When to start Gravity Turns

[thereaverofdarkness]

I find that gravity turns only save me maybe 10% fuel at best, and I'm talking about rockets that launch at approximately 1G initial acceleration. Since it's not a lot, I like to keep it simple. For rockets that are agile and can turn easily enough, I go straight up till 10km, then turn to 45º prograde (right side from initial facing direction, going in the direction of Kerbin's rotation) until my apoapsis is just above the atmosphere (1000m/s surface velocity if I have the map closed). Then shut off engines and coast to appoapsis, where I point straight level (90º) and circularize the orbit.

When my ship is too rigid or flimsy, I just burn straight up until I have the apoapsis above the atmosphere (with enough extra to turn this sluggish beast before apoapsis), then coast up, turn slowly, and thrust to circularize. It doesn't seem to waste much more than trying to get into a retrograde orbit will. The time you save not overthinking this can be spent launching more payload and/or fuel.

[TronX33]

Wow I'm an idiot. Before I just tipped all the way over to 45 degrees at 20km. Now, (it's probably also inefficient) I tilt to 5 degrees at 5 km altitude, 15 degrees at 10km, 25 degrees at 15, then 45 at 20 km. You guys must think I'm horrible.

[cantab]

Necro much?

[Sirine]

Wow I'm an idiot. Before I just tipped all the way over to 45 degrees at 20km. Now, (it's probably also inefficient) I tilt to 5 degrees at 5 km altitude, 15 degrees at 10km, 25 degrees at 15, then 45 at 20 km. You guys must think I'm horrible.

You are digging up Sep-2013 thread. Anyhow if you are not using Mechjeb.

Here are "my-way" of doing the gravity turn.

1) Click on the 'surface' label on top the navball, so it turn to 'orbit' at launch pad. It now shows 90 degree.

2) Launch your rocket. On the ascends, the 90 degree marker will slowly going upward. (See for yourself).

3) Once the marker hit 45 Degree. Start turning your craft "press D", slowly, and "maintain-the-marker" for 45 Degree. (Turn your craft "Press D or A" to maintain it.)

4) Until you reach the altitude high of your desire, Stop engines, and wait till the marker slowly move down to 90 degree again.

5) Fire your engines, and "maintain-the-marker" at 90 degree. (Turn your craft "Press D or A" to maintain it.) Until you are circularizes orbit to Kerbin.

If you understand the logic behind, congratulation, you have understand on how to read the navball. And it apply to almost ALL situation, including RSS. (Real Earth Scale mods).

Read The Navball.

Read The Navball.

Read The Navball.

Read The Navball.

Read The Navball.

Edited August 22, 2014 by Sirine

[Vectura]

I tend to go for ~14 k and up. Any lower and my rocket just starts to drop back to kerbin when I start turning. I normally go for orbits around 500-600km.

[Dispatcher]

If you've got a ridiculously overpowered rocket running at full throttle, starting your turn lower than the commonly used 10K altitude will allow your ship to shed some of its excess velocity due to the drag experienced in the lower atmosphere. However, if you have the opposite problem; a ridiculously underpowered rocket, chances are that you will not achieve orbit regardless of how you fly or when you turn.

I've experimented with turn altitudes and for reasonably capable ships (TWR between 1.1 and 2.0) and dV of at least near 4500 m/s, starting my turn tends to work well between 10K and 12K altitude.

These comments have been interesting reads, even if most are old.

[cantab]

I have found that rockets with low TWR upper stages need a steeper ascent profile, which I believe is to reduce drag losses when said upper stage is firing. I still start the turns at 10 km but might only pitch to 60 degrees or so.

[Jett_Quasar]

The standard practice is to start at 10Km, but basically for the best efficiency you need to turn as soon as your rocket has enough acceleration to push against gravity while still gaining forward momentum. It has to do with vector graphics. The least efficient is burning straight up, then once you hit altitude turn 90 degrees and burn horizontal - so what you want to do is cut that corner as much as possible. In simplest terms that means burning at a 45 degree all the way, but it's also complicated by the atmospheric affects and the curvature of the planet.

There are diagrams that you can look up for optimal gravity turns on different planets, but the rule of thumb I use it to get your rocket up to about 200m/s and an acceleration of about 1.5g, then do a 45 degree turn after which you slowly nose down to horizontal as your craft burns off fuel and increases acceleration.

JR

[rkman]

what you want to do is cut that corner as much as possible. In simplest terms that means burning at a 45 degree all the way, but it's also complicated by the atmospheric affects and the curvature of the planet.

On planets/moons without atmosphere it is most efficient to turn to horizontal as much as you can without hitting mountains, that way gravity losses are minimized.

Ascent on a body with an atmosphere is a compromise between that and minimization of drag losses.

[GoSlash27]

I start my gravity kick earlier than most here; 7 KM. I then let it rotate slowly and try to hit the following marks:

68* pitch at 15KM

45* pitch at 25KM

30* pitch at 35KM

Best,

-Slashy

[Dispatcher]

... It has to do with vector graphics. ... JR

I don't think you meant "graphics"; perhaps you meant "thrust vectoring."