Preforming gravity turns

[M1_A4_Abrams]

So I've been playing KSP since just after 1.0.0, but I haven't been on the forums until just a few days ago. Hi, I guess? 

Anyways I've got a decent handle on most aspects of spaceflight but I've always had problems during launch/ascent. I mainly just over design most vehicals and brute force my way through atmo before starting my turn at 40k~60k meters. Obviously thtas not very efficient. Can anyone help me figure out a better ascent path or is the optimal path entirely dependent on the vehical design?

 

edit: Wow, I was expevting one, maybe two responses, not eight. Thanks for all the info.

Edited February 18, 2017 by M1_A4_Abrams

[foamyesque]

16 minutes ago, M1_A4_Abrams said:

So I've been playing KSP since just after 1.0.0, but I haven't been on the forums until just a few days ago. Hi, I guess? 

Anyways I've got a decent handle on most aspects of spaceflight but I've always had problems during launch/ascent. I mainly just over design most vehicals and brute force my way through atmo before starting my turn at 40k~60k meters. Obviously thtas not very efficient. Can anyone help me figure out a better ascent path or is the optimal path entirely dependent on the vehical design?

 

The exact flight path does depend on vehicle design -- usually the higher the TWR you maintain during a launch, the sooner and harder you turn, limited by things like thermal effects -- but in general you want to smack your rocket five degrees or so eastward as soon as you get up to around 50m/s, and then more or less ride your prograde vector until you're horizontal.

[bewing]

Hi?

Turning at 40k - 60k isn't as inefficient as people make it out to be. But yes, you can certainly do better. The optimal path depends on the TWR of your rocket during the first 10km of ascent, mostly. But unless you are using a mod such as MechJeb to fly your rocket for you (and it doesn't sound like you are) -- then the very best you can do is either guess, or try the same launch repeatedly to see how much guidance you need to provide.

So there are basically two ways to do it.

1) use launch clamps on the pad to pre-tilt your rocket to the proper angle. You need a pretty stiff rocket for this to work, and your clamps should be around the CoM if possible. Turn on SAS before launch, and immediately click SAS to prograde when you have 1 m/s of velocity.

2) before launch, turn on SAS. Launch vertically and immediately click SAS to prograde. Tap/mash the D key to get your rocket to turn a couple degrees to the East. This may cause some roll (and yaw into the N/S direction) -- which you really can't fix.

After that, the ship will gravity turn by itself with SAS on prograde. The only question is whether it's turning too little or too much. I like to be at about 45 degrees by 8km, because I usually have a lot more TWR than most of these guys use. If it's turning too much, then click SAS to Stability for a bit. If it's too little, then either re-launch, or make a note to turn a little more at liftoff next time. Once you are around 25km, if you want to be more horizontal than you already are, you can click the guidance mode from Surface to Orbit earlier than it would change automatically.

 

Edited February 17, 2017 by bewing

[HebaruSan]

First, you need aerodynamic stability so you can steer without flipping. This is accomplished in two main ways, adding mass at the front, and adding drag/lift at the back. A set of 4 fins at the bottom chosen to fit the rocket size will generally do the trick. (I think real rockets rely more on gimbal, but SAS isn't quite up to that most of the time.)

Then, I tend to think of the part of the navball between vertical and 45° as a progress bar; it should be at 0% at launch and 100% when you get to an altitude of 10 kilometers (picture in linked post below). Go vertical for a short while after launch, to maybe 1 km / 60 m/s, and then tilt slightly to turn your prograde vector a few degrees, then lock prograde and make small adjustments to fit the "progress bar" metaphor. You can safely be horizontal by 50 km most of the time.

 

Edited February 18, 2017 by HebaruSan

[A_name]

Have a look at my gravity turn tutorial. I haven't properly updated the thread for 1.2 but the methods described still work:

 

 

[astroheiko]

Hello and welcome to the forum.

I am certainly not the best to answer this question, as there will be some really experienced players give answers. But I'll try it. Edit: already happened, i was to slow

In order to create an effective start, it is important to compensate as soon as possible for the losses caused by the gravity. It would be ideal if you could start right in the direction of Hörizont. Of course this is not possible.

I always do this:
At first -The rocket should have a TWR of 1.3 to 1.4 on the launchpad. TWR means thrust weight ratio. The rocket does not lift under 1. There are mods that show this - Mech Jeb and Kerbal Engineer Redux (KER).

