Preheat oven to 350°F. Butter a 9-inch pie plate. In food processor, grind graham crackers into crumbs, about 1 minute. Add sugar, butter, and salt and pulse motor until mixture becomes fine crumbs, about 2 to 3 minutes. Press mixture evenly onto bottom and up sides of pie plate. Bake in middle of oven until golden brown, about 10 minutes. Cool crust in pan on rack. In 2-quart saucepan, whisk together sugar, cornstarch, and 1 1/2 cups water. Set over moderate heat and simmer, whisking constantly until mixture becomes thick and clear, about 6 to 7 minutes. Add Jell-O and whisk until dissolved, about 1 minute. Remove from heat and let cool to room temperature. Using sharp knife, cut peel, including all white pith, from grapefruit and cut segments from membranes. Add segments to filling, then pour into crust and chill until firm, at least 3 hours and up to 24 hours. In medium bowl, using electric mixer, beat cream with sugar until it holds soft peaks. Spread evenly over filling and serve.