Thermal Louvers: Passive Bimetallic Radiator Control

Thermal Load

Internal heat load 70 W

Louver Bimetallic Spring

Set-point temperature 20 °C

Spring response time τ 8 s

Control Mode

Auto lets the bimetallic spring drive the blade angle from the baseplate temperature, exactly like a real passive louver. The locked modes bypass the spring so you can compare against the uncontrolled radiator. Drag the orbit dial (top-right of the view) to scrub the spacecraft to any point in its orbit.

Live Readouts

Baseplate temp

20.0 °C

Blade angle

45°

Effective ε

0.47

Net heat flow

0 W

How it works

Each louver blade is closed by a bimetallic coil spring that senses the radiator baseplate temperature directly — no sensor, no electronics, no moving parts beyond the spring itself. As the plate warms, the spring uncoils and rotates the blades open; as it cools, the spring recoils and closes them. This exact mechanism flew on Pioneer, Voyager and Cassini.

Blade target angle (0-90°):
  θ_target = clamp( 90° · (T − T_closed) / (T_open − T_closed), 0°, 90° )

Spring lag (first-order):
  dθ/dt = (θ_target − θ) / τ

Effective radiator emissivity:
  ε_eff(θ) = ε_closed + (ε_open − ε_closed) · (θ / 90°)

Baseplate energy balance:
  C · dT/dt = Q_internal + Q_solar(sunlit) − ε_eff(θ) · σ · A · T⁴

Internal heat load — electronics dissipation, always on (orange arrow into the plate).
Q_solar — absorbed sunlight/albedo; switches off the instant the orbit dial shows the spacecraft crossing into Earth's shadow (yellow arrow, only while sunlit).
ε_eff — blends between a nearly-reflective closed louver (ε≈0.12) and the fully exposed black radiator coating (ε≈0.82) as the blades rotate 0°→90° (red/blue arrows out through the louvers, scaled by radiated power).
Because ε_eff rises with T, this is a negative-feedback loop: a hot plate opens the louvers and radiates harder, a cold plate closes them and conserves heat — entirely passive, with the set-point and τ sliders standing in for the spring's calibration and stiffness. The strip chart below the diagram plots baseplate temperature against the closed/open band so you can watch the loop settle.

☀ SUNLIT

Radiator baseplate Louver blades

drag to scrub orbit