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Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
They vent hydrogen during reentry for cooling.
Skylon probably is viable, I've attended a lecture on it, and it's been looked at by the EU, and they and I found no show-stoppers (financials are a bit marginal in my opinion though, but probably much better than the Shuttle, which isn't saying much).
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
The Space Shuttle was a crap design built entirely on American ego and pride, (of course those thieving Russians stole enough to build thiers, which they quickly abandoned).
why else would we be going back to the old apollo/soyuz type capsules!!!
All the heat (that is, anything significant IE anything worth considering) comes from friction.
You would talk about friction, if the air molecules would hit the crafts surface mostly unstopped by the shock wave in front of the craft. They would then transfer their energy directly to the shield. The mean free path of air molecules at room temperature and pressure is around 70nm. Temperature and pressure of a shockwave are far away from that. But I think it is conceivable, that most air molecules will be thermalized before hitting the surface.
BigBad wrote:
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.
Sounds plausible. It probably depends on the altitude. Very high up you won't get much of a shockwave.
Check out the Schlieren photograph on the right, you can actually see the shock waves; they're quite well spaced.
wrote ... Very high up you won't get much of a shockwave.
Pretty much. You still get a shockwave, and it's the same temperature, but the pressure and forces generated are lower, and the heat is transferred more slowly.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
im going to agree with the latter comments. And Ill go out on a limb, but i think NASA over-simplifies these scientific matters for the benefit of the "lay people" (Prolitariates) by rolling it [the complex matters of fluid behavior under dynamic circumstance] all up into the supposed "friction".
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
Meatball wrote ...
All the heat (that is, anything significant IE anything worth considering) comes from friction.
Source: My roommate was on the thermal team for orbiter Discovery and is currently at Stennis space center for engine R&D.
Sounds like Stennis is fucked then because this is completely untrue.
The temperature rise is nearly exclusively due to compression - this is one of the first lessons you will learn in a compressible flow class. The temperature reached is called the "stagnation temperature" and is a function of the Mach number.
He's an Embry Riddle graduate. BSE Aerospace Eng and Masters in Aerospace Eng. He was a part of some 14 missions even outside of Discovery's routine. Did Discovery have any thermal issues upon re-entry? Nah. I'm sure he understands all the workings. But it seems as though when he is asked about it, the sum of it all comes back to friction. Compression is not a hard concept to understand. There is no reason why NASA would even need to over-simplify the matters.
My roommate doesn't consider me just "another mind of the public eye". We're both in engineering school, so if compression is really so significant of an issue, I would've heard it from him. Believe me. I hear plenty about the space center from him.
If he says "Oh yeah, its friction", I believe him.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
It's not friction in the sense of fluid shear, that plays only a minor effect, but if you want to look at the whole vehicle, rather than looking at the cause of the friction, then the vehicle is clearly experiencing 'friction' because it's slowing down.
FWIW I have actually read the original H. Julian Allen paper, who was the guy that worked it all out first.
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
Meatball wrote ...
He's an Embry Riddle graduate. BSE Aerospace Eng and Masters in Aerospace Eng. He was a part of some 14 missions even outside of Discovery's routine. Did Discovery have any thermal issues upon re-entry? Nah. I'm sure he understands all the workings. But it seems as though when he is asked about it, the sum of it all comes back to friction. Compression is not a hard concept to understand. There is no reason why NASA would even need to over-simplify the matters.
My roommate doesn't consider me just "another mind of the public eye". We're both in engineering school, so if compression is really so significant of an issue, I would've heard it from him. Believe me. I hear plenty about the space center from him.
If he says "Oh yeah, its friction", I believe him.
If we're going to play the credentials game then I have a BS in Aero Eng, a BS in Applied Physics, and an MS in Aero Eng. from Purdue. I've also worked in commercial spaceflight for six+ years now. When you and your friend had this discussion he either didn't know the right answer or didn't care to give it to you.
NASA certainly agrees; check their own isentropic flow page for the equation to predict the temperature rise at the stagnation points. You want the T/Tt equation, where T is the ambient temp and solve for the stagnation temp Tt (aka the "total" temperature).
In reality all sorts of icky complications like shocks and dissociation enter the picture and friction does play a role in coupling heat from the omg-so-fucking-hot gas layer around the vehicle to the vehicle body. But that omg-so-fucking-hot gas layer is created by the compression in front of the vehicle and dominates the design.
But hell, don't take my word for it, try the arbiter of truth itself, wikipedia, where the article on aerodynamic heating contains not a single utterance of the word "friction":
Or the article for atmospheric entry, which explicitly states:
(Direct friction upon the reentry object is not the main cause of shock-layer heating. It is caused mainly from isentropic heating of the air molecules within the compression wave. Friction based entropy increases of the molecules within the wave also account for some heating.)
Or the article for the Space Shuttle's own thermal protection system, of which your friend must be well acquainted:
Over 80% of the heating the Orbiter experiences during reentry is caused by compression of the air ahead of the hypersonic vehicle, in accordance with the basic thermodynamic relation between pressure and temperature.
Wikipedia isn't much of a reference, granted, so while I haven't researched the provenance of this page specifically, it seems quite well researched:
Stagnation heating: The point right at dead center front of the shock wave is called the stagnation point, see Fig. C5. This is where the highest temperatures occur and they are known as stagnation temperatures and or stagnation heating. ...The areas of the shuttle that have stagnation points are the nose, the leading edges of the wings and the leading edge of the tail.
Wing leading edge and nose: The leading edge of the wings and the nose of the shuttle see the most heat by far of any other part of the orbiter. These areas are at or near the stagnation points of the shockwaves created by the orbiter and are subject to stagnation heating. The temperatures seen here are well over 2500°F for much of the reentry period. The Thermal Protection System (TPS) is considerably more robust in these areas as well. A location along the leading edge designated the 55% semispan location sees the highest temperatures due to an unusual crossing of shock waves from different parts of the orbiter, (a double shock region). The absolutely highest temperature at the leading edge occurs 5 inches below the midline of the wing at that point due to the shuttles 40° angle of attack as pointed out on Page C, "The Effects of Hypersonic Flow During Reentry of the Space Shuttle".
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burn up on reentry
