Lunar Lander Demo Spec

Goal

Build a Lunar Lander-style physics demo that exercises gravity, thrust, rotation, fuel, terrain collision, landing evaluation, scoring, restart flow, and persistent interactive rendering.

Player Experience

The game opens to a side-view lunar terrain with a lander above the surface, a marked landing pad, and HUD. The player rotates, applies thrust, manages fuel, and attempts to land gently on the pad with acceptable speed and angle.

World

• Use a 2D side-view playfield with uneven terrain and one clearly marked landing pad.
• The lander starts above the terrain with downward or neutral velocity.
• Gravity constantly accelerates the lander downward.
• Terrain and pad remain visible at all times in the base demo.
• World edges may clamp or wrap horizontally; choose one and show it consistently. Prefer horizontal wrapping for classic feel.

Controls

• Left or A: rotate lander counterclockwise.
• Right or D: rotate lander clockwise.
• Up, W, or Space: main thrust while held.
• Down or S: optional weaker retro-thrust if implemented.
• P: pause or resume.
• R: restart after landing or crash.
• Esc: request close when hosted interactively.

Physics

• Lander has position, velocity, rotation, angular velocity if used, mass-like tuning, and fuel.
• Gravity is constant.
• Main thrust applies acceleration in the lander's facing direction and consumes fuel.
• Rotation changes orientation at a fixed rate or through angular velocity.
• When fuel reaches zero, thrust input has no effect.
• Collision with terrain evaluates crash or landing.
• Landing succeeds only on the pad, with vertical speed, horizontal speed, and angle within thresholds.

Landing Rules

Default safe landing thresholds:

• Vertical speed at contact: no more than 35 units per second.
• Horizontal speed at contact: no more than 20 units per second.
• Angle from upright: no more than 12 degrees.
• Contact point must overlap the landing pad.

Anything outside those thresholds is a crash.

Scoring

• Successful landing base score: 1000.
• Fuel bonus: remaining fuel rounded down times 5.
• Softness bonus: higher bonus for lower landing speed.
• Crash score: 0 for that attempt.
• Show attempt count.

Visual Requirements

• Render dark space background, terrain line or filled terrain, landing pad, lander, thrust flame, and HUD.
• Lander orientation must be visually obvious.
• Thrust flame appears only while thrust is active and fuel remains.
• HUD shows fuel, vertical speed, horizontal speed, angle, score, attempt, and state.
• Crash or landed state should be clear without hiding the lander and pad.

Game State

Track at minimum:

• Lander position, velocity, rotation, thrusting flag, fuel, alive/landed/crashed state.
• Terrain points, pad bounds, gravity, and physics tuning constants.
• Score, attempt count, paused flag, elapsed time, and random seed if terrain can vary.

Determinism and Evidence

• Accept an optional seed if terrain varies; fixed terrain is also acceptable.
• Evidence mode should inject rotation and thrust inputs, run long enough to show lander motion and thrust, and exit explicitly.
• Evidence outcome should include frame count, input count, lander position, velocity, rotation, fuel, state, score, and close reason.
• Screenshot evidence should show the lander, terrain, landing pad, thrust or HUD evidence of motion, and flight metrics.

Acceptance Criteria

• Gravity changes vertical velocity over time.
• Thrust consumes fuel and changes velocity in the lander's facing direction.
• Rotation changes lander orientation.
• Fuel depletion disables thrust.
• Terrain collision evaluates crash.
• Pad contact with safe speed and angle evaluates successful landing.
• Restart resets the attempt while preserving deterministic terrain.
• Pause freezes physics and timers.
• Interactive mode remains open until explicitly closed.

Out of Scope

• Orbital mechanics.
• Multiple landing pads.
• Procedural terrain requirement.
• Complex particle effects.
• Required audio.