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

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