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M A Y 2 0 1 4 | 7 9 (Denver, Colo.). Because Lockheed Mar- tin had built all the aeroshells for JPL's six previous Mars rovers, the MSL engi- neers were able to call on a considerable design heritage. Richard Hund, Lockheed Martin's MSL aeroshell program manager, clari- fes that Lockheed Martin considered all the loads that the aeroshell would see throughout its life, including test loads as well as launch and entry loads. Differ- ent load cases and different events infu- ence the design of different parts of the structure, he explains. For example, the design of the aeroshell structure's parts that are closest to the launch vehicle is typically driven by the launch loads. The primary design loads, however, are sustained during entry into Mars' atmosphere. "The aeroshell is traveling at a certain velocity," says Hund. "When it hits the Mars atmosphere, the rover wants to keep driving in at velocity, but friction in the atmosphere is slamming on the brakes." These pressure loads are transmitted along the primary load path from the heat shield up into the backshell, through the backshell and back down to the rover inside. Entry test loads, then, were more than 100,000 lb- force/45,359 kg-force, enough to safely withstand an entry velocity of ~13,200 mph (5,900 m/sec). According to David Scholtz, Lockheed's heat shield principal engineer, the MSL team employed NX NASTRAN to perform fnite element analysis of the structure, and MSC Adams dynamic simulation software for Mar's entry/landing compo- nent-separation analyses (MSC Software Corp., Newport Beach, Calif.), Biconic backshell Scholtz says the biggest challenge in de- signing the MLS aeroshell was to "provide the volume [space] in the aeroshell that JPL needed for the rover within the con- straints of the Atlas V launch vehicle and the payload fairing [the expendable clam- shell that protects the spacecraft during launch]." The solution was a 15-ft/4.5m maximum diameter asymmetrical biconic backshell and heat shield. Biconic refers to two cones joined to- gether at their perimeters, but in this case, composites processing enabled Lockheed Martin to build the backshell structure in one large piece by using sandwich construction. The backshell's structural sandwich is formed with pre- preg made with a satin-weave fabric wo- ven from Toray M55J, an intermediate modulus carbon fber from Toray Carbon Fibers America (Flower Mound, Texas), in a BTCy-1A cyanate ester prepreg from TenCate Advanced Composites (Morgan Hill, Calif.), with a Flex-Core aluminum vented honeycomb core. BTCy-1A is a toughened resin that offers the advan- tages of low-pressure 350°F/177°C pro- cessing, low moisture absorption and negligible volatile emission in space. It has characteristics similar to epoxy, but was developed specifcally for service in vacuum as an alternative to epoxy, which is more hygroscopic (water-absorbent) and tends to outgas in a vacuum. Lockheed Martin hand layed the skins and core over a male mold. The prepreg's satin weave helped it conform to the part contours. The ply schedule was tailored for stiffness and strength to accommo- date the expected loads. Six-inch/152- mm prepreg tape was added in areas that required local reinforcement (e.g., load points for interfaces to other hardware). A proprietary fastening system was de- signed specifcally for the shell's access doors, to facilitate quick removal and re- placement on the launch pad. Notably, the part was oven-cured at 350°F/177°C for about two hours. Although the backshell is not the frst line of defense against the extreme heat generated during entry into the Martian atmosphere, it is in the eddy of the heat wave. For thermal protection, therefore, Lockheed Martin covered the cured back- shell with its Super Light Ablator SLA- 561V system, a ground cork, silica and phenolic mixture in a silicone binder that is hand packed into a phenolic hon- eycomb, developed by Lockheed Martin (Bethesda, Md.). After ablator applica- tion, the backshell was painted white. Heat shield But the backshell was only half of the aeroshell solution. The other half, the heat shield, entailed more than its share of the design challenges, in part, be- ROVER TO A SAFE MARS LANDING Packed and ready for launch With Curiosity securely seated inside, the heat shield (shown here on top) and the backshell are joined. The phenolic impregnated carbon ablator (PICA) heat shield protected the MSL from extreme entry conditions. The phenolic resin ablates as it undergoes pyrolysis and forms a carbonaceous char. Source: NASA/JPL by donna dawson illustration / karl reque stuck the landing! 0514hpc FOD-OK.indd 79 4/22/2014 3:40:28 PM