Reentry Heating Calculator
Simplified estimate using the Sutton-Graves stagnation-point approximation. Real reentry heating depends on many additional factors.
Why Reentry Vehicles Are Blunt, Not Pointed
Counterintuitively, a blunt reentry capsule survives atmospheric entry better than a sharp, pointed one. A blunt nose pushes the shock wave further away from the vehicle surface, and the Sutton-Graves relationship this calculator implements shows why: stagnation-point heat flux scales inversely with the square root of nose radius, so a larger nose radius meaningfully reduces peak heating even as velocity stays fixed.
The Formula
This is the Sutton-Graves approximation for simplified stagnation-point convective heat flux, where k is the Sutton-Graves constant (1.7415 × 10⁻⁴ in SI units), ρ is local atmospheric density, Rₘ is nose radius, and v is velocity.
Where This Calculation Matters
- Thermal protection system sizing — peak heat flux estimates inform how much ablative or reusable heat shield material a vehicle needs at its stagnation point.
- Nose radius tradeoffs — the cube relationship with velocity and inverse-square-root relationship with nose radius together explain why reentry capsules (Apollo, Dragon, Orion) use large blunt heat shields rather than aerodynamically sharp shapes.
- Comparing entry velocities — because heating scales with velocity cubed, a modest increase in entry speed (such as a lunar-return trajectory versus a low-orbit return) produces a dramatic increase in peak heating.
- Trajectory shaping — entry trajectories are designed to control the velocity-density profile the vehicle experiences, directly trading peak heat flux against total heat load.
Heat Flux Sensitivity to Velocity
| Velocity | Entry scenario | Heat flux |
|---|---|---|
| 3,000 m/s | Suborbital return | 0.27 MW/m² |
| 7,800 m/s | Low Earth orbit return | 4.77 MW/m² |
| 11,000 m/s | Lunar or deep-space return | 13.38 MW/m² |
Roughly a 3.7x increase in velocity from the first to the third row produces close to a 50x increase in heat flux, illustrating the cubic velocity dependence.
How to Use This Calculator
- Enter the local atmospheric density in kg/m³.
- Enter the vehicle's nose radius in meters.
- Enter the velocity in m/s.
- Select Calculate to get the estimated stagnation-point heat flux.
Related Calculations
Check the entry velocity itself with the Mach Number Calculator, or trace it back to the deorbit maneuver with the Delta-V Calculator.