Performance characterisation of metal additives in paraffin wax hybrid rocket fuel grains.

Abstract

The Aerospace Systems Research Group (ASReG) at the University of KwaZulu Natal is actively developing sounding rockets in the Phoenix Hybrid Sounding Rocket Programme, for use by the South African scientific community. These sub-orbital launch vehicles use nitrous oxide and paraffin wax as propellants. While paraffin wax offers large performance gains over typical polymeric fuels, due to its high regression rate, further performance gains can be achieved via the use of metal additives such as aluminium powder. The main advantage of using additives such as aluminium is the ability to create a smaller, more compact launch vehicle. This is due to a decrease in the optimal oxidiser-to-fuel ratio brought about by metallisation, which increases overall propellant density. Theoretically, an added advantage is the higher heat of combustion as a result of aluminium combustion. This added heat further increases the regression rate of the solid fuel grain. In order to realise these performance gains, various challenges need to be overcome. Some of these include delayed combustion due to the alumina layer that naturally coats the aluminium particles, slag formation and nozzle erosion. In this study, a laboratory scale hybrid rocket motor was developed to test aluminised paraffin wax fuel grains via a series of hot fire tests. A nitrous oxide feed system was developed, as well as a computer program and associated electronics to control the system remotely and capture data from an array of sensor equipment. Due to time constraints placed on the project, only pure paraffin wax and fuel grains comprising 40 % aluminium by mass were tested. Using specific impulse and characteristic velocity as performance metrics, preliminary data shows little to no gain in performance with aluminised fuel grains due to incomplete combustion of the aluminium. Substantial erosion of the copper nozzles that were used in the aluminium grain tests, due to localised melting, was also noted. Large amounts of aluminium and alumina slag was also found on the nozzles converging face. In order to seek maximum performance gains from aluminium as an additive, it was recommended that the particle size be reduced and stripped of its oxide layer before addition into the solid fuel grain. This will ensure more complete and rapid combustion of the particles before being ejected from the combustion chamber.