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Gravity turn


SnappingTurtle
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I finally looked up what a gravity turn actually is. It seems it's the use of gravity to turn the rocket so that the engine gimbals are not required and the thrust vector stays aligned with the center of mass for maximum acceleration. Does this mean that for an efficient launch I should start the turn by disabling SAS, tipping some degrees off the vertical and waiting for gravity to pull the rocket to the horizontal?

Assuming a zero-lift rocket, surely any vertical velocity gained will, in the end, not be converted into tangential velocity. Is this true and if so does this mean that if I continue to launch as I have been by using the gimbals that the gravity turn is not providing any benefit versus burning vertically then again tangentially at apoapsis?

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I finally looked up what a gravity turn actually is. It seems it's the use of gravity to turn the rocket so that the engine gimbals are not required and the thrust vector stays aligned with the center of mass for maximum acceleration.

... does not compute ...

Actually the gravity turn is when you switch from straight vertical ascent into a ballistic trajectory. In zero atmosphere, you can launch directly into a ballistic trajectory, but this doesn't make sense in atmosphere that will slow you down, hence the initial vertical ascent phase.

Does this mean that for an efficient launch I should start the turn by disabling SAS, tipping some degrees off the vertical and waiting for gravity to pull the rocket to the horizontal?

No, you need to turn it yourself. First go to about 45 degrees at 10 km (if launching from Kerbin), then stay a bit "before" the prograde marker and let it chase you.

(I realize this might not be the most efficient way to do it, but it works just fine for me.)

As for turning off SAS, this is only necessary in KSP < 0.21.

does this mean that if I continue to launch as I have been by using the gimbals that the gravity turn is not providing any benefit versus burning vertically then again tangentially at apoapsis?

Well orbiting means to go around the planet sideways, so this is were you want to go. By launching straight up, then turning 90 degrees to the side will cost you more fuel than "bending" your trajectory to the side while going upwards by gradually tipping to the side.

Edited by blizzy78
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Does this mean that for an efficient launch I should start the turn by disabling SAS, tipping some degrees off the vertical and waiting for gravity to pull the rocket to the horizontal?

Ideally, yes. However in practice, the turn will almost always require some steering corrections (unless you plan the turn and your rocket perfectly or get really lucky). Using gimballed engines helps give your craft control authority, but does not negate the benefits of performing a gravity turn. If you were to burn directly vertical until the craft is beyond the atmosphere and then turn it horizontal and add velocity, you would see huge fuel losses fighting gravity the whole way up instead of allowing it to bend your path and add horizontal velocity for you.

Assuming a zero-lift rocket, surely any vertical velocity gained will, in the end, not be converted into tangential velocity.

Some of the vertical velocity will be converted by the gravity turn and some won't. The velocity that isn't will become what we refer to as "gravity losses." They can be minimized, but never eliminated entirely. Don't ask me about the math, I'm not that proficient yet. :)

Edited by Tarmenius
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If you were to burn directly vertical until the craft is beyond the atmosphere and then turn it horizontal and add velocity, you would see huge fuel losses fighting gravity the whole way up instead of allowing it to bend your path and add horizontal velocity for you.

The most extreme example of this would look like this: Go all the way straight vertical up to 100 km, then turn east, then burn for orbital velocity (about 2.2 km/s). You can clearly see the disadvantage of doing that.

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On the subject of G turns I've always performed mine to the east (this was how I learnt it from tutorial videos) - do you HAVE to turn east or could you perform the turn to the west, north or south and suffer no detrimental effects (other than your orbit path being different)?

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The reason why everyone launches to the east is because you get a little boost from the planet's rotation which is also traveling East. In practice Retrograde launches (those launching to the west) take a bit more Delta V to achiveve because not only are you not getting the little boost from the planet's rotation, you actually have to kill all of that initial velocity the rotation gives you. That said it's not a whole lot extra as far as launch delta v is concerned, I belive it's only about an additional 250m/s total, just be aware that your assent stage should have that little extra amount of fuel to compensate.

As for Gravity turns, for the most part the sooner you get horizontal the better, just be aware that planets with atmospheres do slow you down. Typically since I use FAR I pitch over about 5 to 10 degrees at the 1000 meter mark with the goal of being completely horizontal by about 35 to 40,000 meters up.

