Kerbin atmosphere and drag

[Noobton]

Going by the information on the wiki http://wiki.kerbalspaceprogram.com/wiki/Kerbin , Kerbin's atmosphere goes up to 70Km, from my experience, this is what I've learned:

Anything under 70K will experience drag and its apoapsis will slowly decay.
Between 40Km and 70Km the effects of drag and atmospheric heating are very subtle, when launching once I'm over 40K I go full throttle and not worry too much about my vertical rate of ascend but rather try to start circularizing, when I'm descending I usually don't take much care until I break the 40Km barrier.
Between 20Km and 40Km while ascending I try to stay under 900 m/s to avoid seeing "flames" until I reach 40K, I also try to keep my ascending profile over 60 degrees to try and leave this part as soon as possible, on reinsertion this is the most dangerous part, where my capsules can go Kaboom if they flip to the wrong side and overheat.
Under 20K the atmosphere seems to be thicker, and I begin to see drag effects around 500 m/s I try to keep my speed under that mark and ascending at around 45/50 degrees maximum, for reinsertion if I reach this part I usually consider them safe unless I have messed up and make the inertion too steep but I usually have the opposite problem and make them too shallow.

Question 1. do the visual flames effect indicate that I'm generating too much drag / I'm way over my terminal velocity?
Question 2. what is the reason behind these "atmospheric bands"? looking at the atmosphere information I can see that over 40K there's less than 0.01 atm of pressure, but I can't identify anything that makes sense for the second band.

Thanks
 

[FullMetalMachinist]

1). No, as you gain altitude, terminal velocity gets really fast. It's unlikely that you've gone past it. The flames happening on the way up is a result of the scaled-down model of KSP. You might be in danger of overheating, but you're almost certainly not going faster than terminal velocity just because you see flames. 

2). No idea, sorry. 

[numerobis]

I'm usually at 60-degree pitch around 4 to 6km, 45 degrees somewhere between 6 and 10 km, and 30 degrees not long after 10 km, and as shallow a climb as possible after that -- but without overheating.

That sets me up for circularizing around 40 km, where it's safe to reach orbital speed even with airplane parts. Doing it this way minimizes gravity loss, without incurring much drag loss -- unless you have a particularly draggy payload, drag is not a huge problem.

I build rockets with TWR about 1.5 to 3 (preferably about 2) until about 10km, then TWR about 1.

[Noobton]

1 minute ago, numerobis said:

unless you have a particularly draggy payload, drag is not a huge problem.

this also reminded me, I drop fairings above 40k, should I wait until I'm over 70?

[numerobis]

If you're thrusting, probably drop them ASAP: they have mass, which is probably more expensive to accelerate than the drag. If not, keep the fairings: the drag benefits are positive and the mass doesn't matter.

[Noobton]

3 minutes ago, numerobis said:

If you're thrusting, probably drop them ASAP

ok, but that's precisely what I'm trying to figure out (Edit:I'm trying to figure out when is the soonest I can drop them) haha, when is too soon? also I'm trying to figure out why, so I can deduce this for other bodies.

Edited March 31, 2016 by Noobton

[N_Danger]

I think I can answer #2. It gives a rough guide to pressure which affects ISP, Jet engine performance and aerodynamics. I'm not a numbers guy myself so the visual representation is helpful. I'll watch the indicator measuring the bands when testing new boosters.

I know that below the 20 KM band I have to be careful about light payloads in a fairing when staging at a some angles of attack.  The aerodynamic pressure on the fairing of the rocket below that band along with the shifting CG at staging can over come my control from SAS, gimbal, fins and hopes thus flipping my rocket.

I haven't had any thing explode yet from the flame effect and I've hit over 1200 m/s at 35 KM altitude. I've wondered where the setting is to turn the flaming visual effects down as they seem over done IMO.

[Plusck]

Goo cannisters are among the things that tend to explode quite quickly if you go too fast in the lower atmosphere.

That also makes them a good indicator of what is "too fast" on test runs; I seem to recall that speeds of 1200m/s at 20km and 1600 m/s at 28km are survivable for nose-mounted parachutes, but deadly for Goo.

