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FOCUS ON DESIGN 8 0 | H I G H - P E R F O R M A N C E C O M P O S I T E S L E A R N M O R E @ w w w. c o m p o s i t e s w o r l d . c o m Read this article online at short.compositesworld.com/Curiosity1. Several parts on the descent stage and rover itself were built from composites. See HPC's online report, "The Curiosity Mars rover: Decent stage composites," at short.compositesworld.com/Curiosity2. cause the increased size and mass of the MSL mandated a different angle of entry than past Mars missions. This resulted in a higher velocity and a more extreme ae- ro-thermal entry environment than previ- ous heat shields had experienced. So the heat shield and its thermal protection system (TPS) were designed to withstand the G-forces and up to 250 watts/cm 2 (1,613 watts/in 2 or 1.53 BTU/s/in 2 ) of heat, "about 2,500 times what we feel when we walk out into the bright sunshine," says JPL's principal engineer Eric Slimko. SLA-561V was previously used on the heat shields of the two Viking Landers in 1976. Although early tests indicated this TPS solution would also work for the MSL heat shield, Slimko says parameters of shear stress, turbulence and enthalpy (a measure of the total energy of the ther- modynamic system) had not been fac- tored into the testing process. "Unfortunately, matching the four pa- rameters of heat fux, shear stress, en- thalpy and turbulence is nearly impos- sible to obtain in ground test facilities," Slimko explains. To validate the tests, Slimko's team developed an approach of matching one or two parameters while straddling the remaining parameters. When enthalpy was matched with shear stress and heat fux, the SLA-561V failed test requirements for MSL entry. Suddenly, the heat shield material that had served so well during previous safe entries into Martian airspace was deemed inadequate for the landing environment this larger, heavier rover would face. A search for alternatives produced candi- dates that included a modifed SLA 561V, a carbon-carbon with carbon fber insula- tion, a fully dense carbon phenolic, and the fnalist, phenolic-impregnated car- bon ablator (PICA, pronounced peak-a). Invented at NASA Ames in the early 1990s, PICA comprises a fbrous carbon substrate. Trademarked as FiberForm by Fiber Materials Inc. (Biddeford, Maine), PICA is impregnated with phenolic resin inside a custom-built vessel under pro- prietary temperature and pressure con- ditions. During high heat (in this case, entry into Mars atmosphere), the pheno- lic resin ablates as it undergoes pyrolysis and forms a carbonaceous char. The PICA was cut into tiles, which were arranged in a structural architecture to meet design specifcations for entry forces. Then, the tiles were bonded on- to the outer face of the composite heat shield, using HT424 adhesive flm, an aluminum-flled, modifed epoxy/pheno- lic resin from Cytec Aerospace Materials (Tempe, Ariz.) and grouted using RTV 560, a low-temperature, two-part sili- cone rubber compound from Momentive Specialty Chemicals (Columbus, Ohio). The result was the largest — and the frst tiled — ablative heat shield ever built. Before fnal assembly of the aeroshell, Curiosity was tucked into the heat shield, tethered to the JPL-patented sky crane, for fnal lowering of the rover safely to Mars' surface after separation from the heat shield, backshell and parachute. At- tached to the descent stage by a bridle, or harness, the sky crane, was also connect- ed to the backshell, along with a para- chute and parachute support structure (see drawing, p. 78). The backshell and heat shield were joined by nine separa- tion fttings. These and other attachment fttings featured pyrotechnically released separation bolts so the heat shield could be separated from the rover and back shell during the Mars descent and the backshell and other descent stage com- ponents could be jettisoned during the landing sequence (see graphic, at left). Satisfying Curiosity The aeroshell delivered Curiosity, the larg- est rover yet landed on any planet, un- scathed to Mars' surface in August 2012. In its frst year, the MSL established that ancient Mars offered a wet habitat with conditions favorable to microbial life. The rover is now traveling to Gale Cra- ter, where geological layers might yield further evidence of ancient habitability. When asked if the landing could have been done without composites, Slimko says defnitely not. Designers cite their low weight, high stiffness, low coeffcient of thermal expansion, exceptional produc- ibility and their capacity to form large, complex shapes and biconic angles. Cruise, entry and landing As the MSL Flight System began its descent, the Cruise Stage separated from the aeroshell, which entered Mars' atmosphere 78 miles from its surface, at 13,200 mph (5,900 m/sec). The heat shield endured 3800°F/2100°C temperature. After deceleration aided by onboard engines, the parachute was deployed and the heat shield was jettisoned at the five-mile mark. One mile up, the backshell was jettisoned. At 66 ft/20m elevation, the rover separated from the descent stage but remained attached to the sky crane bridle. At touch down, connecting cords were severed and the descent stage flew clear to avoid collision. Source: NASA/JPL 0514hpc FOD-OK.indd 80 4/22/2014 3:40:48 PM