Hypergolic solid propellant using NaBH₄/KNO₃ with paraffin for HTP-based hybrid rockets

To improve the ignition system of a hybrid rocket engine, this paper reports the experimental study of the performance of the hypergolic solid fuel grain (HSF) consisting of a mixture of NaBH₄/KNO₃ with paraffin for a hybrid rocket engine using 90 % HTP as the oxidizer. Drop tests were conducted to examine the ignition delay time (IDT) of HSF compositions with varying ratios of sodium borohydride (NaBH₄) and potassium nitrate (KNO₃) mixing with paraffin. The results showed that IDT decreased with increasing amount of NaBH₄, and decreased further with KNO₃ addition. For comparison purpose, two formulations were selected for the tests: HSF01 (80 wt% paraffin, 20 wt% NaBH₄) and HSF02 (65 wt% paraffin, 20 wt% NaBH₄, 15 wt% KNO₃). A series of image visualizations using high-resolution and high-speed cameras, and event cameras revealed similar ignition mechanisms for both tests, in which flamelets were formed upon the contact of HTP droplet, leading to explosive combustion. However, KNO₃ in HSF02 consumed the smoke-like mixture produced during the explosion, in which behavior was absent in HSF01. In addition, atmospheric combustion tests (ACT) were conducted in a PMMA tube (10 mm in diameter and 100 mm in length) and the results showed that only HSF02 was successfully ignited, with flamelets forming ∼4 ms after HTP injection and full combustion was established at ∼0.7 s. The test results indicated that the KNO₃ as an oxidant additive added to the hypergolic solid fuel grain (HSF) plays an important role in the HSF ignition mechanism. It decreases not only the IDT of the fuel grain, but also increases the flammability of the HSF.