Maximus_Meridius said:
The criticality of any TPS tile depends on its location on Starship, as well as the angle of attack taken during reentry. And while stainless is certainly better at high temp than aluminum, you can't tell me that losing a tile near the centerline between the flaps isn't concerning. Or on one of the aerocovers near the flap hinge. The melting point for austenitic stainless (which is what they're using if I remember correctly) is roughly 1400 C. The Shuttle supposedly saw temps up to 1600 C. At ~3.5 mm wall thickness…you'll have a problem in seconds.
I'm sure they've done lots of analysis, but simulations are only as good as the assumptions made when setting them up. This will remain my biggest fear until we actually see a couple make it through in one piece.
The higher melting point of 304L SS vs Aluminum is just one of a multitude of factors that decrease the probability of failure in the case of a tile missing.
Stainless reaches half of its mechanical strength at around 700C compared to aluminum at 200C. The shear strength and modulus is three times higher. The tensile strength is over two times higher. So mechanically superior. The specific heat capacity for stainless is less than half of that of aluminum but that is a stat on a per unit mass basis. The density of stainless is over 3 times that of aluminum. So it takes longer for the unprotected portion of the stainless surface to heat up than the same unit area of aluminum. Couple that with stainless having a little more than one tenth of the thermal conductivity compared to aluminum and a significantly lower thermal expansion characteristics and you have tremendously decreased the odds of failure.
Remember metal does not simply melt when it is exposed to temperatures above it's melting point, it melts when the material itself reaches those temps. That takes time. Much longer for stainless.
Also consider that the max temperature exposure during re-entry is not as important as how long the vehicle is exposed to these temperatures.
I also understand the surface loading of the cylindrical body of starship is less than that of the shuttle even though they are the same dry mass. Starship also has a larger horizontal projection due to its size compared to the shuttle. It lends itself to experience a greater deceleration during descent.
There is no telling whether or not we will see shuttle like re-entry temperature exposure or more but it seems as though it will experience a shorter duration of this intense heat and the spacecraft is better suited to handle intense heat. As it is designed now, the TPS system is not seamless or sealed up like it was on STS. There is space between the tiles and just behind them as well. There is a thermal blanket system that is sufficient to protect the small exposed seams between the tiles. Elon has even spoken about cryogenic purges around and near seals and important systems that have thermal vulnerabilities.
Like I posted before, there is a lot to learn. Elon has mentioned that there will be sensors and thermal cameras placed on the inside of the ship to monitor heat on every point of starship during this first orbital test flight.