The development of a paraffin wax/nitrous oxide hybrid rocket slab motor.

Abstract

Slab motors are used to determine and investigate the regression rate characteristics of hybrid rocket propellant combinations. This information is fundamental to the overall design and thus used to determine the payload, altitude and thrust parameters of a rocket. The Phoenix Hybrid Sounding Rocket Programme in the University of KwaZulu-Natal’s (UKZN) Mechanical Engineering Department uses paraffin wax and nitrous oxide in their series of hybrid sounding rockets. The regression rate behaviour of paraffin wax with nitrous oxide has not previously been investigated in slab motors. This study focused on the regression rate behaviour and entrainment mechanism with regards to non-classical fuels including those with metal additives. This was used to gain a greater understanding of the increased regression rates associated with these fuels. The addition of metal additives, such as that of aluminium to fuel grains, was explored since the research suggested that it increases the regression rate of pure paraffin wax by 30%. A hybrid rocket slab motor visualisation test stand was developed to observe and obtain regression rate data. The stand includes a feed system, injector and a combustion chamber. All the components were manufactured using brass and stainless steel materials for their nitrous oxide compatibility, strength, and thermal resistance. Quartz glass windows were incorporated into the combustion chamber design for visualisation purposes. Due to the presence of quartz glass the use of finite element analyses became critical and more complex in order to ensure that the glass could withstand the operating conditions of the slab motor. A side-glass spacer was implemented to minimise the effects of side burning and to observe the influence of regression rate. Tests were conducted at a 130 g/s oxidiser mass flow rate and an atmospheric chamber pressure. A data acquisition system using LabVIEW software was implemented to obtain tank readings for the duration of the burn and to ensure safe motor operation. The regression rate of Sasolwax 0907 fuel was volumetrically determined and observed to be on average 3.74 mm/s. This shows a much higher regression rate than other paraffin wax compositions which have been found to regress at 1.5 mm/s. The characteristics of the entrainment process were validated for the investigated propellants, and the high regression rate mechanism of paraffin wax was observed in the liquid melt layer, droplet entrainment, and roll waves. Tests using aluminised wax fuel grains at atmospheric conditions proved to beunsuccessful with nitrous oxide as the oxidiser. A possible reason for this could be due to the aluminised fuel grains requiring increased heat transfer, therefore not producing sufficient apourisation of the fuel. Moreover, decomposition of the oxidiser appeared to be inhibited by the combination of the oxidiser mass flow rate and the port area which prevented combustion.