Start Date
28-2-2012 11:25 AM
Description
This presentation describes an experiment that solved a mysterious problem affecting the material that protects solid rocket motor cases from burning propellant gases: Why was the ablation of this material in the forward dome region of recovered flight test motors as much as 2x that observed in full-scale ground test firings?
Heat transfer to the forward dome elastomeric insulation is predominantly radiation from the burning propellant. The 15- kW CO2 laser at the Wright Patterson AFB Laser Hardened Material Evaluation Laboratory (LHMEL) was used to provide uniform incident radiation from 200 to 400 W/cm2. We suspected that the rocket acceleration force (which isn’t simulated in static firing tests) was removing char layers, so we used a centrifuge to simulate these forces. Gold-plated mirrors directed the laser beam from the centrifuge centerline onto the outward-facing material specimen mounted on the rotating arm. A pyrometer and video camera were also mounted on the arm to observe the specimen response. Accelerations of 0 to 20 gs were generated by varying the centrifuge RPM, and the specimen velocity provided a rudimentary simulation of the convective environment.
The highlight of this presentation is a video that very clearly shows char layers being removed by acceleration forces. At zero and very low acceleration levels, the char layer is robust. At higher acceleration, char layers sequentially grow and are then pulled off when their mass x acceleration exceeds their tensile strength x area. The removal frequency increases with acceleration and heat flux, and some materials are more susceptible than others. The pyrometer data shows a saw-tooth pattern with abrupt surface temperature decreases when char layers are removed to expose virgin material.
This phenomenon was modeled using the CMA code modified to include effects of acceleration forces, pyrolysis gas porous flow forces, and char strength. Predictions were consistent with experimental data when an appropriate char strength was input, but of course this was essentially a fudge factor since its impractical to directly measure the tensile strength of an ablating char.
Direct Observation of Mechanical Ablation
This presentation describes an experiment that solved a mysterious problem affecting the material that protects solid rocket motor cases from burning propellant gases: Why was the ablation of this material in the forward dome region of recovered flight test motors as much as 2x that observed in full-scale ground test firings?
Heat transfer to the forward dome elastomeric insulation is predominantly radiation from the burning propellant. The 15- kW CO2 laser at the Wright Patterson AFB Laser Hardened Material Evaluation Laboratory (LHMEL) was used to provide uniform incident radiation from 200 to 400 W/cm2. We suspected that the rocket acceleration force (which isn’t simulated in static firing tests) was removing char layers, so we used a centrifuge to simulate these forces. Gold-plated mirrors directed the laser beam from the centrifuge centerline onto the outward-facing material specimen mounted on the rotating arm. A pyrometer and video camera were also mounted on the arm to observe the specimen response. Accelerations of 0 to 20 gs were generated by varying the centrifuge RPM, and the specimen velocity provided a rudimentary simulation of the convective environment.
The highlight of this presentation is a video that very clearly shows char layers being removed by acceleration forces. At zero and very low acceleration levels, the char layer is robust. At higher acceleration, char layers sequentially grow and are then pulled off when their mass x acceleration exceeds their tensile strength x area. The removal frequency increases with acceleration and heat flux, and some materials are more susceptible than others. The pyrometer data shows a saw-tooth pattern with abrupt surface temperature decreases when char layers are removed to expose virgin material.
This phenomenon was modeled using the CMA code modified to include effects of acceleration forces, pyrolysis gas porous flow forces, and char strength. Predictions were consistent with experimental data when an appropriate char strength was input, but of course this was essentially a fudge factor since its impractical to directly measure the tensile strength of an ablating char.