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Earlier, there was news about a proposed concept for Starship to include "transpirational cooling" in its reentry heat shield design.

These days there is less news about this and tests so far don't show evidence of "bleeding water".

Is this technique now off the table for Starship? If so, is it because of design challenges, unnecessary complexity not absolutely necessary, something else?

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    $\begingroup$ Could you please cite your source that SPaceX ever considered such a thing? Or are you maybe meaning to ask what happened to the "Transpiration" cooling concept? (the two words mean vastly different things) $\endgroup$ Oct 30, 2021 at 8:10
  • $\begingroup$ Yes I mean the transpiration cooling concept $\endgroup$
    – Mhd Afz
    Oct 30, 2021 at 9:15
  • $\begingroup$ I have changed your question title to match your intent. If you wish rollback, please state so. I do suggest you flesh out your question body with more detail on the planned systems and/or what research you have done in this regards already. $\endgroup$ Oct 30, 2021 at 11:16
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    $\begingroup$ @MhdAfz I've added some sources to your question supporting that SpaceX ever considered such a thing. It's generally a good idea to support the premise of a Stack Exchange question with sources; we should not assume that every reader has read or seen everything that we have. Thanks! $\endgroup$
    – uhoh
    Oct 30, 2021 at 23:30
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    $\begingroup$ What does this have to do with water? $\endgroup$ Oct 31, 2021 at 9:01

3 Answers 3

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Despite SpaceX's high-tech reputation, SpaceX aren't where they are today because they made massive technological leaps or pushed the envelope of science--after all, just doing science doesn't earn money. SpaceX doesn't have some "secret sauce" of IP or some secret, hyper-advanced engine technology that makes their engines an order of magnitude better than the competition.

In fact, with only very few exceptions, basically all of SpaceX's successes have come from streamlining the production of known technology to a ridiculous degree and getting the assembly lines and processes down to a science, which is, in Elon's own words, where most of the effort is (in an interview, he states that compared to production, design and development is simple). SpaceX primarily optimizes to price and dollar effectiveness, not individual rocket efficiency.

Throughout SpaceX history, they've consistently dropped advanced and cutting edge technologies, favoring simpler and straight-forward solutions under their "the best part is no part" design philosophy. One good example of this is how the ITS/BFR/Starship system used to be conceptualized as a massive carbon fiber construction, before they decided that the gains achievable with this material wouldn't outweigh the development, research, and other costs associated with it--they figured it would be simpler to just eat the minor hit to mass and make the whole thing out of steel, and so they did.

I suspect a similar thing has happened with the transpiration cooling. Elon likes to kick around wacky ideas, and that's his role as the CEO: he provides vision and fulfills the 'dreamer' role. Doing so causes lots of people to talk and think about the issue, and then a more informed decision can be made later. (Ablative) heat shield tiles are the simple solution in this case. No moving parts, no valves, no complex fluid dynamics, decades of experience and flight heritage, etc. When there's a tried and tested method of doing something, there has to be a very good reason to go and reinvent the wheel if your primary goal is profit.

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  • $\begingroup$ Aka why all jetliners (except a few tail engine types) look exactly like up-scaled ME262’s. $\endgroup$ Nov 1, 2021 at 0:36
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    $\begingroup$ "SpaceX's successes have come from streamlining the production of known technology to a ridiculous degree [...]" That description is a striking parallel to Ford, early in its history. $\endgroup$
    – jpaugh
    Nov 1, 2021 at 2:45
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    $\begingroup$ "The best part is no part" ironically, this is how many feel about billionaires and space tourism in the first place. $\endgroup$
    – corsiKa
    Nov 1, 2021 at 4:52
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As of a year ago in a thread discussing the initial tiles appearing on SNs, Elon said that they were still considering using it in some locations.

https://twitter.com/elonmusk/status/1314689966870425605

Tiles will be on hot side of flaps too. A very tough problem is sealing the moving flap to body joint without melting or shredding the seal.

https://twitter.com/flcnhvy/status/1314693111193513986

Are you still considering transpiration cooling for most vulnerable hotspots or are the heat shield tiles tough enough?

https://twitter.com/elonmusk/status/1314741760568954880

It might be used in some areas. ITAR laws prevent us from being too specific about solutions.

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From the moment that E.Musk announced the idea of Starship transpiration cooling with liquid CH4, many people had doubts that it was good idea. Transpiration cooling of any spacecraft during orbital reentry was never tried yet and it wasn't considered 100% failure resistant even for spacecraft with heatshield size of Soyuz reentry capsule. Compare to Soyuz reentry capsule, Starship has massive surface area and most of the Starship bottom part (including bottom part of 4 large flaps) would require transpiration cooling. There were also concerns that large quantity of liquid CH4 freely floating under Starship could have negative impact on stability during reentry, belly flop and landing. At the end, SpaceX dropped the idea of transpiration cooling and chose rather option of ceramic TPS for Starship reentry side (including 4 flaps) how many people expected. Using ceramic TPS means higher weight penalty, but should be much safer option for Starship difficult reentry. Don't have really time to search for good sources to back this up, it is mostly based on comments from REDDIT and Starship YouTube channels. Anyway, since reentry TC of any spacecraft was never tried yet, it was only everyone opinion. Didn't hear any comments from SpaceX about why Starship TC was dropped. If somebody heard about different reasons from SpaceX or other sources, they can post them. But until it will be really tested during actual reentry, we couldn't know how safe it will be.

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