Characterization of Green Hypergolic Propellants for Hybrid Rocket Applications

Figure 1. Schematic diagram of a hybrid rocket motors

As seen above, the solid fuel and the liquid propellants are separately stored. During ignition, the liquid oxidizer is pumped into the fuel chamber which also serves as the combustion chamber. Hot gases resulting from the fuel-oxidizer combustion are expelled from the nozzle, propelling the rocket forward.

Hybrid rockets have the following three major advantages:

1. They are safe: Compared to solid rockets, in hybrids, the propellants are stored separately, reducing the probability of accidental ignition. Additionally, as compared to both solid and liquid rockets, hybrids can use more inert, and hence safer, fuels.
2. They are cheap and simple: As opposed to liquid rockets, only one reactant is in the fluid state. Hence, the plumbing and pumping complexity and costs are halved for hybrids as compared to liquid motors.
3. Their output can be throttled: By controlling the flow rate of the fluid oxidizer, the thrust level can be controlled, based on mission requirements. This capability is absent in solid motors since its combustion cannot be controlled post-ignition.

However, they suffer from two major drawbacks:

1. They are difficult to ignite: Partly because hybrid propellants are more inert than solid and liquid rocket propellants, they are harder to ignite. Conventional hybrid rocket ignition systems either (like a heating element or plasma-arc igniters) add weight and complexity to the system or degrade over time (like catalytic igniters), compromising re-ignition capabilities.
2. They suffer from lower regression rates: The regression rate (and hence, thrust per given size of motor) of hybrids is atleast an order of magnitude lower than solid rocket motors. This leads to increased size and greater weight or more atmospheric drag.

Overall, the project’s focus is to research hypergolic propellants and understand their performance with a view to eventually use them in hybrid rockets to overcome the drawbacks of conventional ignition systems. Hypergolic propellants are those that ignite upon contact without the need for external heat/energy sources. They solve ignition problems by firstly removing the need for a separate, intricate ignition system and secondly by ensuring great re-ignition capabilities. Conventional hypergolic propellants like MMH (monomethyl hydrazine) and RFNA (Red fuming nitric acid) are toxic: this research focuses on studying a promising class of non-toxic or “green” hypergolic hybrid rocket propellants.

The project is currently focusing on a propellant consisting of Rocket Grade Hydrogen Peroxide (RGHP) as oxidizer, polyethylene (HDPE) as fuel and sodium borohydride or SBH (NaBH₄) as an energetic additive embedded in the HDPE fuel matrix. Figure 2 below shows the ignition progress with this propellant.

Figure 2. Ignition process for RGHP-HDPE-SBH propellant

As shown in the picture above, ignition proceeds when the RGHP droplet impinges on the solid surface. After impact, the liquid RGHP undergoes a rapid exothermic reaction with the exposed SBH particles on the fuel sample surface. The heat release pyrolyzes the HDPE fuel. This heat also provides the energy required for igniting the flammable pyrolysis products, with combustion proceeding in the gas-phase. Thus, ignition is achieved without the need for a separate igniter; the ignition system is “built into” the reactants.

Figure 3 shows ignition of the HDPE + SBH fuel with just one drop of hydrogen peroxide.

Figure 3. Ignition of fuel with one drop of oxidizer

As can be seen in the above figure, ignition is rapid and energetic with the entire ignition event wrapping up in less than 10ms. For hypergolic propellants, fast ignition is important; in a real hypergolic hybrid rocket motor, slow ignition can cause propellants to accumulate in the combustion chamber which can simultaneously ignite, causing undesirable pressure spikes. In general, faster ignition is generally desired also because in applications like satellite propulsion, quick ignition and re-ignition implies accurate positioning and maneuvering.

1. Conduct broad parametric studies. We vary various parameters like pressure, reactant concentration, drop impact velocity etc to see how they influence ignition delay and the general ignition process. This provides insights into the principal factors and processes that likely affect ignition behavior of this propellant.
2. Conduct detailed studies to determine the mechanisms that influence ignition. Trends observed in broad parametric studies are studied in-depth to reveal the underlying mechanisms and parameters that influence ignition.
3. Study flame structure. The temperature, chemical species involved and other facets of the flame are studied to gain a better understanding of the ignition and combustion behavior of this propellant.

To aid in these aforementioned aims, we use cutting-edge diagnostic tools and techniques. We use high-speed visible imaging to study visible emissions from the sparks and flames, as shown in Fig. 3. High-speed infrared imaging provides insight into heat-release trends associated with ignition, as shown in Figure 4 below.

Figure 4. Heat-release due to hypergolic reaction. Darker regions correspond to lower emission intensity

Infrared imaging is also utilized to estimate surface temperatures associated with the ignition event of the propellant as shown in Figure 5 below.

Figure 5. Estimation of surface temperature using infrared imaging. Data shows that prior to and during ignition, surface temperatures on the fuel surface reach about 140^oC

Finally, we can use our high-speed infrared camera in conjunction with an optical filter to study emissions from specific chemical species resulting from propellant ignition and combustion. An example of this application is shown in Figure 6 below.

Figure 6. Infrared imaging detecting emissions from CO2 as a result of combustion of the HDPE + SBH propellant grain due to sprays of RGHP

In the figure above, we can see emissions from carbon di-oxide in the flame caused by the impinging of sprays of RGHP on the fuel sample surface. Filters that selectively allow for the imaging of emissions from species like H₂O, BO₂ etc can also be used in this manner. Detection of chemical species and information on temperature can provide information on flame structure.

Taken together, the aforementioned techniques and diagnostic methods can lead to a deeper understanding of the ignition and combustion behavior of this class of “green” hypergolic propellants. This understanding can be utilized to design better and more efficient hybrid rocket propulsion systems while overcoming the ignition challenges that plague this otherwise promising class of rocket motors.