טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentWeinstein Alexandra
SubjectParametric Investigation of a Hybrid Motor Using a Paraffin-
Based Fuel
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Alon Gany
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Hybrid motors consist of fuel and oxidizer components in different physical states. The combination of safety, possibility of on/off operations, and high energetic performance, makes them attractive for large launch boosters that require high thrust levels. However, classical hybrid systems employing polymeric fuels are characterized by relatively low thrust levels resulting from their low fuel regression rate.  

This work investigates both experimentally and theoretically the combustion phenomena of a new class of high regression rate liquefying fuels, particularly paraffin-based fuels.  

The experimental part focused on measuring fuel regression rate, motor energetic performance, and combustion efficiency of paraffin-based fuels, providing a broad data base consisting of hundreds of stating firing tests in a lab-scale motor employing gaseous oxygen as the oxidizer. Plain paraffin undergoes severe melting during combustion causing up to 6-fold regression rate increase compared to polymeric fuels. However, its excessive melting may lead to incomplete combustion and its mechanical properties are poor, particularly at high ambient temperature. Hence, plain paraffin may not be adequate for practical motors. An original contribution of the present study has been the investigation of fuel grains consisting of solid paraffin granules mixed within a polymeric HTPB binder. It was found that such a fuel has improved mechanical properties and combustion efficiency (over 95%) as well as less melting during combustion. Hence, though such fuel increases regression rate only about 3-fold compared to polymeric fuels, it may be a very promising solution, combining high regression rate with good mechanical properties.    

The theoretical model developed in this research aimed at predicting the overall regression rate (equal to the melting rate), the contributions of the different mass loss mechanisms, and the thickness and velocity of the liquid (melt) layer formed during the combustion of liquefying (paraffin) fuels. Besides the mass loss mechanisms of fuel evaporation and liquid droplet entrainment into the core gas flow, reported in the literature, the present work proposed and analyzed an additional novel mechanism of melt layer flowing along the burning fuel surface. It was shown that in the range of operating conditions investigated in this research, evaporation has the largest contribution to the overall regression rate, whereas liquid drop entrainment and melt flow have comparable contributions. At higher oxidizer fluxes entrainment is predicted to become more dominant.  A good agreement has been demonstrated between the model prediction and test results.