Chapter 15: Investigating XDT in Rocket Propellants

Unknown (X) to Detonation Transition (XDT) is a significant concern in the design, assessment and use of rocket motors. The threat is the motor being hit by a fragment or other projectile which is not travelling sufficiently fast to cause Shock to Detonation Transition but causes damaged propellant to spall across the bore and detonate on impact with the propellant on the other side of that bore. QinetiQ have developed, and have been using, a small scale test to determine propensity of propellant to XDT and better understand the mechanisms that cause it. The use of high speed video and other instrumentation has been critical to this effort. It is found that detonation at the second surface is always initiated away from the point of impact. X-ray imagery has revealed that the impacting fragment is retarded in materials where XDT can occur and simulations have indicated that this retardation is not due to mechanical interaction with the propellant. It is proposed that an impactinduced reaction in the propellant target produces high pressure gas, contained by an intact bulge of propellant, that provides a retarding force on the fragment and that this pressure subsequently plays a part in causing XDT when the propellant bulge impacts against the second surface.