maneuver question

[EeeLee]

Hello everyone,

I've been sending my very first mission to Duna, but cannot get the final maneuver(s) right. I understand that for an aerobreaking maneuver I've got to come in at below 40km. I set the flight path accordingly, but end up at a very different position once I get close to the planet.

1. The craft is 3 days away, maneuver node set to a 24km Periapsis "in front" of Duna

2. Maneuver performed (well enough)

3. Craft arrives with a 464km Periapsis "behind" Duna

I'm obviously missing something. I guess flying to Duna is different from flying to Minimus or the Mun, but in terms of projected flight path my ships always ended up exactly where they were supposed to. So I dont quite understand why that is not the case now. Hopefully some nice Kerbal with experience in interplanetary navigation can give me a hint.

Edited October 9, 2014 by EeeLee
solved

[Taki117]

if you are timewarping when you enter the SoI it can have an effect on your orbit. also, because of rounding errors the farther you are from your target the more your orbit will shift.

[capi3101]

I should mention that you're going to want to come in closer - much closer - to Duna's surface than 25,000 if you want a successful aerobraking. Myself, I haven't successfully aerobraked if my periapsis was above 15,000. It doesn't have that much of an atmosphere.

Are you performing your correction burn before or after you enter Duna's SOI?

[Taki117]

I should mention that you're going to want to come in closer - much closer - to Duna's surface than 25,000 if you want a successful aerobraking. Myself, I haven't successfully aerobraked if my periapsis was above 15,000. It doesn't have that much of an atmosphere.

Are you performing your correction burn before or after you enter Duna's SOI?

Piggy backing on that, here's a handy calculator for calculating how deep you should go. It assumes stock aero so if you are using NEAR, FAR, or Stock Drag Fix, it won't work.

[EeeLee]

First of all big thanks for the help, it was indeed the timewarp that screwed with my orbit.

I performed the correction burn far outside Duna's SoI. My understanding of orbital mechanics approaches zero, so far I've been going by the rule that the further away from a target a correction burn is performed, the less energy is required.

As for the aerobreaking, I just tried 17km and didnt get much closer to establishing an orbit. Looks like I'll have to almost scratch the surface as I got absolutely no fuel to spare. Thanks for the link to the calculator!

[EdFred]

First of all big thanks for the help, it was indeed the timewarp that screwed with my orbit.

I performed the correction burn far outside Duna's SoI. My understanding of orbital mechanics approaches zero, so far I've been going by the rule that the further away from a target a correction burn is performed, the less energy is required.

As for the aerobreaking, I just tried 17km and didnt get much closer to establishing an orbit. Looks like I'll have to almost scratch the surface as I got absolutely no fuel to spare. Thanks for the link to the calculator!

You are correct that he further away you are the less dV you have to expend, so you aren't exactly approaching zero.