Orbiting

[Scarredclown]

When is it the best time to retrograde back to kerbin ? Apoapsis or periapsis ? Also, in the career mode i have the orbit kerbin mission. i'm orbiting and i dont unlock it ? is it because my orbit isnt perfect ?

Edited August 26, 2015 by Scarredclown

[Xavven]

Let me make sure I understand correctly. You are in an orbit around Kerbin and you want to know the most efficient time in your orbit to burn regrade so that you can re-enter the atmosphere and land on Kerbin? If so, then it is most efficient (i.e. takes the least amount of fuel and delta-v) to burn retrograde at your Apoapsis. This is because making a prograde or retrograde burn raises or lowers the altitude on the opposite side of your orbit.

Regarding your second question, could I ask if your periapsis is above 71,000 meters? If your periapsis dips into the atmosphere (which is at 71km) then the game does not consider your craft to be "in orbit."

[Snark]

+1 to Xavven's answer, though I believe Kerbin's atmosphere stops at 70km, not 71km. Your periapsis has to be above that.

When you are ready to go back home to Kerbin, the most efficient time to do your retro-burn is at apoapsis, as Xavven points out. You just need to lower your periapsis into atmosphere and you'll eventually get home-- whether you reenter and land in a single pass, or end up doing multiple aerobraking passes, will depend on how low you lower your periapsis.

When you're in LKO, lowering your periapsis to around 35km generally works pretty well-- that will give you a nice long time to aerobrake (so that it's not too traumatic to your ship), but will generally bring you home in a single pass (so it doesn't tax your patience). Be careful not to lower your periapsis too much, or you could come in too steeply and burn up (even with a heat shield). If you keep periapsis above 30km when you do your retro-burn, you should be fine.

[Kelderek]

Also, keep in mind that the higher your apoapsis is the faster you will be going at the periapsis or when you re-enter the atmosphere. If you are concerned about the re-entry heat being too much for your vessel, then you might want to shrink your orbit some (burn retrograde at the periapsis) before you do a de-orbit burn at apoapsis. That would probably only be a concern if your apoapsis is well over 1,000km.

[maceemiller]

Ill be honest, when I want to deorbit I do anywhere I see fit. I only ever come back home after a successful mission so to me efficiency is outta the window by then

[Warzouz]

True, when you return from a mission, the "efficiency" of the deoribit is mostly meaning-less. You can reenter atmo at a very low dV from nearly anywhere. Any fuel you don't burn on reentry will increase you weight and make you arrive faster on the ground.

Don't hesitate to burn ALL your fuel (even monoprop) in atmo.

When I return from a mission, I usally go straight to the atmo. Moho and Eeloo are exceptions because of the periaps speed : you could vaporize in atmo. So I slow down to around 4000m/s and I'm safe.

[Kryxal]

Doing a skip reentry is also useful if you don't trust your craft to handle the heat ... multiple braking passes means less heat generated per pass.