Wrong mass of Moho?

[dozer]

Hi all!

I've made a spreadsheet that helps me plan my missions. I just went to Moho, established a polar orbit and turned on my Mapsat. While waiting I noticed that my circular orbit velocity calculation does not match the orbit speed KSP reports. I checked my calculations again, tested them with Kerbin and Eve, 100% correct.. I suspect something wrong with the Wiki numbers regarding Moho. The "Constant of Gravitation" is of course correct... (6,67259E-11) This means that the mass of Moho in the Wiki (3.6747079E+21) cannot be correct (or have I misunderstood something?).

I'm at 100km orbit around Moho. KSP reports 694,1m/s. My spreadsheet suggest 836,04m/s.

If KSP speed is right, Moho has a mass of about: 2,527E+21

This is quite different from the Wiki..

I'm no rocket scientist, can anyone shed some light on this?

EDIT: From some more calculations, it could also be that the radius of Moho actually is around 207600m (I think??)

Thanks!

Edited May 5, 2013 by dozer
More info

[Oggula]

Moho's mass was changed after 0.17, it was originally a lot heavier, the wiki probably has the old figures

[dozer]

Ah! Nice, thank you for your fast reply! Do you know how to get the 100% figure?

[Oggula]

Yeah I'm no rocket scientist either. Surely you could work it back from your orbital velocity in the same formula to find the gravitational parameter? I remember doing roughly the same thing for Kerbin around v0.9 but I don't have the formula on me...

[Oggula]

Gravitational parameter is orbital radius times orbital speed squared according to wiki, giving a parameter of 69410000 for Moho by your figures. Thus the mass is 69410000 divided by the gravitational constant, 6.67384*10^(-11), giving a mass of 1.04003E+18. Hopefully that's about right.

[EndlessWaves]

Do you know how to get the 100% figure?

Use a plugin to ask the game:

foreach(CelestialBody aBody in FlightGlobals.Bodies)
{
	if(aBody.name == "Moho")
	{
		print(aBody.Mass);
		print(aBody.Radius);
	}
}

produces:

2.52636168196748E+21

250000

[Oggula]

Yeah I'm completely wrong, ignore me I'm a cretin

[dozer]

Use a plugin to ask the game:

foreach(CelestialBody aBody in FlightGlobals.Bodies)
{
	if(aBody.name == "Moho")
	{
		print(aBody.Mass);
		print(aBody.Radius);
	}
}

produces:

2.52636168196748E+21

250000

Nice trick! I guess I'll have to learn how to make plugins when I'm done with my "Mission datacard" project. Excellent tip!

BTW I'll attach an image of my project. The idea is to plan a mission using an excel file so I know what to build, then put the requred delta V into it, print it, and track my progress compared to the calculations.

It has become a huge file with lots of calculations. Next to the "datacard" are several preformatted areas ready to be copied and pasted to build your mission from different "events"(liftoff, escape, transfer etc.). Hopefully I'll make it more user friendly somtime in the future