Benefit of intial retrograde burn

[kahlzun]

Hi guys!

Ever since i was first learning how to intercept other planets in 0.17, I was always told to launch into a 90 degree orbit if planning to transfer to 'outside' planets, and a 270 degree orbit for 'inside' planets.

I was thinking about it the other day, and realised it shouldn't make any difference to your orbital velocity, it would just change what side of the planet you burn from, unless I'm missing something.

So, kerbonauts, I ask you: Is there any benefit to doing this, or am I just wasting 178 ms of fuel getting into a 270 orbit?

I mean, on that calculator thing it says to get into a retrograde orbit for Moho or Eve... how much difference is there really?

Cheers,

Sam

[Bothersome]

No I think you mis-understood whomever it was trying to explain or tell you what was needed. Always launch to 90 degrees (due East) from the launch pad. But, I think they were trying to say go prograde of the planet for outer planets and retrograde of the planet for inner planets. This means you want you ejection to be prograde or retrograde of the planet you are leaving. So, if you wanted to go inward, you want to leave your current planet on the sunny side. Making your ship travel around the sun in a lower orbit around the sun. But, this is not alway the best practice. There are better ejection angles that will optimize your fuel/DV needed. But for general, off the cuff, ejections, just orbit counter-clockwise as you normally do but when you "break away" from the planet's SOI, you want to have less energy (lower orbit) for inner planets or more energy (higher orbit) for out planets.

If that's not too muddy, maybe it helped. Someone else might be better at explaining what I'm trying to say better than me. A teacher, I am not.

[kahlzun]

Alright, so that's one vote stating "makes no difference".

Two, if you count my confusion.

Are there any rebuttals from the retrograde launch crowd?

[ComradeGoat]

It does make a difference. Launching into a retrograde orbit wastes several hundred m/s of delta V because Kerbin's spin is against you rather than with you.

[RoboRay]

Your intuition is correct. While the early versions of the OLEX guide specifically directed you to launch westward into a retrograde orbit for inner destinations, this error was quickly pointed out and eventually corrected in the guide.

You actually save over 300m/sec by launching east... double the planet's rotational velocity. The only difference between going to inner and outer destinations is which side of the planet you are over when you make your injection burn.

[SRV Ron]

For visiting either the outer or inner planets, just get into orbit the normal way, 90* orbital insertion. From there to visit the outer planets, plan your escape burn, usually the night side of Kerbal, when you are traveling pro-grade to Kerbal's orbit. For the inner planets, do the escape burn, usually from the daylight side of Kerbal, when you are traveling retrograde to Kerbal's orbit. The burn should be done when you are parallel to Kerbal's orbital path around the sun. Pro-grade equals the same direction of travel, retrograde is the opposite direction of travel. In either case, beware of a possible encounter with Mum's gravity field which can throw you way off and even change your orbital plane.

What is interesting, due to orbital mechanics, is that your path is actually backwards from your direction of acceleration when you aim to visit the inner planets. That is because you are burning retrograde in relation to Kerban's orbital velocity thus going slower then Kerbal is going around its sun which causes you to fall towards it's sun to reach a new stable solar orbit.

Edited June 26, 2013 by SRV Ron

[Paul Kingtiger]

You can use the Mun to show how this works. Apollo entered a retrograde orbit around the moon (orbiting backwards compared to it's rotation). If you've seen Apollo 13 you'll see this as the classic figure of eight flight plan. Why do they do this?

So if you fly to the Mun and orbit prograde, then when you come to leave you'll be burning prograde to break Munar orbit, but this prograde burn will also increase your orbit around Kerbin, so the burn to exit the Mun also takes you further from Kerbin. That's the opposite of what you want to do.

If you orbit the Mun retrograde like they did with Apollo then the burn to escape the Mun is the same direction as a burn to lower your orbit around Kerbin, which means a single burn will escape the Mun and return you to Kerbin. That means less DeltaV to get where you want to go! In my experiments it's about 300m/s less which it substantial.

Exactly the same logic applies when breaking orbit from Kerbin to travel to Duna or Eve. Prograde orbit will send you away from the sun, Retrograde will send you towards it. Is the delta v saved worth the extra cost of launching the wrong way? I don't know, you'll have to experiment.

