Unfortunately the theory of molecular sticking is not well developed. The sticking fraction for return flux is usually calculated from a simple temperature relationship derived from limited materials testing. However, the collision processes and their impact on the ability of the contaminant molecule to adhere must be accounted for. The sensitivity of any technique used to measure sticking fraction is limited by the signal to noise ratio, the total mass flow in the plume gases over the sensor, and the resolution of the sensor. Most sticking fractions are measured with larger engines, cooler sensors, and dirtier thrust gases than are appropriate for the Genesis mission.
Bipropellant exhaust constitutes a larger contamination concern than hydrazine monopropellant.
Existing test data is inadequate to establish a general relationship for the persistent sticking fraction. It is, therefore, necessary to select a value for this parameter based upon scientific judgment of the analysis being conducted. Table B gives the sticking fractions used for this analysis. In determining the contamination associated with the ultra-pure hydrazine thrusters I have assumed a fuel burn of 20 grams for the short, daily attitude control burns and 100 grams for the longer monthly state adjustment burns. As examples of the geometry, figure 3 below shows the distances and angles associated with the axial thrusters.