minecraft-assets/data/minecraft/recipe/firework_rocket_simple.json

{
  "type": "minecraft:crafting_shapeless",
  "category": "misc",
  "ingredients": [
    "minecraft:gunpowder",
    "minecraft:paper"
  ],
  "result": {
    "count": 3,
    "id": "minecraft:firework_rocket"
  }
}