*Abstract of Proposed Technical Paper*

*2^nd^ International Planetary Probe Workshop*

**Family System of Advanced Charring Ablators**

**for Planetary Exploration Missions**

William M. Congdon

Applied Research Associates, Inc.

Donald M. Curry

NASA Johnson Space Center

Under NASA's In-Space Propulsion Program, extensive thermal testing was
performed during the past two years to characterize and qualify selected
ARA ablative materials for NASA planetary exploration missions.
Principal facilities were the IHF arc-jet at Ames Research Center for
convective heating tests and the Solar Tower Facility at Sandia National
Laboratories for thermal radiation tests. Missions of interest included
spacecraft aerocapture at Titan and direct entry into Mars. The primary
ARA ablators were low-density silicones and phenolics, all under 20
lb/ft^3^ (0.32 g/cm^3^). Planned aerocapture at Titan has shown
relatively mild levels of convective heating up to approximately 41
W/cm^2^ but more severe radiative heating up to about 150 W/cm^2^. Mars
entry for upcoming missions may involve convective heating to as high as
300 W/cm^2^. This proposed paper focuses of finalized test results from
the arc-jet and solar facilities and the resulting thermal protection
system designs for several planned missions. TPS thickness and weight as
a function of alternative aeroshell structures are also addressed.

Because of Titan's relatively high radiation environment, some initial
concerns were that: 1) radiation might penetrate in-depth within the
ablator, thus increasing its "apparent" thermal conductivity and
degrading insulation performance; 2) radiation might cause high rates of
subsurface pyrolysis and generation of high-pressure gasses sufficient
to cause ablator spallation; and 3) some radiation might actually
penetrate/transmit to the bondline resulting in early and/or significant
temperature increase of the aeroshell structure. During thermal
radiation testing that was conducted in the Solar Tower all of these
concerns were addressed and found to not be a problem for the ARA
ablator formulations. The materials were subjected to square pulse
environments at flux levels of 100 and 150 W/cm^2^ for times that far
exceeded predicted flight durations for such high heat heating. They
were also subjected to "exact" flux vs. time environments in test using
programmed heliostat focusing in the Solar Tower facility. All of the
candidate materials showed no spallation and very good thermal
performance to these imposed environments. The figure below shows
posttest photographs of the ARA SRAM-17 silicone ablator after exposure
to the worst-case predicted radiation environment for the Titan mission,
which is a 145-second shaped pulse that peaks at 150 W/cm^2^ with total
integrated heating of 10,610 J/cm^2^.

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SRAM-17 Sample 4056 After Solar Tower Testing to

Titan's 150 W/cm^2^ Shaped Radiation Pulse