Study of Thermo-Mechanical Properties of HTPB–Paraffin Solid Fuel

Abstract

The classic hybrid rocket which employs hydroxyl-terminated polybutadiene (HTPB) fuel is characterized by low ablation rate. To enhance the ablation rate, paraffin was added to the HTPB. The enhancement is due to the low viscosity and low surface tension of the surface melt layer of the paraffin during combustion. This paper reports the thermal properties, mechanical properties and ablation rate when HTPB was blended with different percentage of paraffin. The thermal stability of the blends was investigated using differential scanning calorimetry (DSC) and thermogravimetry analysis with constant heating rate 27Kmin−1 under argon atmosphere. The thermal stability of the blends decreased with the increase in paraffin content. The constant pressure specific heat capacities ( $${C_{\rm p})}$$ of the fuel blends were measured using DSC in the temperature range of 80–250 $${{\circ}}$$ C. The theoretical model of the ablation rate of HTPB–paraffin blends using activation energy, pre-exponential factor and $${C_{\rm p}}$$ was studied. The results indicated a higher regression rate of 6–33% with the increase in paraffin. The combustion enthalpy of HTPB blended with paraffin is determined by oxygen bomb calorimeter at a constant volume. The standard combustion enthalpy of pure HTPB and paraffin was found to be 37.9 and 41.9MJkg−1. The data revealed that blending of HTPB with paraffin, resulted an increase in the heat of combustion by 2–9%. The blending of the paraffin will reduce the mechanical strength. Studies were also carried out to find out the dynamic mechanical properties as the result of blending paraffin to HTPB using dynamic mechanical analysis. The temperature was held constant at room temperature and the frequency scan ranged from 1 to 10Hz in steps of 1Hz. The results show that the presence of paraffin in HTPB binder has considerable effect on the dynamic response of the material.