Ending hohmann transfer in prograde vs retrograde orbits

[THX1138]

I think that sometimes I end my hohmann transfers in prograde and sometimes retrograde orbits. I was thinking maybe this is because I sometimes make my transfer orbit overshoot my destination planet and other times I make it undershoot. I was hoping someone could tell me whether that is what is happening. Maybe I always end up in a retrograde orbit and just haven't noticed it because I rarely go to the moons of anything.

Edited April 15, 2015 by THX1138

[TheXRuler]

*snip* I was thinking maybe this is because I sometimes make my transfer orbit overshoot my destination planet and other times I make it undershoot.

If I understand you correctly then you are right. If you overshoot the transfer a little your trajectory might go through the body and out the other side. This will put you on either a pro or a retrograde orbit.

I reccommend just setting up a regular LKO and then hohmann transfer to the mun. As you edit the manouvre no you will notice that you can either come in on the Kerbin side of the mun, or on the "outer side". This is what makes the difference.

Edited April 15, 2015 by TheXRuler

[THX1138]

Thanks a lot. This has been an expensive lesson! Franklin's five-year trip to Bop has been a waste of time and money! (fortunately I have plenty of funds to waste).

[smjjames]

You should be able to use the other moons gravity to flip the orbit back to prograde. Or just do a cheap inclination flip after you stop at Bop.

[TheXRuler]

Always happy to help Also, this is one of the things that is used for gravity assists. If you pass before the body it will decelerate you, if you pass behind it will accelarate you. Similarily if you pass above or below it will change your inclination. Of course you might know all this, I just thought it's kinda relevant since it ties into your question.

[Superfluous J]

If you're daring, you can try to flip your orbit around using gravity assists from Laythe, Vall, and Tylo. Or use them to fling your ship way far from Jool (but still in its SOI and way out at your apoapsis, a burn to reverse your orbit would only take a drop or two of fuel. Then fall back in and use Laythe, Vall, or Tylo gravity assists to lower your orbit to a more normal one.

Or aerobrake STRONGLY at Laythe (use quicksaves and quickloads here, it's hard to nail this heavy an aerobrake) so you still escape Laythe, but are now going prograde around Jool. That'd be freaky but fun.

[smjjames]

Yep, using gravity to swing around the Jool system while using as little fuel as possible is fun.

[THX1138]

You should be able to use the other moons gravity to flip the orbit back to prograde. Or just do a cheap inclination flip after you stop at Bop.

I can't stop at Bop because I don't have enough fuel to reverse my orbit. I'm looking into using the other moons' gravity but I'm not sure I can get them to flip my orbit. The best I'm getting is small reductions in the size of my orbit. If I keep doing that, surely I'm still going to have to come up with the bulk of the energy myself. I don't see how I'm going to get orbital velocity from a single encounter and I can't do it over the course of multiple encounters because somewhere in the transition I would surely collide with Jool.

Always happy to help Also, this is one of the things that is used for gravity assists. If you pass before the body it will decelerate you, if you pass behind it will accelarate you. Similarily if you pass above or below it will change your inclination. Of course you might know all this, I just thought it's kinda relevant since it ties into your question.

I knew some kind of interaction like that was possible and the basis of gravity assists but I never knew what the rules were exactly so thanks again. Are these rules reversed if the body you encounter is travelling in the opposite direction?

[Reactordrone]

And just in case the OP doesn't know about it, if you focus on the targeted object your trajectory will be displayed relative to that object so you'll be able to see which way you're entering and leaving its sphere of influence. You can then do a minor course correction if you're coming in the wrong way.

[Yasmy]

When you perform your transfer burn or mid-course plane change burn, focus on the target planet. Then you will see your trajectory in the planet's frame of reference. Looking at the SOI exit marker on your trajectory, you can tell if the orbit is prograde or retrograde. Then make tiny adjustments to set up your desired encounter. (When you are done, hit Backspace to return the camera to your vessel.)

<ninja'd!>

[THX1138]

Good golly! You really can flip an orbit! This is mental! For the low, low cost of ~14 m/s. More people need to know about this!

[Xavven]

When you perform your transfer burn or mid-course plane change burn, focus on the target planet. Then you will see your trajectory in the planet's frame of reference. Looking at the SOI exit marker on your trajectory, you can tell if the orbit is prograde or retrograde. Then make tiny adjustments to set up your desired encounter. (When you are done, hit Backspace to return the camera to your vessel.)

<ninja'd!>

This is definitely the solution. Use a combination of prograde/retrograde and radial/anti-radial to tweak your encounter so you end up going the right direction to begin with. The earlier you correct (in general) the less delta-v it takes. In fact, for interplanetary transfers, I often fine tune with monopropellant (RCS translation controls: H and N keys) because the main engines are too powerful and move my encounter too much.

[celem]

I knew some kind of interaction like that was possible and the basis of gravity assists but I never knew what the rules were exactly so thanks again. Are these rules reversed if the body you encounter is travelling in the opposite direction?

No the rules dont reverse. Just remember that when we say 'pass in front of the body to slow down, behind to speed up', we are talking about the direction the moon or whatever is going in, its own prograde. Your own direction is fairly moot, as long as you pass on the aft-side of the gravity well you will gain energy and visa-versa.

Glad you managed to flip at a moon, its very cost effective. I use it on Ike at Duna a lot too. Longrun the best bet is as stated to tweak your inbound trajectory while still way out of Jool SoI, but knowing how to reverse on a moon is a handy skill. (as is using moon slingshots to achieve huge apoapsis and performing a straight-up reverse out where you have hardly any orbital-velocity, then grav-braking back in.)

Edited April 17, 2015 by celem