Shuttle Launch 2006
As very small particles with high surface area are mixed into a liquid, the viscosity of this liquid increases. A typical sugar propellant with a 65:35 ratio of oxidizer to fuel can have a varying degree of melt viscosities depending on the size of the oxidizer particle mixed with the fuel. If particle sizes are roughly 50 to100 mesh, the melt viscosity of a propellant mix can be reasonably low, roughly on the borderline of being pourable. When particle sizes are 200 mesh or higher the melt viscosity of a propellant mix is much like that of thick mashed potatoes. Such a hot, high viscosity mixture becomes somewhat difficult to work with when casting into propellant grains.
Based on some work done in years past on high solids coatings I was familiar with the use of surfactants as a means of lowering the viscosity of these high solid content liquids. I decided to investigate the possibility that surfactants could be used to lower the viscosity of hot propellant mixtures enough to allow them to be poured directly into grain casting tubes.
Samples of a large number of different types of surfactants were obtained and evaluated for their effect on reducing the viscosity of a standard 63:35 KNO3/sorbitol propellant mixture. In general the following observations can be made from this study:
·A reduction in viscosity can be realized using very small amounts of certain surfactants
·The use of too much surfactant will reduce the burn rate of the propellant
·The burn rate under pressure can be
significantly different than that
of non-surfactant doped propellant
·The addition of a second small particle
size solid, other than oxidizer, such as
Iron Oxide, Al, or Ti dust, even in very small quantities, seems to negate
the unusual burn rate under pressure properties seen in these surfactant containing sugar propellants
The following links discus, in greater detail, my findings while evaluating various surfactants.