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Due to re-entry heating and the sheer velocity when coming in from solar orbit into the Earth's SOI, will Starship do a capture burn to slow it down before entering the atmosphere or just go for a direct encounter?

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    $\begingroup$ You might want to make it explicit that this question is about return from outside LEO, presumably from Mars? Adding some velocity numbers for LEO re-entry and Mars return re-entry would also make it clearer. $\endgroup$ Apr 20 at 5:48

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According to a chart in a report from NASA, a reusable Thermal Protection System (TPS) is suitable for returning from LEO, but insufficient for protecting a vehicle returning from the moon or another planet in the solar system.

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I also learned that roughly 15% of the Space Shuttle orbiter's empty weight was TPS. Whereas, in Starship's case, the mass of the TPS is 5-8 tons (ref), and its mass at reentry will likely be closer to ~150 tons, so the TPS mass percentage is only around 3-5%. Percentage-wise, that's a lot less thermal mass than the Shuttle had. So, presumably, with less thermal mass per kg of reentry mass, Starship's tiles will heat up quite a bit more than the Space Shuttle's tiles did, even if Starship is only returning from LEO.

Also, according to the presentation, radiative heating is proportional to $v^8$ (see slide 8). This means that if a vehicle enters twice as fast it experiences 256 times as much radiative heating.

Stan Bouslog, TPS Senior Discipline Expert at NASA JSC, speaking in 2024, also commented on this during his presentation and also during the Q&A. He said

... if you remember at the very beginning of this talk I mentioned the heat fluxes and how the heat fluxes scale with velocity to the cube for convective and by velocity to the eight power for shock layer radiation. So the heat fluxes are going to be high when you come back from the moon so probably going to need ablators because a lot of the other material systems are not going to be able to handle the heat fluxes or temperatures.

...right now to come back from the Moon you ... need that ablator just because it's such high energy re-entry - unless you can figure out a different mission set where you can potentially repulsively slow yourself down enough that it's just a LEO re-entry. That's a way to get away with maybe an alternate TPS system.

I don't think he would have made these statements if he knew of a promising near-term non-ablative solution for faster than LEO reentry speeds.

Based on this data, if Starship were returning to Earth from another planet, as currently designed, it would need to somehow slow itself down to LEO orbital velocities before it could reenter Earth's atmosphere. A direct encounter may not end well.

(Some useful additional reading about the $v^{8th}$ relationship.) (Elon Musk answering a question on this during a Q&A in 2018.)

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  • $\begingroup$ Probably no surprise about the lower mass percentage of Starship TPS compared to Shuttle, when they switched to stainless steel Elon pointed out that one of the advantages of stainless steel was that it handled heat better, and thus required less TPS mass, helping to offset the additional weight of the steel. $\endgroup$ Apr 20 at 13:39
  • $\begingroup$ The first link you've provided is to a local file on your computer. $\endgroup$
    – Erin Anne
    Apr 20 at 18:34
  • $\begingroup$ @ErinAnne Thanks! I updated the link. $\endgroup$
    – phil1008
    Apr 20 at 21:45
  • $\begingroup$ After having read the first link's slides, it's not really clear to me that we've even tried to engineer a reusable TPS for beyond LEO (barring Starship, which isn't in the presentation's scope). Is there anything else in the slides that leads you to conclude that that chart indicates that reusable TPS is necessarily insufficient for beyond LEO? $\endgroup$
    – Erin Anne
    Apr 21 at 1:18
  • $\begingroup$ @ErinAnne In addition to the slides, in the last link of the answer, "here too", a NASA expert on TPS, speaking in 2024, during the Q&A portion of his presentation states, "...when you come back from the moon ... going to need ablators because a lot of the other material systems are not going to be able to handle the heat fluxes or temperatures." I don't think he would have said that if he knew of a promising non-ablative solution. But that's just the state of things today. Next year things could be different. $\endgroup$
    – phil1008
    Apr 21 at 1:32

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