V0.2 - 17-04-2020
Thermal Protection systems
During atmospheric entry, the vehicle heats up due to aerothermal effects. These scale with the velocity to the power of three. To ensure the payload or crew is safe during entry, some form of thermal protection is needed. This Thermal Protection System (TPS) ensures the heat of entry does not reach the payload.
The most used form of thermal protection is an ablative thermal protection system. This uses an ablative that burns away. The energy required to shift the state of the material from solid to gas is not used to heat the vehicle and thus prevents the vehicle from warming up. Most crewed missions use or used this type of heat shield. A noticeable exception is the Space Shuttle. Examples of ablative thermal protectors are Phenolic-Impregnated Carbon Ablator (PICA) used on Dragon, AVCOAT used on Orion and Apollo, and SLA-561V used on most Mars missions.
Ceramics are a form of non-ablative heat shields that found their application onboard the US Space Shuttle. As they do not ablate, they can be reused. However, experience showed that ceramics are very fragile and prone to damage. This leads to long inspection times post landing and can thus increase the thermal protection system's cost. Another significant disadvantage of ceramic or non-ablative systems is that one cannot perform a post-flight validation. For other crewed capsules, a post-flight inspection can be a validation for the aerothermal and ablative simulations. For the Space Shuttle, this was limited to thermocouples and FLIR data.
A passively cooled TPS relies on heat absorption when needed and allowing for heat dissipation when the external heat flux is lower. The only space missions that use this are suborbital missions such as SuperMAX and SPEAR. These vehicles do not fly fast enough for more complex heat shields to be required. Other non-space examples are high-performance jet aircraft such as the SR-71. The only human-rated spacecraft to use a passively cooled heat shield was the suborbital Mercury capsule. It was possible to use a passively cooled system as the flight was only suborbital, and thus the entry velocity was much lower. Later Mercury flights went orbital and did carry an ablative heat shield.
The last category is an actively cooled system. This system relies on actively cooling the vehicle or actively diverting the heat flow away from the payload. Two significant subcategories can be identified: Internal cooling and external cooling. The first method uses an active method to divert the heat flux into a different location inside the vehicle. This decreases the temperature on the wind side of the vehicle and distributes the temperature over the vehicle. The second method uses an external liquid that is ejected through small holes in the vehicle. This liquid creates a film over the vehicle which evaporates, reducing the temperature. There are no actively cooled missions at the moment; the SpaceX Starship was supposed to use cooling but it has switched to a ceramic-like system.
A separate system that can be used for both thermal protection and deceleration at super/hypersonic conditions is a Hypersonic Inflatable Aerodynamic Decelerator. These inflatable cones can be manufactured with a thermal protection system on the outside. This thermal protection layer is flexible as it has to be inflated with the system. Therefore a high-strength material is needed that is thermally stable usually, this is either aramids or Zylon.