In the effort of increasing the notoriously slow fuel-regression rate of hybrid rockets two approaches are investigated in this paper: addition of metal particles to HTPB grains against the use of paraffin-based fuels, which are expected to raise more significantly the regression rate. A series of firing tests on two lab-scales hybrid rocket engines have been carried out with gaseous nitrous oxide and pure HTPB, aluminum-loaded HTPB or paraffin wax. Scale effects are examined. Outcomes are discussed in terms of regression rate and combustion efficiency. A comparison with the data retrieved from pure HTPB or metal-loaded HTPB burnt with gaseous oxygen is also drawn. Both aluminum powder addition to HTPB and the use of paraffin-based fuel appear to increase regression rate, the latter showing the major gain. In particular aluminum powder addition seems to increase regression rate more than observed with oxygen.
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