|M.Sc Student||Shani Sisi|
|Subject||Parametric Investigation of a Hybrid Motor Using Paraffin|
and Nitrous Oxide
|Department||Department of Aerospace Engineering||Supervisor||Professor Emeritus Gany Alon|
|Full Thesis text - in Hebrew|
Hybrid rockets may present a good propulsion alternative for different missions, in particular for the emerging space tourism, mainly due to the safety of the motor both during development and operation. Nevertheless, for certain applications the low regression rate of conventional polymeric fuels implies too low thrust. In recent years, paraffin (wax)-based fuels have been investigated because of their much higher regression rate compared to that of polymeric fuels. Paraffin, however, presents relatively poor mechanical properties.
The study includes an experimental investigation of plain paraffin fuels and HTPB-reinforced paraffin fuels (which improve the mechanical properties) with gaseous nitrous oxide (N2O) oxidizer via static firing tests of a lab scale hybrid motor. Polymethyl-methacrylate (PMMA, Plexiglas) fuel was tested as well for reference data. A comparison to results from the literature for similar fuels with oxygen as the oxidizer was also done to indicate influencing properties of the specific oxidizer. It was found that regression rate of paraffin is about five times higher than that of PMMA (polymer). A mixed paraffin-polymer (HTPB) fuel gives a lower regression rate than pure paraffin, yet it is higher by about 2-3 fold than that of PMMA. Characteristic velocity efficiency obtained was in the range of 80-100%, with an average of about 90%. The use of oxygen as the oxidizer was found to yield higher regression rate due to its higher flame temperature.
A theoretical study presents a physical-mathematical model of the combustion of liquefying fuels in hybrid combustion chambers, accounting for blowing effect on the heat transfer. It predicts the overall regression rate (melting rate) of the fuel and the different mechanisms involved, including evaporation, entrainment, and mass loss due to melt flow on the burning fuel surface, as well as the thickness and velocity of the liquid (melt) layer. In the range of oxidizer mass fluxes tested, both experiments and model revealed a significant mass loss (as high as 50%) due to melt flow along the surface, which may lead to reduction in overall motor performance. However, for higher oxidizer mass fluxes, which are typical to practical motors, entrainment becomes the dominant mass los mechanism. The model predictions reveal good agreement with experimental results.