Jool's reported gravity stats don't make sense

[cephalo]

I thought it would be fun to try and get an EVA report from Jool's lower atmosphere. The reported 'surface gravity' in the wiki, and also what KER indicates is quite lower than one would expect for such a high orbital velocity. Can anyone explain to me why these stats are correct? Also, how would I guess the TWR needed to hover around 119,000 meters above Jool? Don't wanna get that wrong.

Edited August 1, 2015 by cephalo

[Hcube]

well, at 119000m above jool the force acting on you is ((jool mass*ship mass)/(119000^2))*(6.67*10^-11) newtons N, so your thrust should be equal to that... but then your mass will decrease as you burn propellant so the force of jool acting on you will decrease and thus you will need to throttle down the engine that's about it ! no need for surface gravity value, just jool's mass

I don't know why the gravity stats ae off, but since Jool is gaseous it may have a surface that is not defined very well and that could be the cause of discrepancies... anyway the wiki is old and rarely updated !

[Tank Buddy]

The force of gravity (and acceleration) follows the inverse square law: the force of gravity exponentially decreases the further you get from the mass. (Because you are dividing by the square of the distance between the objects)

Equation:

The gravitational force decreases significantly the further away you are from the center of Jool. Because your spacecraft is 119 Km from the surface of Jool (and however many more from the center), the local gravity is much less than you'd expect. KER actually changes your TWR value based on this.

Also note that even though Jupiter is more than 200 times more massive than Earth its surface gravity is only a little over 2g. This is for the same reason; Its diameter is so great (11x Earth's) that you are far enough from the center that the gravity is diminished because of the inverse square law.

Edited August 1, 2015 by Tank Buddy

[Padishar]

They are correct because they are very simple to derive from the planet's standard gravitational parameter as is the orbital velocity. No need to guess at all, you should be able to set KER to Jool and atmospheric mode and then set the altitude slider to 119km. It will then do all the calculations for you, including accounting for the effect of the atmospheric pressure on the ISP, and hence thrust, of your engines...

[cephalo]

thanks everyone.