Review of Solid-Propellant Ignition Studies

Abstract

AN extensive review of the literature on solid-propell ant ignition was made to establish the state-of-the-art. Various ignition theories, experimental measurements, and ignition criteria were critically examined. The review was summarized in easy-to-read tabular form to facilitate comparison between various studies. The effects of important parameters on ignition processes were also discussed. Major technological gaps were identified and areas for future studies recommended. Contents The study of the ignition processes of solid propellants is important for many combustion and propulsion applications. An extensive review of research work performed in this area was conducted 14 years ago by Price et al. * Because many ignition studies have been conducted in the interim, a detailed survey of literature subsequent to the review paper of Price et al.1 is presented by the authors in Ref. 2. This synoptic of Ref. 2 (in which over 100 publications are cited) brings together the developments to date and the difficulties encountered under a unified view in order to establish the stateof-the-art in solid-propellant ignition. In general, ignition of a solid propellant is a complex phenomenon which involves many physicochemical processes, as depicted in Fig. 1. The ignition consists of the following sequence of events: 1) energy transfer to the propellant by an external stimulus which can be thermal, chemical, or mechanical; 2) heating and subsequent decomposition of the solid phase; 3) diffusion of vaporized gases into the surrounding atmosphere; and 4) subsurface, heterogeneous, and/or gas-phase reactions. When the net heat evolved from chemical reactions overcomes heat losses, sustained ignition is achieved. It is generally understood that ignition is incomplete if steady-state combustion does not follow the ignition event after the removal of external energy stimulus. The time period from the start of external stimulus to the instant of sustained ignition is called ignition delay^. Generally, it is controlled by three characteristic time intervals, viz., inert heating time, mixing (diffusion plus convection) time, and reaction time. Ignition delay, however, is not simply the algebraic sum of these three characteristic time intervals since there is no clear demarcation between the mixing process and the chemical reactions; these processes may have some overlapping periods. Ignition delay is one of the most important parameters in the study of ignition. However, it is very difficult to identify precisely the instant of sustained ignition.