What is the entry corridor, and why was Apollo's so tight?

Enter too shallow and the capsule does not slow enough in the thin upper air; with lift it bounces back into space (skip-out). Enter too steep and it decelerates violently in thick air, exceeding g and heating limits. The range of allowed entry angles between the two is the entry corridor. Returning from the Moon at about 11 km/s, 40% faster than from low orbit, Apollo's corridor was only about 2° wide and needed precise mid-course corrections. The tool runs a full simulation for each entry angle to draw the window.

What do the ballistic coefficient and L/D do?

The ballistic coefficient β = m/(C_D·A) is mass per unit of frontal drag area. A low β slows down earlier, higher up in thinner air, so heating and g-loads are lower. Lift (L/D) lets the vehicle hold altitude and stretch out the deceleration: Soyuz pulls about 4 g on a normal entry but 8–9 g when it falls back to a ballistic entry. The bank angle tilts the lift sideways to tune its vertical component, which is the main way to control the landing point and the g-load.

Why does heat flux go with the cube of speed?

The Sutton–Graves formula q = k√(ρ/r_n)·V³ describes convective heating at the stagnation point of a blunt body: the kinetic energy flux of the oncoming air scales as ρV³ and part of it reaches the surface through the boundary layer. Heat flux also falls with the square root of the nose radius r_n, which is why capsules are big and blunt: the shock stands farther off and spreads the heating. Going from 7.8 to 11 km/s nearly triples the heat flux.

How does a heat shield work?

Most capsules use an ablator. As the surface heats it pyrolyses, chars and is carried away by the flow; the decomposition absorbs heat, the gases injected into the boundary layer block more, and the hot char radiates a lot away. The tool divides the total heat load by an effective heat of ablation to get the ablator mass needed, a rough upper bound that ignores re-radiation; real shields also allow for heat soak and margins. The Space Shuttle used reusable ceramic tiles instead, which insulate and radiate rather than ablate.

How is Mars entry different?

Mars' atmosphere is only about 1% as dense as Earth's, so deceleration happens very low, the parachute must open at supersonic speed and the vehicle is still falling at close to 100 m/s under it. That is why Curiosity also needed retro-rockets and a sky crane. Entry speed is about 5.8 km/s, so heating is lower than an Earth return, but the timeline is tight. The Mars atmosphere here is an exponential model with an 11 km scale height, an approximation.

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☄️Reentry Simulator - Ballistic & Lifting Entry, g-Load, Heat Flux, Corridor

Pick a preset (Apollo lunar return, Soyuz, Space Shuttle, Dragon, Starship, a meteoroid or Curiosity's Mars entry) or set the entry speed, entry angle, ballistic coefficient, lift-to-drag ratio and bank angle yourself. The tool integrates the planar entry equations through a standard atmosphere and plots altitude, speed, deceleration and stagnation heat flux against time. In the animation the path is coloured by heat flux, the vehicle glows with shock-layer plasma and parachutes appear when they open. The corridor tab scans entry angles to find the window between skipping out and exceeding the g-limit, and the heat shield tab sketches how ablation works and roughly sizes the ablator.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

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How to use Atmospheric Reentry Simulator

1Choose a preset, or adjust entry speed, entry angle, ballistic coefficient, L/D, bank angle and nose radius
2Play the animation on Entry trajectory and read the live state, the four time charts and the altitude–speed plot
3Check peak deceleration, peak heat flux, total heat load, the blackout window and touchdown speed
4Set a g-limit on Entry corridor to see the corridor width, and estimate ablator thickness on Heat shield

FAQ

What is the entry corridor, and why was Apollo's so tight?: Enter too shallow and the capsule does not slow enough in the thin upper air; with lift it bounces back into space (skip-out). Enter too steep and it decelerates violently in thick air, exceeding g and heating limits. The range of allowed entry angles between the two is the entry corridor. Returning from the Moon at about 11 km/s, 40% faster than from low orbit, Apollo's corridor was only about 2° wide and needed precise mid-course corrections. The tool runs a full simulation for each entry angle to draw the window.
What do the ballistic coefficient and L/D do?: The ballistic coefficient β = m/(C_D·A) is mass per unit of frontal drag area. A low β slows down earlier, higher up in thinner air, so heating and g-loads are lower. Lift (L/D) lets the vehicle hold altitude and stretch out the deceleration: Soyuz pulls about 4 g on a normal entry but 8–9 g when it falls back to a ballistic entry. The bank angle tilts the lift sideways to tune its vertical component, which is the main way to control the landing point and the g-load.
Why does heat flux go with the cube of speed?: The Sutton–Graves formula q = k√(ρ/r_n)·V³ describes convective heating at the stagnation point of a blunt body: the kinetic energy flux of the oncoming air scales as ρV³ and part of it reaches the surface through the boundary layer. Heat flux also falls with the square root of the nose radius r_n, which is why capsules are big and blunt: the shock stands farther off and spreads the heating. Going from 7.8 to 11 km/s nearly triples the heat flux.
How does a heat shield work?: Most capsules use an ablator. As the surface heats it pyrolyses, chars and is carried away by the flow; the decomposition absorbs heat, the gases injected into the boundary layer block more, and the hot char radiates a lot away. The tool divides the total heat load by an effective heat of ablation to get the ablator mass needed, a rough upper bound that ignores re-radiation; real shields also allow for heat soak and margins. The Space Shuttle used reusable ceramic tiles instead, which insulate and radiate rather than ablate.
How is Mars entry different?: Mars' atmosphere is only about 1% as dense as Earth's, so deceleration happens very low, the parachute must open at supersonic speed and the vehicle is still falling at close to 100 m/s under it. That is why Curiosity also needed retro-rockets and a sky crane. Entry speed is about 5.8 km/s, so heating is lower than an Earth return, but the timeline is tight. The Mars atmosphere here is an exponential model with an 11 km scale height, an approximation.

Features

RK4 integration of the planar point-mass entry equations, ballistic or lifting (L/D plus bank angle)
1976 standard atmosphere for Earth (tabulated above 86 km) and an exponential atmosphere for Mars
Sutton–Graves stagnation heat flux, total heat load and radiative-equilibrium wall temperature
Multi-stage parachutes (drogue then main, with inflation) and meteoroid ablation mass loss
Entry corridor scan with skip-out boundary, g-limit boundary and corridor width
Heat-coloured trajectory animation with plasma glow and a heat shield cross-section sketch

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