Calculating Landing Speeds For Different Planets

[The Flying Kerbal]

For some reason I've got a funny feeling this isn't that complicated, but I'm just not seeing it.

I'm experimenting with a small non-powered probe to be dropped onto different planets by parachute.  In tests at Kerbin, dropping it in from around 250K, it's landing speed is 4.3m/s in the desert.  Knowing this, is there a way to calculate what the landing speed might be on other worlds?

The surface gravity would play a role, but then so would the density of the atmosphere, and I can't figure out how to calculate for both of these, maybe it's not as easy as I think it is?

Any help will be much appreciated.

[Jurassic kerbal]

Can’t help you but thanks that’s very interesting 

[catloaf]

Tylo: 10¹⁰  m/s

I'm sure there's a formula, but its probably easier and safer to just test it in sandbox mode via cheat menu or hyper-edit.

Edited July 7, 2020 by catloaf

[king of nowhere]

since speed is proportional to gravity, and drag is proportional to atmospheric density times speed squared, I think in your case a simple empyrical formula can be constructed as

landing speed = 4,3 m/s *((planet gravity / kerbin gravity) / (atmosphere density/kerbin density))

if you land somewhere with twice the gravity and the same atmosphere, you're going to be 1.41 (square root of 2) times faster. if you land somewhere with half the atmosphere and the same gravity, you're also 1.41 times faster. twice the gravity and half the atmosphere means twice as fast - twice as fast generates four times the drag, which in half atmosphere means twice the drag on kerbin, which compensates the higher gravity. the formula should work.

in practice, though, you don't really need any formula, because there aren't many bodies with an atmosphere. if i'm not forgetting anything there are exactly 3: kerbin, eve, laythe. and duna, but i'm not counting it because the atmosphere is too thin to land with that.

eve has moderately higher gravity and atmosphere 5 times more dense, so you will be slower. laythe has less gravity and less atmosphere, you should have about the same speed. duna has a very thin atmosphere, you're gonna fall at over 100 m/s.

[Grogs]

This tutorial has the parachute math that you can use to calculate landing speed. I used it in 1.7, and it was pretty accurate. As far as I know, they haven't changed anything in the model since then.