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1.3 Rocket Ascent Profile and Gravity Turn


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1 hour ago, grosser_Salat said:

But I also got a question: There are of course different ISP-values for atmosphere and vacuum. In which altitude do I have to switch between them on Kerbin (and how about Earth-RSS) to calculate the real amount of DeltaV I have / need? It feel like we are reaching vacuumvalues on 10km allready on Kerbin.

And a follow-up question: Is this relevant enough for the ascendpath? Should I go straight up in the beginning, if my rocket has 1.5 times better ISP in vacuum?

You're right, 10km is about the level that you can consider ">95% vacuum", so a Terrier at 10km is worth more than a Reliant if you don't need the extra thrust.

As Warzouz has said, it's a matter of balance.

I'd simply say you should never go straight up except in two cases:

- you are certifiably mad and/or overcompensating, and have a ridiculous TWR, like more than 8, and want to burn in a straight line to Moho or Jool;

- your payload is so terribly unaerodynamic and light that you cannot make any kind of control input without flipping, and are prepared to pay for that payload with vastly excessive amounts of fuel and engine weight (since you'll need a higher TWR to get into orbit without falling back down again later).

Even in the second case, there is bound to be a better solution (such as hanging the payload lower between two large boosters, putting fuel tanks above the payload or whatever).

Edited by Plusck
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Thank you very much for your advice, Warzouz and Plusck, I will consider it in upcoming launches.

But I got another question: What if I do not want to go into orbit, but to leave Kerbin straight for Duna let's say. Isn't it better (or equal) to burn against gravity only, for you want to leave the system "asap" and you need to fight all that gravity anyway?

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On 16/4/2016 at 3:19 AM, grosser_Salat said:

But I got another question: What if I do not want to go into orbit, but to leave Kerbin straight for Duna let's say. Isn't it better (or equal) to burn against gravity only, for you want to leave the system "asap" and you need to fight all that gravity anyway?

Well, I don't think this makes much difference of circularizing or not. Either way you'll always want to gain speed by burning horizontal, not vertical. By going vertical, you'll expose yourself to more gravity loss.

By burning horizontal, you use gravity as a slingshot, instead of just throwing the rock.

I'm not that the gain from burning straight to Duna is worth the complexity and timing of the manoeuvre. If yout trajectory isn't perfectly right, you'll have a more expensive course correction. Don't forget that direction change burns (radial, normal) are more costly when your speed is high. The best is to plan that sooner.

I only do direct burn for the Mun because it's very easy to encounter. In the end, it doesn't change much on dV

 

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9 hours ago, Warzouz said:

Well, I don't think this makes much difference of circularizing or not. Either way you'll always want to gain speed by burning horizontal, not vertical. By going vertical, you'll expose yourself to more gravity loss.

By burning horizontal, you use gravity as a slingshot, instead of just throwing the rock.

[...]

 

This sounds very reasonable. So if you can restart your engine, it is adviced to make an orbit as low as possible and than decide, where to go. One revelation aroud Kerbin/Earth should not be a problem, even if you are dealing with life support constrictions.

I tried the previous advice. Turning SAS of and letting the rocket do the job is a great experience, :) thank you!

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Th

10 hours ago, grosser_Salat said:

...

I tried the previous advice. Turning SAS of and letting the rocket do the job is a great experience, :) thank you!

You're welcome.:wink:

SAS keep your rocket straight, but if you"r rocket is going sideways (such as gravity turn), the SAS will fight that and create cosinus loss (not burning prograde). Furhter more, you'll go away from the trajectory you should use.

To fix that, you have to turn the rocket yourself, and as Stock SAS overcompensate, it's not very pleasant. Further more, if your rocket is structurally weak, it's induce heavy wobble.

Usually I reactivate SAS to follow orbital prograde around 25km, or I activate regular SAS around 35km.

 

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I am trying to do this as a kOS script - so far steps 1 - 4 seem to work for the Kerbal X as my testing rocket.

But I have some questions about the last step:
If I got this right, I shall try to point activly towards the horizon or ajust the throttle, when getting into the upper athmosphere.
I assume, that would be okay, if I start this at about 30km.
Now I should try to ajust my ETA to Apo at about 45secs, as long as it takes me to get to my destinated Apo?
What way is to be prefered? Ajusting the pitch or the throttle? And for those of you, working with kOS: Would this be a case for a PID-Loop?

