Does orbiting Kerbin in an orbit at 270 degrees help when visiting inner planets?

[Quintic]

I have been watching some videos on youtube about interplanetary missions, and I am confused about something. In many of the videos they say that you should take off in a west facing direction (270 degrees) instead of the typical east when visiting inner planets. I have not seen one where they explain why that is better.

I know exiting Kerbin's local system in the retrograde direction (from Kerbin's velocity vector) will help visit inner planets because it slows down your orbit with relation to the Sun, but I don't think that leaving Kerbin in a west facing orbit will affect that at all. It just changes the position you need to burn on your orbit to exit Kerbin's system.

If I am right, then isn't orbiting west just a waste of fuel since you already have velocity in the east direction from the planet's rotation?

Am I mistaken?

Thanks for clarification.

Edited July 11, 2013 by Quintic

[Xaiier]

No, don't do that. It costs much more DV to get into orbit and has no real DV gain for the interplanetary transfer.

[Quintic]

Thanks, that is what I thought. I figured I was right, but I usually end up forgetting some small detail. heh.

[knotbeer]

Thanks, that is what I thought. I figured I was right, but I usually end up forgetting some small detail. heh.

Quintic, I completely thank you for asking this question because I was just asking that my own self. Somehow I managed to get a satellite at 2,768 km for a geosynchronous orbit but realized last night that it is actually going in the opposite direction! :facepalm:

Xaiier, thank you too!

[CCKinnison]

The only reason some people think it's an improvement for interplanetary transfers is if you're using Ion engines and solar panels, you get to make more of your burn in sunlight, for consistent power flow. After having done it a few times, I don't see the benefit being that great. if you're using Ions, you're expecting massively long burns, or MANY burns, anyway. And the fuel lasts nearly forever.

[PixelStache]

For a while, there was an orbital calculator out that suggested going towards a heading of 180 for inferior planets. I'm not exactly sure why, but everyone just assumed that was the way to do things.

Launching towards 90 gives you some extra DV, but it's only about 175m/s iirc, so it wouldn't be the worst thing in the world to launch to 180, but there really isn't any benefit. I've heard of some countries here on earth launching towards the west, but that's only to avoid flying over enemy territory or no-fly zones. The only reason I can remember doing it in game was because I wanted to rendezvous with a target that was already orbiting in that direction.

You'll be fine always launching to 90, the only thing that should change from superior to inferior planets is your escape angle. My rule is always to burn at sunset for superior planets like Duna, and sunrise for inferior planets like Eve. It seems to get me close enough.

[Kialar]

Try the following:

Build two probes, exactly identical.

Get them in to orbit, one prograde, one retrograde. Build a carrier system that delivers them to a high orbit around Kerbin, let's say just outside of the Minmus Orbit. Establishing the retrograde orbiting Sat will cost you something like 250-300 m/s deltaV more.

Now head for an kerbolar Orbit with both satellites, with a Sun PE of 500.000 km and an AE near Kerbins Orbit, thus going system inwards relative to Kerbin. This for a quick and dirty Test and that you have not to wait for proper planetary alignment.

You will get the following result:

The prograde orbiting Satellite will need something like 7500-8000 m/s deltaV to get such close to the Sun.

The retrograde orbiting Satellite will need 5000-5500 m/s deltaV to achieve that.

The effect for Eve or Moho will be smaller but you will get back your deltaV, that you invested for retrograde orbiting.

-Kia

[kahlzun]

I had a similar question a few weeks ago

The general consensus is that there is no difference between them: IP efficiency depends on your ejection angle, which is dependant on your burn time, so launching retrograde just burns more dV.

[kahlzun]

Build a carrier system that delivers them to a high orbit around Kerbin, let's say just outside of the Minmus Orbit.

You should never transfer from such a high orbit if you can avoid it, its much less efficient as you lose much of the Oberth effect.

[Kialar]

You should never transfer from such a high orbit if you can avoid it, its much less efficient as you lose much of the Oberth effect.

I know. This was just for the test described above. Not for a "real" mission.

[maltesh]

Try the following:

Build two probes, exactly identical.

