Interplanetary Transfer

[Dizzle]

Hey guys. After a long time of not being able to get interplanetary encounters beyond getting lucky, I just sent an orbiter to Duna. Through the initial burn, and an adjustment when I was about midway there, I was able to get it down to about 5,072,000 meters. Still, when I got there, I had to burn off almost 1km/s delta-v, and I wanted to know if there's a better way. Should the encounter be from as far a possible, meaning slower speed and therefore less delta-v to burn off? Or try to get it in-atmosphere for an aerobraking maneuver?

If it's the latter, how? 5,072,000 was about as close as I could get it.

[relicen]

To get a really close encounter you may need to do adjustments on the way to your target. Also, get the protractor mod to get around the "luck" part of searching for interplanetary transfer windows.

[ComradeGoat]

You have 3 axes to correct. The first two can be done anywhere, and are cheaper the further away you are. The other axis can only be fixed fully at an ascending or descending node:

From as far out as possible (i.e. once leaving kerbin SOI), try thrusting prograde and retrograde to get the encounter down to as low a value as possible. Then turn to either the "north pole" or "south pole" on the navball and fix it in the other axis.

At some point you will encounter an ascending node or a descending node. At an ascending node, thrust due south on the navball's horizon. At a descending node, thrust due north.

By fixing all 3 axes, X, Y and Z, you can get your encounter down to zero. Once you've done this, wait until you enter Duna's sphere of influence, and then thrust normal to your approach line, so that you will swing round the planet and turn left as you do so. Set your Duna periapsis to between 12,000 and 14,000m. This should aerocapture you into Duna orbit. Beware Ike encounters!

Once aerocaptured, remember to burn prograde at apoapsis to raise your periapsis out of the atmosphere, otherwise you will end up landing, whether you like it or not.

The protractor mod is very helpful, because it will show your closest approach at any given point.

[tntristan12]

I highly recommend using the protractor mod to plan your launch windows. I never miss when I do it that way.

[Paul Kingtiger]

Kerbal alarm clock is another great tool for planning injection burns. I have more success with it that Protractor but YMMV

[Spatzimaus]

Should the encounter be from as far a possible, meaning slower speed and therefore less delta-v to burn off? Or try to get it in-atmosphere for an aerobraking maneuver?

If you can pull off an aerobrake intercept from a different planet's orbit, then by all means do so. But realistically, your chances of getting the right height to aerobrake are not great (too low and you'll spiral in, too high and you won't get much of a slowdown and will just fly right back out of the SOI), so I only aerobrake once I'm already in a stable orbit inside an SOI. Beyond that, there's not much difference; a far intercept requires less delta-V to decelerate into a circular orbit, but you'll then have to spend more velocity (almost exactly the amount you saved) getting that orbit down to a useful distance, unless you're trying to intercept one of the planet's moons or something.

In most non-aerobrake cases, what I've done is this:

1> Aim for a very close intercept, as close as possible. I don't use mods to help calculate this, but it's not hard to get close.

2> Upon reaching periapsis after entering the SOI, decelerate until the apoapsis just barely stays inside the SOI, giving a very elongated orbit.

3> Fast-forward to apoapsis, where your velocity will now be very low (less than if you'd tried to circularize at that outer distance from the start). Do any major orbit orientation changes there, where it'd take very little effort to shift from a polar orbit to an equatorial one. If you're trying to land on a body with an atmosphere, just burn retrograde a bit here to intersect the atmosphere; if it's airless, or you only want to orbit, then try to keep the periapsis barely above the surface/atmosphere.

4> Fast-forward again to periapsis. Circularize as necessary by burning retrograde.

You can also use a little aerobraking in step 3 to save fuel in step 4; once you've got that really elongated (but stable) orbit, lower the periapsis just inside the atmosphere. On each pass you'll lose a little orbital energy from the apoapsis, without having to worry about escaping if you don't brake enough. That's how I landed on Duna my last time there.

[Stealth2668]

2 questions; what is an injection burn? And the descending node is the intersection of your orbit when it goes under your target? (vice versa for ascending node)

[ComradeGoat]

2 questions; what is an injection burn? And the descending node is the intersection of your orbit when it goes under your target? (vice versa for ascending node)

Set Duna as your target in the map. You will see two markers appear: "An" and "Dn".

"An" is the ascending node, when you cross the plane of your target's orbit going "up". "Dn" is when you cross going down.

Obviously these are the only two points where it's possible to align your orbital plane.

[Spatzimaus]

2 questions; what is an injection burn? And the descending node is the intersection of your orbit when it goes under your target? (vice versa for ascending node)

By "injection burn" I assume he means orbital insertion burns, as in a burn that transfers you from a trajectory that'll pass through an SOI and escape out the other side, to a stable orbit around that body.

The Ascending Node and Descending Node designate the points where your current orbit passes above/below the plane of the target's orbit. ("Above" and "below" are defined using the right-hand rule, where up/north is the direction around which an orbit/object rotates in a counter-clockwise direction.) If you're trying to match an orbit, it's quite a bit easier (although often more fuel-consuming) to cancel out this inclination first by burning up/down at those locations, and then find the location on your now-coplanar orbit where a simple prograde/retrograde burn creates an intersection with the target. Besides being easier to plan, this also tends to minimize the amount of delta-V you'll need to expend at the target, which is important for objects with small spheres of influence (Moho, Gilly, etc.) when using low-thrust, high-efficiency engines.