[Flowz0r]

The picture in my signature should make the prograde/retrograte Kerbin SOI escape a little easier to understand.

/Edit: Scratch that, looks like I removed that at some point, will post as soon as I dig it up again.

It has nothing to do with your orbit around kerbin, you can escape it pro- or retrograde no matter if you're orbiting pro- or retrograde.

Setting up your orbit in line with Kerbin's rotation just saves a little bit of fuel because you're using Kerbin's rotation to get delta-v instead of fighting it.

[Bothersome]

No, that's not correct. They used the figure 8 because they wanted a free return trajectory if things went wrong like the service pod engine not starting.

For absolute best landing and takoff from the Mun would be also in the way it rotates. For best exit from the Mun back to Kerbin, would be to burn on the far side of the Mun and make your apoapsis rise on the near side (closest to Kerbin). Then when it breaks open, you are then in a lesser energy orbit around Kerbin. If you keep burning a bit more you will decrease you orbit even further. But, eventaully, you will add energy if you burn too long. However, that would be a serious over-burn.

But you are right in that it is the same principal when dealing with the sun and the planets. Just think of them as a larger Kerbin and Moons.

[csiler2]

You can use the Mun to show how this works. Apollo entered a retrograde orbit around the moon (orbiting backwards compared to it's rotation). If you've seen Apollo 13 you'll see this as the classic figure of eight flight plan. Why do they do this?

So if you fly to the Mun and orbit prograde, then when you come to leave you'll be burning prograde to break Munar orbit, but this prograde burn will also increase your orbit around Kerbin, so the burn to exit the Mun also takes you further from Kerbin. That's the opposite of what you want to do.

If you orbit the Mun retrograde like they did with Apollo then the burn to escape the Mun is the same direction as a burn to lower your orbit around Kerbin, which means a single burn will escape the Mun and return you to Kerbin. That means less DeltaV to get where you want to go! In my experiments it's about 300m/s less which it substantial.

Exactly the same logic applies when breaking orbit from Kerbin to travel to Duna or Eve. Prograde orbit will send you away from the sun, Retrograde will send you towards it. Is the delta v saved worth the extra cost of launching the wrong way? I don't know, you'll have to experiment.

I believe the main reason Apollo missions used the figure eight was for delta V savings. They burned just enough to get into the Moon's SOI and then let gravity do the rest until they circularized. Getting into a retrograde Earth orbit before departing would cost a lot of delta V and burning to get into a prograde lunar orbit would require a more direct path (also more delta V).

Regardless, you can always burn prograde or retrograde to the orbit of your parent body regardless of whether or not your orbit is prograde or retro grade. The only thing that changes is where you start your burn. Ships in prograde orbits want to burn near-side for access to the inner bodies and far-side for the outer bodies. Ships in retrograde orbits will do the opposite; near-side for access to outer bodies, far-side for inner bodies.

e.g. A ship in prograde Kerbin orbit will want to burn dark-side (far-side) to get to Jool, while a ship in prograde orbit of the mün will want to burn on the near-side to get back to Kerbin.

Since Apollo missions were in retrograde lunar orbit they burned on the far-side to get back to Earth. It was risky having their major burns for Lunar orbital maneuvers on the far side, but it was much cheaper in Delta V.

[CoriW]

It is indeed true that whether you are going to the inner or outer planets, the most efficient way to approach it is with a 90 degree (East) heading orbit. Now since the explanation of this can sometimes be a little bit confusing, I have taken some time to draw up a visual representation of how you want to approach interplanetary transfer burns.

I hope that helps.

Edited June 26, 2013 by CoriW

[Eric S]

The one time a retrograde orbit makes sense is if you're doing your planetary transfer burn with an ion drive and you're transferring to a planet farther out. A retrograde orbit in this case means that you can use solar panels to power the ion drive for the burn, rather than being in Kerbin's shadow when you want to burn.

[CCKinnison]

For Ions, I like polar orbits above the terminator of the body being orbited as a starting point. But yeah, it can be a higher cost to GET into that orbit to begin with.