Demonstration of 500 N scale bipropellant thruster using non-toxic hypergolic fuel and hydrogen peroxide
Introduction
Liquid-fueled propulsion systems have been developed over the past 100 years, and monopropellant or bipropellant chemical propulsion thruster systems have undergone several specific changes. The latter is the most commonly used in a variety of civilian and military applications, and in an attempt to increase its performance, various propellant combinations have been tested since Robert Goddard's first rocket. Above all, the hypergolic ignition system has been widely adapted for space propulsion applications, where reliable and repetitive ignition is essential.
A hypergolic bipropellant is a particular combination of fuel and oxidizer that ignites spontaneously upon contact. However, in the existing hypergolic propulsion systems, the use of hydrazine or its derivatives as a fuel, along with nitrogen tetroxide or nitric acids as an oxidizer, is indispensable, even if they are extremely toxic, corrosive, potently carcinogenic to humans and detonable. These utmost dangers that stem from the toxicity of the propellants significantly increase the cost and time required for development and launch of a rocket. The cost penalties are made up of diverse parts: the propellant, its handling, health surveillance, decontamination and disposal of residuals [1], [2]. Furthermore, a curtailment in the budget of national space agencies for space missions, along with stricter environmental and safety regulations, emphasizes the need for non-toxic hypergolic propellants. Thus, extensive research has been conducted on non-toxic hypergolic bipropellants to remedy the drawbacks reported from the existing hypergolic propulsion systems, although follow-up implementation continues to remain relatively scarce [3], [4], [5].
The principal goal of this research is to demonstrate a novel concept of 500 N scale bipropellant thruster using a non-toxic hypergolic bipropellant combination. This paper then presents experimental demonstrations with respect to hypergolic interactions and thruster operation. The focus is placed on the use of high test hydrogen peroxide (HTP) as a green oxidizer, considering its environmentally benign nature, and the search for non-toxic energetic hypergolic fuels comparable to conventional toxic fuels.
In general, there are two different types of additives for granting hypergolicity in a rocket fuel: catalyst or strong reducer. A catalyst dispersed in fuel can vigorously decompose an oxidizer when they are brought into contact. This process represents a rapid exothermic reaction that generates enough energy to ignite the reactants. However, this catalyst additive is not directly involved in the combustion process, which could result in increasing ignition delay and in degrading performance. Unlike the catalytic additive, once a strong reducer, such as metal hydrides, comes into contact with an oxidizer, ignition occurs immediately because the strong reducer itself can easily combust with a strong oxidizer. Due to its active participation in the ignition process, the fuel promoted by a strong reducer, commonly referred to as a reactive fuel, provides faster and more reliable ignition. Therefore, although hypergolic interactions can be caused by the two different types of additives, reactive fuels promoted by strong reducers are typically preferred.
Three reactive fuels were produced by blending sodium borohydride (NaBH4) powder, a strong reducing agent, into energetic hydrocarbon mixtures. Sodium borohydride was previously used as an ignition promoter for 90 wt.% hydrogen peroxide by R. Mahakali et al. [6]. In the previous work, although most of the fuels demonstrated hypergolic properties with the oxidizer, they were potentially not suitable for practical applications because the solvents used for sodium borohydride had low chemical potential energy. In addition, there was no attempt to evaluate the non-toxic hypergolic bipropellant in actual conditions. This work is dedicated to developing enhanced reactive fuels by using more energetic solvents compatible with sodium borohydride and applying them into a 500 N scale hypergolic bipropellant thruster with 90 wt.% hydrogen peroxide as an oxidizer. Through ground hot-fire tests, the feasibility and performance of the non-toxic hypergolic bipropellant thruster were evaluated.
Section snippets
Materials
Metal hydrides are typically introduced as reducing agents in chemical synthesis and in materials for hydrogen storage applications. Light metal hydrides, such as lithium or beryllium compounds, are preferred as additives in rocket fuels due to their low molecular weight, enhancing the rocket performance. They are, however, extremely expensive and difficult to handle; lithium compounds react explosively with moisture and water. Practically, sodium borohydride is a promising alternative when
Results and discussion
The static firing test was conducted for a total of 3.5 s with the objective of demonstrating the feasibility of 500 N scale bipropellant thruster using the novel non-toxic hypergolic propellant combination. As shown in Fig. 8, hypergolic ignition was achieved without the use of an external ignition system, and the rocket exhaust plume was stably formed. Mach diamonds in the plume were clear evidence that the nozzle throat was choked; therefore, the combustion gas flew at supersonic speed. Fig.
Conclusions
This research was devoted to finding a novel combination of non-toxic hypergolic bipropellants and evaluating the performance of 500 N scale non-toxic hypergolic bipropellant thruster. The propellant combination of energetic reactive fuel (Stock 2) and 90 wt.% H2O2 was selected for the static firing test. This work successfully demonstrated the steady-state operation of the thruster for 3.5 s, which implies that the concept of the green hypergolic bipropellant thruster is feasible. However, to
Conflict of interest statement
There is no conflict of interest statement.
Acknowledgement
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (NRF-2014M1A3A3A02034777).
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