PICA (phenolic impregnated carbon ablator) was developed for the forebody heat shield of the Stardust Return Capsule. Conventional thermal protection system (TPS) materials of the time (primarily carbon phenolics) had high densities and thermal conductivities, yielding a TPS mass fraction that exceeded mission constraints. PICA was developed in the 1980s and consists of a rigid carbon fibrous substrate infiltrated with phenolic resin, yielding a TPS with good ablation and pyrolysis behavior. In addition, PICA has the advantages of low density coupled with efficient ablative capability at high heat fluxes. Limitations of PICA include relatively low mechanical properties, high recession rates, and poor handling, as the material sheds phenolic powder and is prone to damage from low-velocity impacts.
An approach was developed to apply a phenolic surface densification at a desired loading and density gradation, leading to a surface densification tailorable to a given mission/application. It is not necessary that the same densification be applied to all areas of the PICA forebody TPS material; in fact, optimization of surface densification is recommended as needed.
In its current state, PICA has limitations in terms of shedding of phenolic powder from the virgin material, less-than-desired inner mold line integrity that results in PICA failure at a PICA billet/ SIP interface, and larger-than-desired differential recession downstream of features such as compression pads. Application of a phenolic surface densification treatment can eliminate/ reduce all of these issues. In addition, application of a surface densification layer results in a graded TPS system with lower density than comparable state-of-the-art TPS systems operating at similar maximum heating conditions.
The approach is straightforward in terms of processing and surface treatment application, and can be applied to as-machined PICA materials without having an effect on the final tolerance of the part.