The current baseline ablator material for the Advanced Development Program (ADP) for the thermal protection system (TPS) of the Orion heat shield is phenolic impregnated carbon ablator (PICA). PICA is a low-density, low-strength material that must be isolated from mechanically and thermally induced deformations and strains of the underlying heat shield carrier structure. The current invention is being developed to provide a means of eliminating gaps between adjacent PICA blocks by filling the gaps with a compatible, relatively soft material that alleviates thermal and mechanical stresses that would occur in rigidly bonded PICA blocks. An ideal gap material should have comparable thermal and ablative performance to PICA, and have low enough porosity to prevent hot gas flow in the gap. It must be compliant enough that adjacent PICA blocks can move somewhat independently of each other and the underlying carrier structure to reduce thermal and mechanical stresses to acceptable levels.

PICA-on-Edge uses strips of preconditioned PICA in an edgewise orientation to fill gaps between adjacent PICA blocks, yet allows the blocks to be structurally uncoupled. Joints made of this material can be wider than competing concepts, which can accommodate larger joint motions. PICA-on-Edge can also provide a verified bond between PICA blocks that provides a secondary attachment to prevent a PICA block from detaching from the heat shield, even if its primary bond to the structure fails. Other gap fillers either provide no secondary attachment, or their secondary attachment cannot be verified.

During through-thickness comparison testing, PICA was found to spring back after large compressive strains with little visible damage. Typical stress-strain curve shows nonlinear behavior, a large amount of hysteresis, and a very soft unloading curve.

Additional through-thickness cyclic compression tests were performed on two PICA specimens to determine whether the initial compression cycle affected the structural properties. The specimens were subjected to an initial compression cycle to 25% strain and four subsequent cycles to 15% strain. The first compression cycle was consistent with results from other tests. Cycles 2 through 4 exhibited repeatable, much softer behavior than the first cycle. The initial stiffness dropped more than an order of magnitude from 3,800 to 268 psi (≈26.2 to 1.8 MPa). These results indicate that edgewise PICA is an excellent candidate for use as a gap filler because it has repeatable soft behavior over a large strain range.

The two compressively cycled PICA specimens were then tested in tension to see if the through-thickness tensile properties had changed. The through-thickness tensile modulus is reduced by 5,000 psi to approximately 100 psi (≈34.5 to 0.7 MPa). The strain to failure is increased from 0.6% to approximately 6%. These results indicate that preconditioned PICA has soft behavior and a useful strain range in tension.

This work was done by Max L. Blosser and Carl C. Poteet of Langley Research Center, and Stan A. Bouslog of Johnson Space Center. LAR-17636-1

NASA Tech Briefs Magazine

This article first appeared in the October, 2014 issue of NASA Tech Briefs Magazine.

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