From surface temperatures as high as ≈3,000 °C, the measured back temperature is only 50 °C
Ablative materials are required to protect a space vehicle from the extreme temperatures encountered during the most demanding (hyperbolic) atmospheric entry velocities, either for probes launched toward other celestial bodies, or coming back to Earth from deep space missions. To that effect, the resinimpregnated carbon ablator (RICA) is a high-temperature carbon/phenolic ablative thermal protection system (TPS) material designed to use modern and commercially viable components in its manufacture. Heritage carbon/phenolic ablators intended for this use rely on materials that are no longer in production (i.e., Galileo, Pioneer Venus); hence the development of alternatives such as RICA is necessary for future NASA planetary entry and Earth re-entry missions. RICA’s capabilities were initially measured in air for Earth re-entry applications, where it was exposed to a heat flux of 14 MW/m2 for 22 seconds. Methane tests were also carried out for potential application in Saturn’s moon Titan, with a nominal heat flux of 1.4 MW/m2for up to 478 seconds. Three slightly different material formulations were manufactured and subsequently tested at the Plasma Wind Tunnel of the University of Stuttgart in Germany (PWK1) in the summer and fall of 2010. The TPS’ integrity was well preserved in most cases, and results show great promise.
There are several major elements involved in the creation of a successful ablative TPS material: the choice of fabric and resin formulation is only the beginning. The actual processing involved in manufacturing involves a careful choice of temperature, pressure, and time. This manufacturing process must result in a material that survives heat loads with no de-lamination or spallation. Several techniques have been developed to achieve this robustness. Variants of RICA’s material showed no delamination or spallation at intended heat flux levels, and their potential thermal protection capability was demonstrated. Three resin formulations were tested in two separate samples each manufactured under slightly different conditions. A total of six samples was eventually chosen for test at the PWK1. Material performance properties and results for five of those are shown in the table. In the most extreme case, the temperature dropped from ≈3,000 to 50 °C across 1.8 cm, demonstrating the material’s effectiveness in protecting a spacecraft’s structure from the searing heat of entry.
With a manufacturing process that can be easily re-created, RICA has proven to be a viable choice for highspeed hyperbolic entry trajectories, both in methane (Titan) as well as in air (Earth) atmospheres. Further assessment and characterization of spallation and an exact determination of its onset heat flux (if present for intended applications) still remain to be measured.
This work was done by Jaime Esper of Goddard Space Flight Center and Michael Lengowski of the University of Stuttgart. GSC-16183-1