Atmospheric drag and heating

[volcanicshrimp]

I've been trying to build some fast planes in 1.2 (a lot of my ksp craft are high speed planes of some type, and very few of them ever go to space), and testing them at low and high altitude to see how much altitude affects their performance. My assumption was that, due to the thicker atmosphere, planes at lower altitude would experience more drag and atmospheric heating. However, I didn't notice as much difference between the performance at low and high altitude as I expected (high altitude heating was if anything higher than at low altitude and drag only seemed to be noticeably higher at low altitude with a fairly flat nose). The heating difference might be due to residual heat from lower altitude flight though and I might just not have gone fast enough to encounter higher drag with the pointy version at low altitude.

Is this normal and I'm just an idiot, or is this a bug, or something else? My guess is that it's normal behaviour, just not as pronounced as I was expecting/have encountered in earlier versions of KSP (not that I've done much low-altitude high-speed flight before)

(I don't now anything much about aerodynamics and atmospheric heating except what I've learned from far too many hours building aircraft in KSP (with and without FAR) and a little forum-browsing)

P.S. The testing wasn't especially scientific as I have only tested 2 variants of 1 craft (with blunt and pointy noses - NCS with 0.625m nosecone or 0.625m ablator-free heatshield on the end)

http://imgur.com/a/lkmQr - various pictures with varying degrees of helpfulness.

EDIT: The pointed nose seems to be able to achieve about 1600 m/s at sea level, 10km and 20km, with the heating being slightly lower at 20km. The blunt nose can get to 1175 m/s at sea level and 1600 at 20km, without much heating showing up at all.

Edited October 19, 2016 by volcanicshrimp
grammar and more info

[Ripper2900]

I saw somewhere that the drag of pointy objects in general has been reduces, while blunt objects has increased drag. I've done only a few planes so far, but my experience with reentry vehicles, is that they generally starts to slow down at lower altitudes than in v1.1, and that they then slow much quicker. 
This means that i've begun using steeper descents. For you, i guess you'd have to move your "high altitude" higher to see the real benefit. The 10-12K is not very high in my world. I flew some tests earlier with a panther engine, and that plane only really started to accelerate at 14-15K and continued nicely to at least 20-22k approx.

[Streetwind]

According to your description, atmospheric heating works just as intended. There are just more factors involved than you think there are

To begin with, drag scales much more strongly with atmospheric pressure than heating. I don't know the exact formulas, but this is the main reason why reentry trajectories that are too flat kill you just as surely as reentry trajectories that are too steep (or would, if stock KSP didn't have such big margins built-in). Really high up in the atmosphere, you still have medium heating, but pretty much no drag at all. As you go lower, heating only increases moderately, while drag really spikes up. So you want to get through the upper reaches fast, then stabilize in the medium layers of the atmosphere for an ideal reentry. Otherwise, your craft will just soak heat without slowing down much.

Additionally, there's convective cooling. Around 5 to 10 kilometers up, the air is pretty cold, but there is still a lot of it. So the heating that does occur is actually lessened just a bit by that.

[KerikBalm]

Whereas at higher altitudes, the air is actually pretty hot again.

The greeks weren't so far off as its commonly thought when they told the story of Icarus flying too high and having the wax on his wings melt....

Sure, distance to the sun doesn't change noticeably, and for practical flight altitudes, going higher gets colder... but the thermosphere is a real thing:

https://en.wikipedia.org/wiki/Thermosphere

The middle line is temperature:

KSP version:

 

[KSK]

Well you live and learn -  +rep for the interesting information!

Thanks.

[Daripuff]

6 hours ago, KerikBalm said:

Whereas at higher altitudes, the air is actually pretty hot again.

The greeks weren't so far off as its commonly thought when they told the story of Icarus flying too high and having the wax on his wings melt....

Sure, distance to the sun doesn't change noticeably, and for practical flight altitudes, going higher gets colder... but the thermosphere is a real thing:

https://en.wikipedia.org/wiki/Thermosphere

The middle line is temperature:

KSP version:

 

The only problem with that statement is that you really can't call the thermosphere "air". 

The thermosphere is "space".   

The molecules of gas do in fact have a lot of thermal energy per molocule,  but there are so few of them that it is actually brutally cold. 

A billion molecules at 300° K has a lot more thermal energy than a thousand molocules at 3000° K. 

Edited October 19, 2016 by Daripuff

[volcanicshrimp]

Thanks for the info!

I agree, 10km up isn't particularly high. I can't actually remember why I wasn't at closer to 20km.

I had heard about temperature varying differently to drag but certainly not in as much detail as this, so thanks for that!

Back to plane building! I'll need to test my Mach 3 seaplane (the one in my forum pics) and other planes from 1.1 to see how they get on with the drag alterations.

Edited October 19, 2016 by volcanicshrimp

[Norcalplanner]

2 minutes ago, volcanicshrimp said:

Thanks for the info!

I agree, 10km up isn't particularly high. I can't actually remember why I wasn't at closer to 20km.

