Tank Head Startup Analysis of a Gas Generator Cycle Liquid Rocket Engine

Startup transients are critical in gas generator cycle liquid rocket engines because pressure overshoot, mixture-ratio excursions, and turbopump instability can occur before steady operation. This study compares conventional pyrostarter and tank head start sequences using a system-level transient model of propellant feed lines, valve motion, gas generator ignition, turbopump acceleration, and chamber pressurization. The engine configuration is used as a representative system-level case rather than a detailed reproduction of a specific flight engine. The integrated solver was checked against coupled startup data reported by Moon et al. using hydrogen as the turbine driving gas. Both sequences were evaluated under identical engine requirements using sustained convergence of pressure, speed, and flow as the startup-completion criterion. The pyrostarter and tank head cases reached steady operation at 2.41 s and 3.73 s, respectively, while discharging 123.596 kg and 95.134 kg of propellant through the combustors. The tank head sequence was further evaluated over a pump-inlet-pressure range using valve commands adjusted to preserve the target propellant arrival order in the gas generator and main chamber. Under these sequence-preserving schedules, increasing pump inlet pressure reduced steady-state arrival time from 3.80 s to 3.23 s but increased cumulative combustor discharge from 84.013 kg to 130.526 kg. These results characterize tank head start performance under physically admissible valve schedules, with startup time and propellant consumption forming competing design objectives.