Duna's Atmosphere in 1.x

[Geschosskopf]

How so? as already mentioned, Mars has a larger scale height than Earth, not smaller.

Scale height is not an independent variable, it's just a shorthand to make certain calculations easier. It is essentially arbitrary and therefore meaningless.

The thing is, Duna used to have a Mars-like atmosphere, an evil thing that was always exactly the opposite of what was helpful no matter what you were trying to do. Now Duna's atmosphere is a benign, helpful thing. You don't need a heatshield to aerocapture or land but it still slows you down quite nicely, and you can rocket up through it without paying it any real attention. OTOH, if you want to use it for parachutes or flying, it helps with both hands. It's the exact opposite of which it was and not at all like the Mars analog it used to be.

I don't care at all WHY this happened, or how it might be the logical result of the new 1.x aero stuff. The bottom line is gameplay, and gameplay has changed significantly when it comes to Duna. The question, therefore, is whether this is desirable or not. I say not.

[KerikBalm]

The thing is, Duna used to have a Mars-like atmosphere, an evil thing that was always exactly the opposite of what was helpful no matter what you were trying to do. Now Duna's atmosphere is a benign, helpful thing. You don't need a heatshield to aerocapture or land but it still slows you down quite nicely, and you can rocket up through it without paying it any real attention. OTOH, if you want to use it for parachutes or flying, it helps with both hands. It's the exact opposite of which it was and not at all like the Mars analog it used to be.

It already did all of that before, except for the flying part... It slowed you down significantly for landings, parachutes were very helpful for landing, its atmosphere didn't matter much during ascents...

Wings were very bad because they didn't follow the v^2 relationship

The bottom line is gameplay, and gameplay has changed significantly when it comes to Duna. The question, therefore, is whether this is desirable or not. I say not.

I don't think it has changed so much.

If you were landing at 8km before - then yea, its a big change.

If you were landing at a lower altitudes... then its qualitatively very similar

[Geschosskopf]

I don't think it has changed so much.

If you were landing at 8km before - then yea, its a big change.

If you were landing at a lower altitudes... then its qualitatively very similar

Hmmm. Used to be, if you wanted to get any real benefit from parachutes, your descent trajectory needed to be very flat, like aiming 1/4 to 1/3 around the planet starting from just above the atmosphere. Then you popped the chutes at like 15-17km and streamered them forever. And you needed literally tons of parachutes, so much that it was often prohibitive to carry that many, so you still needed a fair amount of thrust to land safely. Either that or use a mod to envelop your lander in airbags so it could hit the ground with high horizontal velocity and bounce along a ways. You know, kinda like how things work on the real Mars.

Nowadays, landing on Duna is pretty much the same as landing on Kerbin. You can come down just as steeply yet the chutes still have time to work. You don't need nearly as many chutes as before for the same mass of lander, not really much more than Kerbin, meaning even heavy landers can now rely solely on chutes. And you don't even need a heat shield.

So once again, a very significant gameplay change. Duna landings with chutes used to be "5 minutes of terror". Now they're a doddle.

[KerikBalm]

Hmmm. Used to be, if you wanted to get any real benefit from parachutes, your descent trajectory needed to be very flat, like aiming 1/4 to 1/3 around the planet starting from just above the atmosphere.

Yea, so? parachutes still worked quite well in those conditions.

And that was mainly a consequence of its low scale height, - which, being smaller than Kerbin's, is not analogous to Mars.

Then you popped the chutes at like 15-17km and streamered them forever.

Initially, drogues wouldn't semi deploy until 10km, and normal chutes until 9.

Then they added tweakables, and it was much easier, they'd semi deploy sooner, and you could just set them to open at 5km (before it was much harder, a 500m fixed opening distance left little time for deceleration and then retrothrust).

And you needed literally tons of parachutes, so much that it was often prohibitive to carry that many, so you still needed a fair amount of thrust to land safely.

It was quite easy to calculate that adding enough parachutes for a soft landing was not efficient.

