Getting close intercept with planet

[Nyksy]

Hi,

I manage to do interplanetary transfers quite reliably with close to optimal dV (according to http://ksp.olex.biz/), but the problem is that when I get to the destination planet's SOI, I'am very far away and then I need to burn much fuel to get to orbit. For example, when I get to Duna's SOI I'm usually somewhere between 30 and 40 million kilometers away and then I need to burn over 1k dV to to the atmosphere to perform aerobraking. I'm guessing that I should do some corrections before I get into SOI, but at which point?

Also, it seems that maneuver planner does not work correctly when successive revolutions trajectories are close together but not exactly the same, so it makes it difficult to plan any corrections before entering the SOI. Any tips?

[Vanamonde]

There is a line in the settings file you can change from "CONIC_PATCH_DRAW_MODE = 3" to "CONIC_PATCH_DRAW_MODE = 0". You will then see the project path your ship will take, depicted within the target world's SOI. You can then set a manuever node long before you arrive there, move the node direction markers around, and find a couse correction that will bring you close to the world upon arrival.

[Merinsan]

A lot of people recommend halfway there, ie halfway from the burn to the intercept.

In fact the best time is the same time you do the transfer burn. However it's extremely difficult to work out where you are when you have the encounter due to the camera controls.

Maneuver nodes help in that you can set one up and play around trying to get it right without using fuel. I will actually spend a fair amount of time trying to set up a reasonable encounter before even burning, and generally get close to what I set up. I'll then do another burn around 15 days from the SOI transfer if I think I need it.

Also, don't have time acceleration running when the SOI change happens, and the predicted periapsis doesn't change.

[falofonos]

The earlier you make your course correction burn, the less deltaV you need but the less accurate the prediction will be on the map.

Ideally your orbit intercept should be at kerbol periapsis so that orbital velocity around Kerbol is as close to Duna's as possible.

You can make as many correction burns as you need, tweaking to get a first path that skims the upper duna atmosphere would be worth the fuel spent earlier to get a little aerobreaking.

[DMagic]

The default maneuver node editor is really tough to use for fine tuning transfer orbits like this. It tends to close and be hard to reopen, it hard to make very small changes (sometimes 0.05 m/s changes make a big difference when you are so far away), and, like you mentioned, when you have closely overlapping orbits you have to be careful to set the node for your current orbit, not the next one.

But in general I just try to get an encounter with the initial burn. Usually I am a little above or below the planets orbit, so there is an ascending or descending node somewhere along the way. That's a good place to do your correction burn (if it's not too close to the target planet). Sometimes just a tiny adjustment in your inclination can get you much closer to the planet.

Sometimes when I get tired of dealing with nodes I just do little random burns to get as close as I can. Just point in a random direction, throttle up to maybe 3-5% and see whether your periapsis around the target planet increases or decreases, it usually increases. I generally only do this with RCS, or ion engines, but if you are careful and persistant you can get really close.

[Nyksy]

Thanks you for the answers. I was able to get to 60x60 orbit around Duna with less than 1500 dV. Still not optimal, but much better than before.

[maro]

I'm usually somewhere between 30 and 40 million kilometers away.....

Hi

For the "initial burn thats ok - my results are not better

what i do then is

- imediately after the burn i get back to plan a correction burn within Kerbins SOI. ususally ~30min. after the 1st burn i make a second one - with main engines very tiny or with the RCS.

- there is an option for another correction whne you have left Kerbins SOI...probably you got some bad influence from Mun or Minmus

- next fixed time for a correction is "midcourse" half way to your target - you should there be able to get a very precise result.

- next corrections i make some 5-10 days prior to encounter. There you can manage again your periapsis

- and the last corrections i make immediately at reaching SOI - placing my periapsis the last time to where i want it

BAsic principle behind ist - as earlier you make corrections as cheaper they are.....as longer is your "lever"

[MockKnizzle]

When transferring to Duna, I usually make all of my corrections as soon as I'm out of Kerbin's SoI. By using MechJeb to align my craft in the cardinal directions, I just use a little trial and error to adjust my intercept. Typically I'll watch the approach distance/periapse and burn prograde/retrograde as necessary until I can't get it any lower, then I'll re-orient the ship radially and do the same thing, and then back to prograde/retrograde and so on until I've gotten as low as I can. If the inclination difference is small (fractions of a degree) then I can adjust it as well, otherwise I'll wait for a convenient node and make another round of corrections there.

This lets me get 100km or lower periapses with great consistency, usually with only one set of correction burns right at the start of the trip. It might not be the most efficient, but I try to design my spacecraft with large enough fuel margins that it doesn't really matter.

Once I'm within Duna/wherever's SoI, then I'll immediately make any final corrections necessary to set up Ike avoidance/aerobraking/capture.