When to start Gravity Turns

[3_bit]

I am a real pro at KSP, but have this simple question:

How do I calculate when to start my Gravity-turns in KSP? I typically aim for a 75x75km or 100x100km orbit, and want to make somewhat more efficient rockets.

If I'm flying a heavy slow one I'll wait until ~17km if I really need the vertical speed, most I start at 11km, and the lightest at 10km.

Expert-level question I guess.

[The Jedi Master]

Easy, at 10,000 meters.

[astropapi1]

it depends on your rocket. If you're on a fast rocket, start your gravity turn at 9.5 -10 km, but if you're on a slow one, start turning very slowly at ~15, so you're gaining horizontal speed while gaining vertical speed.

I may recommend tipping over just a bit after takeoff.

[RoboRay]

You never "need the vertical speed." If you produce enough horizontal speed, it will quickly become vertical speed as the curved surface drops away beneath you. And you need the horizontal speed... you're going to have to burn to produce it later if you don't go ahead and do it now. You may as well build speed and move your Ap upward with the same fuel, unless you've simply got a lot of extra fuel to get rid of.

At 10km, you've left 90% of the atmospheric drag behind. Continuing upward at the expense of horizontal velocity is counterproductive. Go ahead and start a gradual turn, keeping your nose within a few degrees of your prograde marker for maximum efficiency.

Edited May 16, 2013 by RoboRay

[UmbralRaptor]

Calculating the best time is... non-trivial. How good are you at numeric integration?

I tend to start at ~5-10 km.

[harharluke]

I use MechJeb for ascent (Feel free to harass me) and it defaulted to 12km for my gravity turns and it works well as it's when my launcher's first stage is ejected

[Fyrem]

between 5 and 10k is generally what everyone says is right. It sort of depends on the weight and isp of your specific rocket as to when in that window you would want to turn. (however, 10k is a good baseline if you want just one number).

Also, depending on if you have a stage separation at lower alt (say, between 5k and 10k) you may not want your craft making hard turns at the moment of separation, so turn AFTER any delicate separation event.

[Hooligan Labs]

I use MechJeb for ascent (Feel free to harass me) and it defaulted to 12km for my gravity turns and it works well as it's when my launcher's first stage is ejected

I think this is based on atmospheric density. It starts around when you are past the "thickly" colored part of the atmosphere gauge.

Atmosphere is a real drag.

[Guest]

I typically start the gravity turn at ~8km. It seems to be the sweet spot for the majority of my designs.

[JaySmoka]

I wait to be at least a 150 m/s to begin my gravity turn. Slow rockets mean higher, fast rocket mean lower.

[Conarr]

I am a real pro at KSP, but have this simple question:

How do I calculate when to start my Gravity-turns in KSP? I typically aim for a 75x75km or 100x100km orbit, and want to make somewhat more efficient rockets.

If I'm flying a heavy slow one I'll wait until ~17km if I really need the vertical speed, most I start at 11km, and the lightest at 10km.

Expert-level question I guess.

It really depends on your rocket, but I conducted about 25 test flights last night using MechJeb to create consistent data. For the rocket I used going to a 100km circular orbit, starting the gravity turn at 11km into a 0 degree orbit topping out the turn at 70km with a 30% shape yielded the best results. I'd reckon anywhere close in there would be okay. I can post the results in their entirety when I get home if you're dying to see something REALLY boring

[3_bit]

Easy, at 10,000 meters.

Absolutely wrong. Scott Manley said it varies per Launch Vehicle but is mostly at 10km. There's more science than that. Sure, 10km is standard, but I'm looking for the science and formulas behind it.

[Francesco]

I don't know if starting a gravity turn at 17 km qualifies as being "a real KSP pro".

the usual recommended gap on Kerbin is between 8km (where atmospheric pressure is ~0.2) and 12 km (~0.1).

average = 10km.

[Eric S]

I usually start very low, 1k or 2k, but don't tip more than a few degrees until I get over 10K, but that's so that I don't drop things on the launch pad rather than efficiency.

Yes, it's going to depend on the craft, though if you're not doing anything too unusual and have a "normal" TWR, 10K is the normal place to start it.

