Best ascent profile

[uncle_jew]

Hello,

this question has been probably asked 1000 times before: What is the best ascent profile from Kerbin?

I searched for answers, and most answers are like "turn 5-10 degrees after reaching 100m/s, then do a gravity turn". But the gravity turn will work very differently after tilting 5 and 10 degrees! It also depends a lot on whether or not I throttle down at max-Q.

Is there a detailed table (or a calculator) somewhere for e.g. the best Kerbin ascent profile? Like:

altitude speed pitch

altitude speed pitch

altitude speed pitch

...

So that you could actually follow a trajectory, instead of blindly tilting "5-10 degrees" and hoping for the best?

Also, how do you figure out the total delta v your rocket produced? Is a mod required for this?

[cantab]

What you describe is my generic ascent - vertical to 100 m/s, pitchover, then hold prograde. How much the pitchover needs to be indeed depends on the rocket, so I just use a bit of trial and error. In general the higher the TWR the more aggresive the pitch over.

[Retread]

I agree that the initial nudge has a great deal to do with how a gravity turn will turn out. You can control the rate at which your rocket tips towards the horizon to some degree with the throttle, however. As you ascend, keep a close eye on your TWR; you'll need a mod like mechjob or kerbal engineer for this. (They will also calculate your delta V for you). If you find the nose of the ship not dropping to the horizon fast enough, throttle back to reduce your TWR. If it is dropping too quickly, increase your throttle. This technique does not give you a huge amount of control, and will not overcome poor design decisions, but it does allow some control over the flight path.

[Taki117]

A well designed rocket, (and I mean on the order of NASA level design) once you pitch over ever so slightly, you can turn off SAS and it will perform about as optimal an ascent as you can get. Cantab is right, pitch at 100m/s, then follow prograde (or depending on your TWR keep slightly below prograde) and you will be fine.

[Reddeyfish]

rocket delta-v can be calculated manually, but most people use Kerbal Engineer.

[Alshain]

I just typed this the other day so forgive me if I don't want to do it again lol. The thread I'm linking has a lot of good info in it anyway. It does differ based on the rocket, but it's not hard to design all your rockets to be relatively consistent and you should do that. None of my launches will ever be exact, you would need Mechjeb or kOS to do that. But the procedure in that post has a list of 'milestones' that I try to be somewhat close to. You don't have to be perfect.

http://forum.kerbalspaceprogram.com/threads/137567-Why-is-creating-a-Kerbin-orbit-so-unpredictable?p=2262719&viewfull=1#post2262719

Edited October 27, 2015 by Alshain

[aluc24]

I have a generic ascent profile which works well for me: pitch down 2 degrees every 1000m. So at 1km you pitch to 88°, at 2km - 86°, and so forth. At 30km you'll have 30° pitch, hold it until apoapsis reaches 70km, then pitch down to 0. Burn until desired apoapsis is reached, and do circularization burn.

Works for most rockets I designed, gives a very smooth ascent.

[Mastikator]

The higher the TWR the more you turn. A normal 1.4 TWR should do a 10-20 degree turn when you get to 100m/s (depending on the TWR of later stages). You pay less gravity drag when you burn horizontally.

[Snark]

The amount of pitch really does depend on ship's TWR. It also depends on what your planned velocity profile is, for managing aerodynamic drag. The issue is that aero drag spikes upward sharply at Mach 1, which is a bit over 300 m/s at sea level on Kerbin.

I've seen a couple of schools of thought on this:

Option #1: Take it slow. Avoid going over Mach 1 until you're high enough in the atmosphere that aero drag isn't such an issue. With this approach, keep speed under 300 m/s until you're over 10 km, then punch it hard.

Option #2: Bat out of hell. If you have a high enough TWR, just blast right through Mach 1 and head out of the atmosphere as fast as you possibly can.

I tend to go with option #1 myself (primarily because to use #2, I'd have to have a wastefully high TWR, with too much mass dedicated to engines).

This affects ascent profile, because the "take it slow" approach means that your gravity turn will bend harder, so you wouldn't pitch over as much. Whereas the #2 approach would need to pitch harder sooner, because its pitch won't drop by as much in the gravity turn because it'll be going so fast.

[uncle_jew]

Thanks for the feedback. How big are the delta-V losses due to the atmosphere?

I keep the speed under 300m/s until 10km. But there is still some air friction above 10km. If I have the rocket tilted by 45 degrees when I hit the 10km mark, the rocket will be glowing red hot between 20km and 30km altitude - but how much delta V does it lose due to that? If I keep it tilted at 25-30 degrees between 10-20km, I get to orbit without any (visible) air friction effects.

I use engine gimbaling with the SAS to control the tilt - the SAS is in the prograde mode, with occasional manual inputs.

[Retread]

Once you're above 10km, friction losses are pretty minimal, despite the fireworks going on around your rocket.

With the right rocket design, you can actually turn SAS right off, and let the rocket fly itself to space on a gravity turn, naturally following the prograde marker. It takes some practice adjusting your TWR in flight, but I do most of my launches without SAS now.

[FancyMouse]

Not glowing red is a sign of not enough gravity turn. Drag loss is a lot less than it seems from visual effects.