Characterization and Improvement of Aluminum Combustion in Solid Propellants

Yinon Yavor

Department of Mechanical Engineering - Mcgill University, Montreal, Canada

Solid rocket motors use composite propellants containing up to 20% aluminum powder for the enhancement of energetic performance. Aluminized solid propellant tends to experience the undesirable phenomena of agglomeration – merging of small aluminum particles to a large particle, which doesn’t fully burn inside the motor. This could lead to incomplete combustion, higher two-phase flow losses and the accumulation of slag – which might damage motor insulation, and increase significantly the weight of the empty motor.

This work suggests two in-house-developed options to reduce agglomeration:

(a)    Nickel-coating of the aluminum particles: Nickel-coated particles ignite faster and at a lower temperature than regular aluminum. This leads to reduced Agglomeration, since particles ignite prior to their accumulation to large agglomerates.

(b)    Using porous aluminum: Porous aluminum has large specific surface (similar to that of nano-aluminum), leading to high reactivity and short ignition time. As a result, the agglomerates ejected from the burning surface are smaller.

Experiments of aluminized solid propellant combustion were conducted in a wide spectrum of pressures and propellant compositions, and analyzed in various methods. It has been found that the use of both nickel coated and porous aluminum decreases the mean agglomerate diameter dramatically (65-75% mass wise).

A mathematical model was developed, explaining the agglomeration phenomena and predicting its reduction resulting from the use of nickel-coated or porous aluminum. The model's predictions regarding ejected agglomerate diameter are in good agreement with experimental results.