Scientists from the Institute of Chemical Kinetics and Combustion of the Siberian Branch of the Russian Academy of Sciences (SB RAS), Novosibirsk State Technical University and the Lavrentyev Institute of Hydrodynamics SB RAS have found a way to improve the combustion of solid rocket propellant by reducing the formation of large aluminum agglomerates, lumps of molten metal that emerge during combustion and reduce engine efficiency. In order to achieve this, they proposed adding titanium dioxide (TiO₂) to the propellant.
Aluminum is used in solid rocket propellant because it increases density and specific impulse, making engines more efficient and powerful. However, it has a shortcoming: during combustion, its metal particles stick together, forming large agglomerates. As these lumps do not have the time to burn completely, they fly out of the nozzle, cause erosion, pollute the environment and reduce the propellant’s efficiency.
Scientists have long been looking for ways to suppress agglomeration, but no universal solution has been found thus far. One promising approach is to add special modifiers to the propellant in order to alter the combustion process, making it more complete and controllable.
The Russian researchers have decided to focus on titanium dioxide. This material is widely known for its catalytic properties and is used in, among other things, air and water purification systems. However, its use in rocket propellants has been relatively underexplored, with a focus on the way it affects the combustion rate. The scientists looked into whether it could also affect the formation of aluminum agglomerates. For that purpose, they used four types of powder: three commercial samples from different manufacturers and one sample that had been synthesized in a specially designed laboratory unit. The unit was a vertical glass tube in which titanium particles were burned, with the resulting smoke settling on the tube walls. The result was titanium dioxide with a specific ratio of crystalline phases (rutile and anatase).
The experiments were conducted on a model paste-like propellant that consisted of ammonium perchlorate, aluminum powder and an active binder. The samples were burned in two modes: in nitrogen in a high-pressure chamber at a pressure of about 0.35 megapascals and in air at atmospheric pressure. The combustion process was captured on video, and solid combustion products were collected and later examined under a microscope. The scientists analyzed the shape, size and mass of the particles, paying particular attention to particles larger than 80 micrometers, which are considered the primary agglomerates.
The analysis showed that the addition of titanium dioxide does affect aluminum agglomeration. Nearly all TiO₂ variants reduced the mass of large agglomerates compared to the propellant without additives. However, the fuel combustion rate remained virtually unchanged in most experiments. An especially noticeable effect was demonstrated by a commercial powder with very fine particles measuring less than 25 nanometers. With this powder, the mass of agglomerates was reduced by almost 60%.
Microscopic analysis showed that the combustion products are a complex mixture of particles. They include silvery metallic spheres resembling the original aluminum, light-colored matte particles and typical large agglomerates that look like small acorns in which the molten metal is partially coated with an oxide shell. The ratio of these particle types varied significantly depending on the specific titanium dioxide that was added to the propellant. This means that the additive does influence the processes occurring on the surface of the burning propellant.
The researchers emphasize that these results are only preliminary. They need to find out why different forms of titanium dioxide behave differently, how they interact with other propellant components and how they behave under higher pressures typical of real rocket engines. Nevertheless, it is already clear that titanium dioxide can become a useful tool for controlling the combustion process of aluminized fuel.
