JETSAM

JETSAM is the first Channel crossing by air, played as a weight problem. 7 January 1785: Jean-Pierre Blanchard and Dr John Jeffries lift from the cliffs of Dover in a hydrogen balloon, carrying the first letters ever to cross a frontier by air. A gas balloon has no throttle and no burner. Lift fades all afternoon — the winter sun weakens, hydrogen creeps out through the varnished silk — and halfway over, a bank of cloud takes the sun off the envelope, the gas cools and contracts, and the Channel rises to meet you. What you have is a valve you will regret opening, and a basket full of things that can be made to leave.

How to play. Four keys. W vents gas — she sinks, and the gas never comes back. Space heaves the next item overboard, in fixed order: sandbags first, the mail last. A/D work the aerial oars and the moulinet for a few honest newtons — enough for the last hundred yards, no more, exactly as history found. Ride the wind layers: the surface air drifts you backward, the cruise layer runs for France, the high air is faster but carries you off-course. One sandbag buys about +1.9 m/s of climb; two seconds on the valve costs about −1.1. Land on French soil, gently, with as much still aboard as you can bear.

The twist is the manifest. Every item thrown is score forfeited, and the wardrobe is on it. Sandbags are free — that is what they are for. Then it gets expensive: the grapnel (which would have softened your landing), the oars (which are your only propulsion), the instruments, the brandy, the coats, and finally Blanchard's trousers. The real crew threw it all and arrived over Calais half-dressed with the mail intact. The hidden badge is named accordingly.

Under the silk. The envelope is a real soft body: a 64-particle ring solved by XPBD at a fixed 1/240 s timestep, near-inextensible in tension but slack in compression, so half-empty silk folds and droops instead of holding a drawn shape. Interior pressure is hydrostatic about the simulated gas/air interface, which makes shape and buoyancy literally the same force — the pear-shaped sag of a spent balloon and the lift that keeps you dry come out of one integral. The gas follows the gas law with automatic spill at pressure height; the mid-Channel descent is not scripted, it is simply what cold hydrogen does. Layered winds with Ornstein–Uhlenbeck gusts visibly buffet the fabric, and the basket ships seawater by the kilogram if you touch the swell.

How it was made this run: the concept and every word here were written by Claude Fable 5; the whole simulation was built, profiled and tuned in a single HTML file by a Claude Opus 5 agent, which flew its own scripted pilots across 24 weather seeds (competent play wins 23 of them; hugging the waves ditches a third of the way across, every time), calibrated the sandbag against the valve with twin-world tests, and passed a 68-point self-review before the file left the bench.