Retrograde orbit after interplanetary transfer

[KerBlammo]

I often seem to end up in a retrograde orbit after an interplanetary transfer (i.e. going to Duna). For instance in my recently successfully completed Duna crewed landing campaign (woo hoo!!) out of 4 interplanetary transfers (base, habitat & lander & crew to Duna and crew return from Duna) 2 of them ended up in a retrograde orbit.

This happens if I plot the transfer by hand or use a tool to compute the node. When I enter the SOI of the target I burn retrograde until I achieve capture. Obviously this is sub-optimal as I have to expend dV just to get orbiting in the correct direction.

Anyone have any tips on how to prevent this from happening?

Edited October 20, 2015 by KerBlammo

[SpaceplaneAddict]

Hmm, never actually ran into this... maybe try aerocapture?

[Starhawk]

It just requires tweaking the maneuver node for the transfer. After you set up the node, change your focus to your destination planet. You can then see the trajectory you will eventually take when you arrive. By tweaking the maneuver node you can adjust that trajectory and determine which side of the planet you arrive at and the altitude of your periapsis, as well.

Happy landings!

[tomf]

If you find you are in a retrograde orbit after a transfer it is because you got your encounter slightly wrong. It is important to set up your encounter so that your periapsis is on the correct side. There are a couple of ways to check.

When fine tuning my encounter I click on the target and zoom in. If you have a low PE you should be able to see some deflection of the orbit. This should always be in the anti clockwise direction (to the left) when viewing from above the north pole. If you are coming in so fast that there is no deflection you can still see which direction you are traveling from the escape node at the end of the orbit segment and ensure that the planet is on the left.

[KerBlammo]

Wow thanks for the quick replies. This forum never ceases to impress!

Especially when plotting the maneuver by hand I'm just happy to get an intercept, so it it never occurred to me to zoom in and look at the details!

[Mastikator]

If you're going to land using airbrakes and parachutes then it won't really matter which direction you're going. The sidereal rotation speed at the equator is only 30m/s, so the difference is only 60m/s if you're going retrograde on Duna.

Hell, if you use aerocapture you actually get more drag going in against the rotation, it's probably more efficient to do it that way.

My advice, if correcting it costs more than 60m/s then it's not worth correcting. (and probably not even then)

[moogoob]

You can also adjust this for very little delta V right as you enter the destination's sphere of influence. Just point your ship at the right side of the planet and burn until your periapsis is on that side, rather than the left, so that when you capture you end up in a proper counter-clockwise prograde orbit.

It'll cost you more delta-v than adjusting before hand, but I found it easier and less fiddly to do it that way, especially if I'm in a lazy mood.

[sal_vager]

I better just mention that if you do aerobrake retrograde you'll be hitting an atmosphere that is moving towards you at the planets rotational speed, rather than away, this will heat you up more at the same altitude than a prograde entry.

I found this out when returning to Kerbin after a retrograde rescue contract (I couldn't reach the rescuee in time before the Mun helpfully inverted the orbit).

[FyunchClick]

Especially when plotting the maneuver by hand I'm just happy to get an intercept, so it it never occurred to me to zoom in and look at the details!

When setting up the capture burn maneuver node from Kerbin, cycle the focus through the planets by using the tab key untill you hit your destination planet/moon, that way the node doesn't collapse and you can keep tweaking it. If you set the focus using the mouse, it'll collapse and you first have to fiddle clicking it open again before you can tweak it.

I've found that you can usually get a pretty accurate node set up for nearby worlds like Duna, setting a PE up untill a dozen km or so of what you're looking for with with some accuracy. Of course, hitting that exactly is a lot harder since you'll be burning for a while but it doesn't hurt to try and aim as close as you can.