Aerodynamics

Archean simulates aerodynamic forces that automatically apply to any vehicle moving through a fluid medium — whether it's air or water. These forces include drag (resistance to motion), lift (perpendicular force from thin surfaces), and buoyancy (upward force from fluid displacement). Understanding how these systems work is key to designing efficient aircraft, boats, submarines, and any other moving creation.

How It Works

Fluid Medium

The physics engine queries the environment at each relevant point on your vehicle to determine the local fluid properties:

• In air, density decreases with altitude. Higher altitude means less drag and lift.
• In water, density is roughly 800× greater than air — aerodynamic forces are dramatically stronger.
• In space (vacuum), density is 0 — no aerodynamic forces apply at all.

Aerodynamic forces only activate when a vehicle's speed exceeds 0.1 m/s. Below that threshold, forces are not computed.

Drag

Drag is the force that opposes a vehicle's motion through a fluid. It acts in the opposite direction of the velocity.

The drag force on each exposed surface follows the standard aerodynamic equation:

F = ½ × C_d × ρ × v² × A

Key points:

• Drag grows with the square of speed — doubling your speed quadruples the drag
• Only exposed surfaces contribute to drag (see Occlusion)
• The force is computed per surface, at each surface's position, which means drag can also induce torque (rotation) if applied off-center

Lift

Lift is generated automatically by thin, flat structures — such as wings or fins — that the physics engine detects based on geometry.

A surface is classified as a lift surface when all of the following conditions are met:

When a lift surface is detected:

• The lift coefficient depends on the angle of attack: C_l = sin(|angle_of_attack| × π/2)
• The drag coefficient is very low: only 0.01 (compared to 0.4 for regular surfaces)
• Lift force is perpendicular to the velocity, pushing the vehicle in the direction of the surface normal

To build wings that generate lift, use flat arrangements of blocks at least 4 meters long and less than 0.3 meters thick. Slopes can be used to shape the leading and trailing edges.

Buoyancy

Buoyancy is the upward force exerted on a submerged or partially submerged object. It opposes gravity and depends on how much fluid the vehicle's blocks displace.

F_buoyancy = V_displaced × ρ_fluid × g

• The engine samples at least 16 random points across all colliders to handle partial submersion smoothly
• Each block's contribution depends on its material's volumeDisplacementRatio (see Materials)
• Buoyancy is applied at each sample point, so a vehicle can tilt based on uneven submersion

Blocks and Shapes

Block Shapes

Different block shapes have different occupancy ratios, which directly affect drag calculations:

The occupancy ratio scales the calculated drag area — a slope block facing the wind produces roughly half the drag of a cube in the same position.

Materials

Each block material has different physical properties that affect aerodynamics, buoyancy, and mass:

The volume displacement ratio determines how much a block contributes to buoyancy and how visible it is to the aerodynamic surface detection:

• Lead (1.0) fully displaces fluid — maximum buoyancy force but also very heavy, so it sinks
• Steel/Aluminium (0.01) barely displace fluid — they contribute almost no buoyancy
• Composite (0.2) offers a moderate balance between buoyancy and light weight

Occlusion and Exposed Surfaces

The aerodynamic system uses raycasting to determine which surfaces are actually exposed to the airflow:

1. For each block collider, the engine identifies the surface facing the velocity direction
2. A ray is cast from that surface outward in the velocity direction
3. If the ray hits another block of the same vehicle, that surface is considered occluded and does not contribute to drag or lift
4. Only truly exposed surfaces generate aerodynamic forces

This means:

• Internal blocks inside a hull add no drag — only the outer shell matters
• A compact vehicle with fewer exposed faces has less drag than a spread-out structure
• When a group of blocks has an occupancy ratio below 0.9, the system recursively examines the individual child blocks to find the actual exposed surfaces

This is an important optimization point: two vehicles with the same outer shape but different internal structures will experience the same aerodynamic drag. Fill interiors freely without worrying about added drag.

