what's a sensible pressure for atmosphere cut-off?

[jedidia]

I finally got a formula together that lets me enter an atmospheric pressure and get an altitude out (i.e. at which altitude on that particular planet that pressure would persist). I'll use that to calculate the atmospheric AltLimit that gets written in the config file, so Orbiter knows when to stop processing an Atmosphere.

First tests have been run with 0.1 milibar (the surface value where a planet is recognised as having no atmosphere by Orbiter Galaxy), but the cut-off altitudes get pretty big then. What would be a sensible pressure to cutt off atmospheric calculation?

 

[Urwumpe]

Use density as reference. The Karman line is defined as the altitude, where you would need to reach local orbital speed for countering gravity.

The speed to produce enough lift is a function of density, so if you calculate the current velocity and then compare it to the circular orbit velocity, it should work.

 

[jedidia]

Sorry, I think you misunderstood. The question is not when to cut my engines. I'm exporting config files for planets, and Orbiter wants an altitude limit where it can ignore atmospheric influence. I'm trying to calculate a sensible limit for individual planets, so I'd like to know at what pressure the atmosphere calculation can be comfortably ignored because it has next to no influence (in the short run, anyways).

 

[RisingFury]

0.1 mbar is still a bit high...
I'd go with around 0.01 Pa.
1 bar = 100 000 Pa, so that's 10e-7 bar.

 

[jedidia]

Oh wait... just now I noticed that I don't have the logarithms in my code... that's probably why I'm getting that large results. I'll be back.

---------- Post added at 09:55 PM ---------- Previous post was at 08:40 PM ----------

Yes, that's much better! :lol:

The corrected formula paired with the cut off pressure provided by rising fury seems to return quite reasonable results when compared to the default sol cfg files.

 

[RisingFury]

I'd go as low as possible. I know that Martian cutoff altitude in either the default or one of the scenery addons was around 80 or 90 km altitude, where density was still decently high. I dropped out of a parking orbit that way one time...

 

[mjessick]

Density is probably the most appropriate criteria to use for determining the atmosphere limit. Density affects the trajectory through atmospheric lift and drag. Density affects the heating (most models seem to use density times velocity to some power). Density is used to scale "reentry flames".

What does pressure affect? (Engine thrust is affected by pressure, but this is only important at high pressures. At the high altitudes we are discussing, the pressure is so low that this effect is not important.) I'm not aware of any other effect of pressure, but there may be one. Anyone?

 

[jedidia]

What does pressure affect? (Engine thrust is affected by pressure, but this is only important at high pressures. At the high altitudes we are discussing, the pressure is so low that this effect is not important.) I'm not aware of any other effect of pressure, but there may be one. Anyone?

The converted formula works with pressure at a given height, surface pressure, surface density and surface acceleration, so the density is in there somewhere...

 

[Cairan]

Density, not pressure, is the way to go... As mentionned above, if the density is too low to provide enough lift even at orbital velocity (that density-altitude being called the Karman line), the atmospheric effects begin to loose their importance. However, this is not quite the altitude you should consider for a cut-off.

For a circular orbit, even above the Karman line, you'd still experience some lift, just not enough to support the ship... Hence, you'd still experience drag, dependant on the lift-drag ratio. I'd set the atmospheric cut-off at an altitude where the density is 1/10 of the density at the Karman line. I think it would then be safe to assume the atmospheric effects are negligible.