Thermal Materials

Coatings, insulation, heaters and phase change materials selected by flown and qualified planetary robotics programs.

A dusty environment inverts the usual radiator design. IPEx cannot leave a radiator exposed, so it closes an actuated cover over it during excavation, dumps the waste heat into a phase change material, and opens the cover to reject the stored energy and refreeze the material only when operations pause [1]. That turns a passive surface into a mechanism with a seal, a motor, an insulation blanket acting as a thermal switch, and a tribo-charging problem where the cover separates from the radiator.

Thermal materials selected and flown

Part	Manufacturer	Used by	Source
S13GLO white thermal paint	Not named	ipex (radiator plate)	[1]
n-Hexadecane, 99+ percent	KULR	ipex (avionics heat sink)	[1]
Polyetherimide (Ultem), Cerakote coated	Not named	ipex (radiator cover)	[1]
Etched-foil Kapton heaters	Not named	perseverance (Mastcam-Z)	[2]
Aluminized Kapton thermal bag	Not named	Apollo 17 lunar surface receiver	[3]
Thermal control coatings	Not named	Apollo scientific instrument module	[4]
Chromel-R fabric, RTV-630 facing	Not named	Apollo suit boots and gloves	[5]

Ratings and qualification results

• S13GLO white thermal paint, vendor not named. Zinc oxide pigment in an RTV602 silicone binder. Ratings: Applied to the 6061 aluminum cover that forms the top of the phase change material housing and doubles as the radiator surface [1]. Qualification: Chosen specifically as the low-outgassing variant of the S13G family [1]. Measured in a tribo-charging test at 1e-5 Torr against an MLI sheet on a 100 pF capacitor: peak 0.004 V and 0.006 V, giving surface charge densities of 5.25e-6 and 7.87e-6 microcoulomb per square meter. Those two values fed a COMSOL particle trajectory model of whether separation of the cover lofts dust onto the radiator [1].
• n-Hexadecane, 99+ percent, an organic phase change material filled by KULR into a housing built at Kennedy Space Center [1]. Organic phase change material. Ratings: Melting temperature 18 C, calculated latent heat capacity 300.5 kJ [1]. Housed in 6061 aluminum with internal aluminum fins bonded with thermal epoxy to distribute the load as the material expands and contracts; the assembly weighs 2.5 kg and its base is the conductive interface to the avionics enclosure [1]. Qualification: Sized by transient Thermal Desktop analysis of the whole mission profile. Vacuum chamber testing to confirm even heat transfer under the time-varying non-uniform avionics load was still planned when the paper was written [1].
• Polyetherimide (Ultem), Cerakote coated, vendor not named. Non-metallic actuated dust cover carrying a wireless charging antenna. Ratings: Chosen non-metallic so it can hold the wireless charger antenna; a felt seal seats under it against the radiator, and an MLI blanket under the cover works as a thermal switch preventing heat transfer to the phase change material during operation [1]. Qualification: Ballistic particle test with silica sand distributed over the cover and the cover actuated 90 degrees: particles flowed off as expected, with minor accumulation on the radiator cover motor and on the radiator edge nearest the motor, caused by particles deflecting off the motor. That is recorded as a design change item for the next iteration [1].
• Etched-foil Kapton heaters, vendor not named. Thermostatically controlled surface heaters in redundant pairs. Ratings: A redundant pair per camera head with an additional pair of platinum resistance thermometers, controlled by the rover rather than by the camera [2]. Qualification: Flown since February 2021 to hold camera head temperature within a nominal Mars operating range of -40 to +40 C [2].
• Aluminized Kapton thermal bag, vendor not named. Multilayer thermal bag whose flaps are retained by Velcro hook and pile bonded with polyurethane FR-127 A and B. Ratings: Velcro pile pads bonded to the bag, Velcro hook straps bonded to the flaps [3]. Qualification: Flight failure. The pad bonds on both flaps had already failed before the Lunar Module Pilot configured the receiver at the end of the first EVA, allowing dust to accumulate on the mirror surface under both flaps. The pads separated leaving no trace of adhesive on the Kapton. FR-127 A and B was assessed as acceptable and remains recommended for bonding Velcro to Kapton; the failure was attributed to bonding preparation, with mixing, timing and surface cleanliness identified as the controlled parameters [3].
• Thermal control coatings, vendor not named. Applied finishes at 0.3/0.85 and 0.05/0.4 solar absorptivity to infrared emissivity [4]. Ratings: The inside surfaces of the module were coated at 0.3/0.85; the surfaces facing the radial beams, and the beams themselves, at 0.05/0.4. Qualification: Flown on the Apollo J-series service modules, with protective covers and thermal blankets providing individual instrument control on top of the module finishes [4].
• Chromel-R fabric, RTV-630 facing, vendor not named. Abrasion-resistant metallic fabric with a silicone facing, in the extravehicular mobility unit boots and gloves. Ratings: Chromel-R woven into the boots and gloves, RTV-630 used for soles and finger tips [5]. Qualification: Introduced because the super beta cloth outer covering was not abrasion resistant against sharp lunar rock. Dust control on the same suits relied on a single nylon bristle brush, which removed coarse grains but not fines, and which may have transferred nearly as much dust as it removed by the later parts of a mission [5].

