Why turn so early during ascent?

[THX1138]

When I launch a vehicle capable of quick and controlled turning, I'll usually keep it oriented vertically until past 15 km and sometimes even past 20 km. Why do people seem to recommend turning much earlier (as early as 8-10 km)?

[worir4]

The sooner you start turning, the less you loose delta-v due to gravity pulling you down. However you have to find a balence as you don't want to waste fuel flying in thick air. I find a more gradual turn is best.

[OhioBob]

I start turning at about 5 km, which is early then what most people seem to do. The reason is to make it a slow gradual turn and to keep the angle of attack small. (The angle of attack is represented by the angular distance between the level indicator and the prograde indicator on the Navball.) This is more efficient than making sudden sharp turns. Consider the following:

In figure A a right-angle turn is made using a single sharp 90-degree turn. In figure B the turn is made using two 45 degree turns. In figure C a steady gradual turn is made. In all cases the total ÃŽâ€V provided is 3000 m/s (black). However, you can see that the resultant velocity vector (red) is greatest with a slow steady turn.

By starting the turn early and keeping the angle of attack as small as possible, a more efficient turn is possible.

[xrayfishx]

For maximum efficiency you should start turning pretty much as soon as you launch. The "Turn at 10k" ascent path is a terrible thing that is there just to help new players get something into orbit, but really even with the stock aerodynamics this isn't very efficient. Yeah, you can get to orbit with all kinds of ascent paths, but the earlier you start turning the more efficient. Of course, I don't mean 45 degrees at launch. It should just be a very gradual turn that reaches 45 degrees somewhere around 15-20k (I haven't played stock since 0.22 at least so I don't really know that well, but it's somewhere there). Learning and using the efficient ascent path is a bad idea actually, because your eyes will bleed every time you see someone do the "10k then turn" ascent path.

Tl;dr - It's more efficient.

[Majorjim!]

When I launch a vehicle capable of quick and controlled turning, I'll usually keep it oriented vertically until past 15 km and sometimes even past 20 km. Why do people seem to recommend turning much earlier (as early as 8-10 km)?

I find its much more helpful to keep an eye on speed. I turn @ 200m/s and my rate of turn depends on the TWR. All this can be learned without using mod tools.

[Paul G]

It's to do with the trade off between how long you want to fight gravity (going straight up) verses how much air resistance you have to fight when gaining speed.

Getting into orbit is not so much about going high, it's about going sideways fast. Below 8,000ft you are in the thick bit of the atmosphere, so you want to get above that before speeding up (othwise you are wasting effort pushing fast through the atmosphere). Remember, if you go straight up, unless you can gain escape velocity, you will just fall down, not going into orbit.

Once above this height you can make better use of your thrust by turning, so you are spending less effort fighting gravity, and more effort going sideways to gain enough speed to get into orbit.

That's how I imagine it anyway.

Paul

[Sirrobert]

Alternatively, in realistic aerodynamics such as FAR, a very sharp turn will cause your rocket to flip out of control.

Starting your gravity turn early will also mean you will input most of your speed during the gravity turn, which means you don't have to burn as much in orbit to finalize the orbit. It's much easier than burning 1,3km/s deltaV at apoaps, hoping you don't lose to much altitude before you can reach orbital velocity

[GoSlash27]

^ What they said.

If you start your turn early (I generally begin around 6km), you can keep your nose pointed prograde throughout the ascent. Not only does this save DV by rounding off the corners (as shown above), but it also increases efficiency by keeping your thrust aligned with your velocity vector.

Finally, you want to perfectly balance your gravity losses with your drag losses. This profile does a good job of that.

Best,

-Slashy

[eddiew]

FAR makes it more favourable to turn early as well, since attempting a 45 degree pitch/yaw manoeuvre at 10km (the classic mislabelled 'gravity turn' in KSP) will almost always flip the rocket, unless you're already in vacuum.

Personally I put all my rockets on launch clamps and tilt them 1 notch east in the VAB, and calibrate for ~2.0 TWR. At launch I put them on prograde SAS from the start, and they gracefully get to orbit with decent efficiency and very little interraction

[Laie]

Somewhat related and worthwhile: http://what-if.xkcd.com/58/

Getting to space is easy and cheap. Staying in space requires you to move sideways so fast that you miss the planet as you fall down to it. Vertical speed will be eaten by gravity: what goes up must come down. Horizontal speed, on the other hand, is yours to keep forever. You preferably want to spend as much of your fuel on the latter as possible, while keeping the former to the bare minimum that is necessary.

On a body without an atmosphere, the most energy-efficient ascent would consist of barely getting the feet off the ground, then zooming off sideways. You can try that on one of Minmus' flats. But even on such a flat surface and with such a low gravity, you'll probably have to point your thrusters a little downwards to keep you up until you're fast enough.

[Sirrobert]

FAR makes it more favourable to turn early as well, since attempting a 45 degree pitch/yaw manoeuvre at 10km (the classic mislabelled 'gravity turn' in KSP) will almost always flip the rocket, unless you're already in vacuum.

Personally I put all my rockets on launch clamps and tilt them 1 notch east in the VAB, and calibrate for ~2.0 TWR. At launch I put them on prograde SAS from the start, and they gracefully get to orbit with decent efficiency and very little interraction

I can't believe I never tought of that. Genius, I'm stealing that idea

[eddiew]

Lol, you're welcome to do so

Most of my rockets look like this on the launchpad (terrible aerodynamics I know, this one never actually went into service). Trying to tilt them manually during ascent always introduces some roll wobble, but pre-inclining them seems to avoid that altogether and everything goes up smoothly.