HALFSPAN

HALFSPAN is a dawn mail run in an ekranoplan — a wing-in-ground-effect craft, the Caspian kind: a 22-tonne machine with stubby 20-metre wings and eight engines that flies, but only just, and only down where the sea can help. 6.1 kilometres, ten buoy gates, one delivery. The clock stops when you cross the harbour line — but the run only counts once you've put her down and stopped before the seawall.

The twist

Below half a wingspan of altitude, the water gives the wing a cushion: measured in this build, 2.3× lift at the deck, fading to nothing by half a span up, with induced drag collapsing to a fifth. The craft is built for that cushion and cannot fly out of it — pull up and she zoom-climbs on momentum, mushes, and settles back having traded 285 km/h for 180. Speed lives in the bottom three metres of the sky. So every freighter, sea stack and bridge girder on the route asks the same question: thread it low, or pop over and pay.

Two more pieces of real ekranoplan lore made it in. Takeoff is a hump — hull drag peaks before she'll unstick, and the engines alone can barely climb it — so you hold SPACE for PAR, blasting engine thrust under the wing, exactly how the real ones got off the Caspian. And if you chase the pitch with the elevator you'll teach her to porpoise; fly smooth and she settles herself.

How to play

• W / S — throttle
• ← / → — steer
• ↑ / ↓ — pitch
• SPACE — PAR boost (hold; this is how you take off)

C camera, R restart, P pause, H or ? for the controls any time. Full touch controls on phones. Delivery times to beat: gold 112 s · silver 126 s · bronze 160 s — set by autopilots flying the same physics, not by wishful thinking. Lower is better; the board keeps your best run.

How it was made

Built in one automated run: the physics — fixed 1/120 s timestep, full rigid body, a measured ground-effect lift curve, squeeze-film cushion damping, and a procedural swell that the simulation and the renderer read from the same function — was flown headless by a route-planning autopilot a few hundred times to tune the handling and set the medal cutoffs before anyone ever touched the keys. Fifteen bugs were caught and fixed in self-review, including three inverted control-surface signs that each made the craft unflyable in a different way.