A few questions

[ghostbuck]

Hey everyone,

I started supporting KSP back in .16 I think (whenever it was first launched on steam, but .16 is the earliest version number I remember) but just came back to it as I saw it entered beta....congrats squad!! .90 is awesome with career mode, the contracts are the best tutorial as they have you start focusing small and working your way towards stuff.

I walked through some forum posts and i have installed remotetech, kas, boxsat, and others based on requirements.

Now to my questions, I have attempted launch my first two satellites. Unfortunately both have failed but surprisingly, my second attempt did not crash down back to Kerbin. I have read about the calculations used to get into orbit and saw for Kerbin, a minimum altitude (is that what you call it in space?) is 70km. However, my sat went all the way down to 44km at its periapsis and then ascended all the way to its apoapis of approx 565km (why my this attempt failed, was waiting to watch the Drainex1 trigger the stage for the first time and not paying attention to the orbital view ). Will the sat eventually fall as it continues to dip below 70km hitting friction slowing down the orbital speed? Or is the minimum altitude for orbit dependent on mass and speed (all thought I am sure there has to be some absolute floor)?

Kind of going along the lines with the question above, I have learned how to calculate my deltaV. I read that 4500/ms is required to orbit Kerbin. I know people have spelled it out already but can someone provide the equation to determine minimum deltaV needed to execute maneuvers? ie, how was 4500/ms determined as minimum deltaV to orbit Kerbin?

Part of the deltaV equation is the gravity factor, not sure what the actual name is. The tutorials all said to use the constant 9.8. However, in other reading I saw that is what earth's factor is. Is it a constant or dependent on the gravity of a planet? If not a constant, how do I find the gravity factor for different planets?

Finally, on a complete side note, are forum accounts wiped after a certain amount of activity? I did an initial introduction and asked some similar questions to above along time ago. My account was no longer active and I cant find my forum post from last year.

Edited February 11, 2015 by ghostbuck
answered

[nekogod]

With an AP of 565km you'll have hit the atmosphere at some speed a PE of 44km would not have been low enough, ie the atmosphere is still too thin at that point to have slowed you down into a sub orbital trajectory.

Some real life craft do this on purpose, skimming the atmosphere and bouncing back to space, they do this for multiple orbits to slow down gradually to reduce stress and heating of the spacecraft.

As for an absolute floor pretty much any PE under 30km will take you sub orbital, at least at the speeds you would normally see in the game, 2-3kms from the mun/minus 3-4kms from interplanetary.

I'm not 100% sure how the delta v calculation works for bodies with an atmosphere, Kerbins orbital velocity at 70km is about 2200kms I would assume you take the delta v needed to get to 2200kms at the ground and in space and take the average between the 2, this I'd have thought would get you a figure roughly in the right ballpark.

9.8 is correct for the gravity, it's also true this is earths, but that is because kerbin is modelled on earth, they have the same gravity.

Essentially it means that were there no atmosphere to create drag an object dropped on earth/kerbin would accelerate at 9.8m/s^2, the gravity of the other planets is on the KSP wiki http://wiki.kerbalspaceprogram.com/wiki/Main_Page

[tomf]

The calculations for delta-v to orbit for kerbin are discussed in many threads but the basic answer is that it is difficult to model mathematically and the quoted 4500 is simply from experience. For other bodies that don't have atmospheres the maths is much simpler. If you are interested you can look here http://forum.kerbalspaceprogram.com/threads/27171-Calculating-interplanetary-delta-v. If you aren't interested the wiki has a chart that can tell you the delta v to get to any body from kerbin.

The delta-v equation has g as a constant mostly so the resulting answer for ISP is in seconds and therefor is the same for mertic and american engineers. it isn't related to the surface gravity of any body you might be on.

You can find out the surface gravity for every body on the wiki page for that body e.g. http://wiki.kerbalspaceprogram.com/wiki/Mun lists a surface gravity of 1.63

With a PE of less than 69km your craft will be being slowed al littel each time it is in the atmosphere and will eventually crash. However, KSP doens't model drag of craft that don't have focus so if you leave it and fly somethign else it will stay there indefinately.

[nekogod]

I could be wrong but, I didn't think g was actually used in the delta v calculation. Mostly because 100% g is only subtracted from the delta v budget when the rocket is pointing straight up.

Edited February 11, 2015 by nekogod

[MircoMars]

...my sat went all the way down to 44km at its periapsis and then ascended all the way to its apoapis of approx 565km (why my this attempt failed, was waiting to watch the Drainex1 trigger the stage for the first time and not paying attention to the orbital view ). Will the sat eventually fall as it continues to dip below 70km hitting friction slowing down the orbital speed? Or is the minimum altitude for orbit dependent on mass and speed (all thought I am sure there has to be some absolute floor)?

minimal orbit is 69.5 x 69.5 km.

If your PE is under 69km, you'll lose velocity on every pass. that is called aerobraking and is what saves you loads of fuel whenever you need to brake down to achieve orbital velocity (eg. returning from mun). test different speeds and heights with F5 and F9 to get a feeling for that. Afaik if you don't use FAR, size and shape doesn't matter, only speed and height. ah and deactivate SAS while aerobraking.

Finally, on a complete side note, are forum accounts wiped after a certain amount of activity? I did an initial introduction and asked some similar questions to above along time ago. My account was no longer active and I cant find my forum post from last year.

there was a big forum wipe /crash about one and a half years ago, dunno when exactly. I guess your account fell victim to that too...

[tomf]

The form normally used in KSP is delta-v = ISP*g*ln(m0/m1)

[nekogod]

The form normally used in KSP is delta-v = ISP*g*ln(m0/m1)

Ah my bad, it's not one I'm overly familar with, I've been using KER to work it out for some time.

[ghostbuck]

Thank you everyone for the explanations!

[Renegrade]

Note that in KSP, for delta-v calculations, g₀ is 9.82 (instead of 9.80665). Harv made a booboo somehow