The Fallacy of Altitude-Dependent Rocket Thrust for Thrust/Drag Accounting

摘要

The concept of altitude-dependent rocket thrust is shown to unnecessarily complicate thrust/drag accounting for computing rocket motion within the atmosphere. The standard approach is shown to involve non-physical adjustments to both thrust and drag. A control volume formulation is developed that relies completely on the physical forces acting on the rocket. The development of the alternative approach reveals potential sources of errors that may occur when consolidating aerodynamic and propulsive models into a single model of rocket motion. Contradicting standard rocket propulsion texts, the expansion ratio that matches the nozzle exit pressure to the ambient static pressure is shown not to be the optimum expansion ratio for flight conditions. A rational approach is developed to unify the static and dynamic cases, showing that the system response (measured scale force for static conditions and vehicle change of momentum for dynamic conditions) is always given simply by "thrust minus drag", as long as thrust and drag are defined by physical forces acting on the vehicle. Concepts such as "vacuum thrust" and "altitude thrust" are shown to be unnecessary and misleading.