Edited by Horman
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My problem with turning off SAS is that the craft often likes to start to rotate or deviate sideways. Controlling a craft in this situation manually is very difficult. That´s where I learned to appreciate the advanced pilot/autopilot "prograde" setting. That´s why I always have the SAS module on board; it also works in conjunction with a simple remote control unit. I basically do it exactly like the OP, just that I click on "prograde" once my vessel has achieved 100m/s (which is roughly at about 6-8km altitude). If for some reason, the prograde marker drops too fast downwards, I can manually correct this by switching from "prograde" to "hold heading" for some moments. My aim is that the rocket ist almost horizontal at about 42km, but not earlier.

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4 hours ago, Falkenherz said:

My problem with turning off SAS is that the craft often likes to start to rotate or deviate sideways. Controlling a craft in this situation manually is very difficult. That´s where I learned to appreciate the advanced pilot/autopilot "prograde" setting. That´s why I always have the SAS module on board; it also works in conjunction with a simple remote control unit. I basically do it exactly like the OP, just that I click on "prograde" once my vessel has achieved 100m/s (which is roughly at about 6-8km altitude). If for some reason, the prograde marker drops too fast downwards, I can manually correct this by switching from "prograde" to "hold heading" for some moments. My aim is that the rocket ist almost horizontal at about 42km, but not earlier.

That's because your rocket is probably not balanced. You should check it with KER. there is 2 values which are interesting (I don't remember their name precisely, it should be

  • Torque offset (the torque power needed to keep the rocket straight - with SAS on)
  • Thrust angle offset (the engine angle to keep the rocket straight - with SAS on)

Those 2 values should be as near 0 as possible. If you can't, activate SAS on start, but i'll have to fight SAS to turn and it may increase wobble a lot, so you should try to deactivate it as soon as possible (around 100m/s).

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On 4/22/2016 at 2:56 AM, Falkenherz said:

My problem with turning off SAS is that the craft often likes to start to rotate or deviate sideways. Controlling a craft in this situation manually is very difficult. That´s where I learned to appreciate the advanced pilot/autopilot "prograde" setting. That´s why I always have the SAS module on board; it also works in conjunction with a simple remote control unit. I basically do it exactly like the OP, just that I click on "prograde" once my vessel has achieved 100m/s (which is roughly at about 6-8km altitude). If for some reason, the prograde marker drops too fast downwards, I can manually correct this by switching from "prograde" to "hold heading" for some moments. My aim is that the rocket ist almost horizontal at about 42km, but not earlier.

If your rocket doesn't naturally hold the prograde without SAS on, it is not aerodynamically stable enough. Of course, it's a trade-off between spending time tweaking your rocket and just using SAS to get into orbit already. With the method you mention, the main disadvantage is that with the help of the SAS the prograde will indeed tend to sink much faster, forcing you to do exactly what you describe (holding SAS on stability for a while). Usually you can get away with it, but any time you burn away from directly prograde (because the prograde will inevitably continue to sin while you keep burning straight) your are wasting fuel to drag and exposing your craft to more heat and the possibility of losing control. Whatever works best for you, in any case.

Oh and horizontal at 42km is pretty good, just have to mind the throttle to not overheat.

Edited by A_name
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Yes, it is a nice shortcut if you do not want to tweak a rocket up to perfection. Even if my vessel is basically stable, I found that manually controlling heading in conjunction with SAS on stability is just not a fine enough control, oftenly overshooting or leading to pendulum movements. Alternating between stability and prograde SAS just does the job for me.

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I've seen tutorials like this a lot, quite simply it very rarely works.  It's not worth the hassle of reverting a dozen times to get it to perfection over just flying with SAS.

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1 hour ago, Alshain said:

I've seen tutorials like this a lot, quite simply it very rarely works.  It's not worth the hassle of reverting a dozen times to get it to perfection over just flying with SAS.

It depends of what's your goal...

Judging from this post and previous posts you've made, it looks like you think everyone want and should have a playstyle similar to yours.

You just want to throw your stuff in space, and if it doesn't work, just add moar booster. It's your playstyle, and I'm ok with it.

But if people want to precisely dimension their rocket and get a perfect launch NASA style, what's the problem ? This tutorial is perfect for that.

When you say something like "it's not worth the hassle", I'd like to say "It's not worth the hassle for you". It might be worth for other people. Please quit disregarding other players playstyle.

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51 minutes ago, Tatonf said:

It depends of what's your goal...

Judging from this post and previous posts you've made, it looks like you think everyone want and should have a playstyle similar to yours.

You just want to throw your stuff in space, and if it doesn't work, just add moar booster. It's your playstyle, and I'm ok with it.