Get them in to orbit, one prograde, one retrograde. Build a carrier system that delivers them to a high orbit around Kerbin, let's say just outside of the Minmus Orbit. Establishing the retrograde orbiting Sat will cost you something like 250-300 m/s deltaV more.

Now head for an kerbolar Orbit with both satellites, with a Sun PE of 500.000 km and an AE near Kerbins Orbit, thus going system inwards relative to Kerbin. This for a quick and dirty Test and that you have not to wait for proper planetary alignment.

You will get the following result:

The prograde orbiting Satellite will need something like 7500-8000 m/s deltaV to get such close to the Sun.

The retrograde orbiting Satellite will need 5000-5500 m/s deltaV to achieve that.

The effect for Eve or Moho will be smaller but you will get back your deltaV, that you invested for retrograde orbiting.

-Kia

Done.

Both satellites are in circular orbits around Kerbin, with radius 47 000 km, and maneuver nodes are placed by hand to push solar periapse down to 500 000 km.

Spacecraft Prograde (inclination 0°): Delta-V on Maneuver Node: 5988.7 m/s.

http://i.imgur.com/gRIWPhi.png

http://i.imgur.com/sBU1c16.png

Spacecraft Retrograde (inclination 180°): Delta-V on maneuver node: 6040.7 m/s.

http://i.imgur.com/T5ZqA1Y.png

Delta-V requirement on the retrograde-orbiting craft is higher by roughly 60 m/s, and then there's the extra delta v to get into the retrograde orbit in the first place. I suspect that, in your test, you placed the maneuver node for the prograde craft in a suboptimal location.

(Edited to fix screenshots; should have labeled the bloody things)

Edited July 11, 2013 by maltesh
Bah, Even more problems with the advanced editor. Screenshots should be fixed now.

[Kialar]

OK. Then I'll try again in the evening. Maybe I am doing that terribly wrong.

[Kialar]

OK. I give in. I really cant't reproduce what I have done. Stupid me (it must be my second personallity, Alpert Keinstein the Khysicist, who misslead me). ^^

[Xaiier]

The difference in maltesh's findings is likely due to how the two ships are exiting Kerbin SOI at different altitudes relative to the sun. Kialar, sorry, but you must have put one of the nodes at a non optimal point. For the prograde ship, the optimal location of the maneuver would be at the point on the orbit closest to the sun, and for the retrograde ship, the optimal location would be at the point on the orbit farthest from the sun.

EDIT: No, thats wrong, it should be at the point where the ship's motion direction is pointing towards the sun...maybe a little off of there based on some other factors.

Edited July 11, 2013 by Xaiier

[Mr Shifty]

The difference in maltesh's findings is likely due to how the two ships are exiting Kerbin SOI at different altitudes relative to the sun. Kialar, sorry, but you must have put one of the nodes at a non optimal point. For the prograde ship, the optimal location of the maneuver would be at the point on the orbit closest to the sun, and for the retrograde ship, the optimal location would be at the point on the orbit farthest from the sun.

EDIT: No, thats wrong, it should be at the point where the ship's motion direction is pointing towards the sun...maybe a little off of there based on some other factors.

Still wrong. It's at the point where your escape will be parallel to Kebin's orbit, which depends on your velocity. (At higher velocities, Kerbin curves your trajectory less.)

You should never transfer from such a high orbit if you can avoid it, its much less efficient as you lose much of the Oberth effect.

The fact that your orbit has so much lower energy (because you are so much further up the sides of Kerbin's gravity well) more than negate any benefit you get from the Oberth effect. It's generally better to launch from lower orbit not because of the Oberth effect, but because the benefits of a higher orbit are offset by the fuel cost to get up there.

I don't think that leaving Kerbin in a west facing orbit will affect that at all. It just changes the position you need to burn on your orbit to exit Kerbin's system.

This.

[SRV Ron]

From a normal 90* launch to orbit, To visit the inner planets, do the escape burn on the sunny side of the planet, for the outer planets, do the escape burn on the night side. You can also get a boost from Mum if you plan the maneuver right.

Quick, dirty, and probably less efficient is a dawn or dusk launch straight up to escape velocity. No gravity turn. Dawn gets you to the outer planets, dusk to the inner ones.

[Capt'n Skunky]

Happy Launching!

Arrr!

Capt'n Skunky

KSP Community Manager