Current Scale Heights? (also, terminal velocities plots)

[Sharpy]

What are current Scale Heights of planet atmospheres? As I understand, they are currently functions rather than constants? (still, a decent approximation would be sufficient).

I'm trying to create a graph of terminal velocities for various planets, and using the Atmosphere article I got to create a script...

As you see below, "Hw" - scale heights - exist in two variants. The first is data gathered from the wiki. The other I got from this article.

Both are dubious. The Wiki data for Jool means you're getting a sharp cut-off at its 200,000km atmospheric border, terminal velocity at some 560m/s turning into nothing. Also, Duna would be about the same as Kerbin in terms of terminal velocity, despite the atmosphere being much thinner, and similar Laythe hardly better than Eve. The other link data seems like nice, round numbers (too nice?) with believable curves, but dates back to 2013.

Could someone give me current Scale Height values? (and also, check if I didn't goof somewhere else)?

# drag coefficient, vehicle dependent
d=0.2

names = "Duna Laythe Kerbin Eve Jool"

# zero altitude pressure
p0w = "0.2 0.6 1 5 15"

# Scale Height
#Hw = "6000 8000 5600 7300 31000"
Hw = "3000 4000 5000 7000 10000"

# Planet masses
Mw = "4.5154812e21 2.9397663e22 5.2915793e22 1.2244127e23 4.2332635e24"

#Planet radii
rw = "320000 500000 600000 700000 6000000"

#Atmospheric heights (graph cut-off)
mhw = "50000 50000 70000 90000 200000"

p0(n) = 1.0*word(p0w,n)
H(n) = 1.0*word(Hw,n)
M(n) = 1.0*word(Mw,n)
r(n) = 1.0*word(rw,n)
mw(n) = 1.0*word(mhw,n)

Vtn(h,n) = sqrt((250 * 6.67384e-11 * M(n)) / (((r(n) + h)**2) * 1.2230948554874 * p0(n) * exp(-h/H(n)) * d ))

set dummy h
set xlabel "Altitude [m]"
set ylabel "Terminal velocity [m/s]"

set xrange [1 : 90000]
set yrange [0 : 5000]
set grid x y

plot for [i=1:words(names)] (h > mw(i))?NaN:Vtn(h,i) title word(names, i) 

Edited June 3, 2015 by Sharpy

[Redhaze]

In general, with the new atmospheric model, you're goofing already, simply because of making the assumption that terminal velocity still remains the same regardless of shape, volume and weight. You're better off mapping atmospheric density, which at least remains somewhat the same, assuming conditions (temperature) remain the same.

About Kerbin and Duna: That's actually about right. Duna's thinner atmosphere is offset by it's lower gravity. (and lower temperature)

TL,DR: The atmospheric model doesn't work that way any more.

[Murph]

The model has completely changed. We updated the scale heights on the wiki, but they are now no longer high precision, and not accurate over the entire altitude range. The numbers on the wiki for "scale height' are probably as good as you will get, but are basically a quite rough approximation over the majority of the altitude range, and particularly inaccurate at the upper limit of the atmospheres. They are just a very rough approximation to the completely new model. At the very upper end, Squad's new model basically forces the pressure rapidly to zero (off the top of my head, that's somewhere around 62.5km upwards on Kerbin). The new model does not have a single equation from sea level to space.

That Wikia article is from 2013, so completely out of date for KSP 1.0. The official KSP wiki's info on atmospheres on each celestial body's individual page should now be mostly up to date, I believe, thanks to some extensive work by one of the contributors (although I believe he's still working on it). Check the revision history on each page, and it should be quite obvious which articles have been heavily worked on over the last month.

(The "Atmosphere" article on the KSP wiki is indeed outdated at present, as noted in the prominent notice at the top of the article.)

Edited June 3, 2015 by Murph

[Chaos_Klaus]

This: http://forum.kerbalspaceprogram.com/threads/117069-Great-New-Physics-Thread%21?p=1894742&viewfull=1#post1894742

and

this: http://forum.kerbalspaceprogram.com/threads/117069-Great-New-Physics-Thread%21?p=1940443&viewfull=1#post1940443

Also, terminal velocity is no longer important for launching rockets. Terminal velocity is very high now. So high that it makes for impractical ascent trajectories.

[Sharpy]

The numbers on the wiki for "scale height' are probably as good as you will get, but are basically a quite rough approximation over the majority of the altitude range, and particularly inaccurate at the upper limit of the atmospheres.

They are pretty much moot for upper atmosphere (concerning terminal velocities) as you'll never exceed them (for longer than a couple seconds anyway). either escaping the atmosphere or descending into lower layers - if you move at terminal speed through upper atmosphere, you're probably moving way too fast anyway.

Also, terminal velocity is no longer important for launching rockets. Terminal velocity is very high now. So high that it makes for impractical ascent trajectories.

I'm still getting flames surrounding my rocket and overheating my systems if I'm using my "big asparagus" subassembly (7 Twin Boars) for a load that really doesn't require so much much. Generally, exceeding 1000m/s in the 20000-30000m altitude range results in these effects. Are they related to terminal velocity?

[OhioBob]

I'm trying to create a graph of terminal velocities for various planets...

Your premise is wrong right from the start. Under the old aero model, every vehicle, regardless of its size and configuration, had the exact same ballistic coefficient (it was a universal constant in the game)*. Also, the air density at any given altitude on any given planet was a constant. It was therefore possible to compute an exact and consistent value of the terminal velocity at a given altitude. This is no longer true. Ballistic coefficient is now dependent on the shape, size, and speed of the vehicle - it is highly variable. Furthermore, air density is now a function of temperature, which varies by latitude and time of day. All these variable factors now make it impossible to produce a table or graph of terminal velocity with any reliability.

As Murph explained, the scale heights listed in the Wiki article are just a rough approximation, and not even a very close approximation in some cases. Scale height now varies with altitude. It is no longer possible to compute pressure using the old method. If an approximate value of pressure is close enough, I recommend the following: Each planet's Wiki article now contains a table of altitude (Z) and pressure (P). Create a graph in which you plot LN(P) versus Z. Fit a trendline to the data points so that you now have LN(P) as a function of Z. The pressure is simply the exponential of LN(P), that is, P = EXP(f(Z)).

(edit)

* Correction, ballistic coefficient was a constant for a given drag coefficient. However, drag coefficient varied very little from one vehicle to another, with a value of about 0.2. For most computations it was assumed that Cd = 0.2, thus the ballistic coefficient was also assumes to have a constant value of 625 kg/m².

Edited June 3, 2015 by OhioBob

[maccollo]

I'm still getting flames surrounding my rocket and overheating my systems if I'm using my "big asparagus" subassembly (7 Twin Boars) for a load that really doesn't require so much much. Generally, exceeding 1000m/s in the 20000-30000m altitude range results in these effects. Are they related to terminal velocity?

Mach effects is simply a function of atmospheric density and speed. All objects will create the same atmospheric effects at the same speed, while their terminal velocity will vary to a great degree.

Your typical rocket will be so aerodynamic that it simply can't catch up with it's TC. In this example I have a first stage that is 90% powered by solids (low ISP, TWR increases rapidly) with an initial TWR of 2.0. Despite the ludicrous amount of mach effects you can see that the atmospheric efficiency is only 69%, which means the rocket is only at about half TC. Even in the transonic region the atmospheric efficiency only goes up to about 82%.