Constant Dynamic Pressure Trajectory Simulation in POST

Abstract

Future space transportation vehicles may well rely on high speed airbreathing propulsion (ramjets and scramjets) to supply much of their motive power. Because of the tradeoff relationship between engine thrust and vehicle airframe weight, ascent trajectories are typically simulated using a constant dynamic pressure phase during airbreathing acceleration; dynamic pressure is increased to benefit vehicle thrust up to some fixed limit imposed by the vehicle structure. The constant dynamic pressure portion of the trajectory typically begins around Mach 2-3 and continues to the maximum airbreathing Mach number or until some convective aeroheating limit is reached. We summarize comparative research on three candidate guidance methods suitable for simulating constant dynamic pressure trajectories. These are generalized acceleration steering, linear feeedback control, and cubic polynomial control. All methods were implemented in POST (Program to Optimize Simulated Trajectories), an industry standard trajectory simulation code. Both quantitative and qualitative comparisons of these methods (i.e. in terms of computer processing time, number of required iterations for convergence, sensitivity to quality of initial values, accuracy and program robustness) are presented. Of the three methods, the linear feedback control approach is found to be the most efficient and robust, with good accuracy.