Aerodynamic heating now more sophisticated?

[AeroGav]

This is a tale of two aircraft.    The kerballed version has a flimsy mark 1 cockpit and crew cabin.   Both have FAT455 airliner wings, which as you know are made of white chocolate.   The kerballed version weighs 30 tons and has a lift rating of 25.

 

This unmanned probe launcher has 10 tons less weight and the lift rating is just 2 less.

I have taken screenshots during 2 different launches in the kerballed version, capturing altitude and mach numbers and the verified non exploded status of the airplane.  Some data points  

Alt   24km   Mach 4.33

Alt   25km   Mach 4.52

Alt   27km   Mach 4.65

Alt   30km   Mach 4.82

Alt   32km   Mach 5.18

Alt   36km   Mach 5.71

Alt   40km   Mach 5.97

Alt   43km   Mach 6.28

Alt   47km   Mach 6.67

The second ship, by comparison, is an absolute nightmare.  I could not exceed 3.75 mach on the Rapier airbreathing without blowing up.  Heat levels remained horrendous even when closed cycle.  There was some weird effect where allowing the angle of attack to decrease below a critical value (18 degrees) would make kerbal engineer's critical thermal percentage increase 30% in less than 3 seconds. If i was fast enough to the S key,  they'd instantly drop as soon as i nosed up , but obviously there were occasions (10 in fact) where I was not and found myself starting over.  The critical part was always the wings, it must have been related to skin temperature because everything changed so quick, and obviously at 43km altitude, raising the nose from 16 to 18 degrees does not produce much of a change in speed or altitude within the space of 2 seconds. 

I was forced to fly a very inefficient profile with a hugely draggy AoA, massive cosine losses and could not exceed mach 5 till over 50km.

Why is one set of wings so much more durable than the other? 

Are they being shielded by the manned aircraft's longer nose or its forward strakes?  I had no idea the thermal model was that sophisticated !

[p1t1o]

Not sure if this helps but this is what I was thinking, hope it at least makes sense:

You're going too fast, too low. When your AoA drops, you speed up more and overspeed. Second craft is worse off in this situation due to its lower mass.

Mach5 at 50k seems normal.

Make any sense? Its the only reason I could think of that a *drop* in aoa would cause catastrophic things.

[AeroGav]

34 minutes ago, p1t1o said:

You're going too fast, too low. Mach5 at 50k seems normal.

 

Sorry if it wasn't clearer but the larger version of this ship did mach 5.18 @ 32km without melting.   That list of mach numbers and heights is basically how fast it was going at each altitude as it accelerated to orbit, slowly, on the incredible thrust of 2 NERVs.

 

Quote

When your AoA drops, you speed up more and overspeed. Its the only reason I could think of that a *drop* in aoa would cause catastrophic things.

Indeed,  that is the normal run of things I'm sorry if I didn't make that more clear.   If not using mechjeb, i fly my spaceplanes with pitch trim and more AoA means more lift but more drag, you climb quicker but accelerate slower.   However, as I said

 

Quote

obviously at 43km altitude, raising the nose from 16 to 18 degrees does not produce much of a change in speed or altitude within the space of 2 seconds.

Which is why i thought there must be some shielding occuring from other parts  - alternatively, the wings are able to reradiate their heat to space or to other parts at some pitch angles, but not others.

 

[Mat2ch]

This is a nice looking craft, but you're carrying much too much wing area around with you.

[AeroGav]

@ Mat2ch

The unmanned ship - possibly.  Though the mk 2 version has even more lift.

 I used the huge wings so i could have a single large LF tank for the NERV on my CG, only have one pair of fuel lines, and not have to worry about CG shifting as the NERV did its thing.

The manned version has another reason for needing large wings btw

 

 

[KerikBalm]

My guess is it has to do with an attached vs detached shock cone, which I've been told by a dev (nathan I think) *is* modeled by KSP

 

Found it:

If you present a blunt face to the bow shock, the shockwave is morelikely to remain detached... heating at the edges of wings, particularly with low angle of attacks, will have an attached box shock.

The shockwave is the area where compression and heating of the air is highest, and it would help immensely to avoid having that right up against the skin of the aircraft

https://en.wikipedia.org/wiki/Bow_shock_%28aerodynamics%29

"Unlike an oblique shock, the bow shock is not necessarily attached to the tip of the body. Oblique shock angles are limited in formation and are based on the flow deflection angle, upstream Mach number. When these limitations are exceeded (greater deflection angle or lower Mach number), a detached bow shock forms instead of an oblique shock. As bow shocks form for high flow deflection angles, they are often seen forming around blunt objects. In other words, when the needed rotation of the fluid exceeds the maximum achievable rotation angle for an oblique attached shock, the shock detaches from the body. Downstream of the shock, the flow-field is subsonic, and the boundary condition can be respected at the stagnation point.

The bow shock significantly increases the drag in a vehicle traveling at a supersonic speed. This property was utilized in the design of the return capsules during space missions such as the Apollo program, which need a high amount of drag in order to slow down during atmospheric reentry."

So... uh yea... to reduce heating, you do need to fly a draggy profile. When the AoA decreases, the deflection angle decreases, and the detached bow shock becomes an oblique shock... the zone of maximum heating is then pressed right up against the skin of the craft... which is why the temperature spikes.

In short, those wings won't survive without a high enough AoA, or the game thinking that they are being shielded from heating by a part in front

 

Edited May 25, 2016 by KerikBalm