Transfer Questions

[Biggen]

I'm trying to get more comfortable and gain more experience with KSP but transfers to other planets are still a bit confusing for me.  I wanted to lay out the steps I use to reach other planets and see if it makes sense the way I'm doing it.

1. Use Alex Moon's launch window calculator and time warp to a day or two before the ideal window.
2.  Plug in the amount of prograde and normal/anti-normal the window says I need with the correct ejection angle.
3. Look at map mode for closest approach markers.  Fine tweak maneuver gizmos to either get an encounter or at least keep markers as close as possible before ejection burn.
4. Burn at node when I'm happy with closest approach markers
5. Exit Kerbin SOI, move 1/4 to 1/2 way to destination and drop another node for mid-course correction.  Here, I'm just trying to get the encounter finalized and get my Pe at the right altitude and on the equator in order to set myself up for an easy burn for equatorial orbit once I'm captured.
6. Arrive at destination SOI, burn at Pe to barely capture, coast to Ap and burn normal/anti-normal to reduce inclination to ~0, coast back to Pe circularize.

Does the above sound like the correct way to go about most transfers (Moho excluded)?  I never end up burning at a AN/DN node this way during a mid course correction burn which I see people saying they do during transfers.  It seems that the launch window is already directly on top of one of the nodes when I leave Kerbin and the other is usually right outside the target SOI.  (Well, I guess technically I am burning at an AN/DN node since the node is where Kerbin is when I do the ejection burn). 

Also, does the higher I setup my Pe for the encounter mean the more dV I need to circularize once in the SOI?  I was going to Dres this morning and my encounter Pe was 250km.  Once I was within the SOI I dropped a test node at my Pe to check and see how much dV was required to circularize and it was ~1800dV+.  The cheat sheet said it should have only been 1290dV from a 12km Pe.  Is the difference because my Pe was so much higher??

 

Edited February 20, 2017 by Biggen

[michal.don]

1 hour ago, Biggen said:

Does the above sound like the correct way to go about most transfers (Moho excluded)?

Yes, that should be it. Or, at least, it is quite similar to what I'm doing  The only thing I do differently is the inclination correction - I rather set another course correction burn exactly at AN/DN and set my flyby to be an equatorial one. In most cases, you spend just a few m/s worth of fuel. Just try to fiddle with the normal/antinormal vectors.

1 hour ago, Biggen said:

Also, does the higher I setup my Pe for the encounter mean the more dV I need to circularize once in the SOI?

Yes. And, as you observed, this makes a big difference

Generally, you should aim for your periapsis to be as low as possible to be the most fuel efficient. If you are landing on the planet in question, you need to get as low as it gets, so why not begin as low as possible from the start?  Again, if needed, you can do a cheap burn to fine tune your PE height a few days befere arriving to the planet.

Michal.don

[Biggen]

6 minutes ago, michal.don said:

Yes, that should be it. Or, at least, it is quite similar to what I'm doing  The only thing I do differently is the inclination correction - I rather set another course correction burn exactly at AN/DN and set my flyby to be an equatorial one. In most cases, you spend just a few m/s worth of fuel. Just try to fiddle with the normal/antinormal vectors.

Yes. And, as you observed, this makes a big difference

Generally, you should aim for your periapsis to be as low as possible to be the most fuel efficient. If you are landing on the planet in question, you need to get as low as it gets, so why not begin as low as possible from the start?  Again, if needed, you can do a cheap burn to fine tune your PE height a few days befere arriving to the planet.

Michal.don

Thanks Michal.  It seems that when I leave Kerbin, I'm already at an inclination node.  At least, when using the launch window planner it seems that is always the case from what I remember.  The next node is, of course, 180 degrees opposite which generally sits just outside the SOI of the target.  

So, are you doing a midcourse burn to get your encounter Pe setup at the right altitude, then coasting to the AN/DN node, and then finally doing another burn at the node right before the target SOI to reduce inclination to zero?

Edited February 20, 2017 by Biggen

[michal.don]

Yes, most of the celestial bodies in stock are orbiting in the same plane as Kerbin, so in most cases, you can do just one correction burn. If you did the ascent and the transfer burn 100% exactly easwards, you would arrive in a perfect equatorial orbit. But, at least for me, that is never the case  So a slight correction is needed.

So to sum it up, when going to Duna/Eve, one correction burn halfway there to correct both Pe and inclination is usually enough. When going to Dres or Eeloo, things are more complicated because of the different orbital plane, so you might need more correction burns in order to arrive exactly as you need. When going to Jool, you might want so tweak your flyby to get some nice gravity assists, so again, more than one burns might be needed. And finally, when going to Moho, good luck to you, because going there is a serious pain in the rear end

Michal.don