Comparative Evaluation of Ablating Materials in Arc Plasma Jets

In a round-robin ablation study monitored by Stanford Research Institute in 1964, Teflon and high-density phenolic-nylon models were evaluated at various enthalpies and heating rates under supersonic conditions. The results of that study, published in NASA Contractor Report CR-379, showed that the best description of the test environments for twelve different plasma arc heater facilities was given by the stagnation point heating rate, and pressure. The mass loss rates from all facilities could be correlated in terms of these parameters with a standard deviation of approximately 11 percent for both the Teflon and phenolic-nylon material.

The second phase of this study described in this report, also involved twelve facilities, most of them the same as in the first phase. The same two high-density materials, Teflon and phenolic nylon, were evaluated at stagnation pressures up to 10 atmospheres for the former and 30 atmospheres for the latter. The effect of model size was also evaluated using both hemispherical and flat faced Teflon models having effective radii varying from one-quarter to four times the radius used previously. The mass loss rates again correlated with the results from the earlier study, except that high density phenolic nylon models showed a rapid increase in ablation above a stagnation pressure of about 2.5 atmospheres. These higher rates were also correlated, and found to agree, with literature data for this same pressure regime.

Five new low-density materials-Langley phenolic-nylon, Hughes phenolic nylon, Avcoat epoxy novalac-filled honeycomb, Langley silicone elastomer, and General Electric silicone elastomer-were also studied in the second phase at enthalpies from 2500 to 34,000 Btu/lb and stagnation pressures from 0.004 to 2 atmospheres. Mass loss rates, front surface, and internal temperatures were measured for these materials.

Mass loss rate correlations similar to those developed in the first phase of the study were satisfactory except for the silicone materials, which may suggest that the ablation mechanism varies for these in the range of conditions studied. Dimensional analysis was used to develop new correlations for interrelating front surface, internal temperature rise, and test environment. The resulting relations show that data from the various facilities could be satisfactorily compared.