Gear models are also seen thanks to Dr. Granin's colleagues in direct sequels; Metal Gear Solid: Portable Ops features the quadrupedal design Intercontinental Ballistic Metal Gear by Dr. Nikolai Stepanovich Sokolov and Metal Gear Solid: Peace Walker features the first bipedal design Metal Gear ZEKE by Dr. Huey Emmerich. Models in the canonical series Metal Gear TX-55 Metal Gear is the Metal Gear model's designation in the original Metal Gear game, although it is simply referred as "Metal Gear" and no particular meaning is given behind this codename. Within Outer Heaven, Solid Snake learns Metal Gear's weakness after rescuing creator Dr. Drago Pettrovich Madnar and his daughter Ellen Madnar. The player must place a certain number of plastic explosives on Metal Gear's feet in the order stated by Dr. Madnar. Metal Gear is stored in a room guarded by two laser cams and never actually becomes operational; the final challenge is instead a battle with Outer Heaven's mystery leader and a race against time to escape the self-destructing base. Metal Gear D in Metal Gear 2: Solid Snake is a revision of the original Metal Gear prototype. Metal Gear D is equipped with a Vulcan cannon and a six-missile pod. The improved model was also built by Dr. Madnar, willingly this time after his radical ideas were rejected by the Western scientific community. The Metal Gear is piloted by Gray Fox and can only be destroyed by Solid Snake throwing grenades over its feet. This Metal Gear's mecha design was chosen through an internal contest between Konami's artists, with the primary criterion being realism. The final design is credited to Tomohiro Nishio. In the non-canonical sequel Snake's Revenge (that was released in the west for the NES), mass-produced versions of the original (dubbed "Metal Gear 1" in the game) are stored inside an enemy cargo ship. The wreckage of the original makes a cameo in Metal Gear: Ghost Babel, an alternate sequel to the first game in the Metal Gear Solid series set in a rebuilt Outer Heaven. Both Metal Gear TX-55 and Metal Gear D make cameo appearances during the Metal Gear Solid games: D is visible as Dr. Granin's blueprint shown to Naked Snake in Metal Gear Solid 3: Snake Eater, and TX-55 is seen in a psychic vision along with Big Boss and his three "Les Enfants Terribles" sons in Metal Gear Solid: Portable Ops. Metal Gear