Ignition Delay in solid-fuel ramjet combustor

Abstract

The ignition delay requirement and ignition characteristics of a solid fuel in the ramjet combustor were investigated. The shock in a simple diffuser geometry was calculated to obtain the upper limit of ignition delay, and it was determined to be shorter than 1 s. To achieve this short ignition delay, a high-density polyethylene (HDPE) fuel grain coated with an ignition support material and a fuel-rich propellant (FRP) were prepared. The ignition support materials were nitrocellulose with boron potassium nitrate (NC/BKNO3) and an ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene (HTPB)-based composite propellant. For testing purposes, a compact gas generator was built to simulate the conditions in the ramjet combustor. The simulation results revealed that the NC/BKNO3 ignition support material showed the shortest ignition delay of 1.27 s, but the flame in this case was not sufficient to ignite the HDPE. In a demonstration test, an ignition support material composed of NC/BKNO3 and the AP/HTPB-based composite propellant was applied to the FRP fuel grain, in which case a stably sustained flame was obtained. The test results demonstrated that ignition of the FRP and that of the ignition support material occurred simultaneously with an ignition delay of 1.74 s.