Calculating Orbital Velocity =(

[GateCrasherVI]

EDIT: Obviously flawed data in my calculation was pointed out.

Can anyone tell me, for sure, what the value for G is in KSP? Defaulting to wiki mu values feels like cheating the last 2% of what has been many weeks of study for me.

Anyone passing through that can help illustrate the KSP values for a 100km altitude circular orbit?

v^2 = GM (1/a) is the proper equation?

v^2 = (1 / 700,000,000 ) for a 100km circular orbit

* G (would that be 6.67384x10e-11?)

* M (Kerbin = 5.2915793×1022 kg)

Does all that check out?

Kinda writing that backwards for the sake of reading in the order I'm assuming it would be carried out correctly.

Because my result NEVER matches what I can actually pull up and see in-game. And at first, I suspected maybe I need to reduce my result by the speed at which the planet rotates, but when I toggle surface / orbit speed displayed on the nav-ball, it kinda defeats the purpose of that and shows that niether value matches my result. ='(

If anyone could ever so kindly, just once show me the definitive correct values for gravity / mass, it's going to spring-board me through calculations I've already theoretically solved, including TWR, orbital period, transfer time and deltaV, orbital darkness period for various planets. I've made tons of progress, all thats missing is accurate data. The math works just fine. But....=( i'm sure you see my point

I keep finding crumbs of evidence square root is supposed to be used somewhere in this process but get drastically negative results and haven't the faintest idea of what to do with those values.

Edited March 13, 2014 by GateCrasherVI

[Padishar]

Well, the first thing that jumps out at me is that your value for a is in the wrong units. 700km is only 700,000m.

v^2 = G * M / a

v^2 = 3528430000000 / 700000 (only used 2 decimal places for G and M)

v^2 = 5040614.3

v = 2245 m/s

Edit: G*M is the "Gravitational parameter" value quoted on the wiki's various planet pages (for Kerbin this is 3.5316000x10^12 m^3/s^2) so all you need is this...

v = sqrt(3.5316x10^12 / 700000)

v = 2246.1 m/s (this should be more accurate than the above)

Edited March 13, 2014 by Padishar

[GateCrasherVI]

Thank you so much! Sometimes the numbers are just numbers, I'm sure you know how it goes.

I knew it had to be a problem with either exponents or conversion that the actual math was being done properly, and equation was built properly (at least one of the more than one dozen pages of attempts I've written out).

I appreciate the time and it'll no doubt be the piece I was missing for all this.

EDIT= Also, your 2246 is right on the money.

I knew the mu was shown on the website but really hoped to have a value for G so that I'm never stuck saying "Well, I'm not around Kerbin anymore"

Can you tell me how the gravitational parameter is derived? I was really hoping to understand enough to me fine no matter where I find myself, as long as I have my calculator, pen, paper, and the basic object data and position.

Edited March 13, 2014 by GateCrasherVI

[Padishar]

Well, mu = GM and both mu and the mass are given on the wiki so you can calculate G if you need to but it should be the same as our standard G. Using the values from the wiki gives:

G = 6.6739999530952885842606573050885e-11

The reason this is slightly different is that the gravitational parameter on the wiki is only given with 5 significant digits and was presumably calculated using the real value of G and the "real" mass of Kerbin...

[GateCrasherVI]

Thanks! Thats what I was using, but because of the other errors, I wasn't sure if it was correct.

I had seen the mu or "GM" on the wiki, and figured I could probably reverse-calculate what was used to arrive at the various values for any of the planets and determine what value was being used for G. But really hoped someone would save me from that headache and just confirm through thier own previous successes what the correct G was. It's been so long since highschool, and I've literally not once since then needed to or been inclined to do these kinds of calculations by hand.

KSP certainly sparks the most healthy of interests I've seen from a game in a long time. The last time I remember really deconstructing something this real-world applicable, it was the 3d vectoring used in the Doom2 level designer on our old Apple Performa2. =) Good times.

Greatly appreciated Padishar!

[Padishar]

No problem, glad I could help...