Mechanical properties of HTPB propellants: Recent advances and underlying mechanisms

Hydroxyl-terminated polybutadiene (HTPB)-based composite solid propellants (CSPs) are widely used in solid rocket motors due to their excellent mechanical properties and low cost. This review synthesizes recent progress on how strain rate, temperature, confinement, aging, and compositional design govern the tensile/compressive strength, Young's modulus, and maximum strain of HTPB-based CSPs. Compiled room-temperature data show that as tensile strain rate increases from ∼10⁻⁴–10⁻¹ s⁻¹, tensile strength typically rises from ∼0.15 to ∼0.96 MPa, Young's modulus from ∼2.45 to ∼8.79 MPa, while maximum strain decreases from ∼0.73 to ∼0.28 MPa. At ∼0.03 s⁻¹, lowering the temperature from 293 K to 223 K increases strength (∼0.37 → 1.48 MPa) and modulus (∼7.07 → 35.30 MPa) while reducing maximum strain (∼0.30–0.61 → 0.15–0.22), consistent with thermo-softening and brittle-to-ductile transitions. This review also delves into the underlying mechanisms driving these property changes, focusing on the roles of binder systems, solid fillers, plasticizers, bonding agents, and core-shell structured fillers. Interfacial engineering further tailors properties, such as MD (molecular dynamics) simulation-guided cyclic borate ester bonding agents increased adhesion work by up to ∼17 % relative to HTPB–RDX baselines, and core–shell fillers (e.g., AP@Al, Al@RDX) enable formulation-level control of rate/temperature responses. The review consolidates these trends and maps multi-factor interactions to guide the design of tougher, more reliable HTPB-based CSPs.