Excess drag in Duna's upper atmosphere

[ftunk70]

Hi all,

I had this problem yesterday when leaving Duna. Even in the high upper atmosphere (>30 km), I was losing Ap height at an unexpected rate. Much more so than in Kerbins upper atmosphere.

Shouldn't the upper atmosphere on Duna be much thinner than Kerbin's, since it is thinner at sea level. I'd expect the atmo tapering off slower on Duna, but the initial density is so low, the drag should not be measurable at 30+ km height.

What do you think?

Edit: It has been verified that the atmosphere of Duna has higher drag at 30000 m than that of Kerbin's at 40000 m. These heights are both roughly 60% up to vacuum from sea level.

Edited May 26, 2015 by ftunk70
Added info

[Idrryl]

I have aerobraked on Duna in 1.0.2 with a periaps at about 20km and now that I think about it, the drag was a bit more than I expected. But nothing so extreme as Kerbin's 30km atmosphere. I guess it depends on the look of your spacecraft, if it's wider then of course the drag is higher.

[Cebi]

I was telling myself this first time i was ascending from Duna surface too. Then i realized that Lander Can without any aerodinamic parts is not so good idea.

[Chaos_Klaus]

Drag will also depend on the shape of your craft now.

[magnemoe]

High atmosphere drag is larger than in 0.90, mechjeb landing autopilot is very inaccurate here.

braking force depend on drag from ship and also its weight, an heavy ship will require more force to slow down

[ftunk70]

The drag on Duna is indeed higher than on Kerbin above roughly 35000 m.

This is a little bit surprising to me since the pressure is so low on Duna sea level. I guess it would have to do with Duna being smaller and thus not compressing its thin atmosphere at a very high rate. But then I think it is probably premature to end Duna's atmosphere at 50000 m. It should probably extend to 100-150 km.

Edit: This also means that the pressure should be higher at those altitudes, since drag is proportional to density, which is proportional to pressure. I realise I should have used the presbat instead of the accelerometer for my measurements, it would have been much easier to measure the pressure rather than the drag directly.

Edited May 22, 2015 by ftunk70

[Kuu Lightwing]

Maybe Duna atmosphere consists of a heavier gas?

[Janos1986]

Maybe the fact that Duna's gravity is lower than Kerbin's means that the gases are more evenly distributed in the atmosphere...?

Dunno, I'm a plump neckbeard, not a physicist.

[ftunk70]

Kuu, I think that if the gas is heavier (say 100% Radon?), then the atmosphere will be more concentrated to lower altitudes?

Janos, I think you're probably right.

The drag is proportional to air density, but pressure and density goes hand in hand.

I guess Duna's atmo could be more viscous? Maybe it's all Higgs?

Edited May 22, 2015 by ftunk70
Answered Janos

[Cebi]

Will try today to monitor presure during ascent of my probe. Will post readings later today.

[Cebi]

Ok so here is what i found out.

https://docs.google.com/spreadsheets/d/1JdYXcMcwQA3NNzd08sNx7npjDPSgkETCp18grYnDw28/pubchart?oid=1346886800&format=interactive

Blue line is Kerbin and orange is Duna. Bumps are errors in my measurement.

There is a diference but even when there is preasure at 30000 aprox. 3x lower you wont "feel" it with such small numbers involved.

Edited May 22, 2015 by Cebi

[Chris_2]

Doesn't Duna have a surface pressure of ~0.2 bar? This is still quite high, and much higher than (real life) Mars for example. So Aerodynamics of the craft will still matter probably.

Ok so here is what i found out.

https://docs.google.com/spreadsheets/d/1JdYXcMcwQA3NNzd08sNx7npjDPSgkETCp18grYnDw28/pubchart?oid=1346886800&format=interactive

Blue line is Kerbin and orange is Duna. Bumps are errors in my measurement.

There is a diference but even when there is preasure at 30000 aprox. 3x lower you wont "feel" it with such small numbers involved.

Nice measurements. What are the units?

[Cebi]

@Chris...honestly dont know what those units might be but i measured 20-ish units 550ASL on Duna. I used redings that presmat barmometr gives.

Edit: is it posible that barometer is using hPa/milibar?

Edited May 22, 2015 by Cebi

[OhioBob]

Edit: It has been verified that the atmosphere of Duna has higher drag at 30000 m than that of Kerbin's at 40000 m. These heights are both roughly 60% up to vacuum from sea level.

Yes, the density of Duna air at 30 km is indeed greater than Kerbin air at 40 km.

At 40 km on Kerbin, the air pressure is 78.89 Pa. Temperature varies, but over the equator it is about 275 K at this altitude. The mean molecular weight of Kerbin air is 28.9644 g/mol. These give us a density of 0.0009994 kg/m³.

At 30 km on Duna, the air pressure is 108.0 Pa. The temperature at this altitude is 150 K. The mean molecular weight of Duna air is 14 g/mol. These give us a density of 0.001212 kg/m³.

Maybe Duna atmosphere consists of a heavier gas?

Duna's atmosphere is actually composed of lighter gases, having a mean molecular weight of 14 g/mol. In real life this doesn't make sense. A small terrestrial planet like Duna could never retain the lightweight gases needed to lower the molecular weight to 14.

BTW, in just the last couple of days I revised the atmosphere section of each planet's Wiki article. There is now a table for each planet that gives the pressure at various altitudes. Some of you might find that information useful.

Edited May 22, 2015 by OhioBob

[Dspan_000]

They changed the Aerodynamics system now so expect stuff that didn't happen before. Such as new drag. (Also Ship size and shape also affect drag)