Nav Ball display

[VoidSquid]

I'm wondering, not for the first time, a vessel about to land at Kerbin:

It says, the G-force is 1g or say 10 m/s2. Hmm... probe is just slowly falling down to Kerbin, even accounting the breaking by air resistance, if I follow the decrease of speed, it's definitely not some 10m/s2 (maybe 2 or 3 m/s2, but not 10, my all means). Why that display then?
Or in broader context: I've seen it a couple of times, when returning to Kerbin, I often get the display of 1g acceleration while in micro-gravity, i.e. zero acceleration.

Question: what is it that I don't understand here?

Edited January 18, 2020 by VoidSquid

[AHHans]

31 minutes ago, VoidSquid said:

Or in broader context: I've seen it a couple of times, when returning to Kerbin, I often get the display of 1g acceleration while in micro-gravity, i.e. zero acceleration.

Ermmm....

You are probably sitting on a chair right now. Are you in micro-gravity? But are you moving - relative to that chair?

Do I need to say more?

[VoidSquid]

Lost me here, AHHans, sorry. Sitting in my chair, nor moving relative to chair (or earth, for the matter), ofc I feel 1g acceleration. Thought experiment: I take a pen in my hand and let it drop.

In micro gravity though, same pen would not fall down due to zero g.

Then why does KSP show me more often than not 1g when my vessel slows down by just a faction if 1g?

I mean, when the chute is fully open and I have a constant velocity of say -6 m/s, of course, the acceleration due to gravity is 1g (dv/dt = 0, no effect from that). But free falling (even taking into account the slight deceleration due to air resistance), it should not show me 1g, should it?

 

EDIT: In other words, you say that the combined effect of deceleration of say 3 m/s are resulting from gravity (which, unchecked, would simply result in a respective acceleration while feeling zero g) plus air resistance, and this combines to that exactly 1g? Possible, yes, just a big chance that both effects combine to exactly 1g. Odd...

Edited January 18, 2020 by VoidSquid

[AHHans]

[Waaaahhhh! This is so simple in my mind, but hard to explain when I don't understand where exactly your problem is.]

What the navball shows you is the force that something (or someone) inside the craft experiences. This is due to the combination of acceleration - as in change of movement - and the attraction due to the mass of a nearby CB (e.g. Kerbin). Please keep in mind that movement is always relative to something (the surface of Kerbin, the CoM of Kerbin, a selected target, you know the drill). Please also note that the speed indicated on the navball does not include the direction, so if you fly in a turn and change the direction of your velocity vector (say relative to the surface) but not the length, then you can have arbitrary high g-forces without the speed displayed on the navball changing. When you are in a stable orbit, then there is a force (Kerbin's gravity) that pushes you inward, but also a change in movement relative to Kerbin's CoM (the curvature of the orbit) both cancel each other out and you are in micro-gravity.

1 hour ago, VoidSquid said:

But free falling (even taking into account the slight deceleration due to air resistance), it should not show me 1g, should it?

What do you mean with "free falling"?

When you are close (less than one Kerbin radius away from the surface of Kerbin) to Kerbin you always have the the 1g from Kerbin's gravity. But if this only results in a change of your movement (like keeping you in orbit) then you don't feel that inside the vessel, so the navball shows 0g.  If you don't have a change of your movement - either because you are still on the surface, or moving at constant speed in a straight line (e.g. falling down at constant speed, flying through the air with constant speed and direction - not being in a stable orbit, that movement is curved) - then you feel that 1 g acceleration inside your vessel, and the navball will show 1g.

So if you mean with "free falling" that you don't have parachutes deployed and your speed (as indicated on the navball) is not changing much, then this could be: a) you are nearly in orbit, in the outer fringes of the atmosphere. In this case the little air resistance there is slows you down a bit and most of Kerbin's gravity is used to curve your velocity to keep you in orbit, so the navball shows only a small g-force. Or b) you are at relatively low speed, deeper in the atmosphere, and moving mostly downwards. Then Kerbin's gravity mostly pulls you in the direction that you are already going, but air resistance keeps you from accelerating faster downwards. This is essentially like hanging from parachutes, so the navball shows you essentially the 1g of Kerbin's gravity. This is what I think is happening in the screenshot above (in the first post), the air resistance pushes you with slightly more than 1g at a 15 deg angle from straight down. So most of that force is offset by Kerbin's gravity and only a few m/s2 change your velocity. Inside the capsule you feel the somewhat more than 1g that is displayed on the navball.

 

 

[VoidSquid]

Never mind, I found my error. Only saw the velocity as a result, din't check the individual horizontal and vertical vector components... duh...  

[VoidSquid]

51 minutes ago, AHHans said:

when I don't understand where exactly your problem is.

Me being a bit dense at times, it is