Test Stand Results
with additives in
Potassium Nitrate/Sugar Formulations
The melt viscosity of propellant formulations based on potassium nitrate and sugars can be dramatically lowered with the use of certain surfactants at low levels. However, the burn rate, under pressure, of many of these propellants containing surfactant is often significantly effected. Most often burn rates are significantly longer at chamber pressures lower than ~700 psi. Traditional Bates 2 grain 38mm motors using KNO3/sugar propellants at (65:35) with 3/8” cores with no surfactant present have burn times between 0.75 and 1.13 seconds according to Nakka’s SRM simulation spread sheets (for sucrose, dextrose, and sorbitol using Aerotech type hardware at 1000 psi max chamber pressure). Personal test stand evaluations gave burn times of ~ 1.7 seconds and Isp engine ratings between 125 and 128 sec in two separate motor burns using sorbitol fuel. This indicates that my personal test stand is generating data that match that modeled by Nakka. It is interesting to note that motor burn times of ~1.7 sec for sorbitol based motors matches those burn times I obtain for high pressure surfactant containing sorbitol motors as well as those sorbitol/surfactant motors using RIO as an additive.
The links below, based on the different sugars used as fuel, connect to pages with data on the use of surfactants such as Polystep B-1 with this basic 65:35 formulation. Links are also present which will detail experiments using other surfactants that exhibit viscosity reduction (other_surfactants). Test stand evaluation of other additives used as burn rate modifiers in combination with surfactant have their own page as well. Although fine Al and Ti powders are really considered fuels rather than additives, links to test stand data on motors with these metals are also present (other_additives). The use of other additives such as RIO in formulations containing surfactant does not affect the viscosity reducing ability of the surfactant. The presence of such an additive does, however, negate the unusual lengthening of the burn rate sometimes seen with some formulations which contain only a viscosity reducing surfactant as an additive.
Test stand results comparing formulations with and without surfactant reveal that sometimes slight reductions in Isp occur and at other times normal normal motor performance is obtained when surfactant is present. When it occurs this reduction appears to be on the order of 5 - 10%. Work is ongoing to determine what variable(s) cause this ISP reduction or why it sometimes does not happen. Most likely this difference is due to increased "flushing" as described on the Nakka web site (http://www.nakka-rocketry.net/knsbchar.html).