Just switch the SAS on, just the stability assistant.
Hit Space and let the rocket rise. When the rocket reaches about 20-30 m / s, I tend to tilt it somewhat towards the east. Approximately 7-8 degrees - a little estimate.
At about 50 m / s I switch the SAS to Prograde. The rocket always follows the marker. Over time, the flight curve becomes flatter, almost like in a real gravity turn.
I always set myself as a target at about 300 m / s at 45 degrees above the horizon and 10000m high.
If you are too flat, click on prograde later, maybe at 75 or 100 m/s.

At an altitude of 15000m and an apoapsis of 25000m I switch from Surface to Orbit.
With an apoapsis of 75000km I switch off the engines and wait until I am at the apoapsis. Then burn the periapsis to over 70000m.

[Tex_NL]

I have often used a rough rule-of-thumb based on the atmosphere gauge at the top center of the screen.
Start your gravity turn early and only make small gentle manoeuvres. When the atmosphere gauge needle passes from light blue to blue you should be at roughly 60 degrees. When it passes from blue to dark blue be near 30 degrees. When passing from dark blue to black be close to zero.

Keep it mind this is a very rough rule-of-thumb but it is very easy to remember and is a reasonable approximation.

Edited February 17, 2017 by Tex_NL

[paulprogart]

So all of the answers thus far are valid, but they refer to math stuff like TWR, or physics stuff like aerodynamics, etc.  I'm going to offer an alternative approach that does none of that!  This is the Average Joe method; the intuitive approach; the "I'm playing this game for fun and don't want to have to learn a bunch of physics and math up front to enjoy myself" technique.

So first, try the Orbiting tutorial in the game.  (Assuming you're now on 1.2.x and not actually still using 1.0.x.)  Repeat it a few times if needed.  It will give you a feel for how to do the proper turn.

The tutorial is great, but the only problem with it is that it's a real nasty tease because it gives you the manoeuvre marker to follow, which of course is nowhere to be found once you try it on your own.  So how do you actually know when and how fast to turn?

When you're first learning a skill you typically want a very simple method even if it's only approximate.  So when I started I just made up a rule-of-thumb based mostly on watching the marker in the tutorial:  Start turning at 60 m/s, and keep the marker at about 10 degrees for every 120 m/s total.  (i.e. 10 degrees by 120 m/s (it's a bit sharp to start), 20 by 240, etc.)  Until you get to around 60 degrees, then hold it there until the Ap is high enough for orbit.

I did dozens of launches using nothing more than this simple guideline.  I'd say it works for pretty much everything you can do in at least the first 1/3rd of career mode, assuming you aren't getting carried away with what you're launching (e.g. too heavy, too unaerodynamic, etc.).  Yes it will waste funds, fuel, and time, but that's part of the learning process.

It will also fail quite badly at times, especially as you get more ambitious, but the nice thing about simple rules is that once you start to see where they go wrong you can enhance and adapt them as you go.  E.g. you might find you need to hold at 50 or 70 degrees to get the Ap up high enough depending on the vessel.  This is indeed largely because of TWR, but you don't even need to know what TWR is or how to calculate it.  All you need to know is "it's a 50-degree vessel" or "it's a 70-degree vessel".

Eventually all of the permutations will start to get too complex (this is rocket science, after all) and you'll wonder "there has to be an easier way" and that's where you'll probably want throw it away and start going for the math and physics stuff to design your rockets more efficiency - the game will have tricked you into wanting to learn the hard stuff!  But by then you'll also be an expert at the gravity turn, so you can shift focus to your designs.

P.S. For bonus points, don't trust my rule-of-thumb, or anyone else's.  Make up your own.

[Tex_NL]

1 hour ago, paulprogart said:

P.S. For bonus points, don't trust my rule-of-thumb, or anyone else's.  Make up your own.

Totally agree. Don't take any rule-of-thumb for granted. But you should give them an honest try none the less. See what works and what doesn't. Then draw your own conclusion.

[Norcalplanner]

I did a couple studies in 1.1 regarding ascent profiles in both stock and in RSS.  Here's the link to the stock version:

Here's the takeaway from the OP in the linked thread:

1. Launch with an initial TWR of at least 1.4.  Piloting gets more difficult when it's over 1.7.

2. Don't worry about drag - gravity losses are much larger and more important, unless you reach orbital velocity below 30 km..

3. Ignore the flame effects.  Pay attention to the temperature gauges.

4. At higher thrust levels, crank it to the east immediately after launching, but be precise about it.

5. Try to keep vertical velocity below 500 m/s.  If it's over 700 m/s, you're going to have noticeably higher gravity losses.

6. Getting a rocket to orbit in stock for less than 3,200 m/s of delta V is very doable.  Using less than 3,100 m/s is a harder but still achievable goal.