The difference between dropping at 20m/s and 10m/s was too much mass in parachutes. There were diminishing returns to adding more, but parachutes were still very effective, even if a retroburn to with a dV of about ~20-30 m/s of was needed.

Either that or use a mod to envelop your lander in airbags so it could hit the ground with high horizontal velocity and bounce along a ways. You know, kinda like how things work on the real Mars.

On mars, no amount of parachutes will get you down safely. On Duna I've done all of the following:

* Spam chutes

* very few chutes, retroburn

* chute + lithobraking... either in the form of shock absorbing struts (I had 1 design where the kerbal's needed to repair the legs after each landing) or parts with a high impact tolerance.

Optional: put wings on your craft to allow it to come in a bit steeper/lessen opening shock of parachutes, etc.

A mars descent has always had:

Heat shield -> supersonic chute -> detach chute and retro burn -> optional lithobraking in the form of airbags.

The retroburns needed were orders of magnitude greater, and cannot be eliminated.

In KSP, they could easily be eliminated before, just as they can now.

Nowadays, landing on Duna is pretty much the same as landing on Kerbin. You can come down just as steeply yet the chutes still have time to work. You don't need nearly as many chutes as before for the same mass of lander, not really much more than Kerbin, meaning even heavy landers can now rely solely on chutes. And you don't even need a heat shield.

Duna landings with chutes used to be "5 minutes of terror". Now they're a doddle.

Duna landings with chutes were always a "doddle."

[OhioBob]

Nowadays, landing on Duna is pretty much the same as landing on Kerbin. You can come down just as steeply yet the chutes still have time to work. You don't need nearly as many chutes as before for the same mass of lander, not really much more than Kerbin, meaning even heavy landers can now rely solely on chutes. And you don't even need a heat shield.

I haven't attempted a Duna landing since v1.0, but, based on the numbers, I don't believe that's the case. Duna's atmosphere at high altitudes is denser that it use to be, but at low altitudes it is less dense. Landing at an elevation <3000 m should be the same or more difficult than previous versions. I estimate that to get a vehicle to descend at the same terminal velocity on Duna that it would have on Kerbin requires about 2.5 times the parachute area.

Below is a graph comparing the old and new atmospheric density of Duna.

[Geschosskopf]

I haven't attempted a Duna landing since v1.0, but, based on the numbers, I don't believe that's the case.

The numbers don't matter if the algorithm that crunches them has problems. Go see for yourself. The air is much better for flying and parachuting both at low and high altitudes. It's like night and day.

Prior to 1.x, I spent much of my KSP time on Duna, so I got very familiar with its old air. Hundreds of aerocaptures, many dozens of landings, flying planes all over the place at all altitudes. I knew Duna better than Kerbin because I was building permanent bases so hardly anything ever came back from there. So I feel pretty confident in saying what I do about Duna's air in 1.0.2 and notice significant changes everywhere.

[OhioBob]

The air is much better for flying and parachuting both at low and high altitudes. It's like night and day.

Then they must have made changes to the parachutes. All other things being equal, lower density = less drag. If parachutes are performing better in thinner air, then it's possible the area and/or drag coefficient of the parachutes has been increased.

[Geschosskopf]

Then they must have made changes to the parachutes. All other things being equal, lower density = less drag. If parachutes are performing better in thinner air, then it's possible the area and/or drag coefficient of the parachutes has been increased.

I think it more that the air is in fact functionally way more dense than the numbers are leading you to believe, because everything is affected consistently. Wings produce way more lift, control surfaces produce way more force, chutes produce much more drag, falling landers slow down way more even before you pop chutes at low altitude, etc. IOW, maybe good numbers are going in, but something in the new system is misapplying them.

[Mesklin]

Now Dune are much better for landing. I easily landed craft with mass in 88 t on Dune only with 2 pairs of big delta wings 3 times from 4 in time of testing my new creation. Last time I simple break my concentration when I tried to make some beauty screen shots.

[KerikBalm]

Wings should produce way more lift than before, because before the linear relationship of lift to speed was *expletive deleted*

Chutes stopped things plenty well in the old aero, unless you were trying to land at a high altitude.

