REPUBLIC OF SERBIA

MINISTRY OF DEFENCE

www.mod.gov.rs

MINISTRY OF DEFENCE

Material Resources Sector

Defensive Technologies Department

Military Technical Institute

www.vti.mod.gov.rs

reliability of solid rocket propellant grain under simultaneous action of multiple types of loads

Nikola GLIGORIJEVIĆ

Military Technical Institute, Rocket Armament Sector, Belgrade, nikola.gligorijevic@gmail.com

SAŠA ŽIVKOVIĆ

Military Technical Institute, Rocket Armament Sector, Belgrade, sasavite@yahoo.com

VESNA RODIĆ

Military Technical Institute, Sector for Materials and Protection, Belgrade, vesna_rodic@vektor.net

SAŠA ANTONOVIĆ

Military Technical Institute, Rocket Armament Sector, Belgrade, saleantonovic82@gmail.com

ALEKSANDAR MILOJKOVIĆ

Military Technical Institute, Sector for Materials and Protection, Belgrade, aleksandar.milojkovic@gmail.com

BOJAN PAVKOVIĆ

Military Technical Institute, Rocket Armament Sector, Belgrade, bjnpav@gmail.com

ZORAN NOVAKOVIĆ

Military Technical Institute, Aeronautical Sector, Belgrade, novakoviczoca@gmail.com

Abstract: A case-bonded solid propellant rocket grain is subjected to many stress-inducing loads during the service life, due to temperature, extended polymerization, transportation, vibration, acceleration, aerodynamic heating etc. and finally due to the operating pressure in the rocket motor. Composite propellant is a viscoelastic material whose mechanical properties highly depend on temperature and strain rate and sometimes may vary in the range of use of rocket motors for several orders of magnitude. Relationships between stresses and strains are much more complex than for the elastic material. Therefore, the stress and strain analysis and estimation of safety factor under the action of each individual load is quite complex and sometimes impossible. An even greater problem occurs when multiple different types of loads act simultaneously. An extreme case occurs in the moment of rocket motor ignition. Then, the very fast load act due to the pressure, at which the propellant tensile strength is high. At the same time, the very slow thermal load act on the grain, and in these conditions the propellant tensile strength is low. The vector addition of the stresses and strains due to different loads is not possible. It is also not possible to define the equivalent or resultant values of tensile strength and allowable strain. The principle of adding the current damages is applied here, similar to the model of cumulative damage. In addition, due to the large variations in mechanical properties of the rocket propellant, it is necessary to apply the methods of mathematical statistics for assessing the propellant grain reliability and service life.

Keywords: Propellant Grain, Viscoelasticity, Stress, Tensile strength, Damage.