Flight Question Skin temperature at mach 0.8 (xr-2)

[Keatah]

I was doing a sea-level highspeed pass at mach 0.8 in the xr-2 and noted the skin temperature was like almost 350`C !! I can't imagine this is correct! There can't be that much heat being generated can there? And what about ram compression? How does that factor in here?

 

[johan]

I was doing a sea-level highspeed pass at mach 0.8 in the xr-2 and noted the skin temperature was like almost 350`C !! I can't imagine this is correct! There can't be that much heat being generated can there? And what about ram compression? How does that factor in here?

The skin temperature will depend on how much air is rubbing against the skin of the craft in a given time, say in a second, and how fast that air is moving. In other words, how many air molecules are you blasting through in a second.

Mach 0.8 at (near) sea level means an enormous indicated airspeed, which means a huge amount of air is moving past. You will also notice the dynamic pressure is a big number, and that even the mighty Altea rocket engines cannot overcome air friction past about Mach 0.85 at sea level.

If you were doing Mach 0.8 at, say, 20km altitude, you'll notice the differences: dynamic pressure is much lower, skin temperature is much lower, and your engines can keep accelerating you strongly. I don't think the XR2 has such an instrument, but if it had, you would also see the indicated airspeed at 20km for Mach 0.8 is much lower, than it is at sea level.

 

[Ripley]

IIRC, Surface MFD has it.

 

[johan]

IIRC, Surface MFD has it.

Hmm.... you may be able to configure it, I haven't checked, I think the default thing it measures is TAS, not IAS.

 

[asbjos]

In SurfaceMFD, you can switch between IAS (indicated air speed), TAS (true air speed), GS (ground speed) and OS (orbital speed).

 

[Keatah]

What about at airshows. With F/A-18's doing these near-supersonic hi-speed passes. Their skin (and paint) is getting up to +300`C ??

 

[johan]

What about at airshows. With F/A-18's doing these near-supersonic hi-speed passes. Their skin (and paint) is getting up to +300`C ??

Hmm... I suspect so... perhaps someone with a strong aviation background can answer that one. My google-fu seems to be too weak today.

You're right though, people who fly high-speed aircraft should surely have this in mind.

 

[Gr_Chris_pilot]

In real life even airliners have [ame="http://en.wikipedia.org/wiki/Total_air_temperature"]SAT and TAT[/ame] indications in cockpit.
For a fighter aircraft after each supersonic flight excessive visual inspection needed and the aircraft is not allowed to pass Mach one many times over a short period of time... One F16 pilot told me that the wings leading edge can be damaged and repaired after some Mach1+ flights.

 

[Keatah]

According to this -- http://www.aerospaceweb.org/design/scripts/atmosphere/ -- I should be seeing an increase of about 40`C above OAT for a near supersonic pass at sea level.

XR-2 heating is way off!

---------- Post added at 09:37 AM ---------- Previous post was at 09:34 AM ----------

http://adg.stanford.edu/aa241/drag/skinfriction.html

 

[Cras]

If you travel at supersonic speeds at sea level, the water under you will boil. Trans sonic, you are not that far off from that. My realism gland also begs me to question why you are burning at over M.80 at a few hundred feet AMSL....

 

[sputnik]

If you travel at supersonic speeds at sea level, the water under you will boil.

No, no, NO! Where did you get this idea?

What Keatah said. Total temperature rise, at the stagnation point only, is on the order of 40C. This is nothing, and the XR-2 heating code IS in error.

 

[garyw]

What about at airshows. With F/A-18's doing these near-supersonic hi-speed passes. Their skin (and paint) is getting up to +300`C ??

No idea about F/A-18's but Concorde would register 120c on the nose when doing mach 2.01 at 58,000ft.

 

[Cairan]

Actually, much of the heating has absolutely nothing to do with friction... It's mostly heat from the compression of the air resulting from the shockwave accompanied by stagnant subsonic flow at supersonic speeds... The heating itself occurs only at the shockwave itself, which then radiates heat to the stagnant air and aircraft/spacecraft surfaces... Besides the surfaces outside the nosetip shockwave cone, which generate their own shockwave fronts, the whole ship is almost exclusively heated radiatively (except where there is subsonic flow recirculation).

One thing we learned from the Cold war during ICBM warhead design was that a blunt nose is better than a sharp one... Conventional wisdom would dictate to minimize resistance with a pointy end to have less friction, but that doesn't prevent the formation of a shockwave, and the closer the shockwave is to the structure, the easier it is to radiate heat to it... So with a blunter nose, you push back the shockwave, with more resistance to airflow, yet you gain a lot by reducing the heat transfer and heating rate with a cushion of stagnant subsonic gas providing -some- insulation...

 

[johan]

Hmm.... you may be able to configure it, I haven't checked, I think the default thing it measures is TAS, not IAS.

Well looky there, a button labeled IAS :lol:

Actually, much of the heating has absolutely nothing to do with friction... It's mostly heat from the compression of the air resulting from ...

Right, that actually makes sense. Shows you how much I know about supersonic flight

So if I understand all of you correctly, it boils down to saying that the skin temperature simulation is way off in Orbiter? At least at near-M1 speeds near sea level on Earth.

 

[statickid]

The skin temperatures are specific to the vessel code aren't they? I thought orbiter only calculates whether there should or shouldn't be reentry effect, but things like hull and wing temperatures are coded into the vessels that "measure" them. i.e. DG-IV has it's own heating code, XR-2 has it's own heating code, and AMSO has it's own failure parameters etc

---------- Post added at 01:59 PM ---------- Previous post was at 01:48 PM ----------

I just went mach .8 in the DG-IV 3 at about 110 meters and the temperature was about 130 C at the nose (something like that, can't remember even though i just did it mere moments ago) the tail section was 111 C.

 

[johan]

The skin temperatures are specific to the vessel code aren't they?

Makes sense that it would work this way - so each craft can have as many or as few "heatable areas" as it likes.

 

[Urwumpe]

looks like the XR family does not handle heat transfer by conduction. Only radiation seems to be there. which is correct for higher altitudes but not enough at low dense atmosphere.

 

[dbeachy1]

Thanks for the feedback, guys. :tiphat: Today I added some conductive heat transfer logic to the XR heating code to make it more realistic when flying in dense atmosphere. I just uploaded some XR Beta-2 DLLs if you want to try them out:

XR1 1.11 Beta-2 DLL
XR2 1.6 Beta-2 DLL
XR5 1.9 Beta-2 DLL

Just drop the updated DLL(s) overtop of your existing XR1 1.10 / XR2 1.5 / XR4 1.8 DLL(s) in $ORBITER_HOME\Modules.

 

[Hlynkacg]

What about at airshows. With F/A-18's doing these near-supersonic hi-speed passes. Their skin (and paint) is getting up to +300`C ??

Yes

 

[Urwumpe]

Yes

No.

You would need a much higher EAS for compensating for higher air density. For 300°C, which is the skin temperature of a SR-71 at 90,000 ft (0.05 atm) and Mach 3.2 (473 EAS), you would need about (Different materials and air temperature ignored) 690 EAS at sea-level, or Mach 1.05.

So, not just near-supersonic, but actually almost at the limit speed possible for a F/A-18 at sea-level with clean config.