Heat oven to 275°F (135°C). Line a rimmed baking sheet with two large sheets of heavy foil for each rack. Pat ribs dry. If a tough membrane remains on the bone side, lift an edge with a blunt utensil and pull it off with a paper towel; do not cut toward your hand.

Mix sugar, paprika, garlic powder, onion powder, salt, pepper and optional cayenne. Rub evenly over both racks. Lay each rack meat-side up on its foil, turn foil edges upward and add half the apple juice around each rack without rinsing off the rub.

Seal packets tightly, allowing a little space above the meat. Place on the rimmed tray and bake 3–4 hours. Begin checking at 3 hours, opening carefully to release steam. Ribs are tender when a skewer slides between bones with little resistance and the rack bends easily; continue covered in 20-minute increments if firm.

Carefully open and drain packets, saving juices if desired. Mix barbecue sauce and vinegar. Brush about two-thirds of the sauce over the ribs. Raise oven to 425°F (220°C) and return ribs uncovered for 10–15 minutes until the glaze is sticky and bubbling, watching closely because sugar burns.

Rest 10 minutes, then cut between bones into portions and serve remaining sauce separately. Pork must reach at least 145°F (63°C) with a 3-minute rest for safety, but ribs generally need a much higher temperature and longer cooking to become tender. Bone weight, rack thickness and sauce brand affect yield and nutrition substantially.