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PostPosted: Tue Dec 05, 2017 8:12 am 
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Here is a tidbit I found on the net that might be useful for determining absolute top speed in an atmosphere. ( no credit to me... I'm not clever enough )
The highest temperature for a melting point so far is just over 4000C. Top speed would be limited to temperatures that the hull can remain solid.
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At lower speeds (below Mach 5-ish), stagnation temperature (TAT) is a very accurate proxy for skin temperature. But at mid/high hypersonic speeds (especially in the thin upper atmosphere where mass flow is low), thermal radiation bleeds off a significant amount of heat, especially as temperatures climb into the thousands of Kelvin.

I've come up with a very crude formula to estimate the skin temperature. It assumes that the power Pabsorbed
to stagnate the oncoming air [intercepted by an area equivalent to the drag area CD Aref] is radiated away over the entire vehicle skin surface area Arad

Pabsorbed=Pradiated

1/2 * mflowrate v2=Arad σ ϵ T4

1/2 * (ρ v CD Aref)v2=Arad σ ϵ T4

T = ( ρ v3 CD Aref / 2 Arad σ ϵ )1/4

(σ is the Stefan-Boltzmann constant, emissivity ϵ is estimated at unity, T is skin temperature)

Here are a couple examples:

# | Airframe | Mach | Speed | Altitude | Drag area | T (DATA) | T (stag) | T (rad)
----|----------|------------|-----------|----------|-----------|----------|--------------------
4. | HTV-2 | Mach 20. | 5,812 m/s | 125k ft? | 0.05 m^2 | 2,200 K | 21,000 K | 2,771 K
8. | X-43A #3 | Mach 9.6 | 3,000 m/s | 109k ft | 0.10 m^2 | 2,255 K | 4,900 K | 2,143 K
8. | X-43A #2 | Mach 6.83 | 2,123 m/s | 109k ft? | 0.10 m^2 | 1,700 K | 2,514 K | 1,650 K
3. | X-51A | Mach 5.1 | 1,500 m/s | 64k ft | 0.10 m^2 | 2,200 K | 1,355 K | 2,058 K
13. | SR-71 | Mach 3.2 | 930 m/s | 79k ft | m^2 | ,640 K | ,651 K | , K

(Data are actual temperatures, where available. Stag is stagnation temperature. Rad is predicted temp using the above formula. Drag areas are pure guesses. Surface areas were 10-12 m^2 for the X-43A, X-51A, and HTV-2. Mass flows were 20-40 kg/s/m^2), except for the X-51A, which encountered 140 kg/s/m^2).

For predicting skin temp, stagnation temperature seems more accurate at lower mach, formula temperatures at higher mach, as expected. Admittedly, I'm pleased (and surprised) that the formula even yields ballpark figures. However, it's a bit sensitive to drag area and radiating surface area, and these are the only data for which I have estimated surface area, so I can't be confident this formula works well for other aircraft.
----------------------------------------------------------
found at https://physics.stackexchange.com/quest ... mperatures

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PostPosted: Tue Dec 05, 2017 1:31 pm 
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Oh Boy, does THIS bring back memories.

I had to go through the derivation of these base equations during my graduate school days in my Hypersonics class.

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PostPosted: Wed Dec 06, 2017 9:40 am 
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All right, the math kind of goes over my head, but there´s something else I wonder about... would the melting point really be the upper limit? AFAIK metals lose strength as they get hot, becoming softer - which is why they are heated in the forge before being hammered into shape.

So, wouldn´t most hulls reach a point where the hot hull deforms under the force of air friction (and thus loses aerodynamic properties, hull integrity or both) significantly before they start melting?

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PostPosted: Wed Dec 06, 2017 1:10 pm 
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I suppose.
Ultra high temp ceramics are good to around 2000C, but temps survived seem be higher than that still. Re-entry goes much higher ( >6000C? ) but probably involves controlled ablation.
Of course giving separate temps for each material in my material list is tiresome, so I guess it should scale with tech level. I'll have to look into that, but I'm wasting my time playing with Pioneer spacesim at the moment.

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