[AeroGav]

I logged some data from one of my space plane ascents, and took some screenshots with aerodynamic forces display (F12) enabled.   

There is definitely a max speed you can do at each altitude ,  if you get too fast too low heat builds up SEVERELY.      At lower altitudes,  aerodynamic drag losses become huge long before anything threatens to melt, you see huge red drag lines on the aero debug.   But above 20km you can melt your ship without hardly experiencing any drag.       I guess what's happening is,   at lower altitudes, yes there is frictional heating , from large quantities of air moving at lower speed,  but there is also a lot of cold air flowing over the part cooling it down.    If you're going twice as fast in air half as thick, you might have the same overall drag force + friction, but there is less air flow to take the heat away.   Also ambient air temperature starts going up in the stratosphere.

Mach 3.7 @ 23km - plenty of lift on my generously winged design.  No drag and thermal is ok (38%)

Mach 4 @ 26.5km  - faster but also higher, only 40% max temperature.  Still no drag.

29KM and Mach 5 UH OH.   Thermal has shot up to 86%.   You can see i added some UP on the pitch trim in response.   Even though we came close to blowing up, drag is still negligible.

That instinctive pull saved our bacon.  38km and mach 5.5 but temperature is dropping below 60% and we'll soon be shaking hands with Jesus.    You can see that lift is starting to fade, but the centrifugal forces from being near orbital velocity are supporting most of our weight now anyhow.

 

Edit - bear in mind this is a pretty slick airplane,  and at greater pitch angles you would see big red drag lines even at this altitude.     That's why it has so much wing area,  so that it does not need large pitch angles to get lift above 20km, and therefore 2 NERVs are able to slowly accelerate it.

Edited March 31, 2016 by AeroGav

[Vanamonde]

The colored bands on the indicator are merely graphic gradations, and no not represent anything physical. Just thirds of the full atmosphere height. 

[Noobton]

30 minutes ago, AeroGav said:

I logged some data from one of my space plane ascents, and took some screenshots with aerodynamic forces display (F12) enabled.   

(...)

This is awesome, thanks for sharing

[Streetwind]

12 hours ago, Noobton said:

ok, but that's precisely what I'm trying to figure out (Edit:I'm trying to figure out when is the soonest I can drop them) haha, when is too soon? also I'm trying to figure out why, so I can deduce this for other bodies.

Someone did some testing (I forget who or where) and the outcome was, if you're thrusting nearly all the way to the edge of the atmosphere (as you should, else you're launching too steeply), your fairings should be staged away somewhere between I think it was 25 and 30 km. It admittedly depends on the dragginess of your fairing, the dragginess of your payload, and the mass you had to devote to the fairing walls.

Personally, I keep my fairings until 50 - 60 km. I don't usually notice the dV difference, and IRL there are good reasons to keep carrying the fairing above the Karman line that aren't modeled in KSP, but could be roleplayed.

Edited April 1, 2016 by Streetwind

[numerobis]

If you install mechjeb, one piece of information you can get out of it is the current acceleration due to drag. Said acceleration points retrograde, so lower numbers are generally better. Try shedding the fairings at various altitudes to see the effect.

Thermal protection is a big problem. For efficiency you want to circularize as low as possible. Spaceplane parts (max temperature 2000K) can handle circularizing as low as 39km before transferring to LKO, or even 38km for an edge-of-the-atmosphere target orbit. A 38x90km orbit will have you overheat. Deep-space pods can handle a bit higher temperature, and you can use various thermal protection tricks (struts, radiators, blunt nose, etc).

The heat you suffer grows as the *cube* of your surface speed, and linearly with density (which is roughly exponential in altitude). Every km you can go about 6.5% faster for the same heating; 5km you can go about 40% faster. Heat takes time to translate into temperature, so if you're climbing quickly, you can go through a spot where the heat would melt you down, but if you're circularizing at low altitude that's not an option.

Thing I don't know: does launching at night help?