[QUOTE=chalsall;431558]Perhaps someone here can shed some light on a question I have...

The recovered Falcon 9s have a great deal of black soot on them, very clearly visible when comparing the white surface protected by the landing legs during re-entry vs. the surfaces not protected. I assume most of this is from the incompletely burned fuel in the gas-generator turbo pump (the Merlin engines are an "Open Cycle" design) which washes back around the rocket during re-entry.

My question is: why is there such a well-defined difference in the bottom 2/5ths of the rocket compared to the top 3/5ths? Is a different material used there, or is it a function of the aerodynamic flow?

[URL="http://spaceflightnow.com/2016/04/13/new-views-of-falcon-9-landing-from-on-board-spacexs-drone-ship/"]These pictures should clearly show[/URL] what I'm wondering about.[/QUOTE]

That is where the RP-1 tank ends (below) and the lox tank starts (above). The LOX tank is very cold and probably gets a layer of frost that prevents the soot from sticking.

[url]http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf[/url]

[QUOTE=Mark Rose;431562][url]http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf[/url][/QUOTE]

Super coolness. Thanks. I was not aware of that document.

[QUOTE=chalsall;431558]Perhaps someone here can shed some light on a question I have...

The recovered Falcon 9s have a great deal of black soot on them, very clearly visible when comparing the white surface protected by the landing legs during re-entry vs. the surfaces not protected. I assume most of this is from the incompletely burned fuel in the gas-generator turbo pump (the Merlin engines are an "Open Cycle" design) which washes back around the rocket during re-entry.

My question is: why is there such a well-defined difference in the bottom 2/5ths of the rocket compared to the top 3/5ths? Is a different material used there, or is it a function of the aerodynamic flow?

[URL="http://spaceflightnow.com/2016/04/13/new-views-of-falcon-9-landing-from-on-board-spacexs-drone-ship/"]These pictures should clearly show[/URL] what I'm wondering about.[/QUOTE]

Is it possible that those particles have a pretty strong charge, relative to the skin? This might remove most of them in the lower sections.

On the other hand, to say that there would be turbulence around the booster, especially at the bottom, would be a bit of an understatement. Could cooling of the hot gases cause them to precipitate onto the (slightly) cooler skin? :confused2:

[QUOTE=kladner;431567]Is it possible that those particles have a pretty strong charge, relative to the skin? This might remove most of them in the lower sections.[/QUOTE]

Probably not.

Even if there is plasma coming out the back, it is likely net charge neutral. This isn't an ion drive, after all.

[QUOTE=chalsall;431568]Probably not.

Even if there is plasma coming out the back, it is likely net charge neutral. This isn't an ion drive, after all.[/QUOTE]The clouds which went past here a few minutes ago were net charge neutral. Nonetheless there were some pretty impressive plasma phenomena. Certainly made the chucks run for cover.

Anyway, ion drives also squirt electrons out the back, otherwise the charge build up on the engine re-attracts the positively charged exhaust. Their exhaust is also net charge neutral.

[QUOTE=xilman;431575]Anyway, ion drives also squirt electrons out the back, otherwise the charge build up on the engine re-attracts the positively charged exhaust. Their exhaust is also net charge neutral.[/QUOTE]

LOL... Yes. Of course. Otherwise the drive wouldn't work.

[QUOTE=Mark Rose;431562]That is where the RP-1 tank ends (below) and the lox tank starts (above). The LOX tank is very cold and probably gets a layer of frost that prevents the soot from sticking.

[URL]http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf[/URL][/QUOTE]
Thanks for the answer, Mark. Facts are much better than speculation. :markrose:

[QUOTE=kladner;431579]Thanks for the answer, Mark. Facts are much better than speculation. :markrose:[/QUOTE]

The frost theory is speculation though.

[QUOTE=Mark Rose;431585]The frost theory is speculation though.[/QUOTE]
Oh. OK. Still, it is the most plausible explanation put forward thus far.

[QUOTE=Mark Rose;431585]The frost theory is speculation though.[/QUOTE]

It's widely held as "the answer" by a number of internet pundits though, and what I would have replied.

(Although I'm not sure how much of that soot is strictly from the pre-burners -- I imagine most of it is from the actual exhaust of the re-entry burn, since there is far more of said exhaust than of pre-burner exhaust and the re-entry burn occurs at mostly supersonic speeds.)

[QUOTE=Dubslow;431587]It's widely held as "the answer" by a number of internet pundits though, and what I would have replied.[/QUOTE]

Are you familiar with the concept of "what a waste of carbon"?