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Not quite, a gravity turn is when you start turning over (usually east to take advantage of the planet's rotational speed) and begin moving your trajectory, like blizzy said, into a ballistic one. You add less speed to your vertical ascent and more towards horizontal so you can create that falling-so-fast-forwards-that-the-ground-moves-out-from-beneath-you effect. You must manually tilt your rocket so that you balance altitude gain and horizontal speed gain. In the end, you loose nearly all of your vertical speed, even though you're still accelerating because you just are adding more horizontal speed. You absolutely do not want to begin your horizontal burn only when at your apoapsis. You want to have a gravity turn and gradually tilt over to the horizon line.

Scott Manley claims that the target altitude to begin the G turn is 10,000 m, but I believe this to be untrue. If your rocket has a low T/W, and may be underpowered, you will need to turn later because you will not have the appropriate apoapsis. I believe the ratio between your altitude and apoapsis should be 1:3.

When you begin your turn, do not tilt further than the 60 degree mark, or halfway to the 45 degree mark. You should not go down to 45 until you have an apoapsis of around 40-50 km, depending on your T/W. When you have reached your desired apoapsis, I usually stop burning and wait until t- 50-30 seconds (depending on T/W) from apoapsis to burn at the horizon and create a stable orbit. I am sure it is not the most efficient method, but it always works for me.

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Thanks for the replies but I wonder which of you actually knows what you're talking about as opposed to just thinking you do. That's not meant to be insulting, I ask because I don't know what I'm talking about either though I thought I did. What makes me question the gravity turn is this line from the Wikipedia entry on gravity turns.

Firstly, the thrust doesn't need to be used to change the ship's direction so more of it can be used to accelerate the vehicle into orbit. Secondly, and more importantly, during the initial ascent phase the vehicle can maintain low or even zero angle of attack. This minimizes transverse aerodynamic stress on the launch vehicle, allowing for a lighter launch vehicle.

I think the second point doesn't apply in KSP. The first point seems to be saying that the benefit comes from not needing to gimbal the engines.

What I mean when I say that vertical velocity would not be changed to tangential velocity for a zero-lift rocket is that there's no component of one in the other, geometrically.

The most extreme example of this would look like this: Go all the way straight vertical up to 100 km, then turn east, then burn for orbital velocity (about 2.2 km/s). You can clearly see the disadvantage of doing that.

I thought about this, but I know that it takes about 5 or 6 km/s of delta V to reach LKO anyway, and I don't know that it takes more than about 2.5 to reach 100 km by launching vertically.

Again, thanks for the replies, but I'm aiming to hear from someone who - unlike me - didn't also learn this term *after* getting this game.

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In real life, aerodynamics of a rocket ascent are more important than in current stock KSP. The zero-lift gravity turn is used in real life to reduce aerodynamic loads on the structure of the rocket, since rockets are built to be as light as possible while holding a given amount of fuel. We don't have to worry about this so much in KSP right now, so our "gravity turns" can have significant angle of attack, which in some cases (unusually high or low TWR's) can lead to better fuel efficiency given KSP's atmospheric model.

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The gravity turn should start after 5k, do more at 10k, gradually get to 45* at around 20k and be on the horizontal marker by the time you reach 70K. (I usually go for 100K.) As always, practice and find out what works best. The turn should be gradual rather then in big steps to be most efficient.

You will probably have to do this in two stages.

1. Get the apogee to the desired altitude with the gravity turn.

2. Cut power and coast till you are about 30 second short of apogee.

3. Burn horizontal till you get orbital paragee of at least 70K.

4. Watch that the apogee doesn't take off to Mun while you are doing that final orbital burn. That will waste fuel and may cause you to run out before achieving stable orbit.

As always, practice, practice, practice.

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If we had as fine a control over the initial engine gimbal as real-world rockets do, we could probably do a gravity turn like that. Since we don't, we have to be more involved in our ascents and can't just let the planet pull our ship over by itself.

Also, make no mistake, if KSP had a realistic aerodynamic model, then most of the nonsense in this thread would be more apparent to more people. More proper gravity turns would practically be forced on us. FAR doesn't let you get much more than 5 degrees or so outside of prograde before bad stuff starts to happen.

Finally, getting to LKO takes about 4500 m/s.

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Is there any way to actually calculate this for a specific rocket?

In all the topics about gravity turns usually everyone just reports their specific experiences with their own rockets. I'd really appreciate some general guidelines or even better, a calculation method.

(currently I'm designing a heavy launcher without asparagus staging and only liquid engines, 100m/s dV make or break the design)

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