I had heard about temperature varying differently to drag but certainly not in as much detail as this, so thanks for that!

Back to plane building!

A slightly blunter nose will reduce heating at the cost of some drag, as the game's thermo model does include detached shock waves. This is one of the reasons why a lot of spaceplanes use the shielded docking port as a nose cone - in addition to having a high temperature tolerance, it's just blunt enough to get the detached shick wave going.

[TheGuyNamedAlan]

i realised that the burn started much higher up in ksp 1.1.3 and its the same for 1.2 does anyone know why's that??

[volcanicshrimp]

3 minutes ago, Norcalplanner said:

A slightly blunter nose will reduce heating at the cost of some drag, as the game's thermo model does include detached shock waves.

That's why I was testing blunt-nosed and pointy-nosed planes in the first place - to see how much drag and heating are affected by whether the shock cone is attached or detached.

[KerikBalm]

3 hours ago, Daripuff said:

The only problem with that statement is that you really can't call the thermosphere "air". 

The thermosphere is "space".   

The molecules of gas do in fact have a lot of thermal energy per molocule,  but there are so few of them that it is actually brutally cold. 

A billion molecules at 300° K has a lot more thermal energy than a thousand molocules at 3000° K. 

Well, now you're getting into definitions. The thermosphere is almost all above 100km, the conventional definition for "space"... but a more physics based definition would be the start of the exosphere:

https://en.wikipedia.org/wiki/Exosphere

The molecules in the thermosphere still behave like a gas, whereas they don't any more in the exosphere "The lower boundary of the exosphere is called the exobase. It is also called exopause and 'critical altitude' as this is the altitude where barometric conditions no longer apply. ... If we define the exobase as the height at which upward-traveling molecules experience one collision on average, then at this position the mean free path of a molecule is equal to one pressure scale height."

KSP doesn't have a thermosphere, as the atmosphere abruptly cuts off at 70km. If you go back to the first graph I posted, its shape is basically that of the "lower atmosphere" and "middle" with the upper part missing. Still, the temperature does increase, although its not so high, it does have an effect.

Its also true that being so diffuse means that it contains very little thermal energy, and won't traansfer much heat to an object. Said object will likely be losing heat faster by black body radiation.

This changes when you compress it... like when you slam into it at a few km/s.... you'll 1) encounter a lot more mass of this air per second and 2) see compression heating.

I'm no expert, but my guess is if the gas undergoing compression heating started a hundred K warmer, that is going to noticeably increase the temperature of the gas/plasma at the shockwave where its getting compressed... probably by less than a hundred K... but still.

I can't really comment in detail on the compression heating vs convection effects (compression heating has V^3 rtelationship, and I think convection is probably V^2?), but the lower density works both ways... affecting heating rate and convection rate.

I doubt the increase in temperature is a major factor, I'm just pointing out that atmospheric temperature increases from about 10 to 40km, and I'm sure this has a non-zero effect on heating.

[Daripuff]

45 minutes ago, KerikBalm said:

KSP doesn't have a thermosphere, as the atmosphere abruptly cuts off at 70km. If you go back to the first graph I posted, its shape is basically that of the "lower atmosphere" and "middle" with the upper part missing. Still, the temperature does increase, although its not so high, it does have an effect.

Its also true that being so diffuse means that it contains very little thermal energy, and won't traansfer much heat to an object. Said object will likely be losing heat faster by black body radiation.

This changes when you compress it... like when you slam into it at a few km/s.... you'll 1) encounter a lot more mass of this air per second and 2) see compression heating.

I'm no expert, but my guess is if the gas undergoing compression heating started a hundred K warmer, that is going to noticeably increase the temperature of the gas/plasma at the shockwave where its getting compressed... probably by less than a hundred K... but still.

The thing is, though, you don't really see much reentry heating while in the thermosphere.

Heck, you don't even really necessarily consider travel through the thermosphere to be reentry.  

I mean, the ISS orbits in the thermosphere, so it's traveling in that "high temp" area at 8km/s, and experiencing no compression heating whatsoever.
Edit: In fact, most LEO orbits are actually in the thermosphere. LEO being defined as approximately 120km-1000km, and thermosphere being approximately 85km-600km

With the space shuttle, it didn't actually get compression heating to the point of plasma generation until it hit roughly an 80km altitude, where the thermosphere ends 5km above that.

The atmosphere of the thermosphere is so negligible that all it requires is the occasional re-boost burn to counteract the drag it causes.

KSP's approximation of the Thermosphere by simply not modeling it is not their least accurate approximation.

Edited October 19, 2016 by Daripuff

[KerikBalm]

I know, I'm not saying the thermosphere cooks you... I'm just pointing out that the temperature gradient that we're all familiar with (higher = colder, that's why mountain may have snow at the top but not at the base) isn't applicable to the really high atmosphere.

Within the range that we care about, on kerbin, for reentry... 10-40 km, the air actually gets colder as one gets lower, and this probably matters, and I though I should point it out... thats all.