What the radiator cover costs

The IPEx thermal design replaces a static radiator with an actuated one and therefore inherits every mechanism failure mode in this database: a seal, a gearmotor whose Hall sensor part number is unidentified, and a separating dielectric interface that tribo-charges [1]. The measured charge densities are small, 5.25e-6 and 7.87e-6 microcoulomb per square meter, but they were measured rather than assumed precisely because the alternative was to accept an unquantified dust attraction mechanism on the one surface that must stay clean.

Thermal control surfaces measured against dust, erosion and orbit

Every coating below has a measured optical property and a measured degradation of it. The degradation is caused by dust coverage or by particle impact rather than by ultraviolet or by dose [6][9].

Part	Manufacturer	Used by	Source
AZ-93 white paint on aluminum	Not named	Glenn dust adhesion bell jar	[6]
AZ-93 white paint on graphite	Not named	Glenn dust adhesion bell jar	[6]
S13GP:6N/LO-I white paint	Not named	Dayton particle erosion rig	[9]
Aluminized Teflon second surface mirror	Not named	Northrop space chamber	[10]
Aluminized Teflon FEP, 5 mil	Not named	MISSE 9 zenith	[12]
Sputtered coatings on Kapton	Not named	LDEF tray S1003	[11]

Ratings and qualification results

• AZ-93 white paint on aluminum. Ratings: pristine emissivity 0.886 plus or minus 0.024 and absorptivity 0.173 plus or minus 0.029, a ratio of 0.195, averaged over four to seven runs [6]. Qualification: extracted by fitting heating and cooling curves to a 743 node Thermal Desktop model in the NASA Glenn Lunar Dust Adhesion Bell Jar, an extraction that assumes a back-solved lamp intensity of 29.1 kW/m2 [6]. The absorptivity to emissivity ratio goes from 0.22 pristine to 0.65 dusted with JSC-1AF, recovering to 0.30 after 20 strokes of a Zephyr fiberglass round brush and 0.26 after 200 [7]. Residual dust coverage after 200 strokes was 3 percent for that brush and 10 percent for an Escoda nylon fan, both leaving about 29 to 31 percent on silvered FEP, with a spread of about a factor of two between samples.
• AZ-93 white paint on graphite. Ratings: pristine emissivity 0.833 plus or minus 0.027 and absorptivity 0.196 plus or minus 0.006, a ratio of 0.235 [6]. Qualification: the substrate is a K-1100 high conductivity graphite surface layer over PAN structural fibers in RS-3 resin, 6.4 mm thick, and it lowers emissivity and raises absorptivity relative to the aluminum case [6]. The same coating therefore has two different ratios depending on what it is painted onto.
• S13GP:6N/LO-I white paint [9]. Ratings: measured with a calibrated TESA 2000 and an AZ Technology portable reflectometer. Qualification: the absorptivity to emissivity ratio rises about 37 percent after regolith exposure at 38 to 44 microns and 250 m/s, as particles embed in the soft silicone binder, and plateaus near 0.55 with increasing mass loading, driven mostly by absorptivity [9]. Under 177 to 250 micron sand at 160 m/s and 30 degrees the erosion is linear until about 18.5 percent of initial weight is lost, at which point the paint separates from the aluminum substrate; the largest recovered chip was about 89 mm2, roughly 24 percent of the exposed area [9]. That failure occurred above the expected landing environment with margin.
• Aluminized Teflon second surface mirror. Ratings: total solar absorptance 0.185 clean [10]. Qualification: 0.864 at full coverage by basalt below 74 microns, through intermediate points of 0.265 at 0.5 coverage, 0.481 at 0.6 and 0.739 at unity, measured on a Gier-Dunkle integrating sphere from 0.3 to 2.45 microns [10]. Absorptance also rises with incidence angle, by 0.208 between 20 and 75 degrees at half coverage against 0.082 to 0.091 on clean samples, so dust roughly doubles the angular sensitivity and reaches 0.875 at 75 degrees. Larger particles rolled off when the sample was mounted vertically, so the dusted state is dominated by fines [10].
• Aluminized Teflon FEP, 5 mil [12]. Ratings: flown on the MISSE Flight Facility with the FEP facing space. Qualification: 67 percent of elongation at failure lost after 0.54 years of direct exposure in the zenith orientation, against 48 percent for the wake samples on the same mission, measured against matched ground controls under ASTM D638-08 Type V [12].
• Sputtered coatings on Kapton, ion beam deposited at 700 angstroms of Al2O3, 650 of SiO2, and 650 of 4 percent PTFE with 96 percent SiO2 [11]. Ratings: flown on Long Duration Exposure Facility tray S1003 at 98 degrees to ram, 4.8e19 atoms/cm2. Qualification: no spalling after about 33600 thermal cycles over 69 months, by post-flight scanning electron microscopy [11]. Inorganic coatings a few hundred angstroms thick held adhesion to a polymer substrate for six years of cycling.

Dust coverage does not have to be complete to matter. On AZ-93 and on silvered FEP under JSC-1AF, solar absorptivity may increase by as much as 50 percent at 12 percent area coverage and emissivity fall by as much as 16 percent at 54 percent coverage, so a sub-monolayer already degrades both [8]. The figure is a cited measurement carried into a design study rather than one taken in it.