Efficient Interplanetary Transfers

[More Boosters]

Hey guys,

I would like to improve the efficiency of my interplanetary transfers after Bob Bill and Jeb got stuck in a solar orbit near Moho on return.

I know about transfer windows and I have the Kerbal Alarm Clock, but I'm not sure what to actually do with them.

I'd like to know how to calculate proper transfers, and maybe gravity assist planning too. I heard someone say that if you can get to Eve, you can go anywhere, well, I can get to Eve.

Would be great if you could point me to a tutorial or give me a crash course yourself, thanks.

[Skorpychan]

Right. Firstly, http://alexmoon.github.io/ksp/ is the way you FIND the windows.

Otherwise, I'm not 100% on it myself.

[Thegamer211]

Use the Launch window planner, or even the in game integration of this, transfer window planner.

They both tell you how much dv it takes to do the transfer, not only the time. Useful for making accuarate maneuver nodes.

[maccollo]

If you are going to Moho then forget about regular transfer windows. Instead just wait until Kerbin is in opposition to Moho's periapsis.

Launch at midday with a heading of something like 110 degrees instead of 90, that's is 20 degrees south of eastwards.

Then just perform your escape burn. When you reach periapsis around kerbol Moho probably wont be there, but you should be able to perform a burn that gives you an encounter one or two orbits later.

Edited August 7, 2015 by maccollo

[Gargamel]

I know You're asking for manual control, but mechjeb now has a wonderful porkchop transfer calculator. It will show you the shortest or most effecient transfer windows. If you want, just use it to show you to what is upcoming. As for gravity assists, that too I would like to learn more on.

[sarbian]

Or go all the way and use KSP Trajectory Optimization Tool

[Warzouz]

Use the Launch window planner, or even the in game integration of this, transfer window planner.

They both tell you how much dv it takes to do the transfer, not only the time. Useful for making accuarate maneuver nodes.

This is valid for Duna, Eve, Jool, but not for Moho, Dres or Eeloo. Those bodies have need a plan change. This can be costly. The best spt is to get the encounter when the body is very near the Kerbin plane.

For Moho, this can go up to 2500m/s more. As you get a log of window for moho, select the correct one with care. The best way is to launch to LKO and check each window until you get a proper encounter.

Meanwhile, you can explore elsewhere.

[cantab]

So the basics:

Moho is an exception to all the rules and needs special treatment.

Go at the right time. Lots of tools will tell you when.

Go in one prograde burn from Low Kerbin Orbit. A relatively small mid-course correction is OK but it's inefficient to make big changes to your solar orbit once you've left Kerbin.

Leave Kerbin's SOI nearly parallel to Kerbin's orbit round the Sun. You want your transfer orbit to just touch Kerbin's orbit, not cut across it. If you're bang on the transfer window it should also just touch the destination planet's orbit, but some variation is common there.

[Tank Buddy]

You can't use conventional Hohmann transfers to get to Moho easily. The best thing to do is wait until Kerbin is at the ascending/descending node of Moho's orbit, then make a transfer burn that matches your periapsis with the orbit of Moho, at the AN/ DN. (This way you are guarenteed to cross Moho's orbit w/o a plane change) You can save some DV by launching into an orbit that matches Moho's inclination at that time or adding in a plane change with your ejection burn. When you reach Pe, set up a maneuver that creates an intercept with Moho, and wala! Should be self explanatory from then on.

[clivman]

Use Moho for gravity assists to lower your orbit and only after you have a low velocity intercept try to orbit capture

[Fearless Son]

I'd like to know how to calculate proper transfers, and maybe gravity assist planning too.

The basics of gravity assistance are thus: if you pass through a planet's sphere of influence in front of its orbit, you will lose some energy, while if you pass through behind its orbit, you will gain some energy. The closer you can get to the center of the sphere of influence without actually crashing (or aerobreaking in the cases of planets with atmosphere) the bigger then change in energy you get from passing by that planet.

The tricky part of gravity assists when planning your course is that you effectively have to try and intercept each planet in orbits between you and your target, assuming they will even be in a position to practically do so by the time you cross their orbits. Make sure to plan your maneuver nodes to sling around the planet very carefully. Treat it like you are planning to visit that planet and want to enter a specific orbit, except you will be leaving again instead of actually orbiting, and get the exit vector to match the general direction you want to go as you leave. Then set up other maneuver nodes on the periapsis of your course around the planet, using that node to adjust your course to target the next planet in the sequence, and maybe do a little forward velocity adding burn while you are at it (lots of bang for your buck at low periapsis.) Repeat as necessary as you cross orbits.

While this method might extend your total range, letting you get out further for less fuel, I have found that in practice it does extend the time the mission takes. You often end up going out of your way to maximize fuel economy, but at the cost of longer total distance you have to coast.

[Vanamonde]

This thread has been moved to the how-to subforum. Carry on.

[Tatonf]

On http://alexmoon.github.io/ksp/ you have to select a Transfer Type, choosing between "Ballistic" ; "Mid-Course plan change" and "Optimal".

I know what a mid-course plan change is, and actually it's the only way I know how to perform an interplanetary transfer, and the only method I ever heard of.

So, does anyone know what the other 2 stand for ?

[Warzouz]

Ballistic is as single burn which will intersect your target SOI without plane change. The burn is usually not prograde only. Ballistic should be more efficient.

As for "optimal", I don't know. It usually gives the same result as "Ballistic".

[Red Iron Crown]

"Optimal" computes both ballistic and mid-course plane change approaches and returns the better result. Ballistic is almost always better, though when transferring outward with a large plane change sometimes mid-course plane change is better (think Kerbin->Eeloo or Moho->Kerbin, for example).

[cantab]

If you do the ballistic plot you will often see a "ridge" cutting through your transfer windows. These appear because if your transfer involves making half an orbit round the Sun - which is the most efficient transfer - then you cannot practically shift your position at arrival north or south, you need to make a mid-course correction to do that.

[Tatonf]

Thanks you, it looks like I'm going to play differently from now on.