If you launch opposite to Kerbin's revolution...

[Tex]

... could you be considered as decelerating?

Think of it this way: By definition of "orbiting", you are technically in free fall, could you have slowed your craft down so much that Kerbin is trying to essentially leave you behind, only to have its gravity at as a bungee cord, putting you in orbit?

Lemme rephrase that in case I lost you. If you look at Kerbin in-game from the Sun, it appears to be traveling In one certain direction in a perfect circle. If you were to apply force in the opposite direction, thereby thrusting opposite to Kerbin's revolution, would you basically be decelerating relative to the Sun?

If you achieved Kerbin Escape by doing this, and then time-warping in Map Mode, you would see that Kerbin speeds away from you. Are you basically just putting on the deep-space brakes (relative to the Sun)?

Discuss!

[Nibb31]

You can orbit the Sun either prograde or retrograde, it doesn't matter. You're still orbiting the Sun at the same speed, only in the other direction. You still accelerate to raise your orbit and decelerate to lower it.

[Karriz]

Actually, it depends on which side of the planet (relative to the Sun) you are. Think about it: on the other side, the planet is rotating to the same direction as the planet is orbiting the Sun, but on the opposite side, it's rotating in the other direction.

So, launching into the usual 90 degree equatorial orbit is always the most efficient way to do an interplanetary transfer, to both the outer and inner solar system.

[K^2]

Lemme rephrase that in case I lost you. If you look at Kerbin in-game from the Sun, it appears to be traveling In one certain direction in a perfect circle. If you were to apply force in the opposite direction, thereby thrusting opposite to Kerbin's revolution, would you basically be decelerating relative to the Sun?

That's a pretty standard miscondeption. It doesn't take a force for something to keep going. If Kerbin pulls on object that's trailing it around Kerbol, that object is accelerating towards Kerbin. It's not going to just hang a constant distance behind.

The reason you are thinking of it in terms of bungie is that usually, when you need to pull something, there is friciton or drag, so you have to constantly supply force to keep the object moving. E.g. dragging a boat by the rope. But there is no friction in space. For an object to move behind Kerbin at the same speed, there has to be no force acting on it. It has to be coasting completely free. But there is pull of Kerbin, so it's going to accelerate towards Kerbin, and eventually fall.

[nothke]

There is nothing to "drag"¹ you in space. So therefore you can go both ways it doesn't matter. The reason why we launch rockets prograde is cause the planet's rotation gives you a "boost". There have been a few satellites that were launched retrograde though, because they orbit, relative to the surface of Earth, faster.

When you are orbiting the Earth, either-grade, you are still orbiting the Sun together with Earth, therefore, there is no difference. The moment you do your escape burn you will change orbit around the sun. Therefore, it is always much more efficient to launch prograde because of the boost.

I would compare this to asking would I get from seaport A to seaport B if I walk around the ship's decks clockwise, or counter-clockwise?

¹Not counting minimal drag caused by high atmosphere, which is possibly greater in retrograde orbit, but still negligible.

Edited December 14, 2013 by nothke

[Superfluous J]

Other than using terms like "drag" and "bungee cord" I think you're essentially correct. You also say "revolution" when I think you mean "orbit." Revolution is generally used to refer to the planet spinning.

While you're orbiting Kerbin (in either direction) you spend about half the time going faster than Kerbin around Sun, and half the time going slower than Kerbin around Sun. There is nothing special about it, it's just the way it works.

When you're driving down the freeway at 100kph, the very bottom of your tires are moving at 0kph (unless you're skidding!) and the very tops of your tires are heading at 200kph, for pretty much the same reason.

[Kerbal01]

That's a pretty standard miscondeption. It doesn't take a force for something to keep going. If Kerbin pulls on object that's trailing it around Kerbol, that object is accelerating towards Kerbin. It's not going to just hang a constant distance behind.

The reason you are thinking of it in terms of bungie is that usually, when you need to pull something, there is friciton or drag, so you have to constantly supply force to keep the object moving. E.g. dragging a boat by the rope. But there is no friction in space. For an object to move behind Kerbin at the same speed, there has to be no force acting on it. It has to be coasting completely free. But there is pull of Kerbin, so it's going to accelerate towards Kerbin, and eventually fall.

This misconception was also touched on in one of the books in the nasa trilogy by Stephen Baxter.

[Tex]

Alright, alright, already. I had known there would be a time when I'd ask a question that was blasted.

[DonLorenzo]

Apart from some muddled terminology you're completely correct.

Consider this, aerobraking at Duna after a hohmann transfer isn't really braking.. it's the planet crashing into you, speeding you up to its orbital velocity. You just use the atmosphere as a cushion because the rock is too hard

[Brotoro]

You also say "revolution" when I think you mean "orbit." Revolution is generally used to refer to the planet spinning.

The term 'rotation' is used to describe a planet spinning on its axis.

The term 'revolution' is used to refer to a planet moving around the Sun (or a satellite moving around a planet or moon).

In the Gemini days, NASA used the term 'revolution' to refer to the spacecraft making one complete path around the Earth and back to the starting longitude. They used the term 'orbit' to refer to the spacecraft making one complete path around the Earth relative to the background stars (so one 'sidereal revolution,' really). One Gemini orbit therefore took less time that one revolution (90 minutes vs 96 minutes) because of the rotation of the Earth (which carries the starting point further to the east during the revolution of the spacecraft).

Edited December 15, 2013 by Brotoro

[Superfluous J]

The term 'rotation' is used to describe a planet spinning on its axis.

The term 'revolution' is used to refer to a planet moving around the Sun (or a satellite moving around a planet or moon).

Weird. I did not know that and it's counter-intuitive when you think of RPMs and whatnot describing things rotating around an axis. But I guess NASA needed a word and they were already using "rotation" for that.

[AmpsterMan]

The term 'rotation' is used to describe a planet spinning on its axis.

The term 'revolution' is used to refer to a planet moving around the Sun (or a satellite moving around a planet or moon).

In the Gemini days, NASA used the term 'revolution' to refer to the spacecraft making one complete path around the Earth and back to the starting longitude. They used the term 'orbit' to refer to the spacecraft making one complete path around the Earth relative to the background stars (so one 'sidereal revolution,' really). One Gemini orbit therefore took less time that one revolution (90 minutes vs 96 minutes) because of the rotation of the Earth (which carries the starting point further to the east during the revolution of the spacecraft).

So that means that a ship in a retrograde orbit has the same Orbital Period, but a different revolutional period... I'll have to find a way to use this somehow.

[Mr Tegu]

The reason you launch pro-grade to the planet's rotation is because you get the rotational velocity of the planet (surface velocity in game) added to your velocity for free, if you launch retrograde you have to overcome this velocity before you achieve orbit.

And yes, if you leave a planet's gravity well retrograde to its orbit you have slowed down relative to the star. This causes you drop down further inside the star's gravity well causing you to speed up again.

Edit: I've basically just repeated what others have already stated in the thread, hopefully a little more clearly otherwise I've contributed nothing useful..

Edited December 15, 2013 by Mr Tegu

[Beowolf]

There have been a few satellites that were launched retrograde though, because they orbit, relative to the surface of Earth, faster.

I didn't know about any retrograde launches. Thanks, nothke!

[3_bit]

Well... Decelerating relative to Kerbin's rotation. But once you completely counteract it it'll be accelerating again.

[rdfox]

NASA still uses the revolution vs. orbit terminology, IIRC; it's critical to be able to distinguish between the two, since orbital velocity and surface-relative velocity are significantly different thanks to planetary rotation.