# Experimental Investigation on a Throttleable Pintle-Centrifugal Injector
## 3 Experimental Investigation on the Pintle-Centrifugal Injector  
### Experimental System  
Experimental investigations are indispensable as a primary research tool to enable a more direct exploration of hybrid rocket motors. Hot commissioning is the primary method of studying hybrid rocket motors. The cold test of the injector is a necessary part of the effective study before the hot commissioning. The cold test enables an effective study of the pressure drop characteristics and even the atomization effect of hybrid rocket motor injectors. The throttleable pintle-centrifugal injector is originally designed to control the oxidizer injection state during a wide range of thrust ratios (flow ratios) in hybrid rocket motors. In the experimental investigation, there is a high risk of damage to the test environment and personnel, if the hydrogen peroxide solution is used as the test medium. This is why our experimental study used deionized water as an alternative working fluid. \begin{table}
\begin{tabular}{c c c c} \hline
**Parameter** & **Unit** & **Injector 1** & **Injector 2** \\ \hline \(s\) & mm & 2 & 2 \\ \(N\) & / & 4 & 4 \\ \(D_{\mathrm{t}}\) & mm & 3 & 2 \\ \(D_{\mathrm{inj}}\) & mm & 20 & 20 \\ \(D_{\mathrm{a}}\) & mm & 22 & 22 \\ \(\theta\) & \(\circ\) & 45 & 45 \\ \hline \end{tabular}
\end{table} 
Table 1: The pintle-centrifugal injector construction parameter values. Figure 3: (**a**) Pintle-centrifugal injector assembly (**b**) pintle (**c**) bottom view of the injector. Figure 2: Construction parameters of the pintle-centrifugal injector. Deionized water, like highly concentrated hydrogen peroxide, is a colorless liquid at room temperature and pressure, and the use of water as an alternative is a common method used in general rocket engine injector cold tests. The focus on hydrogen peroxide in this study leverages its ambient-temperature storability and aligns with the requirements of an ongoing project in our laboratory, though oxidizers like liquid oxygen (LOX), as utilized in prior studies cited in the introduction, remain viable alternatives for hybrid propulsion. The experimental system consists of the throttleable pintle-centrifugal injector, a motor actuator, a propellant delivery system, a measurement control system, and a high-speed imaging system. As shown in Figure 4, the throttleable pintle-centrifugal injector actuator is primarily composed of servo motors, ball screws, a grating measurement system with a resolution of 0.001 mm, and corresponding control software (Written by LabVIEW2020). The grating measurement system, with its high resolution, plays a crucial role in precisely measuring and displaying the stroke of the pintle. This system ensures accurate displacement measurement by providing real-time feedback, allowing for fine control of the pintle position and precise monitoring of its motion throughout the injection process. The propellant delivery system is shown in Figure 5. It consists of a hand valve, safety valve, high-pressure water reservoir, pressure sensor (range: 0-2 MPa, accuracy: \(\pm\)0.25% FS), pneumatic valve, mass flow meter (range: 0-600 g/s, accuracy: \(\pm\)0.1% FS), buffer tank, and cavitation venturi. The cavitation venturi is the flow control element of the delivery system, which accurately controls the liquid flow in the system under defined pressure conditions. The feeding pressure, ranging from 3 MPa to 4 MPa, was supplied upstream of the controllable cavitating venturi. While the cavitating venturi's flow rate depends on upstream pressure (3-4 MPa) to sustain cavitation, the choked throat, under sonic conditions, stabilizes the downstream pressure near the fluid's vapor pressure, providing a consistent boundary for the injector. This isolates the injector's performance--pressure drop and atomization--from upstream pressure variations, aligning with our focus on downstream dynamics.