|Ph.D Student||Zamir Inna|
|Subject||Electric Control of the Burning Rate of a Solid Propellant|
|Department||Department of Aerospace Engineering||Supervisor||Professor Emeritus Alon Gany|
|Full Thesis text - in Hebrew|
Solid propellant motors are simple, easy for handling, and ready for immediate operation. That’s why they are the preferred rocket motors in multiple applications and missions. However, in practice, solid propellant motor thrust cannot be controlled once the propellant has been ignited. The thrust developed during operation depends on the propellant burning surface and burning rate. The burning surface may be changed according to a predesigned geometry of the propellant grain, but, practically, neither the burning surface nor the burning rate can be actively controlled during the propellant combustion. This research investigates the possibility of controlling the burning rate by applying electric field/voltage.
The research concluded that to obtain an electric effect, specific compositions should be used. Of the compositions tested, the common ammonium perchlorate (AP) based propellants were found to be insensitive to electric effect. On the other hand, propellants consisting of ammonium nitrate (AN) as oxidizer and epoxy as binder revealed noticeable electric influence, and their burning rate could be controlled by applying voltage on the propellant. Thus, these propellant formulations have been selected for this research. Controlling the burning rate and thrust is promising for better managing the energy, thrust, and maneuverability of tactical motors. It can also be of advantage for small space motors, where such electric controlled solid motors may replace the traditional hydrazine thrusters, whose use is projected to diminish and even to be banned due to the hydrazine carcinogenic and toxic properties.
The fact that the AN based propellants are electrically affected while AP propellants are unaffected, is explained by Ohmic heat release at the surface layer due to electric current resulting from conductive ionic species formed during combustion. In contrast to AP, which decomposes at about 200℃, AN melts at about 170℃, generating electricity conducting ions.
The research includes both theoretical and experimental works. An experimental system has been designed and constructed. The experiments included investigation of the effect of voltage (up to 250V) on the burning rate for different values of oxidizer mass fraction, strand thickness, storage condition, and pressure. Without voltage application, the propellant barely burned, whereas when applying sufficiently high voltage (above 70V) the flame became large and noisy, and the burning rate increased significantly. Experimental results exhibited approximately a linear effect of the voltage on the burning rate. A noticeable increase in burning rate of approximately 3-5 folds was demonstrated when increasing the voltage from 70 to 200V. Furthermore, at a certain voltage the propellant could be ignited and by lowering the voltage or eliminating the electric field it could be extinguished. The voltage threshold value depended on the propellant composition (fraction of oxidizer), geometry (distance between electrodes), and storage conditions. Thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) were performed for AN, epoxy, and propellants consisting of both of them. The theoretical work included development of a model of steady state combustion with electric Ohmic heating as well as an empirical model. The models showed a good agreement with the experimental results.