Scanning electron microscope examination of JA2 propellant samples exposed to plasma radiation

Some plasma-exposed samples of transparent (graphite-free) JA2 propellant were examined by scanning electron microscopy. It was found that the plasma-exposed surface of these samples was covered with humps or bubble-like structures. When the samples were cryogenically split, the interior was found to contain numerous open cracks and voids; in a very few cases where the split actually went through a hump/bubble, cracks/voids were found immediately under the hump, suggesting a cause and effect relationship. The inside surfaces of the cracks and voids appeared in some cases to be either (a) smooth and undulating, or (b) about as would be expected if the two sides of the crack had been simply pulled apart; or (c) rough and/or debris-covered. In some type (c) cases they appeared to have been initially rough or debris-covered but to have been at least partially smoothed, possibly by processes involving heat, liquefaction and thermal decomposition. These observations seem consistent with an interpretation involving subsurface penetration of the propellant sample by visible and infrared radiation; this radiation then might cause cracks as a result of uneven expansion due to heating and/or internal gasification due to photochemical and/or thermal decomposition, particularly at sites of small patches of unplasticized nitrocellulose fibers. If these changes also included liquefaction or melting, with or without decomposition, this would also help to understand the smoothing process. In any case, these results suggest that interaction of plasma radiation with the sample is a significant effect. An end-exposed grain of standard, semitransparent JA2 was also examined; in this case the cracks were oriented differently; they extended from holes in the surface into the interior in a direction perpendicular to the surface and parallel to the direction of extrusion.