If you install FAR, this changes, but mostly in that you start your turn a bit sooner because you can't turn very far away from the prograde without causing instability in the rocket.

[Giggleplex777]

I usually gravity turn when I hit more 200 m/s. I try to stay below terminal velocity in the thick parts of the atmosphere (though most of my rockets don't even get close to it).

If you are really a pro, try to get this rocket to 73 km orbit and deorbit the booster stage after you reach orbit:

Here's the .craft file

Here are the timings to jettison engines (MET):

1. 1:10

2. 1:40

-decouple-

3. 3:15

4. 3:45

Press 0 to active the retro rockets on the core booster after you decouple and switch to it.

Make sure to do a very good gravity; it can't get to orbit if you don't.

[lobsterbark]

I just build a really small rocket, thats max cargo is about .75 tons, and gave it a 1.2 ton payload. Then I edited the ascent on mechjeb until I found the optimal way to get into orbit, just stay at terminal velocity the entire time, if you can, and start turn at 12 kilometers. Set your turn steepness at 20%, and that seems to work the best.

[Nuke]

i usually start a slow rotation from 90 to about 35 degrees starting at about 12000m or whenever my first stage runs out (not counting any boosters attached) and i time it so that by the time im out of the atmosphere im at 35 degrees. this manuver is made easy because joystick. then as soon as i cross 100km il flatten out to 0 for orbit. the only exception is if i have a slow upper stage such as ion powered, or a heavy nerva ship. in which case i usually stay at 35 (or sometimes higher) for most of the orbital insertion.

[Daze]

Stay under 170 m/s under 7500mt, then start gravity turn and increase gradually thrust

[numerobis]

MechJeb 1 defaults to 10 km, MechJeb 2 defaults to 5 km. I tend to get best results at 8 km.

The optimum turn shape has you pointing at the prograde marker all the way down to the horizontal. If you ever thrust below the prograde marker, that means you wasted some thrust boosting your vertical speed too much.

[RoboRay]

An answer right in the middle of the reasonably optimal range for all but the most unusual designs (8 to 12km) is certainly not "absolutely wrong" Mr. "Real KSP Pro."

But if you want to be absolutely wrong, keep calling it a gravity turn. What you and pretty much all KSP players do is in no way a gravity turn, it's simply a pitch maneuver (assuming you even bothered to orient the craft in a standard flight attitude where pitch controls your climb angle).

A real gravity turn starts as soon as you leave the launch pad.

Edited May 16, 2013 by RoboRay

[3_bit]

An answer right in the middle of the reasonably optimal range for all but the most unusual designs (8 to 12km) is certainly not "absolutely wrong" Mr. "Real KSP Pro."

But if you want to be absolutely wrong, keep calling it a gravity turn. What you and pretty much all KSP players do is in no way a gravity turn, it's simply a pitch maneuver (assuming you even bothered to orient the craft in a standard flight attitude where pitch controls your climb angle).

A real gravity turn starts as soon as you leave the launch pad.

EARM... UH.... I mean I cheat and use launch clamps to raise it that high.... Yea that's right!

[Merinsan]

There is no simple answer.

I have a variety of ships I launch on a regular basis and they all have different launch profiles. However, I never start turning before 10km, simply because I am getting out of the think atmosphere. I also like to get to 200m/s before I start - some of my heavier ships don't get that until 11-12km. The main reason for waiting is because of the game mechanics - it's hard to do gradual turns, so I end up turning a bit later than I should.

[Francesco]

<snip>

(assuming you even bothered to orient the craft in a standard flight attitude where pitch controls your climb angle).

<snip>

since you brought this up... why in the world do rocket face west while building in the VAB, while they face north on the launchpad???

I have to turn every damn ship of 90° (Q) before launch.

[RoboRay]

I kind of like it that way. It gives me an excuse to do a little more flying, with a shuttle-style roll maneuver.

[sumghai]

MechJeb 1.9.8's default of 10km seemed to work well for me - when I upgraded to 2.0.7, however, the default became 5km, so I set it back to the 10km I was accustomed to.

So it appears that, at least for Kerbin, anywhere between 5~10km is good for the general case.

Edited May 17, 2013 by sumghai