Effects of Gas Generator Burn Rate Variability on Variable Flow Ducted Rocket Design and Performance

Abstract

Ramjets are a favored method of propulsion for missile systems needing to achieve the long ranges and high flight speeds required for advanced missiles. Ducted rockets in particular are especially favored over liquid fueled ramjets in that they offer ‘wooden round’ maintenance constructs, higher reliability, and less servicing requirements than the latter. Ducted rockets employ solid propellant gas generators that provide fuel-rich combustion by products that mix with a diffused airstream for combustion. Variable Flow Ducted Rockets (VFDR) employ a variable flow valve that controls the amount of fuel-rich byproducts (i.e., fuel flow) to the combustion chamber so that the VFDR can be throttled to maximize performance. Thus a VFDR is a coupled propulsion system consisting of an interaction of solid propellant, flow control, and ramjet combustion subsystems. Therefore, an understanding of gas generator performance and its impact on the ducted rocket is essential to proper design and safe operation of the VFDR. This paper investigates the impact of gas generator propellant burn rate variability on the design and performance of VFDR propulsion systems. Design impacts investigated include sensitivities of burn rate variability on: 1) gas generator chamber pressure and fuel mass flow rate given constant VFDR valve throat area; 2) gas generator fuel flow and valve throat area given constant chamber pressure; and 3) gas generator chamber pressure and valve throat area given constant fuel flow. The approach taken to evaluate the sensitivities on impacts to VFDR performance can be used to assess alternate fuels for existing VFDR designs and/or assess the design requirements for new VFDR designs.