But if people want to precisely dimension their rocket and get a perfect launch NASA style, what's the problem ? This tutorial is perfect for that.

When you say something like "it's not worth the hassle", I'd like to say "It's not worth the hassle for you". It might be worth for other people. Please quit disregarding other players playstyle.

ROFL, your analyst skills fail you.  Pretty much everything you said there is wrong.  I'd be very curious what posts led you to these conclusions.

So lets start with the first one, I absolutely don't believe everyone's playstyle is similar to mine.  I'm merely expressing an opinion from my point of view, and also cautioning beginners that this may not be the ultimate solution.

For the second one, I'm certain you didn't get that from any of my posts.  It's not even my playstyle, I'm typically here suggesting people aim for more cost efficient rockets rather than "moar boosters".  Even then, it's just a suggestion.  If really you want to throw 100 LFO engines on a rocket, why would I care?

The 3rd one, well there is nothing wrong with that, again offering a warning to other that it is more difficult to accomplish that the first post makes it seem does not necessarily mean you shouldn't do it.  I've done it before too, but in the end we come to your last point...

Finally, yes, it is implied that it is not worth the hassle "for me".  How could I possibly be making that assessment for anyone else?

So my suggestion would be to step down off your all might high horse and stop telling people their opinions don't matter.

Edited by Alshain
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I've been playing a lot with ascent lately. I have absolutely no understanding of how to optimize the first 10km, where the atmosphere is thick. Sure you want to be pointing prograde, but how much should you tip over at the start and where should you be pointing at the end?

After 10km, I've had excellent success with low-thrust craft where I pitch at a consistent pitch. I solve an equation that says, essentially, that I need to be at a given apoapsis (42km seems to work well) at the same time that I reach orbit speed. Works quite well!

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  • 4 weeks later...
On 6/30/2015 at 7:10 PM, A_name said:

turn off the SAS

Thank you. It was a glorious feeling to see a rocket fly to (almost) orbit without my input. One question though - the rockets seem to verge pretty heavily from the eastwardly course even with stabilizers. Is there anything I can do to prevent that? Fixing that through manual steering feels contrary to this hands-off approach.

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37 minutes ago, TheBedla said:

Thank you. It was a glorious feeling to see a rocket fly to (almost) orbit without my input. One question though - the rockets seem to verge pretty heavily from the eastwardly course even with stabilizers. Is there anything I can do to prevent that? Fixing that through manual steering feels contrary to this hands-off approach.

If it turns eastward too fast (or too slow) SAS won't help you. There is 2 main reason why rocket turn too fast

  • You started you gravity turn too soon
  • Your TWR isn't high enough for your rocket drag.

Even with lower TWR, you can go to LKO, but you'll need to start turn when your speed is higher (so later)

On the opposite, if your rocket torn to slowly. Reduce you TWR (size down your engines) or turn sooner (you may burn in atmosphere though)

Finally, if you have a hard time initiate your gravity turn, you haven't enough control authority.

EDIT : I've a rocket which can go to LKO with 2850m/s of fuel (including orbital insertion) without touching any key (except cutting down engines to avoid blowing in atmosphere). No pitching, no yawing, no rolling (except for orbital insertion). It's a proof of concept which has very little use : the rocket has a TWR = 6 and has no payload

Edited by Warzouz
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Hope you don't mind, I've made a quick reference from your very nice accent instructions for quick reference and reminder the first few times going into orbit. Thanks for this, things have chanced a bit since I last went into orbit. 

This is of course only a supplement and anybody using this will want to read and understand the original post.

Ascent:

1. Turn ON SAS and set throttle to give TWR of ~1.5.

2. Launch!

3. At 50 m/s, perform a pitch over maneuver (Tip towards East until pointing between 5° to 10°).

4. When SAS stabilizes turn OFF (i.e. the control input arrows on the bottom left are all centered) .

5. At ~40 km, turn SAS ON, start steering manually and begin throttling up

6. Keep your Ap around 45 seconds in front of you.

7. Adjusting pitch and throttle until your Ap reaches desired altitude, then cut your engines, and coast to Ap and circularize.

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On 5/27/2016 at 8:18 AM, TheBedla said:

Thank you. It was a glorious feeling to see a rocket fly to (almost) orbit without my input. One question though - the rockets seem to verge pretty heavily from the eastwardly course even with stabilizers. Is there anything I can do to prevent that? Fixing that through manual steering feels contrary to this hands-off approach.

