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burn up on reentry

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IamSmooth

Fri Oct 19 2012, 04:53PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

When a object falls from space to the earth at high speed, what is the cause for the heating? Is it the friction of the air; or is it due to the compression of the air, resulting in the high temperature?

Marko

Fri Oct 19 2012, 05:10PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi

I'm fairly sure it is primarily due to extreme compression, caused by the intertia of the air itself. I would limit the term "friction" only to solids though; with movement in fluids there are several forms of drag such as drag due to adhesion and drag caused by turbulence in the object's wake, or generation of shock waves for transonic objects... and probably some other I don't even know of.

Marko

Tetris

Sat Oct 20 2012, 08:56PM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

Living in Florida and having been to the Space Coast, I was always told that it was due to friction upon re-entry.

Edit: You made me dream about this last night. And in my dream I was thinking that I should remember what was going on to tell you... hehe...

IamSmooth

Sat Oct 20 2012, 11:48PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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From my source
90% from extreme compression
10% from friction

BigBad

Thu Oct 25 2012, 04:06PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Mostly compression, some friction.

There's a shock wave that builds up from the high compression that forms just ahead of the reentry flight article, and behind that shock wave is subsonic air.

About 1% of the heat in that shock wave convects to the reentering object and heats it up. The convecting currents will also cause frictional heating, but it's mostly just the very hot air.

IRC as a rule of thumb, the temperature in kelvin of that subsonic hot gas behind the shockwave is equal to the reentry speed, so things can get a bit toasty(!)

Patrick

Thu Oct 25 2012, 06:57PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i too was told that its mostly hypersonic compression, with tiny contributions from adhesion, subsonic friction and turbulence. though ive been told from civilian eng/sci, and other martial test pilots involved in hypersonic drone testing in the 60's (Marqudart corporation), that the sub/transonic air behind the compression, but ahead of the machine surface is only a few 0.001's of an inch thick at most, and on non-swept leading surfaces it can be even thinner.

[Let me look through my older papers, but i think i can justify the following claim] :

given the tiny distance the high heat/temp from hypersonic compression (most of the heating as others have said) should be able to couple quickly and easily to any machine/object surface...

BigBad

Thu Oct 25 2012, 07:33PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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That's probably true for a sharp supersonic surfaces like wings and engine inlets, but hypersonic reentry surfaces are deliberately blunt, and the shock wave standoff is much bigger than that.

Patrick

Thu Oct 25 2012, 08:45PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

BigBad wrote ...

That's probably true for a sharp supersonic surfaces like wings and engine inlets, but hypersonic reentry surfaces are deliberately blunt, and the shock wave standoff is much bigger than that.

[ EDIT: Im thinking there would be multivariable differential calculus to predict distance of standoff vs high entry angle, ro--air density, decent rate vs pressure and a million other factors. (which the American and Russian people paid a great deal to have figured out before anyone else, im sure) ]

Oh yes, thats what i was getting too...

when we look at mach 2, 3, and 4 machines, like the SR71, F-104, Sukhoi T-4, XB-70, and the B-58 hustler, we see very sharp leading edges, very thin foils, and steeply sweept surfaces... but, above mach 4.5 to 5, we start seeing the return of large radius, broad structures, like the side and front view of the space shuttles' cockpits, and the large round inconel X steel ball nose on the
X-15 and RCC nose cap on the shuttles, (though the X-15 also has sharp edges) and the radius on the thick vertical tail of the shuttles... one could also seee similar radius features on the would be Venture Star.

the large radius' dissipates forces, (not just heat, but electrostatic and magnetic fileds too) and as bigbad implies. However, the profiles for sustained flight (like my examples, and the blunt reentry faces BigBad alludes too, are meant for different purposes, the capsules, are meant to see high heat, high temp, on broad areas with thick insulation (like Mars and earth entry), but for short times, and the need for aero braking... sustained flight would try to avoid that kind implimentation, most specifically the need for heavy thick all-round insulation on an aircraft, the shuttle had very shallow, very low pressure, and slow velocity change decent profiles, very different from the aggressive plunge of an apollo/soyuz, meteroid type entry.

I am sure if we look at the physics and math, we'ed see that the makers of these machines are using the properties of hypersonic, low-density slipstreams to optimise the interface between the atmosphere and there chosen insulations' capabilities. covering the space shuttle in centimeter thick low density insulation was no trivial matter. clearly for reasons seen in 2003 and before, any insulation strategy is the limiting factor for a really practical civillian transport or martial vehical.

BigBad

Sun Oct 28 2012, 06:57PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

The shuttle wasn't all that gentle, but it was gentle enough for it to get away with silica tiles over most of the surface (except for the high curvature leadng edge, this needed heavier/stronger/higher temperature carbon-carbon.

The Skylon design is a much gentler reentry, because they have a low ballistic coefficient. I believe that they think can get away with silcon carbide fibre reinforced glass for the surface.

The downside of that is that the heat soak energy is far higher than the shuttle even though the peak surface temperature is much lower, because it's a much longer reentry; so they're having to actively cool.

Patrick

Mon Oct 29 2012, 03:13AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

BigBad wrote ...

The shuttle wasn't all that gentle, but it was gentle enough for it to get away with silica tiles over most of the surface (except for the high curvature leadng edge, this needed heavier/stronger/higher temperature carbon-carbon.

The Skylon design is a much gentler reentry, because they have a low ballistic coefficient. I believe that they think can get away with silcon carbide fibre reinforced glass for the surface.

The downside of that is that the heat soak energy is far higher than the shuttle even though the peak surface temperature is much lower, because it's a much longer reentry; so they're having to actively cool.

Are they pre-heating the liquid-to-gasceous hydrogen using the skin? im skeptical of the SABRE model...

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