Research on the performance enhancement of HRM in low-thrust conditions using fluidic throat secondary injection

With the increasing demand for flexibility in future space missions, Hybrid Rocket Motors (HRMs) with deep-throttling capabilities have garnered significant attention. However, under low-thrust conditions, HRMs often suffer from severe specific impulse (I_sp) degradation, primarily due to the substantial drop in combustion chamber pressure. To address this issue, this paper investigates the potential of fluidic throat technology in enhancing the low-thrust performance of HRM. First, ground firing tests using N₂ as the secondary injectant were conducted to assess the feasibility of fluidic throat technology in HRM. Based on the experimental data, a high-fidelity numerical model of the fluidic-throat HRM was established and validated, yielding a maximum thrust deviation of only 3.52 % compared to the test cases. Subsequently, systematic numerical simulations were carried out using the validated model with three common working fluids: N₂, He, and catalytically decomposed H₂O₂ gas. The results demonstrate that the He jet alters the effective throat area by forming a "virtual throat", which elevates the chamber pressure and thereby achieves a higher I_sp when compared to the no-injection baseline at an equivalent total nozzle exit mass flow rate. Conversely, the N₂ jet leads to an I_sp reduction due to its high molecular weight and low enthalpy. Notably, although the decomposed H₂O₂ gas exhibits lower compressibility than He, it triggers intense secondary combustion upon mixing with the fuel-rich primary flow. This exothermic process significantly increases the flow field enthalpy, achieving the most substantial I_sp gain, even with a minimal mass flow ratio of 0.04, the H₂O₂ injection yields a 5.01 % improvement in I_sp . These findings indicate that fluidic throat technology—particularly when utilizing He or H₂O₂ injectants—provides a viable pathway for enhancing the low-thrust performance of HRMs and optimizing cruising range.