Interceptions 101

[Commissioner Tadpole]

Every time, when I am heading for another planet, my periapsis ends up several kilometers away from the destination. Only in this case, when I was trying to head for Jool, my Periapsis ended up somewhat close, and I'm completely clueless on how I managed to do that.

Seriously, what should I do to have an intercept which begins with a hyperbolic Periapsis close to the planet? it's very easy with the Mun and Minmus, but just impossible with Duna, Eve and Jool.

Edited December 29, 2013 by Commissioner Tadpole

[-ELF-]

Mid-course corrections are your friends.

You see, while you are accelerating near Kerbin, any additional m/s turns into several hundreds to thousands of kilometers at destination. To correct that over shooting, perform mid-course correction:

1) Leave Kerbin with any intercept you can make

2) Get to half-way there

3) Make a maneuer node 15 minutes ahead of you, and play a bit with handles, try to get intercept closer to a planet (you will probably be surprised how much less fuel you will need for such corrections)

4) Perform maneuer in time, carefully, with 5-10% of all your engine power (Or even better with RCS)

5) Profit! Your intercept periapsis is as low as you need.

[Commissioner Tadpole]

Well, problem is it does not allow me to pop in a manuever node in my solar orbit for some reason. And I don't want to risk doing a manuever that I have no clue of what its aftereffect would be.

[LethalDose]

Well, problem is it does not allow me to pop in a manuever node in my solar orbit for some reason. And I don't want to risk doing a manuever that I have no clue of what its aftereffect would be.

I've had issues with maneuver node placement, too. Try placing the node further along the path, and then dragging it back to where you need it.

But yeah, mid- or late-course corrections are your friends. Earlier corrections need to be smaller, and therefore take less fuel.

[Cashen]

ELF has it right with the mid-course corrections. I've also had issues with placing maneuver nodes while in solar orbit. They're best placed when the map view is zoomed out a fair distance. If that fails, try exiting to the Space Center and then going back to the vessel, see if that fixes it. Of course, if you have MechJeb you can have it create the proper maneuver for you, using (I think) the "fine tune closest approach" option with a specified periapsis. This is how I do it, since I use MechJeb to plot nodes in some situations, though I then always execute the node by hand.

[Superfluous J]

If you get a "near encounter" set of markers, you don't have to launch blind. If they're close, you can be sure a mid course correction will be able to nail the planet.

[capi3101]

This may be stupid, but - http://alexmoon.github.io/ksp/ - to make sure you're headed to other places at the right time.

Other than that, only thing I can suggest is what others are saying: you need to make mid-course corrections. I usually do one at the ascending or descending node to do a plane shift; sometimes that's a good time to go ahead and do prograde/retrograde and radial course corrections as well. Of course, the earlier you correct, the better.

What I usually do is zoom way out and note the distance to closest intercept. I then set up a maneuver node; usually a couple of days ahead. I pull one direction and see what that's done to the approach - if the distance has increased, I pull the opposing direction. If the distance has decreased, I continue slow pulls until the distance starts increasing again. Don't be afraid to pull radially; yes it's not the most efficient way of travelling, but it often makes the difference between getting where you're going and spending a long time in Kerbol orbit. Once you've got an encounter indicated, make very slow pulls - you want to get as close as you can, especially if you can aerobrake at the target. If you can't get close with a single correction, go ahead and make the indicated burn when necessary and try again later. Ideally you'll get your desired encounter periapsis in a single, early correction burn, but if it takes more than one it takes more than one.

[Kasuha]

Every time, when I am heading for another planet, my periapsis ends up several kilometers away from the destination. Only in this case, when I was trying to head for Jool, my Periapsis ended up somewhat close, and I'm completely clueless on how I managed to do that.

Seriously, what should I do to have an intercept which begins with a hyperbolic Periapsis close to the planet? it's very easy with the Mun and Minmus, but just impossible with Duna, Eve and Jool.

Rule number 1: Do not trust maneuver nodes. Yes, prepare your maneuver and execute it, but as soon as you get an intercept, delete the maneuver and perform corrections right where you are (e.g. still in Kerbin SOI). RCS is your good friend as you can thrust in any direction there and that way you may bring your intercept periapsis rather low still in the SOI. Zoom into your target system and watch what various RCS directions do with your periapsis. If it moves in the right direction but too slowly, place a new maneuver node ahead of your ship and applying just tenths of m/s you can fine-tune your approach. Do not trust that maneuver - orient your ship along it, then delete the maneuver and apply thrust while watching your real trajectory change.

