Entry Parameters

Entry angle 6.0°

Entry velocity 7800 m/s

Ballistic coefficient β 350 kg/m²

Lift-to-drag ratio 0.0

Flight Status

READY

Altitude

120.0 km

Velocity

— m/s

G-load

0.0 g

Peak G

— g

Heat flux

—

Peak heat

—

How it works

This is a point-mass re-entry model integrated in the downrange/altitude plane. Three real physical relationships drive it:

Density (exponential atmosphere):
  ρ(h) = ρ0 · e^(−h/H),  ρ0 = 1.225 kg/m³, H = 8500 m

Drag deceleration (drag equation, folded into β):
  a_drag = ½·ρ·v² / β,   β = m / (Cd·A)  [ballistic coefficient]

Heating rate (Sutton-Graves, stagnation point):
  q̇ = 1.83×10⁻¹¹ · √ρ · v³   [kW/cm²]

Because drag depends on ρ·v² and heating on √ρ·v³, both climb sharply only once the capsule reaches thick enough air — peak heating and peak g-load happen at a specific mid-altitude band (typically 40–70 km), never at the 120 km entry interface and never at the ground. The strip chart below the trajectory plots g-load and heat flux against time so you can see that spike happen live, not just read a number.

Entry angle sets the corridor: too shallow (under ~2°) and the capsule's path curves back above 120 km before it has scrubbed off enough speed — a skip-out. Too steep (above ~9°) and it dives into dense air too fast, and the g-load or heat flux crosses the destruction threshold before it can decelerate safely. Lift-to-drag ratio adds a perpendicular force that stretches the deceleration over a longer path, widening the safe corridor — exactly why crewed capsules like Apollo and Soyuz fly a lifting, not purely ballistic, re-entry.

Trail: cold → peak heat flux

⚠ Couldn't start the simulationSomething went wrong initializing the canvas. Reload the page.

2D Atmospheric Re-entry Simulator

Watch a capsule cross the 120 km entry interface and descend through a real exponential atmosphere, in a plain 2D side-view. Density follows ρ = ρ0·e^(−h/8500), drag deceleration follows the standard drag equation folded into the ballistic coefficient β, and aerodynamic heating follows a Sutton-Graves stagnation-point relation that scales with the square root of density times velocity cubed. Tune the entry angle, initial velocity, ballistic coefficient and lift-to-drag ratio, then read the live g-load and heat-flux strip chart underneath the flight path to see exactly where — and why — the punishment peaks, and whether the capsule skips off the atmosphere, burns up, or lands safely.

About this simulation

The trajectory is integrated with a fixed sub-step using the same three equations printed in "How it works": exponential density, the drag equation and Sutton-Graves heating. This is the 2D side-view companion to the 3D Atmospheric Re-entry simulator — same underlying physics, rendered as a real-time flight-path plot paired with live g-load/heat-flux strip charts instead of a 3D scene.

Frequently Asked Questions

Why does heating peak partway down, not at the top or the ground?

Heat flux scales with √ρ·v³. At 120 km, ρ is nearly zero, so heating is negligible even though velocity is highest. Near the ground, velocity has already bled off from drag, so heating is low again. The two effects cross over in a mid-altitude band, typically 40–70 km, which is exactly where peak heating occurs.

What happens if the entry angle is too shallow?

Below roughly 2°, the capsule's path curves back upward before drag has removed enough vertical speed, carrying it back above 120 km — the "skip-out" outcome. It leaves the sensible atmosphere still moving fast, which for an uncrewed probe means a lost mission and for a crewed capsule can mean a dangerously extended flight.

What happens if it's too steep?

Above roughly 9°, the capsule reaches dense air while still near entry velocity. Both drag (∝v²) and heating (∝v³) spike hard in a short time, and the g-load or heat flux crosses the destruction threshold before the trajectory has had time to decelerate gradually.

Why does raising the lift-to-drag ratio help?

A non-zero L/D adds a force perpendicular to velocity, letting the capsule "fly" through the upper atmosphere rather than simply falling into it. That stretches deceleration over a longer path and more time, lowering both peak g-load and peak heat flux — the reason Apollo, Soyuz and Shenzhou all use lifting, not purely ballistic, re-entry.