If I understand you correctly, your problem is not related to the gravity turn but to your rockets deviating from the 90° heading. If this is the case, you will indeed need more stabilizers, unless there is an inherent design "feature" in your rocket that is causing an offset between center of mass, center of lift/drag and/or center of thrust. My choice is usually the Tail Fin; plenty of lift and deflection angle. If the Tail Fin doesn't cut it, use the Big-S Spaceplane Tail Fin; it'll look goofy but will work. You want to put 4 on the base of the rocket, making sure they are aligned with the cardinal directions (i.e. not 45° to them). Alternatively, instead of turning off SAS when starting the gravity turn keep it on but on prograde hold; I do that with particularly unwieldy rockets.

On 5/27/2016 at 8:55 AM, Warzouz said:

If it turns eastward too fast (or too slow) SAS won't help you. There is 2 main reason why rocket turn too fast

  • You started you gravity turn too soon
  • Your TWR isn't high enough for your rocket drag.

Even with lower TWR, you can go to LKO, but you'll need to start turn when your speed is higher (so later)

On the opposite, if your rocket torn to slowly. Reduce you TWR (size down your engines) or turn sooner (you may burn in atmosphere though)

Finally, if you have a hard time initiate your gravity turn, you haven't enough control authority.

EDIT : I've a rocket which can go to LKO with 2850m/s of fuel (including orbital insertion) without touching any key (except cutting down engines to avoid blowing in atmosphere). No pitching, no yawing, no rolling (except for orbital insertion). It's a proof of concept which has very little use : the rocket has a TWR = 6 and has no payload

I'd like to add I do not advise on waiting to start the gravity turn. The main criteria for WHEN to start a gravity turn should be your speed, and 50 m/s is my rule of thumb. HOWEVER, if you have a lower TWR, you'll want to do a less aggressive gravity turn, that is only tip over a few degrees at the begining, 5 degrees tops. So TWR should affect HOW MUCH you tip over for the gravity turn, but not WHEN. This is in my experience of course, results may vary.

On 5/28/2016 at 10:25 PM, kBob said:

Hope you don't mind, I've made a quick reference from your very nice accent instructions for quick reference and reminder the first few times going into orbit. Thanks for this, things have chanced a bit since I last went into orbit. 

This is of course only a supplement and anybody using this will want to read and understand the original post.

Ascent:

1. Turn ON SAS and set throttle to give TWR of ~1.5.

2. Launch!

3. At 50 m/s, perform a pitch over maneuver (Tip towards East until pointing between 5° to 10°).

4. When SAS stabilizes turn OFF (i.e. the control input arrows on the bottom left are all centered) .

5. At ~40 km, turn SAS ON, start steering manually and begin throttling up

6. Keep your Ap around 45 seconds in front of you.

7. Adjusting pitch and throttle until your Ap reaches desired altitude, then cut your engines, and coast to Ap and circularize.

Great recap, I'll edit it into the OP if you don't mind :)

Edited by A_name
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9 hours ago, A_name said:

I'd like to add I do not advise on waiting to start the gravity turn. The main criteria for WHEN to start a gravity turn should be your speed, and 50 m/s is my rule of thumb. HOWEVER, if you have a lower TWR, you'll want to do a less aggressive gravity turn, that is only tip over a few degrees at the begining, 5 degrees tops. So TWR should affect HOW MUCH you tip over for the gravity turn, but not WHEN. This is in my experience of course, results may vary.

You're true, but that was what I meant, even though I wasn't precise enough. "Later" meant "when you speed is higher".

I use to start gravity turn at 60m/s for a TWR around 1.5.

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  • 2 months later...

Thanks very much! COMPLETELY fixed my rockets flipping. After knowing that the definition of gravity turn is a near-zero angle of attack, I never had a rocket flip again. However I had to manually point towards the prograde marker, as not using SAS will make it flip to the side and lose control (heading off from 90 degrees). Using only the prograde hold would make the rocket oscillate too much, so i developed a method of alternating between prograde and stabilization periodically. Maybe because I have huge unwieldy rockets. But it still works.

Edited by deepspacecreeper
grammar
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  • 5 months later...
  • 2 weeks later...

Good tutorial, got some insights, thanks! I did find aggressive low ascent more efficient back in versions 0.2x, with that aero model. Now it's a standard procedure, you only have to care of efficient aerodynamics. But I did have troubles controlling the rockets, when CoG gradually moved down towards the engine. This will probably eliminate the problem at all.

Edited by Kulebron
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