4 ms·

Sure, everything going through the atmosphere at supersonic speeds will lose at the very least a couple of atoms here and there, but that was not the point and

Sure, everything going through the atmosphere at supersonic speeds will lose at the very least a couple of atoms here and there, but that was not the point and that does not make thermal protection tiles an ablative heat shield.

avmich 3y ago
Let's remember definitions. Thermal soak heat protection: the protective material absorbs - soaks - the heat, becomes really hot, and after the process need to dissipate the heat. Also - which is important - the heat capacity of the material should be enough to absorb the whole heat flux going into it. Also - thermal conductivity of this protection should be such that the protected material behind it won't get too much heat. For this kind of protection to work you want small temperature differentials - less heat flux - and small time of heating - otherwise the material will become too hot. However - as materials have varying thermal conductivity - the thermal soak is always present to some degree. Radiative protection: the protective material heats up so much it stops accepting more heat, because it radiates heat back with the same speed. After the process the surface of this material is really hot. Important requirement - low thermal conductivity, as the high temperature on the surface shouldn't get behind the protective layer. Also known as refractory protection. Space Shuttle thermal protection is an example. Ablative protection: the surface of the protective material releases its material in the form of gases, and this gasification process cools down the material. The material gets consumed, that's the main problem with this cooling approach - neither big thermal capacity nor small thermal conductivity are needed. Early Merlin engines used ablative cooling. According to Wikipedia, SpaceX uses ablative protection for Crew Dragon - https://en.wikipedia.org/wiki/Atmospheric_entry#Thermal_protection_systems https://en.wikipedia.org/wiki/Atmospheric_entry#Thermal_prot... - "A second enhanced version of PICA—called PICA-3—was developed by SpaceX during the mid-2010s. It was first flight tested on the Crew Dragon spacecraft in 2019 during the flight demonstration mission, in April 2019, and put into regular service on that spacecraft in 2020." . Also from Wikipedia, tiles of Starship are made of silica - https://en.wikipedia.org/wiki/Starship_rocket#Starship_spacecraft https://en.wikipedia.org/wiki/Starship_rocket#Starship_space... - "Starship's heat shield, composed of eighteen thousand[131][132] hexagonal black tiles that can withstand temperatures of 1,400 °C (2,600 °F),[133][134] is designed to protect the vehicle during atmospheric entry and be used multiple times with minimal maintenance between flights.[135] The tiles are made of silica[136] and are attached with pins rather than glued,[134] with small gaps in between to allow for heat expansion.[2]" which makes them a kind of radiative or refractory protection.

avmich 3y ago

Let's remember definitions. Thermal soak heat protection: the protective material absorbs - soaks - the heat, becomes really hot, and after the process need to dissipate the heat. Also - which is important - the heat capacity of the material should be enough to absorb the whole heat flux going into it. Also - thermal conductivity of this protection should be such that the protected material behind it won't get too much heat. For this kind of protection to work you want small temperature differentials - less heat flux - and small time of heating - otherwise the material will become too hot. However - as materials have varying thermal conductivity - the thermal soak is always present to some degree. Radiative protection: the protective material heats up so much it stops accepting more heat, because it radiates heat back with the same speed. After the process the surface of this material is really hot. Important requirement - low thermal conductivity, as the high temperature on the surface shouldn't get behind the protective layer. Also known as refractory protection. Space Shuttle thermal protection is an example. Ablative protection: the surface of the protective material releases its material in the form of gases, and this gasification process cools down the material. The material gets consumed, that's the main problem with this cooling approach - neither big thermal capacity nor small thermal conductivity are needed. Early Merlin engines used ablative cooling. According to Wikipedia, SpaceX uses ablative protection for Crew Dragon - https://en.wikipedia.org/wiki/Atmospheric_entry#Thermal_protection_systems https://en.wikipedia.org/wiki/Atmospheric_entry#Thermal_prot... - "A second enhanced version of PICA—called PICA-3—was developed by SpaceX during the mid-2010s. It was first flight tested on the Crew Dragon spacecraft in 2019 during the flight demonstration mission, in April 2019, and put into regular service on that spacecraft in 2020." . Also from Wikipedia, tiles of Starship are made of silica - https://en.wikipedia.org/wiki/Starship_rocket#Starship_spacecraft https://en.wikipedia.org/wiki/Starship_rocket#Starship_space... - "Starship's heat shield, composed of eighteen thousand[131][132] hexagonal black tiles that can withstand temperatures of 1,400 °C (2,600 °F),[133][134] is designed to protect the vehicle during atmospheric entry and be used multiple times with minimal maintenance between flights.[135] The tiles are made of silica[136] and are attached with pins rather than glued,[134] with small gaps in between to allow for heat expansion.[2]" which makes them a kind of radiative or refractory protection.