Mannitol has a relatively high melting point (167ºC) but is one of the commercially available sugar alcohols, in the same family as sorbitol, xylitol, and erythritol.  As such it will be very thermally stable.  A 65/35 mixture of KNO3 (200 - 300 mesh) and Mannitol (processed at 350ºF) is typical of other sugar alcohol based sugar propellants in that it is of a mashed potato consistency when melted.  The addition of a surfactant (polystep B-1 @ 0.1%) reduces the viscosity an exceptional amount.  In fact it seemed that the propellant melt, after sitting for ~ 30 min at 350ºF, had become translucent in color as if a significant amount of the KNO3 had dissolved in the sugar.  The propellant was easily poured into casting tubes for motor testing.  Because of the high processing temperature of this formulation, a significant amount of shrinkage occurred upon cooling.  This sugar crystallizes very quickly.  Grains are ready to face and use as soon as they are cool enough to touch.  A small amount of molten formulation needed to be ready to use to "top off" the grains as a significant "trough" formed as the grains cooled.  This trough was filled with molten propellant.  Once cool the grain faces were made square using a fine grain chop saw blade.  Motor ignition ocurred with no difficulty using my regular igniters.
238 KN/Mannitol + B-1
Propellant weight = 115.3g = 0.254 lb
Thrust time = 2.73 sec
Nozzle throat = 0.0376 in2
Max thrust ~ 13.2 lbs
Max Chamber pressure ~ 250 psi
Ave Thrust = 11.0 lbs = 49 N
Total impulse = 134 N sec = 30 lb sec
ISP = 118 sec
Motor class = G49
Some noise seems to have been present in the mid point of this test.
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