Some propellants currently used in space propulsion have the disadvantage of being dangerous to humans and the environment. A wellknown example of this are hydrazine and its derivatives which are toxic, corrosive, and carcinogenic. These disadvantages have led to a search for less hazardous storable liquid propellants, often referred to as green propellants. A green propellant is safer to use and handle and will therefore bring down the costs related to production, storage and handling. The main reason for the use of hydrazine is its hypergolicity with common oxidizers like NTO and nitric acid. Hypergolicity is the property that a fuel and oxidizer ignite spontaneously when brought in contact without the need for an external source. This property is beneficial since it eliminates the need for an ignition system, thereby making the propulsion system more simple, reliable, and cheaper. It is therefore desired that a replacement propellant also shows this property with common oxidizers.
During this thesis two methods are explored with the goal of creating a green hypergolic propellant combination. One based on catalytically enhanced ethanol and high concentration hydrogen peroxide. The other based on a pyrophoric liquid that is added to ethanol. There have already been efforts made before by adding catalyst particles or a strong reducer to a hydrocarbon fuel like kerosene or ethanol. The problem with using catalyst or strong reducer particles is the difficulty of creating a homogeneous mixture. Due to the liquid nature of the parent fuel, the particles will start to separate from the fuel and sink to the bottom of the container. To overcome this problem an organic gelling agent is added to the fuel to increase its viscosity, thereby increasing the sedimentation time. This will not only increase the shelf life of the fuel but also decrease the vapor pressure, making it less flammable and safer to work with, and reduce storage problems like propellant sloshing and spilling. Increasing the viscosity also makes it more difficult to transfer the fuel through the feed system and achieve proper atomization. However, by applying shear force on the fuel it shows shear thinning behavior, decreasing the viscosity close to that of the parent properties making it easy to use in existing propulsion systems designed for liquids. A second effect of the organic gelling agent is that due to its energetic nature it participates in the combustion. Therefore, the amount of catalyst needed can be reduced to a negligible amount while still achieving good performance properties like ignition delay time. This is verified by means of a drop test.
As a second approach, instead of adding solid catalyst particles a pyrophoric liquid is added to ethanol. A liquid allows for easier mixing and creating of a homogeneous mixture resulting in a longer shelf life compared to using catalyst particles. The pyrophoric liquid also fully participates in combustion resulting in increased performance compared to catalyst particles that do not combust. Due to the reactive nature of the pyrophoric liquid this propellant formulation is expected to be able to achieve hypergolicity with multiple common oxidizers. This results in a versatile system which can be used in current propulsion systems without extensive modification required. By eliminating the need for an ignition system, the propulsion system is simplified and its reliability is increased.