Using thermal methods to understand the interactions between a rocket propellant and igniter material

Abstract

A number of ageing studies have been performed in order to ascertain the effect that a boron potassium nitrate-containing composition known as SR44 has on a nitroglycerine (NG)-based rocket propellant. These have shown that SR44 causes significant stabiliser decomposition in the propellant and heat flow calorimetry (HFC) has demonstrated that the two materials are incompatible with each other. Previous research had concentrated on heating the two materials in close proximity, even if they were not in direct contact. In this latest study, the propellant has been aged as far apart from the SR44 as possible inside a three-litre Kilner jar. Further to this, the effect of adding a composite propellant to the vessel has also been assessed as this would also normally be found in an igniter. Ageing has been conducted in the presence of different concentrations of relative humidity in order to look at the effect of moisture on the decomposition reactions. A number of thermal techniques have been used to characterise the materials and to try and understand the interactions between them. These have included HFC, differential scanning calorimetry (DSC) and bomb calorimetry. The data illustrate that even when the propellant and igniter material are separated by a notable distance, the SR44 still has an adverse effect on the propellant in terms of stabiliser decomposition. However, the relationship between relative humidity and degradation reactions appears to be different in the propellant when SR44 is present compared with when it is absent. Whilst it is clear that SR44 adversely affects the propellant, it has been more difficult to unequivocally demonstrate the effect of the propellant on the igniter composition. As a result, a new method has been developed which may enable the quantification of potassium nitrate in SR44 to be improved as it has been demonstrated that the presence of boric acids can affect this measurement. Essentially this involves heating the SR44 in a DSC from 100 to 160 °C at 5 °C min−1 before cooling it back to 100 °C and repeating the test. By doing this, the boric acids are removed from the SR44 which allows a more accurate figure for the concentration of potassium nitrate to be obtained. HFC has previously illustrated that it is important to understand if boric acids are forming in the SR44 as these can react with NG even if the materials are not in direct contact (Tunnell et al. in: Proceedings of the 9th international heat flow calorimetry conference, Finland, September, 2015). In the most extreme case where boric oxide forms to a sufficient concentration, this can cause the NG to ignite. It is therefore important to subject any SR44 contained within an igniter which is in a NG-based rocket motor to regular analysis. This is in order to find out if any acidic degradation products are accumulating which can lead to an increase in stabiliser consumption and, ultimately, significant NG decomposition within the propellant.