when to start gravity turn on other planets

[deepspacecreeper]

hello, i'd like to know when do i have to start gravity turn when ascending from other planets or moons?

[kurja]

If there's no atmosphere, you start as soon as you clear the ground. As for taking off from worlds with atmosphere... I can't give any numbers, just go up untill past the "thickest" part of the atmosphere like you would on kerbin, then start turning

[sephirotic]

Eve: 20~23km

Laythe: 6~8km

Kerbin: 7~10km

Duna: Dunno, LOL (since the air friction is already very low, probably by 2~4km)

Any other body with no atmosphere: As soon as you can, just don't crash on the mountais.

[allmhuran]

On planets and moons with no atmosphere, there is no actual "gravity turn" to perform. The optimum launch trajectory in such cases is actually horizontal. Obviously you'll need to aim up a little bit at the start to avoid running into hills and such! But essentially, get off the ground and immediately crank that thing over to as close to horizontal as you dare.

[SaturnV]

My experience is start turn when the acceleration lost of air drag is decreased equals to gravity loss, if you have mechjeb installed. I didn't proved it though, but it works on kerbin.

[Kasuha]

On planets and moons with no atmosphere, there is no actual "gravity turn" to perform. The optimum launch trajectory in such cases is actually horizontal. Obviously you'll need to aim up a little bit at the start to avoid running into hills and such! But essentially, get off the ground and immediately crank that thing over to as close to horizontal as you dare.

Gravity turn is a maneuver which can be executed both on bodies with and without atmosphere.

If you read the definition carefully, gravity turn means that the ship is always thrusting towards its prograde vector except for short moment at the start of the gravity turn when it deviates its trajectory from vertical. But determining how much of the deviation should be introduced at the start is the whole science. Because you need it to end with circular orbit without escaping (that happens when the deviation is too small) or crashing back to the surface (that happens when the deviation is too large).

Gravity turn can be used for landing, too. In that case it is also known as suicide burn.

What people usually do in KSP is not gravity turn.

[allmhuran]

Yes, but that's *not* what you want to do in this case. Doing this would mean continuously thrusting as your rocket pitches over as a result of gravity. But that would be wasteful, because the optimum launch trajectory is, as I said, horizontal. This is why I used the phrase "crank that thing over". You want to actively point your rocket horizontal, not wait for gravity to do it. Hence no gravity turn.

[Rage097]

On Planets or Moon's with a low gravity and no atmosphere you can do a gravity turn almost immediately. These bodies include: Mun, Minmus, Gilly. And probably Ike, Vall, Pol and Bop too.

[Kasuha]

Yes, but that's *not* what you want to do in this case.

That depends on your priorities. Gravity turn is way to conserve fuel. People usually don't care much about fuel when leaving low gravity bodies because their TWR and fuel reserves are high enough. Maybe on Tylo it may play some role. Otherwise what's in KSP usually called "gravity turn" is just very rough approximation of one which is used just to avoid atmospheric drag.

Optimum launch trajectory is not horizontal. Not even close. If you launch horizontally, you crash into terrain way before you get enough speed to overcome gravity. Optimum launch trajectory starts at angle which reserves enough vertical thrust to overcome gravity. And somewhere between this maximum angle and perfectly vertical is the gravity turn which will - after starting at the right angle - bring you to orbit if you thrust prograde all the time.

That's why I am saying that what people do in KSP is not gravity turn.

[allmhuran]

Optimum launch trajectory is not horizontal. Not even close. If you launch horizontally, you crash into terrain way before you get enough speed to overcome gravity.

I guess it was somehow not obvious that I was talking about the theoretical optimum trajectory? If you read my post again (sephirotic's too), we both mention that, of course, you need to get high enough to avoid the terrain.

On a perfect sphere and with infinite TWR, perfectly horizontal is, indeed, the best launch trajectory. In any other case, the optimum launch is to be horizontal as soon as possible. In almost every case in KSP, this will mean actively turning the craft, not relying on gravity. Hence no gravity turn.

Edited October 22, 2013 by allmhuran

[lajoswinkler]

If there's no atmosphere, if the highest terrain spot is at n kilometres, and I don't know where the spot is relative to my position, I will turn horizontal at >n km to avoid collision. It's simple as that.

[allmhuran]

Yep, pretty much!

[numerobis]

Absent any obstacles, the optimal path can be to pitch slightly below the horizon: the horizontal burn has you burning along an arc, so you can get the same net effect by burning along the chord of that arc.

I'd be interested in seeing how complicated it would be to code up an actually optimal ascent that takes into account obstacles along the way.

[Moar Boosters]

On Planets or Moon's with a low gravity and no atmosphere you can do a gravity turn almost immediately. These bodies include: Mun, Minmus, Gilly. And probably Ike, Vall, Pol and Bop too.

The level of gravity is irrelevant. You can start turning almost immediately even on Tylo. The only difference is you'll need to keep the prograde slightly higher on the horizon than other moons, because you need a bit more downwards thrust to stop gravity taking hold of you and pulling you back down.

[wilt57]

The level of gravity is irrelevant. You can start turning almost immediately even on Tylo. The only difference is you'll need to keep the prograde slightly higher on the horizon than other moons, because you need a bit more downwards thrust to stop gravity taking hold of you and pulling you back down.

But gravity is important, in so far as the TWR of your ship. Assuming delta-v is irrelevant (which it isn't, of course) the same lander that can pitch over immediately on Minmus may not have the TWR to make such an aggressive pitch over on Tylo because it won't have the total acceleration to provide an adequate vertical acceleration to counteract Tylo's increased gravity while adding orbital velocity at the same rate. Because Tylo will require a higher orbital velocity than Minmus to achieve orbit, Tylo's greater gravity will have pulled that lander back to the surface before it has achieved near-orbital velocity, given it has performed a fast "gravity turn".

Assuming your Tylo lander has the same excessive TWR that a typical Minmus lander has, yes Tylo would be no different, but most Tylo landers cannot afford to bring so much thrust = engine mass.