Investigation of physical properties of paraffin based hybrid rocket fuels

Hybrid propulsion combines the advantages of liquid and solid propulsion. In particular its main characteristics are: safety thanks to oxydant and fuel stored separately, possibility of multiple shut on/off, thrust modulation regulating the oxidant mass flow. Hybrid systems have specific impulse higher than solids, but the main reason that up to now has delayed the development of hybrid technology is the low regression rate of the fuel. The most promising fuels are those that melt forming a thin liquid layer over the solid fuel, leading to entrainment of droplets effect. For this reason in this work done at DLR - Institute of Space Propulsion in Lampoldshausen, paraffin based fuels, which are liquefying hybrid fuels, are considered. Performances can be enhanced also chemically by using additives capable of increasing the energetic content of the fuel. In this work two different types of additives are used: organic additives as Polyethylene and Stearic Acid and metal additives like Aluminum. The purpose is to characterize different formulations by a rheological point of view. Two main properties are supposed to influence regression rate of paraffin based fuels: surface tension and viscosity. Viscosity behavior is analysed as function of temperature, from 200°C to the melting temperature. The mechanical properties of the formulation concerning on storage and loss modulus are also characterized, in particular it is interesting the behavior across the melting point. Surface tension, a parameter that is generally neglected, is investigated basing on Karabeyoglu researches which consider surface tension an important parameter in determining the regression rate. The behavior of surface tension with increasing temperature for different kind of waxes and additives is investigated. The last part that needs to be delved is the analysis of combustion products: the amount of metal collectable (a mixture made of wax and Aluminum is considered) , the specific impulse and combustion efficiency are estimated theoretically. Tests involving different formulations should be done to get experimental results.