Following a detailed literature survey on the fracture-mechanics properties of solid rocket propellants, this paper reports on an innovative set of fracture tests performed on a composite solid propellant based on ammonium perchlorate hydroxyl-terminated polybutadiene. After a short summary on standard linear–viscoelastic mechanical characterization, results on both linear–elastic fracture-mechanics (characterized by the fracture toughness KIC) and nonlinear fracture-mechanics (characterized by GF) tests are reported. Test results for linear–elastic fracturemechanics simulations have been obtained usingmiddle-tension specimens. A practical methodology to separate the amount of strain energy lost through viscous processes from other sources is given and provides an effective method to apply the toughness-test validity criteria of the American Society for Testing and Materials E399 norm to propellants and other thermoviscoelastic materials. Measurements to determine the linear fracture-mechanics properties of the propellant have been carried out applying the wedge-splitting test methodology. Master curves for the toughness, the critical crack-opening displacement, and the fracture energy have been generated to correlate test data. Results are coherent with Shapery’s theory of fracture for viscoelastic materials. Results can be used within finite element simulations to assess the safety and integrity of a solid-propellant rocket motor under various loads, such as thermal cycling and ignition, assuming stationary conditions.