Calculating Orbital Velocity

[Greyskys85]

I feel stupid for posting this, but this is a question I have been trying to figure out since the game was still new. satellite missions always require a specific orbit and I never know how much gas to use on my ships like I do when going to orbits and other planets.

I generally always have a lot of gas by the time I get there, but I like learning to be as efficient as possible with my builds if I can. Since I recently started coding I wanted to make a calculator that could calculate orbital velocity based off of the equation Sqrt(MG/r) = v.

The problem is the mass is a really big number and the answer I get for my test, (a 70km orbit) ends up being over a million or something which doesn't match the actual orbital velocity of around 2,000 or so for that orbit.

I know this is a long shot and feel dumb asking a math question on a game discussion, but short all google ever gives me is an equation and short of going to NASA or an university, I don't really know who to ask.

I am reaching out to anyone else who is better at math then me.

[MechBFP]

See this post. https://www.reddit.com/r/KerbalAcademy/comments/r61x1r/how_do_i_calculate_the_deltav_needed_to_raise_my/?rdt=42068

You will need to do 2 calculations. One when you raise your apoapsis and another when you raise your periapsis to circulize.

[s_salmon2005]

What are you using for GM? Tracking station says it's 3.523e12 m3/s2. That divided by 670,000 meters (kerbal's radius plus the 70km atmosphere) and square rooted comes to ~2293 m/s. I think you probably plugged in kilometers instead of meters, like I did at first.

There's nothing wrong about asking for help.

[Greyskys85]

Originally posted by s_salmon2005:

What are you using for GM? Tracking station says it's 3.523e12 m3/s2. That divided by 670,000 meters (kerbal's radius plus the 70km atmosphere) and square rooted comes to ~2293 m/s. I think you probably plugged in kilometers instead of meters, like I did at first.

There's nothing wrong about asking for help.

I went with a wiki post that had 5.2915158e22 kilograms so I know what the issue is now. Weird that google is showing such a huge difference. Never occurred to me to use my tracking station.

That makes me feel a little better though, thank you all.

[s_salmon2005]

Originally posted by Greyskys85:

Originally posted by s_salmon2005:

What are you using for GM? Tracking station says it's 3.523e12 m3/s2. That divided by 670,000 meters (kerbal's radius plus the 70km atmosphere) and square rooted comes to ~2293 m/s. I think you probably plugged in kilometers instead of meters, like I did at first.

There's nothing wrong about asking for help.

I went with a wiki post that had 5.2915158e22 kilograms so I know what the issue is now. Weird that google is showing such a huge difference. Never occurred to me to use my tracking station.

That makes me feel a little better though, thank you all.

That's Kerbin's mass, the M in the GM. You can look up the standard gravitational parameter (GM) from the wiki too.

[Greyskys85]

Originally posted by s_salmon2005:

Originally posted by Greyskys85:

I went with a wiki post that had 5.2915158e22 kilograms so I know what the issue is now. Weird that google is showing such a huge difference. Never occurred to me to use my tracking station.

That makes me feel a little better though, thank you all.

That's Kerbin's mass, the M in the GM. You can look up the standard gravitational parameter (GM) from the wiki too.

I guess I am just confused as to how GM which should be gravitational constant times mass or 9.81 times a 22 digit number equals a 12 digit number.

I mean how does 9.81 * 52,915,158,000,000,000,000,000 = 3,523,000,000,000 ?

[MechBFP]

The gravitational constant (G) is not the same thing as gravity of a planet (g). G = 6.67430×10−11

[Greyskys85]

Originally posted by MechBFP:

The gravitational constant (G) is not the same thing as gravity of a planet (g). G = 6.67430×10−11

Okay, now it all makes sense. Thanks again.