How do you measure breaking distance in space with gravity

[Dox Blox]

Me and one of my friends is having truble not knowing the breaking distance when we want to land on a planet or a moon. Short story... The ship goes BOOM! Is there an easy way to calculate your breaking distance with gravity ofcurse?

[Prophet_01]

Nope. There's no easy way. This goes pretty deep into physics and space engineers likes to mix the different units (kilo, tons, kg, etc) so take care of that.

It mostly depends on your ships thrust-to-weight-ratio (TWR). Here's how to calculate it.


-Thrust of your bottom engines in N-
divided by
-the weight of your ship in kg- times -the planet's gravety in m/s2 (1G = 9.81m/s2)-


That's only half of it. Next we'll calculate your upwards acceleration within the gravity well.


-your TWR (the number you just calculated)-
minus
-1-
times
-the planet's gravity in m/s2-


Now you simply devide the max speed by the result of this and you get a number (in seconds). This is how long it takes your ship to break to 0 seconds.
Your max speed tells you pretty exactly how fast you travel, so the rest is rather easy. If your breaking time exceeds -your hight divided by the max speed- you're screwed ;)


Hope that helps. If I messed up somewhere along the way, somebody pls tell me. I'm not used to writing that kind of stuff down in english.

Cheers and KSP is awesome!

[casualsailor]

Prophet is correct and it's glitchy too. One tip is to not tap your down key and rely upon your interial dampners alone. You need to tap you forward or back key after you release the down key.

There is a glitch and I've had more than enough thrust to stop my downward momentum but if I don't tap forward or back I'll keep moving down until I hit hard. I've also been stuck in hover unable to lower my lander until I tap forward or back.

It's like the game as trouble with you move straight down without you moving laterally (if only a fraction).

[Prophet_01]

Yep... the details are kinda messy. I think you also get something like 30% more thrust, if you don't thrust yourself, but let the inertia dampers auto-break to 0 m/s. The variing levels of "efficiency" for engines aren't making it easyer...
Those calculation should be good enough for approximations, but trial and error will be required to iron out the inaccuracies.

[Cres]

For me its gut feeling work every time

[Endevours]

you could calculate the thrust to weight ratio or you can just use an online calculator http://steamcommunity.com/app/244850/discussions/0/364040961451993818/

[plaYer2k]

Originally posted by Prophet_01:

Nope. There's no easy way. This goes pretty deep into physics and space engineers likes to mix the different units (kilo, tons, kg, etc) so take care of that.

It mostly depends on your ships thrust-to-weight-ratio (TWR). Here's how to calculate it.


-Thrust of your bottom engines in N-
divided by
-the weight of your ship in kg- times -the planet's gravety in m/s2 (1G = 9.81m/s2)-

Sorry but that aint accurate or even true.

First of all, "kilo" is just another name for "kilogram" and thus kg.

Most importantly though, weight is a meassure of force and not mass.
Mass is measured in kg and weight in N. It only works on earth with 1g (yes, lower case g because the upper case G is the gravitational constant) that we can use this sloppy terminology of saying "weight" for mass because it is a convention of that mass at 1g.
At different gravities you get different weights.
https://en.wikipedia.org/wiki/Weight

TWR thus is the actual thrust-weight-ratio that uses the proper weight. It is unitless because both thrust and weight got the unit N and thus 1 N/N = 1.
https://en.wikipedia.org/wiki/Thrust-to-weight_ratio

Originally posted by Prophet_01:

Now you simply devide the max speed by the result of this and you get a number (in seconds). This is how long it takes your ship to break to 0 seconds.

That is not true either. Even though it looks comfy to use such a formula, it is wrong.
The correct formula aint:
s = v/a
with:
s - distance
a - acceleration
v - velocity
but instead
s = v² / (2 a)

Thus the velocities impact is quadratic and not linear. At 2x the speed you nee 4x the brake distance.

Originally posted by Prophet_01:

I think you also get something like 30% more thrust, if you don't thrust yourself

That was two years ago when dampening used 100% of the thrust and manual mode only 2/3 the thrust. Sadly this outdated knowledge still gets spread and rumors through different kinds of discussions.


So for the OP:

s - brake distance [m]
v - velocity [m/s]
a - acceleration [m/s²]
F - force [N]
m - mass [kg]
g - gravity [m/s²]

s = v² / (2 a_resulting)
F = m a
a_resulting = F_brake / m - g

-> s = v² / {2 (f_brake / m - g)}

Thus assuming you enter a planet with g = 1 g = 9.81 m/s², the ship flies 100 m/s and got 3 MN brake thrust with a mass of 100 000 kg, we get:
s = v² / {2 (f_brake / m - g)} = (100 m/s)² / {2 (3 000 000 N / 100 000 kg - 9.81m/s²)} = 10 000 m²/s² / {2 (30 m/s² - 9.81 m/s²)} = 10 000 m²/s² / (40.38 m/s²) = 247.65 m.

I hope that little bit of math helps. Just keep in mind that the gravity with symbol g has the unit m/s² and not g. For the unit-conversion g to m/s² you have to multiply it by 9.81.

(If i made an error somewhere please kindly point towards it. It is late and i might have twisted a few numbers/units even though it shouldnt.)

[Prophet_01]

Thx for correcting that!

I was refering to kN though when I menitioned kilo ;)

[Maschinengewehr]

Applying actual entry practice helps. Instead of zooming towards a planetary body at a flat angle, try entering it so that you kinda skim along the surface of it into the atmosphere. It's similiar to orbiting. Turning the prow off into a slight angle so that the front and side dampeners engage at the same time will also help with inertia.

[Young Robinhood]

Turn and burn man...
Turn and burn. 🔥🐕