The low scale height of the previous duna was also a bit *expletive deleted*

I don't see the point of your complaints.

[sdj64]

Duna atmosphere was never exactly like Mars, just as Eve atmosphere was never exactly like Venus. If Mars had an atmosphere like Duna (a 33 times increase), it would be similarly easy to fly in. The mars-like conditions of pre-1.0 were a consequence of bad aerodynamics in KSP. I also don't think the "too thick to ignore" part is really true, yes it was too thick to ignore but that just made it easier to land pre-1.0 without reentry heat and aerodynamic failures. (and I did your challenge all stock so I guess I am qualified to say my opinions here)

Duna = 0.2 atm, Mars = 0.006 atm

Eve = 5 atm, Venus = 90 atm

Edit: Easier to land a rocket with parachutes pre 1.0. Harder to land planes.

Edited May 27, 2015 by sdj64

[OhioBob]

How so? as already mentioned, Mars has a larger scale height than Earth, not smaller.

Add on top of that that its atmosphere is composed of mostly gases heavier than Earth's, and that it is colder... the density drop is much lower than on Earth, proportionately.

You keep talking about what Duna is supposed to have...

If they wanted it to be like mars, they would have given it 0.01 atm of pressure at the datum level.

They didn't, they gave it 20%

That is a significant atmosphere.

What made it seem thin before was the really low scale height.

It doesn't make sense for it to have a much smaller scale height than Kerbin, if it has much lower gravity.

Its not counter intuitive - its what you should expect.

Those are some interesting observations. It's really not that hard to figure out how much pressure should vary with height for each atmosphere. All that's needed are the following information:

1) Planet radius

2) Surface gravity

3) Surface pressure

4) Molar mass of atmosphere

5) Temperature-height profile

All this is given, though the temperature-height profile varies based on latitude and time of day. I derived an approximate global mean temperature-height profile for each body. From the surface conditions I extrapolated the pressure-height profile as if we were dealing with real life planets. As an arbitrary upper boundary, I stopped when the pressure dropped to 1 Pa. In cases were the atmosphere extended beyond the limit defined in the game (e.g. 70 km for Kerbin), I assumed the temperature remained constant at the last known boundary value (assuredly not true in real life).

Below are the altitudes at which the atmospheric pressure becomes 1 Pa.

[TABLE=width: 250]

[TR]

[TD]Eve[/TD]

[TD]41.3 km[/TD]

[/TR]

[TR]

[TD]Kerbin[/TD]

[TD]85.5 km[/TD]

[/TR]

[TR]

[TD]Duna[/TD]

[TD]beyond SOI[/TD]

[/TR]

[TR]

[TD]Jool[/TD]

[TD]2,570 km[/TD]

[/TR]

[TR]

[TD]Laythe[/TD]

[TD]107 km[/TD]

[/TR]

[/TABLE]

As you can see, although Eve has the second deepest atmosphere in the game, in real life it would actually have the shallowest atmosphere. Eve's scale height is very small due in part to its high surface gravity (1.7 g) and to its high molar mass (43 g/mol). Also, despite Eve's high surface temperature, its upper atmosphere (above ≈20 km) it quite cold.

The real oddball is Duna. For some reason Squad gave it a very unrealistic molar mass of 14 g/mol. In real life a small terrestrial planet like Duna could never retain such lightweight gases. This low molar mass, coupled with its low gravity, means that Duna has a very large scale height – about 50 km at its surface. This gives Duna a very deep atmosphere. I extended the atmosphere all the way to Duna's sphere of influence and the pressure had dropped to only 2.8 Pa at this extreme distance. Obviously in real life Duna could never have an atmosphere this thick – the solar wind would surely strip most of it away.

If we gave Duna a more Mars-like molar mass of 43 g/mol, its atmosphere would extend to 180 km (1 Pa level), which would still be the second deepest in the game. If we also gave Duna a Mars-like surface pressure of 6.36 mbar, then its atmosphere would extend to about 97.5 km.

Edited May 28, 2015 by OhioBob