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Josh Creek
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productive_air_touch_fraction sat at exactly 0.0 across nine Stage-5 attempts and 540M timesteps. Two rounds of reward shaping were aimed at it (air_approach_weight, then air_touch_bonus_weight); both worked -- airborne_fraction 0.223->0.258, mean_altitude 2.59->3.25, vertical_thrust_mean 0.004->0.063 -- and the ship now visibly plays the ball in the air. The metric could not see it because it counts only touches with the ball above AIR_TOUCH_HEIGHT (5m), and _place_air_intercept never produced a reachable one. Simulating the spawn distribution against the ship's flight envelope (vertical_thrust 120 / mass 5 = 24 m/s^2 less gravity, drag capping climb near 12 m/s): a ball spawned 6-12m up at 6-11 m/s is above 5m for a median of 0.80s, while the ship spawned 7-13m behind, 3-10m below, and at a dead stop. An ideal interceptor -- point mass, instant attitude, no righting torque, zero reaction delay -- makes that touch in 0.00% of episodes and reaches the ball at all in 0.5%. Retune the drill instead of the reward: ball higher (8-14m) and slower (4-8 m/s), ship closer (4-9m behind), narrower lateral spread, and a 6-14 m/s planar run-up rather than a standing start -- the dead stop was the largest single factor. Ideal interceptor now reaches the ball in ~98% of episodes and above 5m in ~37%, so the 0.005 floor has headroom. AIR_TOUCH_HEIGHT stays 5.0 so the metric remains comparable with earlier generations. Resume from retry2 rather than restarting from Stage 4: that rule guards against a changed reward function invalidating the value function, and the reward function is untouched here -- only the state distribution moved, so the policy that already learned to fly is what should be pointed at a reachable target. Adds a one-shot resume_override to generation5_state.json, consumed on first use.