# Experimental Investigation on a Throttleable Pintle-Centrifugal Injector
## 1 Introduction  
The rapid advancement of space exploration, driven by an escalating demand for satellite deployment, interplanetary missions, and maneuverable launch systems, has placed unprecedented pressure on the development of next-generation rocket propulsion technologies [1]. Among the diverse array of propulsion methods available, chemical rocket engines stand out as the most mature and widely adopted technology, underpinning the majority of operational launch vehicles today. These engines are broadly classified into liquid rocket engines, solid rocket engines, and hybrid rocket motors, distinguished by the physical states and storage methods of their propellants [2, 3]. Hybrid rocket motors, characterized by their integration of a liquid oxidizer with a solid fuel grain, have emerged as a particularly promising solution, striking a unique balance between the operational simplicity and inherent safety of solid propulsion systems and the precise controllability offered by liquid systems [4]. This hybrid configuration typically features a liquid oxidizer delivery system--employing oxidizers such as hydrogen peroxide or liquid oxygen (LOX)--coupled with a thrust chamber housing solid fuel, which generates combustion dynamics distinct from those of fully liquid or solid engines. The resulting diffusion flames provide notable advantages, including ease of thrust adjustment, the ability to shut down and restart, higher propellant energy density, and enhanced stability during storage and transport [5]. Despite these compelling attributes, hybrid rocket motors encounter significant hurdles in achieving the levels of throttling precision and combustion efficiency demanded by contemporary aerospace missions, where adaptability across diverse operational profiles is paramount [6]. The efficiency of combustion in these systems is intricately tied to the injector's design and structure, which directly influences the critical processes of oxidizer-fuel mixing and heat transfer to the solid fuel grain within the thrust chamber [7]. One approach that has gained traction for enhancing combustion performance is swirl injection, a technique that imparts rotational momentum to the oxidizer flow. Research has demonstrated its efficacy in significantly boosting regression rates, with studies reporting increases ranging from 30% to as much as 2.7 times, alongside achieving specific impulses up to 263 s at sea level [8; 9]. Furthermore, swirl injectors have been shown to mitigate pressure oscillations and elevate overall combustion efficiency, offering a pathway to more stable and effective hybrid motor operation [10]. However, despite these advancements, swirl-based designs frequently fall short of delivering the deep throttling capabilities required for missions demanding a wide range of thrust levels, posing a limitation to their broader adoption [11]. In contrast, pintle injectors, which have a storied history in liquid rocket propulsion, excel in providing precise flow control and rapid response under variable thrust conditions [12]. Early innovations by TRW laid the groundwork for pintle technology, culminating in its successful application in the Apollo Lunar Module Descent Engine (LMDE), renowned for its deep throttling and instantaneous shutdown capabilities [13]. Modern developments have extended pintle applications to environmentally friendly propellant combinations such as LOX/ketosene and LOX/alcohol, as seen in engines like the TR-106, capable of delivering thrusts up to 290 tons [14; 15; 16]. The exploration of monopropellant injectors has further broadened the scope of injector design, particularly for hybrid propulsion systems where operational simplicity and safety are prized attributes. For instance, a dual-plate injector developed for a 50 N hydrogen peroxide monopropellant thruster demonstrated a throttling range of 24-64% with characteristic velocities spanning 81-97%, showcasing the potential for fine-tuned control in compact systems [17].