Performance characterisation of metal additives in paraffin wax hybrid rocket fuel grains.
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Date
2018
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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.
Description
Masters Degree. University of KwaZulu-Natal, Durban.