How do I get to Duna without having my orbit go THROUGH Kerman?

[Bigred2989]

As you can see, Mechjeb is having me burn retrograde to my orbit, which is causing me to fall into the planet. It looks to me that by the time I'm halfway done with the burn I'll be a crater. What am I doing wrong or how can I avoid this? This always happens when I try to send large ships to other planets.

Edited August 28, 2013 by Bigred2989

[astropapi1]

*Kerbin.

What's your TWR? If it's too low (say, a single nuclear engine), MJ will try burning through the planet.

Although I really love MJ, you might want to do your IP transfer without it.

[blizzy78]

Perhaps you might want to start off from a higher orbit so it will send you past the planet instead of into it.

[chaos_forge]

If you want to escape from Kerbin, you shouldn't have to burn retrograde . . .

Are you orbiting in the same direction as Kerbin rotates? If not, that might be your problem.

Other than that, it seems like a bug with Mechjeb. I'd suggest doing it manually. Apart from that, you could put yourself in a higher orbit, or building a ship with higher thrusts so the burn doesn't take as long.

[DChurchill]

TWR is part of it. Big ships have a tendency to have low TWR, which means that it has to burn longer and start earlier which means that if you start from a low orbit, it can turn you into a smoking hole before your orbit has time to carry you around to clear the planet. You can either increase the thrust on your ship but it will decrease the amount delta-V available to you, reduce the size of your ship which will increase the TWR and dV available, or you can start from a higher orbit which is sort of counter productive from the perspective of the Oberth effect, but will give you more time for your orbit to come around so you're free of the planet. I'm not sure that what you gain in time from raising your orbit wouldn't be negated due to the fact that your orbit will be slower. People with more current math credits than mine would have to weigh in on that one.

[DChurchill]

If you want to escape from Kerbin, you shouldn't have to burn retrograde . . .

Are you orbiting in the same direction as Kerbin rotates? If not, that might be your problem.

Other than that, it seems like a bug with Mechjeb. I'd suggest doing it manually. Apart from that, you could put yourself in a higher orbit, or building a ship with higher thrusts so the burn doesn't take as long.

No if his TWR is way low then MJ would behave that way. MJ is just setting up a node. Since it tries to burn half in front and half after the node then you could actually have to burn retrograde at the start of it if your TWR is low enough, because the burn time could be a significant fraction of the orbital period. I guess if it was exactly half, then you would be perfectly retrograde. It's just pointing at a node and firing for X amount of dV. Sounds like the TWR is really low to me. I've had sort of the same problem but not nearly of that magnitude.

The more I think about it, the size of the burn as a fraction of your orbital period is the whole thing. The smaller it is, the closer to true prograde you'll be when the burn starts. This may be a "duh" for some of you guys but it's kind of a revelation to me.

Edited August 28, 2013 by DChurchill

[chaos_forge]

No if his TWR is way low then MJ would behave that way. MJ is just setting up a node. Since it tries to burn half in front and half after the node then you could actually have to burn retrograde at the start of it if your TWR is low enough, because the burn time could be a significant fraction of the orbital period. I guess if it was exactly half, then you would be perfectly retrograde. It's just pointing at a node and firing for X amount of dV. Sounds like the TWR is really low to me. I've had sort of the same problem but not nearly of that magnitude.

Oh, I see. The best option, then, is to add thrust to your rocket. Such a low TWR is gonna give you problems for your capture burn too . . .

[RoboRay]

Your burn time is too long for a single-burn injection. If your orbital period is, say 30 minutes, and you need a 20 minute burn to transfer, you're going to have problems. The solution is periapsis kicks, where you break your long burn up into a series of short ones on sequential orbits.

• Set up your transfer node as normal.
  
• When you're two or three minutes from reaching it, start your burn. When you are two or three minutes past it (be sure to burn exactly equal amounts on each side of the node), shut down.
  
• Delete the node.
  
• Each orbit, burn prograde for a few more minutes each time you pass periapsis (which marks where your maneuver node was). The times can vary orbit to orbit, but you need to do equal amounts on each side of the Pe marker every time. Your apoapsis will rise higher and higher each orbit.
  
• When your Ap is about to the Mun's orbit or beyond, set up a new maneuver node at your Pe to make the final transfer burn.
  
• Execute that maneuver normally.
  

Your sequence of orbits will look something like this:

Pe Kicks are the real best option. They are much more efficient than one long burn as all delta V is delivered right at the ejection angle, where all thrust is aligned with both your prograde vector and your desired trajectory and you maximize the Oberth effect by conducting more of the burn near Pe where you're moving faster.

Edited August 28, 2013 by RoboRay

[Bigred2989]

I took the "get to a higher altitude" route and that works. I'm on my way to duna now. Need to figure out the best approach altitude now.

[DChurchill]

Pe Kicks are the real best option. They are much more efficient than one long burn as all delta V is delivered right at the ejection angle, where all thrust is aligned with both your prograde vector and your desired trajectory and you maximize the Oberth effect by conducting more of the burn near Pe where you're moving faster.

From an efficiency standpoint this certainly would be the best. A shade tedious though.

[RoboRay]

From an efficiency standpoint this certainly would be the best. A shade tedious though.

Total time spent at the controls is about the same, since you can timewarp around the various orbits. Your burn-length actually ends up being slightly shorter due to the efficiency gains, but you still have to add some time for the warping around. And you probably only need to make three or four kicks, even for massive vessels, unless you're using ion drives, so it's not as bad as those pictures (from Orbiter) make it look.

Which reminds me of one more note to add...

Pe kicks do take time. If you only push your Ap out to the Mun, the total time is only a day or so. But if you try to push it out to Minmus or to the fringes of the SOI, your final orbit will take a week. This may delay you past your ideal transfer window, which is why I suggest only pushing Ap out to the Mun... The delta V difference between rising to the Mun and rising to Minmus is tiny, but the much longer orbital period can cause you to miss your transfer.

Well, unless you plan for it and start your kicks a week early.

Edited August 28, 2013 by RoboRay

[halbert5150]

There might be another problem I didn't see anyone mention. Does your ship have more than one control module? If so and if your controlling it from the wrong module then the maneuver nodes when setup will tell you to burn in the apposite direction of where you actually need to. Just right click on the other control module and select control from here and you might get something very different.

Maybe thats whats going on.

[DChurchill]

Total time spent at the controls is about the same, since you can timewarp around the various orbits. Your burn-length actually ends up being slightly shorter due to the efficiency gains, but you still have to add some time for the warping around. And you probably only need to make three or four kicks, even for massive vessels, unless you're using ion drives, so it's not as bad as those pictures (from Orbiter) make it look.

Yeah but if I let MJ control the burn, I can go make a sammich and surf the KSP forum.

[allmappedout]

Yeah but if I let MJ control the burn, I can go make a sammich and surf the KSP forum.

Best reason for using Mechjeb, ever