Rule number 2: Do not trust SOI changes. Lower your time warp every time you change SOI and check your intercept after the change. Apply corrections if necessary.

Rule number 3: No fuel pumping around the ship while in interplanetary space. By moving your center of mass around, you change your trajectory. If you need to change your ship in interplanetary space, apply corrections after you're done with it.

[Commissioner Tadpole]

I've had issues with maneuver node placement, too. Try placing the node further along the path, and then dragging it back to where you need it.

But yeah, mid- or late-course corrections are your friends. Earlier corrections need to be smaller, and therefore take less fuel.

Huh, I never knew I could drag manuever nodes. Thanks for the tip.

ELF has it right with the mid-course corrections. I've also had issues with placing maneuver nodes while in solar orbit. They're best placed when the map view is zoomed out a fair distance. If that fails, try exiting to the Space Center and then going back to the vessel, see if that fixes it. Of course, if you have MechJeb you can have it create the proper maneuver for you, using (I think) the "fine tune closest approach" option with a specified periapsis. This is how I do it, since I use MechJeb to plot nodes in some situations, though I then always execute the node by hand.

Hm, I'd normally try to avoid the Switch to Space Center workaround since I like having the ability to revert flight, although I guess that's why Quicksaves exist.

And I'll try to use the MechJeb option.

This may be stupid, but - http://alexmoon.github.io/ksp/ - to make sure you're headed to other places at the right time.

I alterady use Olex's calculator(which is a lot more simple than Alex Moon's), and I also have Kerbal Alarm Clock, so transfer windows are not a problem.

So, the problem of mid course corrections is solved, I think I can now handle Trekker 3 and Eve. But I have another problem; whenever I transfer to planets that are further than my initial destination(e.g. Kerbin -> Jool), I always end in retrograde orbits. This doesn't seem to happen with planets that are before the planet I am on, such as Duna -> Eve; I end in prograde orbits. What gives?

I'm performing the gravity turn towards 90°, in case that helps.

[Superfluous J]

I have another problem; whenever I transfer to planets that are further than my initial destination(e.g. Kerbin -> Jool), I always end in retrograde orbits. This doesn't seem to happen with planets that are before the planet I am on, such as Duna -> Eve; I end in prograde orbits. What gives?

I'm performing the gravity turn towards 90°, in case that helps.

THat has nothing to do with the original burn, gravity turn, or mid-course correction time or angle. That has entirely to do with what side you pass Jool on. You will (almost) always approach Jool and Duna from the FRONT. You're going slower than it is so it's actually catching up to you. When you enter SOI, follow the line from your ship to the planet, and see it curve past its periapsis. If that curve is going CLOCKWISE, you're going the wrong way. If it's going COUNTER-CLOCKWISE, you're going the right way. You can speed up time a bit to see how Laythe (or Ike) is going, to make sure you're going the same way.

If you're not, it's easy to fix so long as you're still near where you entered SOI. Just set up a maneuver node and burn the light blue marker with lines going IN from the circle. You can't make the node do the entire burn, just use it to get the direction. Burn in that direction while in map mode and your periapsis will go into Jool (vanishing) and then come out the other side. Boom, you just reversed your orbit.

[Commissioner Tadpole]

THat has nothing to do with the original burn, gravity turn, or mid-course correction time or angle. That has entirely to do with what side you pass Jool on. You will (almost) always approach Jool and Duna from the FRONT. You're going slower than it is so it's actually catching up to you. When you enter SOI, follow the line from your ship to the planet, and see it curve past its periapsis. If that curve is going CLOCKWISE, you're going the wrong way. If it's going COUNTER-CLOCKWISE, you're going the right way. You can speed up time a bit to see how Laythe (or Ike) is going, to make sure you're going the same way.

If you're not, it's easy to fix so long as you're still near where you entered SOI. Just set up a maneuver node and burn the light blue marker with lines going IN from the circle. You can't make the node do the entire burn, just use it to get the direction. Burn in that direction while in map mode and your periapsis will go into Jool (vanishing) and then come out the other side. Boom, you just reversed your orbit.

Ah, thanks for the tip, that'll help me on the long run for my next Joolian trips!