Solving an irregular burning problem in a small rocket motor

Abstract

During the design and development of a solid rocket motor, intensive irregular burning appeared in the area of high initial temperatures. The cylindrical propellant grain was designed for approximately neutral burning, with a hollow tube and both burning ends as well as with the outer surface inhibited. A simple grain configuration and a propellant with plateau burning were selected, and the operating pressure of the rocket motor was chosen to be in the area of the plateau. Unfortunately, the special conditions of the gas flow through the propellant grain port, caused by the central tube passing through the grain channel, as well as high propellant temperature sensitivity, led to an increased turbulence and a very strong irregular burning occurrence at high temperatures (+50oC). In these conditions the combustion went out of the plateau regime and the pressure exponent rapidly grew up. The problem was solved without changing the double-based propellant composition. The high burning surface to nozzle throat area ratio was decreased below the critical threshold appearance of irregular burning, without significantly affecting the motor performances. The rocket motor nozzle and the propellant grain were redesigned. Despite the limited possibilities of changing the propellant grain geometry, an uncommon solution was found, opening a part of the inhibitor at the outer surface of the grain.

Keywords

• solid rocket propellant
• double-based propellant
• propellant grain
• erosive burning
• surface burning
• pressure coefficient
• thrust