טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentYossi Zamir
SubjectElectromechanical System for Solid Rocket Motor Thrust
Modulation
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Gany Alon


Abstract

There are three main types of chemical rocket motors: solid propellant, liquid propellant, and hybrid. Solid rocket motors (SRMs) have many advantages over the other types, the most significant ones being the relative simplicity, reliability, and immediate operational availability. On the other hand, difficulties regarding real time thrust magnitude control are their inherent disadvantages.

The thrust of a SRM mainly depends on the combustion chamber pressure, nozzle geometry and propellant characteristics. The most commonly used method for a prescribed thrust profile design relies only on propellant grain geometry. This method is limited and does not comply with all the requirements of rocket motors. Real time thrust modulation can significantly extend the mission versatility and performance of SRM. One group of methods for real time thrust modulation is based on modification of the nozzle throat area, either directly by moving parts, or effectively by aerodynamic means. Another group of methods for thrust modulation deals with modification of the propellant grain burn rate by an electric field or other means. Reducing the throat area or increasing the burn rate cause an increase of the pressure and thrust of the motor, and vice versa.

The objective of this work is to design, construct, and test an electromechanical system for real time thrust modulation of a SRM. The work also includes a literature survey, description of requirements, and review of concepts. The design is based on a movable pintle against a fixed nozzle. The system is integrated in an existing generic rocket motor with a hydroxyl terminated polybutadiene - ammonium perchlorate propellant having a relatively high burning rate pressure exponent (n=0.5). The propellant is free of metal particles in order to limit the maximum chamber temperature to 3000K and avoid extensive erosion and particle accumulation on critical surfaces. The system components are thermally insulated to withstand the harsh conditions of the motor operation for a sufficient amount of time.

An electromechanical system was designed to meet the system requirements derived from a generic thrust modulation mission. The system is based on an actuator that includes a brushless DC motor, a planetary gear with a ball screw, and an incremental encoder.

The preliminary stages for static firing demonstration test included experiments of the electromechanical system with a spring load, simulating the aerodynamic forces on the pintle during operation of the thrust modulation system. Experiments with static pintle in cold flow and SRM static firing were conducted as well. Then a thrust modulation system with controlled pintle in cold flow was tested successfully. The force on the pintle calculated via the electric current of the motor, was compared to CFD simulations and to the static cold flow tests that included a load cell instead of the actuator. In the last stage of the research, the system was integrated in a demonstration test that included SRM static firing. The thrust modulation system performed a smooth transition between two different thrust levels (thrust ratio of 1.6). This successful test demonstrated a proof of concept of the system.