.
2.
If a purely spherical burning surface were used in a rocket motor as shown in FIG.
1, the burning surface, increasing with the square of the radius, would result in a final sustain pressure of approximately twice the pressure at the beginning of the sustain phase.
Also, such a surface would not produce the relatively large amount of burning surface for the boost phase with the sharp transition to the sustain level.
The fuel grain, as shown in greater detail in FIGS.
3-8, has regressive elements to compensate for the normal progressive burning surface which would be provided in a spherical motor.
Eight star point segments 22, with a cross section as shown in FIG.
7, form one set of regressive burning surfaces.
Eight slots 24 at the forward end of the chamber extending to the motor casing, as shown in greater detail in FIG.
4, provides additional regressive elements.
An additional regressive element is obtained by providing an unrestricted space 26 underneath the throat insert 20, as shown in FIG.
1.
To provide the large burning surface for the boost phase, eight segments 27, with a cross section as shown in FIG.
6, are positioned between the eight star elements.
The segments 27 have parallel sides 30 to provide a large burning surface for boost operation.
When the burning from the opposite sides 30 of each segment reaches the center of the segments, a sharp transition to sustain thrust will be provided.
The cross section of the fuel grain at the position of slots 33, between star segments 22 and boost segments 27, is shown in FIG.
8.
One propellant which may be used for the fuel grain has 85% solids loaded propellant in a carboxy-terminated polybutadiene binder