Circular orbit - Maneuver nodes

[Space_dude]

My target is to go in circular orbit at around 100 K so I'm using Maneuver nodes to predict circuling orbits.

My question is where exactly are you supposed to place your maneucver node ?  I'm not talking about starting the burn but where do you actually right click and "add manever node".  Before or after the predicted Apoapsis ?

If i get my predicted Apoapsis to 100 Km, cut the burn with X and place my maneuver node straight on the apoapsis, I  slowly, gradually pull the prograde line , it will go 110 and 80 and adjusting it becomes very difficult to get 100 K Apo 100 K Per.

If I place the Maneuver node before the predicted Apoapsis, it will go sideways same as if I place it after.

Maybe there's s way to get it right ?

 

Edited March 27, 2017 by Space_dude

[diomedea]

Let's say you have already set one of the apsides at your intended altitude  (100 Km). Your maneuver node to circularize has to be placed exactly at that apsis. If at apoapsis, you need burn prograde to raise the periapsis; if at periapsis, you need burn retrograde to lower apoapsis, in both cases to change the other apsis altitude. If the maneuver is done exactly at the apsis, that apsis altitude won't change and neither the apsis position; if done earlier or later, you'll also have the apsis position rotate and change altitude. With finite time maneuvers (taking some time to execute) you need to start the burn half the burn duration before reaching the apsis, so thrust  components before and after the apsis balance out deviations.

Should you have an orbit that crosses your intended altitude (100 Km) but neither of the apsides are at that altitude, you may burn radial while at that altitude to change altitude and position of both apsides (that's less efficient however than burning at both apsides to correct the altitude of the opposite one).

[bewing]

Besides what diomedea said -- the reality is that when you burn prograde, you always raise both your Ap and your Pe. If you burn almost exactly at your Ap, your Ap will only go up a little, and your Pe will go up a lot. But your Ap will always go up a little. So if you set your Ap to exactly 100km and you want it to stay exactly there while you burn prograde, you are out of luck. It won't work.

So the way it works is, you start by setting your Ap a few Km below where you want. You set your maneuver node and burn a little before you reach your Ap. You control your burn so the Ap does not move out very far in front of you. The farther it is in front of you, the faster it will increase as you burn. When your Pe gets up to maybe 10km below the final value, you burn just enough to set your Ap to the final value. Then you wait until two seconds before the Ap and burn just enough to get your Pe to the final value.

 

[Kryxal]

If you want to just drop a node and set up a burn, go ahead and drop that node fairly close to AP.  Once you have the burn planned more-or-less, zoom in and slide the node so projected AP matches better with current.