Need help catching Duna orbit

[Moon Goddess]

Ok, noob needing some advice.

I bought the game last week and I've been like completely addicted.

Done the Mun landing as befits my screen name, decided to skip Minimus and for the for the first time ever with a destination I have left Kerbin's Sphere of Influence. I've made my intercept for Duna but it seems I've missed slightly.

So I have a quick save just leaving the Kerbin system and I've made the pass a few times... I don't know where I need to burn to like get into orbit, this is different than going to the Mun.

[Francesco]

you're not that far off, at ~40000km you'll meet Duna's sphere of influence.

play around with the maneuver nodes: from the screenshot you seem to be a little "late", I think a correction burn in a combined prograde plus radial inward direction should help.

do it right away, and then correct the orbit inclination at the AN node.

[ThatBum]

You probably need to burn at the midpoint between Kerbin and Duna along your normal vector, to adjust inclination.

[Eric S]

What I usually do in a case like that is put up a maneuver node just outside Kerbin's SoI and play around with the three directions. In a case like this, you can see the kinds of adjustments you need to make. You're arriving too late (behind Duna, and your solar apoapsis is higher than Duna's orbit. The first is a bigger deal, the second is just something to work on for next time.

If you're arriving too late, like in this case, what I usually do is to burn inward (towards the center of your orbit, the sun in this case), as that will usually cause the eccentricity of your orbit to rotate counter-clockwise in this case, which means you spend more time lower in orbit on the way to Duna, so you're travelling faster.

So try that, play around with your maneuver node and adjust it to see what happens. When you've improved the intercept as best you can, burn it, and then set up another maneuver node about a quarter of the way through the transfer and repeat the process. Repeat until you're happy with your intercept.

Using this technique, my last trip to Duna had me entering the SoI of both Duna and Kerbin so accurately that I had to raise my periapsis to avoid crashing directly into the planets, and I still spent a lot less delta-v correcting my course than my old way of doing it at the midpoint of the transfer. Earlier corrections tend to take less delta-V, and while they're not as accurate as later corrections, they allow the later corrections to be much smaller.

[Jason Patterson]

What I usually do in a case like that is put up a maneuver node just outside Kerbin's SoI and play around with the three directions.

This. If you find that the maneuver node widget controls aren't sensitive enough (so that making any adjustment just messes things up more) then you can try test burns to figure out what you need to do to fix things. You're probably less than 100 m/s off course right now, so a small change can put you right in no time.

To do this:

1. Turn your ship to face east (90 degrees, 0 pitch) and make the gentlest burn you can. If your separation with Duna increases, stop and face west instead (270 degrees, 0 pitch.) Make a very gentle burn until the separation distance stops shrinking.

2. Repeat the same process with north (0 degrees, 0 pitch) and/or south (180 degrees, 0 pitch.)

3. Repeat the same process with up (90 degrees pitch) and down (-90 pitch.)

4. Go back to step 1 and repeat as needed.

This process is not the most efficient way to do it, but it works, and with a small burn like this, it's not going to waste much fuel over a single burn.

[Moon Goddess]

you're not that far off, at ~40000km you'll meet Duna's sphere of influence.

play around with the maneuver nodes: from the screenshot you seem to be a little "late", I think a correction burn in a combined prograde plus radial inward direction should help.

do it right away, and then correct the orbit inclination at the AN node.

What I usually do in a case like that is put up a maneuver node just outside Kerbin's SoI and play around with the three directions. In a case like this, you can see the kinds of adjustments you need to make. You're arriving too late (behind Duna, and your solar apoapsis is higher than Duna's orbit. The first is a bigger deal, the second is just something to work on for next time.

If you're arriving too late, like in this case, what I usually do is to burn inward (towards the center of your orbit, the sun in this case), as that will usually cause the eccentricity of your orbit to rotate counter-clockwise in this case, which means you spend more time lower in orbit on the way to Duna, so you're travelling faster.

So try that, play around with your maneuver node and adjust it to see what happens. When you've improved the intercept as best you can, burn it, and then set up another maneuver node about a quarter of the way through the transfer and repeat the process. Repeat until you're happy with your intercept.

Using this technique, my last trip to Duna had me entering the SoI of both Duna and Kerbin so accurately that I had to raise my periapsis to avoid crashing directly into the planets, and I still spent a lot less delta-v correcting my course than my old way of doing it at the midpoint of the transfer. Earlier corrections tend to take less delta-V, and while they're not as accurate as later corrections, they allow the later corrections to be much smaller.

Thank you both... I was doing maneuvers that were like 8000 Delta V. That got me to SOI for only 77!

A few other maneuvers got me into a stable orbit.....

Well at least we lived over it.

[Eric S]

This. If you find that the maneuver node widget controls aren't sensitive enough

Don't think I've ever had that problem since I found out that you can push the opposite side in, which seems more sensitive than pulling the side you want out. Also keep in mind, the farther you push/pull the knob, the faster the node changes, so be careful with the mouse.

[ThatBum]

You can also grab the entire node from the center and slide it around the orbit, changing the time the burn starts.

[CalculusWarrior]

Also, if you zoom in on your maneuver node, it becomes a lot more precise. However, this means that your destination is no longer in view. I've found myself adjusting the map view so that I can see both at the same time, which usually ends up with the target planet being really close to the camera. It's probably not the best way to go...

I know that there are some mods out there that allow you to adjust the maneuver nodes numerically, if you'd prefer.

[ThatBum]

I know that there are some mods out there that allow you to adjust the maneuver nodes numerically, if you'd prefer.

That would be this. MechJeb 2 also has that functionality, if you're not one of those people who oppose MechJeb.

[mcbmaestro]

Don't forget just cuz a encounter shows up on your maneuver node that you have to settle for it. I just went from Kerbin to Jool in 80 days on my fusion rockets, and it took insane fuel to do it and that's the tradeoff, there's pretty much always a lot of encounters at any given point, and you can choose which one has the best time to fuel ratio that you want by experimenting with the maneuver nodes.

Also for those times when it seems like nothing is working you can manually or use mechjeb rendezvous function to align orbits to give you easier encounters.

Edited April 22, 2013 by mcbmaestro

[Jason Patterson]

Don't think I've ever had that problem since I found out that you can push the opposite side in, which seems more sensitive than pulling the side you want out. Also keep in mind, the farther you push/pull the knob, the faster the node changes, so be careful with the mouse.

Yeah, it's more of a problem with getting a Moho or Dres encounter. It's a reasonably good approach for shrinking a close approach, in any case, particularly with maneuverable ships.

[mcbmaestro]

Oh yeah and lol, don't forget to be orbiting the same direction as the planet. Tried to come in the reverse direction yesterday, 2600ish delta-V to get to Kerbin, not bad I thought. But when I got there I needed to kill a speed of 25,000m/s to slow down because I was approaching the planet head on, haha. Even my super fusion rocket had to emergency land with just 250 left in the tank.

Edited April 24, 2013 by mcbmaestro