Frame Beams

Frame beams (the structural bars at the edges of frames) have a volume displacement ratio of 0. This means:

• They produce no drag
• They produce no lift
• They produce no buoyancy
• They only serve as structural collision geometry

Frame beams are aerodynamically invisible. Use them freely for internal structure without affecting your vehicle's aerodynamic performance.

Aerodynamic Components

Aileron

The Aileron is a control surface that deflects to create forces perpendicular to the airflow. It is used to steer aircraft and watercraft.

• Input: a value between -1.0 and +1.0 through its data port, controlling rotation from -45° to +45°
• Force: proportional to fluid density × speed² × deflection angle
• Does not compute occlusion — unlike blocks, the aileron always generates its full force regardless of surrounding geometry

Because ailerons ignore occlusion, you can hide them inside wings made of blocks. The blocks will have their surfaces occluded (reducing drag), while the ailerons still produce their full control force.

Propeller

The Propeller generates thrust by spinning blades through a fluid medium. It works in both air and water.

Key physics:

• Thrust = ½ × ρ × A_disc × v_effective² × 0.4
• Drag on blades = ½ × ρ × viscosity × A_disc × v_effective² × 10.0
• Ground effect: when a propeller is near the ground and pointing downward, thrust increases by up to +50% (within 3× blade radius of terrain)
• Gyroscopic precession: spinning propellers resist changes in orientation, creating a torque perpendicular to the rotation axis — just like real gyroscopes
• Maximum thrust is capped at 100,000 N

Thruster & RCS

Chemical Thrusters generate thrust through fuel combustion and are not affected by external aerodynamics for their thrust output — they work the same in atmosphere and in vacuum.

RCS (Reaction Control System) thrusters, however, experience atmospheric attenuation:

attenuation = max(e^-ρ×4, 0.01)

RCS thrusters are designed for space maneuvering. In dense atmospheres or water, their effectiveness drops dramatically.

Water Physics

When a vehicle enters water, the physics engine applies additional damping effects beyond standard drag:

Water Detection

The engine detects water by measuring the environment's viscosity. A viscosity between 0.0000151 and 0.000999 kg/(m·s) is classified as water.

Water Damping Effects

The submersion factor is calculated from the average viscosity: submerged = clamp(pow(viscosity × 1000, 0.1), 0.5, 1.0)

Water naturally stabilizes vehicles. A partially submerged vehicle will resist tipping over due to the pitch/roll damping. This makes boats inherently more stable than aircraft.

High-Speed Angular Stability

At speeds above 10 m/s, the physics engine applies an artificial angular damping that simulates pressure buildup on the vehicle's surfaces:

ω -= ω × min(1, ρ) × clamp(Δt × |v| / 25, 0, 0.025)

This means:

• Faster vehicles are more rotationally stable
• Denser fluids (water > air) provide stronger stabilization
• This prevents vehicles from tumbling uncontrollably at high speeds
• In water at high viscosity, an additional angular damping factor is applied

Design Tips

Reducing Drag

• Minimize exposed surface area — a compact, streamlined shape creates less drag
• Use slopes, corners, and bevels on leading edges and noses instead of flat cube faces
• Internal blocks don't add drag — only the outer shell matters, so fill interiors as needed
• Frame beams are aerodynamically invisible — use them freely for internal structure

Building Effective Wings

• Wings must be at least 4 meters long, less than 0.3 meters thick
• A wider wingspan (width ≥ length/4) ensures the surface is classified as a lift surface rather than a drag surface

Watercraft Design

• Composite blocks (ratio 0.2) offer the best buoyancy-to-weight balance for floating
• Steel and Aluminium (ratio 0.01) barely contribute to buoyancy — use them sparingly in boats
• Lead (ratio 1.0) displaces the most fluid, but at 150 kg per unit it will sink rapidly
• Water damping naturally stabilizes your vessel — wide, flat hulls are most stable

Propeller Placement

• Ground effect boosts thrust by up to 50% when close to terrain — useful for hovercraft designs
• Propellers generate gyroscopic torque — counter-rotating propeller pairs cancel this effect
• Propellers work in both air and water, adapting their thrust based on fluid density and viscosity