Re-entry Speed Calculator

A spacecraft at orbital velocity has enormous kinetic energy (about 30 MJ/kg for LEO). This must be dissipated in minutes as the spacecraft decelerates to subsonic speed. The compressed air ahead of the blunt body forms a shock wave. Air in the shock layer is heated to plasma temperatures. The vehicle radiates, ablates, or conducts away this heat through its thermal protection system.

A common misconception: reentry heat is primarily from compression of the atmosphere (a shock wave), not surface friction. The shock wave ahead of the vehicle compresses air to extremely high temperatures and pressures. Less than 1% of heating comes from actual surface friction. This is why blunt bodies (Apollo, SpaceX Dragon) are more efficient for reentry than sharp bodies — they push the shock wave away from the vehicle surface.

Ablative TPS materials protect a vehicle by pyrolysis (charring) and vaporization. As the surface heats up, the ablator material chars and vaporizes, carrying heat away from the vehicle. The char layer insulates the underlying structure. Ablative TPS was used on Mercury, Gemini, Apollo, and is used on SpaceX Dragon (PICA-X), Mars Science Laboratory, and other missions requiring deep/high-speed entry.

The Shuttle used four types: Reinforced Carbon-Carbon (RCC) for the nose and wing leading edges (hottest areas, up to 1650 C); High-temperature Reusable Surface Insulation (HRSI, black tiles) for lower surfaces; Low-temperature RSSI (white tiles) for upper surfaces; and flexible Nomex blankets for lower-heat areas. The Columbia accident (2003) was caused by foam damage to the RCC wing leading edge.

A skip reentry (bouncing reentry) occurs when a spacecraft enters the atmosphere at a shallow angle, decelerates partially in the upper atmosphere, then exits back into space briefly before final reentry. It spreads the heating over a longer period and allows precise landing site targeting. The Apollo command module used a form of skip reentry (lifting reentry) to control landing point.

The entry corridor is the range of flight path angles that leads to successful reentry. Too shallow (less negative): the vehicle bounces off the atmosphere without decelerating enough. Too steep (more negative): the vehicle experiences excessive deceleration g-loads and heating. For crewed Apollo reentries, the corridor was only about 2 degrees wide (between -5.5 and -7.5 degrees below local horizontal).

Ballistic coefficient BC = m/(C_D * A), where m is mass, C_D is drag coefficient, and A is cross-sectional area. Higher BC means the vehicle penetrates deeper into the atmosphere before decelerating significantly — it reaches lower altitudes at higher speed, experiencing more concentrated heating. A low BC (large area, low mass) decelerates high in the atmosphere, spreading heating over a longer time and lower temperatures.

Mars has a thin atmosphere (about 1% of Earth's surface pressure) but reentry still occurs because the entry speeds are high (4-7 km/s from orbit, 5.5 km/s from direct entry). The thin atmosphere makes it harder to decelerate: vehicles must use large aeroshells (high drag) and sometimes supersonic parachutes and retro rockets. The Mars Science Laboratory used a 4.5 m diameter aeroshell and a sky crane system for the 1-tonne Curiosity rover.

Peak heating rate (W/cm^2) is the maximum instantaneous heat flux at the vehicle nose. Total heat load (J/cm^2) is the total energy per unit area deposited during the entire entry. A steep, fast entry has high peak heating rate but short duration (low total load). A shallow entry has lower peak rate but longer duration. Both must be managed: peak rate determines material temperature survival, total load determines ablator thickness needed.